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EFects of Beta'Adrenergic Agonists on Growth and Carcass Characteristics of Animals LARRY A. MUIR Until recently, few mechanisms were known through which a drug could promote the growth performance or improve the carcass characteristics of livestock and poul- try (Muir, 1985~. Antimicrobial agents, such as antibiotics and antibacterials, improve growth performance of livestock and poultry by killing or inhibiting the growth of micro- organisms (Muir et al., 1977~. Estrogenic agents improve growth performance and carcass characteristics of cattle and sheep, but the specific mechanism is not well understood (Burroughs et al., 1954; Dinus- son et al., 1950; Muir et al., 1983~. Proges- tational agents improve the growth perform- ance of cyclic heifers by inhibiting estrus and therefore its adverse affects, such as hyperactivity and reduced feed consump- tion (Davis, 1969~. Androgenic agents im- prove growth performance and carcass char- acteristics of cattle and swine, especially females, supposedly through a direct, re- ceptor-mediated action on skeletal muscle cells (Heitzman, 19804. In addition, exoge- nous growth hormone administration re- portedly improves growth performance and carcass characteristics of livestock (Machlin, 1972; Wagner and Veenhuizen, 19784. 184 Now a new mechanism has been found through which the growth performance and carcass characteristics of all poultry and livestock species are dramatically improved (Baker et al., 1984; Beermann et al., 1986; Dalrymple et al., 1984; Moser et al., 1986; Muir et al., 1985; Ricks et al., 1984~. This mechanism involves the activation by beta- adrenergic agonists (beta-agonists) of spe- cific beta-adrenoceptors on the surface of adipocytes and skeletal muscle cells. This paper describes what is known about beta- agonists and the mechanisms through which they work. WHAT ARE BETA-AGONISTS? Beta-agonists are structural analogs of the catecholamines epinephrine and norepi- nephrine. Epinephrine and norepinephrine are very similar in structure, and both bind to four different cell surface receptors called adrenoceptors (specifically, the alpha l, alphas, betel, and betas receptors). Of special in- terest are the effects of beta-agonists on adipose and muscle tissues. The adipose tissue of most species contains beta-recep- tors that, when activated, stimulate lipoly
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BETA-ADRENERGIC AGONISTS sis. Most muscle tissue contains primarily betas or betas receptors, which, when ac- tivated, cause a specific muscular function. Skeletal muscle is known to have betas receptors, but their response function is not well understood. The structures of the beta-agonists that will be discussed in this paper isoproter- enol, clenbuterol, cimaterol, L-640,033, add BRL3513~are shown in Figure 1. Isopro- tereno} is a very potent beta/beta agonist that is not orally active but is very effective in vitro. Clenbutero} and cimaterol (Amer- ican Cyanamid) ant! L-640,033 (Merck) are orally active beta-agonists that have been shown to stimulate animal growth and change carcass characteristics (Dalrymple et al., 1984; Muir et al., 1985; Ricks et al., 19844. BRL35135 (Beecham) is an orally active HO HO~CH-CH2-NH-CH (CH3)2 C1 a>=\ OH / \ 1 H2N - \ /^ CH-CH2-NH-C (CH3)3 C1 OH H H9N~ C CH2-NH- IC-CH2-CH2-C6H5 C - N i4: OH H2N~CH-CH2-NH-CH (CH3)2 i=: OH C\H3 ~ ~ CH-CH2-NH-C-CH2~ OCH2-CO2-CH3 185 beta-agonist that has been shown to stim- ulate lipolysis (Arch et al., 1983, 1984~. EFFECTS OF BETA-AGONISTS ON GROWTH PERFORMANCE AND CARCASS CHARACTERISTICS Numerous growth trials have been con- ducted with different beta-agonists at vary- ing dose levels in poultry, swine, sheep, and, to a lesser extent, cattle (Baker et al., 1984; Beermann et al., 1986; Dalrymple et al., 1984; Moser et al., 1986; Muir et al., 1985; Ricks et al., 1984~. The results of these trials are summarized in Tables 1 and 2. In general, beta-agonists work best when used during the finisher period, regardless of species. Optimum responses are obtained when these drugs are administered right to Isoproterenol CIonbuterol L - 40,033 Cimaterol BRL35135 FIGURE 1 Structures of the beta-adrenergic agonists isoproterenol, clenbuterol, L-640,033, cimaterol, and BBL35135.
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186 TABLE 1 Profile of a Beta-Adrenergic Agonist Product for Livestock Growth Promotion Growth Performance APPENDIX Characteristic Poultry RuminantSwine Dietary use level (ppm) 0.~2 l_l0a0.2 - Growth rate (% increase) 4 ~20b0~ Feed conversion (% improvement) 5 0-2050 6 a Sheep and cattle data. bSheep data only; cattle data not available. SOURCE: Based on studies by Muir et al. I_,, . _ . ~,, _ Moser et al. (1986), and Dalrymple et al. (1984) using different beta-agonists. the time of marketing. How close to mar- keting time that beta-agonists will actually be used will Repent] on the withdrawal time for each cirug; actual withdrawal times have not yet been established. In poultry, the dietary use levels for beta- agonists range from 0.2 to 2 ppm in the feed. When given during the final 2 to 4 weeks of the 7-week period before slaugh- ter, improvements in growth rate and feed conversion of 4 ant] 5 percent, respectively, are usually obtainer] with broilers. Also, total carcass protein is increased approxi- mately 6 percent, while total carcass fat is reduced. Abdominal fat is reduced, but the reduction is less than expected. In addition, the eject of beta-agonists on abclominal fat appears to differ between sexes, with males showing little or no reduction and females a reduction of 5 to 20 percent. As a result of these changes, carcass yield of broilers is usually increased by approximately 1 per cent. (1985). Ricks et al. (1984). Baker et al. (1984), Beermann et al. (1986), In ruminants, the dietary use levels for beta-agonists range from 1 to 10 ppm in the feed. In sheep, feeding of 1 to 2 ppm for the last 3 to 6 weeks of the finishing period appears to be most elective. In most sheep growth trials, responses in growth rate and feed conversion of 20 percent are obtained, although occasionally no response is ob- servecI. In terms of carcass composition, sheep respond] with a 10 percent increase in total carcass protein and a 15 to 30 percent increase in the loineye area. Total carcass fat is reduced 20 to 30 percent, with even larger decreases in back fat and abdominal fat. Data on the effects of beta-agonists in cattle are extremely limiter! but do show changes in carcass composition that are similar to those observer] for sheep. Swine appear to be more sensitive to beta-agonists than other species, with 0.2 to 4 ppm in the feed appearing to yield optimum results. Unlike other species, swine TABLE 2 Profile of a Beta-Adrenergic Agonist Product for Livestock Growth Promotion Carcass Characteristics Characteristic Poultry Ruminanta Swine Carcass protein (% increase) 6 10 4-8 Loineye area (% increase) 15-20 9-15 Carcass fat (% decrease) 4-8 20~0 10-16 Back fat (% decrease) 2~50 10-17 Abdominal fat (% decrease) 2-8 20~5 aSheep and cattle data. SOURCE: Based on studies by Muir et al. (1985), Ricks et al. (1984), Baker et al. (1984), Beermann et al. (1986), Moser et al. (1986), and Dalrymple et al. (1984) using different beta-agonists.
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BETA-ADRENERG1C AGONISTS have failed in most reported trials to respond with improved growth rate or feed conver- sion. The studies in which improved growth performance was observed have used short- duration treatment (4 weeks or less). Swine do show very consistent improvement in carcass characteristics when medicated with beta-agonists. Total carcass protein is in- creased 4 to 8 percent, and loineye area muscle protein is increaser] 9 to 15 percent. Total carcass fat and back fat are reduced 10 to 17 percent. In addition to food animal species, beta- agonists are also very effective in laboratory animals. For example, clenbuterol has been shown to improve the growth performance and shift the carcass composition of young, rapidly growing male rats (Table 3; Rickes et al., 1985). Apparently, a dose of 10 ppm in the feed produces the maximum re- sponse: a 9 percent improvement in weight gain, a 10 percent improvement in feed conversion, a 9 percent increase in total carcass protein, and a 20 percent reduction in total carcass fat. These responses to beta- agonists in the rat are very similar to those observer! in food-producing animals. Thus, the rat appears to be an excellent model for studying beta-agonists as growth promoters. Beta-agonists have been examined for TABLE 3 Effects of a Beta-Adrenergic Agonist, Clenbuterol, on Growth Performance and Carcass Composition of the Rat (percent change over control) Clenbuterol, ppm in diet Characteristic 21050 Weight gain (g/day) Feed intake (g/day) Feed conversion (g feed/g gain) Carcass protein (g/carcass) 5.1*8.5**9.2** Carcass fat (g/carcass) - 8.5*- 19.9**- 23.3** 4.8* 9.6* 0.4 -3.4 8.5* 1.7 -3.6 - 10.7* - 7.2* *P < 0.05 compared with control. **P < 0.01 compared with control. SOURCE: Ricks et al. (1985). 187 their effects on milk production by dairy cows. Cows producing 17 to 18 kg of milk per day were medicated with the beta- agonists formoterol, zinterol, or Z1170. The beta-agonists were fed at 20 mg per head per day for 10 clays. Milk production on days 5 to 10 of treatment was not different from that of controls or from milk production before or 5 days after treatment. In addition, the composition of the milk was not altered. These data suggest that beta-agonists, unlike growth hormone, apparently are not able to stimulate milk production, even though both beta-agonists and growth hormone ap- pear to function through a repartitioning of nutrients. EFFECTS OF BETA-AGONISTS ON LIPID METABOLISM Free fatty acid (FFA) synthesis is the conversion of glucose, acetate, or both to free fatty acids. Lipogenesis is the sum of FFA synthesis and the esterification of FFAs to triglycerides (TGs). Lipolysis is the break- clown of TGs to FFAs and glycerol. The rate of glycerol production can be used to estimate lipolysis because the glycerol pro- ducecl during lipolysis cannot be reused for FFA esterification since adipocytes lack the necessary enzyme for phosphorylation of glycerol (phosphokinase). A scheme for the regulation of lipolysis by beta-agonists through specific adrenoceptors is shown in Figure 2. The activation of the beta-receptor on the outer surface of the aclipocyte plasma membrane activates the chain of events that eventually leads to the breakdown of stored triglycerides to FFAs and glycerol. Many beta-agonists effectively reduce lipic! accumulation in adipose tissue. The mech- anisms through which they act were studied at Merck in an in vitro system (Duquette and Muir, 1984~. Adipose tissue was taken from an animal source, for example, rat epicliclymal or perirenal fat, using the pro- ceclure of Rodbell (1964~. Aclipocytes were incubated with treatment for 2 hours at
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188 APPENDIX Adipose Plasma Membrane Adrenergic Recept pAC~t~de; a Adrenergic Receptor | Inhibiting Peps - Guanino Nucleo~dde ,4N Phosphodl~terase c IMP ' 5'AMP l ATP Guanine Nucleotide Protein Klnase Active Hormone Inactive Hormone Sensitive Upase ~ Sensitive Upa" TrIglycerld" ~ · - Free Fatty Acids Glycerol FIGURE 2 Scheme for the regulation of lipolysis by beta-agonists through specific adrenoceptors. Source: Adapted from I. A. Garcia-Sarnz and J. N. Fain. 1982. Regulation of adipose tissue metabolism by catechol- amines: Roles of alphas, alphas, and beta-adrenoceptors. Trends Phar- macol. Sci. 3:201. 37°C. Lipolysis was estimated by measuring glycerol production. Glycerol was measured by a fluorometric mollification of the enzy- matic method of Wieland (1974). Lipoge- nesis was estimated by measuring the in- corporation of i4C-acetate into fatty acids from TGs. Isoprotereno} is very effective for stimu- lating lipolysis (glycerol release) in rat a(li- pose tissue in vitro. At a concentration of 0.001 ,uM, isoprotereno} had no effect on the basal rate of glycerol release; maximum stimulation occurred at 1 ,uM. The half- maximal effect dose was 0.017 ,uM. The idea that beta-agonists stimulate lipolysis through a specific beta-receptor is sup- portec! by the observation that the stimu- lation of lipolysis by isoproterenol can be blocked by beta-antagonists, such as pro- pranolol or betaxolol. Isoprotereno} was also studied for its effects on lipogenesis (~4C-acetate incorpo- ration into FFAs in TGs) in rat adipose tissue in vitro (Duquette and Muir, 1985~. Insulin was used in this study to increase lipogenesis and to simulate in viva condi- tions. Isoprotereno! at concentrations of 0. 01, 0. OS, and 0.25 EM reduced i4C-acetate incorporation into TGs in a close-related manner. In addition, when the TGs were hydrolyzes! and the resulting FFAs and glycerol were tested for i4C activity, the results showed that the effect of isoproter- enol was primarily to reduce the incorpo- ration of i4C-acetate into FFAs with only minor effects on glycerol. These observa- tions support the validity of this test system for estimating lipogenesis. These in vitro test systems for lipolysis and lipogenesis were used to compare the activities of four different beta-agonists. Iso- proterenol, clenbuterol, L-640,033, and BRL35135 were dose-titratec! in rat adipose tissue to study their effects on lipolysis and lipogenesis (Duquette ant] Muir, 1985). All four inhibited lipogenesis and stimulated
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BETA-ADRENERGIC AGONISTS lipolysis. The intrinsic activity of all beta- agonists was similar for both actions. This means that the maximum effects of the beta- agonists, inclepenclent of dose, were of sim- ilar magnitude for each eject. Three of the four beta-agonists were 5 to 10 times more potent as inhibitors of lipogenesis than as stimulators of lipolysis; BRL35135 was equally potent for both. A comparison of the poten- cies of these drugs for inhibition of lipoge- nesis, as measured by 50 percent effective concentration, were isoproterenol > BRL35135 > L-640,033 > clenbuterol. Sim- ilar comparisons for stimulation of linolvsis animal were BRL35135 > isoproterenol > L-640,033 > clenbuterol. These observations suggest that in the animal a drug's efficacy for reducing belly lipids may be even more depenclent on that drug's activity for block- ing lipogenesis than for stimulating lipolysis. They also indicate that there is considerable variation between beta-agonists in their po- tencies for blocking lipogenesis and stimu- lating lipolysis. The ability of aclipocytes from different species to initiate lipolysis in re- sponse to similar concentrations of isopro- terenol was investigated by Muir et al. (1985~. Adipocytes from sheep, pigs, and rats all responder! with increases of 380 to 2,300 percent over their controls. Aclipo- cytes from chickens failer! to respond. Because chicken adipocytes failed to re- spond to isoproterenol, a more cletailec] study was conductecI. The beta-agonists iso- proterenol, clenbuterol, and L-640,033 and the positive control glucagon were all tested at concentrations of 0. 01, 0. 05, 0. 25, 1. 25, and 6.25 EM for effects on lipolysis in isolated chicken adipocytes (Muir et al., 1985~. As expected, the chicken adipocytes responded to glucagon with increased li- polysis. However, none of the three beta- agonists stimulated lipolysis at any of the concentrations tested. Since the chicken, unlike the other spe- cies studied, synthesizes FFAs in the liver, the ejects of beta-agonists on lipogenesis 189 in isolated chicken hepatocytes were inves- tigated. Isoproterenol, clenbuterol, and L- 640,033 and the positive control glucagon were tested on chicken hepatocytes in a design identical to the one describer! for chicken adipocytes (Muir et al., 1985~. As expected, glucagon caused a close-related inhibition in i4C-acetate incorporation into FFAs in TGs (that is, inhibition of lipoge- nesis). All three beta-agonists also caused a dose-relatec] inhibition of lipogenesis. Thus, beta-agonists block body fat accumulation in chickens by inhibiting lipogenesis in the liver, but they are not able to stimulate lipolysis in adipose tissue. In summary, beta-agonists have been shown to stimulate lipolysis and inhibit lipogenesis in the adipose tissue of the rat (Muir et al., 1985). In adipose tissue from sheep and swine, beta-agonists can stimu- late lipolysis, but no data are available regarding their effects on lipogenesis in these species. Beta-agonists are ineffective in chicken adipocytes. However, they in- hibit lipogenesis in chicken hepatocytes, the primary site for fatty acid synthesis in poultry. EFFECTS OF BETA-AGONISTS ON SKELETAL MUSCLE PROTEIN METABOLISM Skeletal muscle celDs have beta-receptors, ant! beta-agonists increase skeletal muscle pro- tein in animals. Ibus, beta-agonists might exert a direct effect on skeletal muscle cells, or their effect could be indirect through changes in plasma hormone concentrations or nutrient partitioning. Herbert et al. (1986) measurer] the effects of clenbutero! on urinary nitrogen excretion by sheep by infusing clenbutero! together with fee(lstuffs directly into the abom- asum. Within 6 hours after the initiation of infusion, nitrogen excretion dropped about 25 percent anc] remained clepresse ;1 over the entire 7-(lay test. These results suggest that clenbu- terol causer! an immediate improvement in nitrogen retention.
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190 Muscle protein accumulation is the net balance of protein synthesis minus protein degradation. A drug like clenbutero} that dramatically increases muscle protein ac- cumulation might be expected to act by altering the rate of muscle protein synthesis, degradation, or both. This was examined by Reeds et al. (1986), who fed clenbuterol at O or 2 ppm in the diet to young male rats. The rates of protein synthesis in two skeletal muscles the gastrocnemius and soleus- were estimates] by the method of Garlick et al. (1980~. Rats were injected! with a large dose of labeled phenylalanine ant] killed 10 minutes later. The rate of protein synthesis was estimated from the amount of labeled phenylalanine in the muscle protein and from the specific activity of the free phen- ylalanine. Protein deposition was calculated from the slope of the line of the log of the protein content versus time. Protein content was estimates! from bocly weight. The rate of protein degradation was estimated from the differences between rates of synthesis and deposition. The effects of clenbuterol on muscle protein mass and rates of protein synthesis and degradation were determiner! on days 4, 11, ant! 21 of treatment. After only 4 days of clenbuterol treat- ment, the protein masses of the gastrocne- mius and soleus muscles were increased 17.7 and 50.6 percent, respectively, over the controls. These larger muscle protein masses were maintained throughout the 21- ciay test period. In both the gastrocnemius and soleus muscles, the rate of protein degradation was decreased on day 4 by 55 percent, with no change in the rate of protein synthesis. By clay 11, this decrease in the rate of protein degraciation relative to the controls was still evident, but the magnitude of the decrease was slightly less (39 en c] 25 percent for the gastrocnemius and soleus muscles, respectively). The rates of protein synthesis were still the same as for the controls. At clay 21, the rates of protein degradation were still reduced (20 related manner (Rickes et al., 1985~. Feed and 30 percent, respectively), but the rates consumption was increased only at the two APPENDIX of protein synthesis had decreased 20 per- cent relative to the controls. These observations suggest that clenbu- tero} increases skeletal muscle protein in the rat by reducing the rate of muscle protein degradation. Apparently, after mus- cle protein mass is increased by a certain amount, the rate of protein synthesis is reduced. At this point, the rate of muscle protein accumulation is reduced to normal, but the extra muscle protein mass is main- tained. SEPARATION OF LIPID AND PROTEIN EFFECTS OF BETA-AGONISTS The information presented thus far clem- onstrates that beta-agonists reduce the con- tent of lipids in the carcass, increase the accumulation of skeletal muscle protein, and improve the growth rate and feed conver- sion of the animal. To understand the mode of action of beta-agonists, studies were un- dertaken to determine whether the growth and feet] conversion responses were asso- ciated with the ejects of the beta-agonists on lipid metabolism, protein metabolism, or both. L-640,033 is an excellent growth-pro- moting beta-agonist. BRL35135 is a potent inhibitor of lipogenesis and a potent stim- ulator of lipolysis, but it does not appear to affect muscle protein metabolism. Both beta- agonists were evaluated in similar rat growth trials. In each trial, 110 young mate rats (10 per treatment and 20 controls) were feel the beta-agonist at 0, 0.25, 1.0, 2.5, 5.0, 10, 15, 25, or 50 ppm for 2 weeks. Clenbuterol at 10 ppm was used as a positive control. Rate of gain, feed intake, and feed conver- sion were determined. In addition, the weights of the gastrocnemius muscle and epididymal fat pads were measured to assess the ejects of the drugs on skeletal muscle protein and carcass fat. L-640,033 increased rate of gain in a dose
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BETA-ADRENERGIC AGONISTS highest doses (25 and 50 ppm). Feed con- version was improved 4 to 6 percent, in- dependent of dose. Also, L-640,033 in- creased gastrocnemius muscle weight and decreased epididymal fat pad weight, both in dose-related manners (Rickes et al., 1985~. Both responses were similar to those ob- served with Clenbuterol at 10 ppm. BRL35135 had no effect on either rate of gain or feed conversion at any of the doses tested. Clenbuterol, the positive control, increased rate of gain and improved feed conversion. As expected, BRL35135 re- duced epididymal fat pad weight in a dose- related manner, with the maximum effect at 5 ppm. The reduction in epididymal fat pad weight with BRL35135 was significantly greater than that with clenbuterol. BRL35135 did not increase gastrocnemius muscle weight, while Clenbuterol increased it 17 percent. Thus, BRL35135 reduced carcass lipids in the rat without increasing skeletal muscle protein or improving rate of gain or feed conversion. These observations suggest that improve- ments in growth rate and feed conversion obtained with beta-agonists are associated with the effects of the drugs on skeletal muscle protein metabolism and not with their effects on lipid metabolism. Thus, while the ability of a beta-agonist to reduce carcass fat is an important benefit, this activity does not appear to be related to any growth-promoting activity. EFFECTS OF BETA-AGONISTS ON PLASMA HORMONES The effect of beta-agonists on plasma hormone levels is an important considera- tion when assessing the possible modes of action of these drugs in promoting growth. Thus, a study was carried out in which 50 young male rats (10 per treatment and 20 controls) were fed Clenbuterol at 10 ppm, BRL35135 at 15 ppm, or L-640,033 at 15 ppm for 2 weeks (L. A. Muir, unpublished data). At necropsy, gastrocnemius muscle 191 and epididymal fat pad weights were meas- ured to assess the effects of the beta-agonists on skeletal muscle protein and carcass lipids. In addition, blood samples were obtained and assayed for plasma growth hormone, insulin, somatomedin-C (SM-C), and glu- cose. All three beta-agonists produced the ex- pected changes in gastrocnemius muscle and epididymal fat pad weights. Clenbuterol and L-640,033 increased gastrocnemius muscle weight and decreased epididymal fat pad weights. BRL35135 decreased epi- didymal fat pad weight, but did not alter gastrocnemius muscle weight. Plasma in- sulin levels were decreased approximately 30 percent by Clenbuterol and L-640,033, but were not decreased by BRL35135. Sim- ilar effects were observed for SM-C. Plasma growth hormone was decreased by all three drugs, especially BRL35135, but the re- sponses were so variable that none of these growth hormone reductions was statistically significant. Clenbuterol decreased plasma glucose, while the other beta-agonists had no effect on glucose. Beermann et al. (1985) reported that cimaterol fed to lambs for 12 weeks reduced plasma insulin and elevated plasma T4 (thy- roid hormone thyroxine) but did not alter plasma levels of T3 (thyroid hormone tri- iodothyronine), cortisol, or prolactin. SUMMARY Beta-adrenergic agonists are analogs of the catecholamines epinephrine and nor- epinephrine. They appear to work through specific beta-adrenoceptors on the surface of adipocytes and skeletal muscle cells. Beta- adrenergic agonists that are known to pro- mote growth, such as clenbuterol, cima- terol, and L-640,033, improve the growth rate and feed conversion of sheep and poul- try. Effects on swine are more variable, while definitive data on cattle are not yet available. These drugs have also been shown to decrease total carcass fat and to increase
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192 total carcass protein in all four animal spe- cies. Many beta-adrenergic agonists reduce carcass lipids by stimulating lipolysis and blocking lipogenesis in adipose tissue. The exception occurs in poultry, where these drugs inhibit lipogenesis in the liver but do not stimulate lipolysis in adipose tissue. Less is known about the effects of beta- adrenergic agonists on protein metabolism in skeletal muscle. However, recent studies suggest that some of these drugs increase skeletal muscle protein accretion by reduc- ing the rate of protein degradation without altering the rate of protein synthesis. Stud- ies in rats comparing the growth-promoting and carcass-altering effects of two beta- acirenergic agonists, L-640,033 and BRL35135, indicate that improvements in growth rate and feed conversion with beta- adrenergic agonists are associates! with im- proved protein accretion rather than altered lipid metabolism. Finally, growth-promot- ing beta-adrenergic agonists were found to reduce plasma levels of insulin and soma- tomedin-C in the rat but did not elevate plasma growth hormone levels. These ob- servations support the concept that growth- promoting beta-adrenergic agonists work directly through skeletal muscle cell recep- tors and not indirectly through the elevation of plasma growth hormone or insulin con- centrations. In addition, beta-adrenergic ag- onists that reduce carcass lipids appear to work directly through beta-adrenoceptors on the surface of aclipocytes in livestock and hepatocytes in poultry. ACKNOWLEDGMENT The author wishes to acknowledge the work of Paul Duquette, Eric Rickes, and Sandra Wien, who contributed to numerous aspects of Me beta-agonist research at Merck, and Dr. Y. T. Yang, whose ideas and re- search finclings supported our beta-agonists research program. APPENDIX REFERENCES Arch, J. R. S., M. A. D. Phil, and A. T. Ainsworth. 1983. Thermogenic and antiobesity activity of a novel beta-adrenoceptor agonist (BRL26830A) in mice and rats. Am. J. Clin. Nutr. 38:549. Arch, J. R. S., A. T. Ainsworth, M. A. Cawthorne, V. Piercy, M. V. Sennitt, V. E. Thody, C. Wilson, and S. Wilson. 1984. Atypical beta-adrenoceptor on brown adipocytes as target for anti-obesity drugs. Nature 309:163. Baker, P. K., R. H. Dalrymple, D. L. Ingle, and C. A. Ricks. 1984. Use of a beta-adrenergic agonist to alter muscle and fat deposition in lambs. J. Anim. Sci. 59:1256. Beermann, D. H., W. R. Butler, D. E. Hogue, R. H. Dalrymple, and C. A. Ricks. 1985. Plasma metabolic hormone, glucose, and free fatty acid concentrations in lambs fed the repartitioning agent, cimaterol (CL 263,780~. J. Anim. Sci. 61(Suppl. 1~:254 (Abstr.). Beermann, D. H., D. E. Hogue, V. K. Fishell, R. H. Dalrymple, and C. A. Ricks. 1986. Effects of cima- terol and fishmeal on performance, carcass charac- teristics and skeletal muscle growth in lambs. J. Anim. Sci. 62:370. Burroughs, W., C. C. Culbertson, J. Kastelic, E. Cheng, and W. H. Hale. 1954. The effects of trace amounts of diethylstilbesterol in rations of fattening steers. Science 120:66. Dalrymple, R. H., P. K. Baker, P. E. Gingher, D. L. Ingle, J. M. Pensack, and C. A. Ricks. 1984. A repartitioning agent to improve performance and carcass composition of broilers. Poultry Sci. 63:2376. Davis, L. W. 1969. MGA- A new concept in heifer feeding. Pp. 72-82 in Proceedings of the 24th Kansas Formula Feed Conference, Manhattan: Kansas State University of Agricultural and Applied Sciences. Dinusson, W. E., F. N. Andrews, and W. M. Beeson. 1950. The effects of stilbesterol, testosterone, thyroid alteration and spaying on the growth and fattening of beef heifers. J. Anim. Sci. 9:321. Duquette, P. F., and L. A. Muir. 1984. Effects of ovine growth hormone and other anterior pituitary hormones on lipolyis of rat and ovine adipose tissue in vitro. J. Anim. Sci. 58:1191. Duquette, P. F., and L. A. Muir. 1985. Effect of the beta-adrenergic agonists isoproterenol, clenbuterol, L-640,033 and BRL35135 on lipolysis and lipogenesis in rat adipose tissue in vitro. J. Anim. Sci. 61(Suppl. 1~:265 (Abstr.). Garcia-Sarnz, J. A., and J. N. Fain. 1982. Regulation of adipose tissue metabolism by catecholamines: Roles of alphas, alphas and beta-adrenoceptors. Trends Pharmacol. Sci. 3:201. Garlick, P. S., M. A. McNurlan, and V. R. Preedy. 1980. A rapid and convenient technique for meas
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BETA-ADRENERGIC AGONISTS uring the rate of protein synthesis in tissues by injection of"3H" phenylalanine. Biochem. J. 192:719. Heitzman, R. J. 1980. Manipulation of protein metab- olism, with special reference to anabolic agent. Pp. 193-203 in Protein Deposition in Animals, P. J. Buttery and D. B. Lindsay, eds. Boston: Butter- worth. Herbert, F., F. D. DeB. Hovell, and P. J. Reeds. 1986. Some preliminary observations on the im- mediate effects of clenbuterol on heart rate, body temperature and nitrogen retention in lambs wholly nourished by intragastric infusion. Br. J. Nutr. 56:483 (Abstain. Machlin, L. J. 1972. Effect of porcine growth hormone on growth and carcass composition of the pig. J. Anim. Sci. 35:794. Moser, R. L., R. H. Dalrymple, S. G. Cornelius, J. E. Pettigrew, and C. E. Allen. 1986. Effect of cimaterol (CL 263,780) as a repartitioning agent in the diet for finishing pigs. J. Anim. Sci. 62:21. Muir, L. A. 1985. Mode of action of exogenous sub- stances on animal growth an overview. J. Anim. Sci. 61(Suppl. 2):154. Muir, L. A., M. W. Stutz, and G. E. Smith. 1977. Feed additives. Pp. 27-37 in Livestock Feeds and Feeding, D. C. Church, ed. Corvallis, Oreg.: ORB Books. Muir, L. A., S. Wien, P. F. Duquette, E. L. Rickes, and E. H. Cordes. 1983. Effects of exogenous growth hormone and diethylstilbesterol on growth and car 193 cass composition of growing lambs. J. Anim. Sci. 56:1315. Muir, L. A., S. Wien, P. F. Duquette, and G. Olson. 1985. Effect of the beta-adrenergic agonist L-640,033 on lipid metabolism, growth and carcass character- istics of female broiler chickens. J. Anim. Sci. 61(Suppl. 1~:263 (Abstr. ). Reeds, P. J., S. M. Hay, P. M. Dorwood, and R. M. Palmer. 1986. Stimulation of muscle growth by clenbuterol: Lack of effect on muscle protein bio- synthesis. Br. J. Nutr. 56:249. Rickes, E. L., L. A. Muir, and P. F. Duquette. 1985. Effect of the beta-adrenergic agonist L-640,033 on growth and carcass composition of growing male rats. J. Anim. Sci. 61(Suppl. 1):264 (Abstr. ). Ricks, C. A., R. H. Dalrymple, P. K. Baker, and D. L. Ingle. 1984. Use of beta-agonist to alter fat and muscle deposition in steers. J. Anim. Sci. 59:1247. Rodbell, M. 1964. Metabolism of isolated fat cells. I. Effects of hormone on glucose metabolism and lipolysis. J. Biol. Chem. 239:375. Wagner, J. F., and E. L. Veenhuizen. 1978. Growth performance, carcass deposition and plasma hor- mone levels in wether lambs when treated with growth hormone and thyroprotein. J. Anim. Sci. 47(Suppl. 1):397. Wieland, O. 1974. Glycerol UV-method. Pp. 140( 1409 in Methods of Enzymatic Analysis, H. U. Bergmeyer, ed. New York: Academic Press.
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Anabolic EFects of Porcine Somatotropin on Pig Growth TERRY D. ETHERTON Animal agriculture must develop ways to enhance the growth performance of animals raised for meat production in order to pro- vide consumers with a product that is leaner and, therefore, more nutritious. Because leaner meat products will be sought by consumers concerned about the relation between the consumption of saturated fatty acids and the incidence of coronary heart disease, strategies to increase growth rate and improve feed efficiency (ratio of feed consumed to bocly weight gained) will eco- nomically benefit producers. The central question is, what research options are avail- able now and in the foreseeable future that may provide effective ways to manipulate meat animal growth performance? This paper focuses on the concept that an elevation of blood concentrations of growth hormone (GH, or somatotropin) in meat animals markedly increases growth rate, improves feed efficiency, and dramatically increases muscle mass while decreasing adi- pose tissue (fat) mass (Chung et al., 1985; Etherton et al., 1986a, 1986b, 1987; Mach- lin, 1972~. Table 1 shows the extent to which growth hormone can affect the growth per- formance of pigs. The stimulatory effects of GH on growth performance have created great interest in developing a GH-based product for practical use in animal agricul- ture. In fact, it is likely that such a product will be available for use within the next 2 to 3 years. The mechanisms by which GH works are discussed in the following section, since a better understancling of them may leac! to ways to improve the effectiveness of GH or of alternative strategies for enhancing growth performance. Growth hormone is a protein that is synthesized in the anterior pituitary gland of mammals. It plays a central role in stimulating normal growth and is both an- abolic ant! catabolic in that it stimulates growth rate ant! muscle accretion ant! con- currently decreases adipose tissue growth (Etherton et al., 1986b, 1987~. The positive effects of GH on growth rate are indirect, being mecliated largely by the GH-clepend- ent insulin-like growth factor I (IGF-I, or somatomedin-C) (Etherton ant! Kensinger, 1984~. The effects of GH on adipose tissue growth and metabolism are direct, not being mediated by IGF-I (Walton and Etherton, 1986; Walton et al., 1986, 1987a). Observations by Etherton and coworkers 194
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COMPOSITION OF MILK FROM DAIRY COWS Biosynthesis The synthesis of milk proteins has been extensively reviewer! (Larson, 1979, 1985; Mercier ancl Gaye, 1983~. In general, pro- tein synthesis in mammary alveolar cells is similar to other protein synthesis systems in which DNA controls protein synthesis. Messenger RNA carries the encoded DNA message from the nucleus to the ribosomes located in the rough encloplasmic reticulum (RER) and cytoplasm. Ribosomes are com- posec3 of ribosomal RNA and several pro- teins combiner! into a ribonucleoprotein complex, which, in conjunction with trans- fer RNA, combines amino acids into peptide chains. As the polypeptide chains are elon- gatecl to form proteins, they pass out of the HER, through the lumen, and into the region of the Golgi apparatus where they accumulate and polymerize into different milk protein molecules. Casein must be phosphorylated, bound with calcium, and stabilized by calcium phosphate linkages and other ionic bonds before being released from the vesicles. The presence of alpha- lactalbumin in the region of the Golgi ap- paratus promotes synthesis of lactose. The secretory vesicles containing essentially nonfat milk constituents leave the cell by moving to the apical surface and fusing with the plasma membrane and discharging the vesicular contents into the cell lumen. Most of the proteins present in milk are synthesized in the mammary gland, al- though some immunoglobulins and albu- mins are transferred from the blood (Larson, 19791. Blood leukocytes can also cross mam- mary barriers either by passing between secretory cells or by pushing secretory cells directly into the lumen. Urea diffuses freely across mammary cells, so there is a high correlation between blood plasma and milk urea concentrations (Thomas, 1980~. The synthesis of milk protein requires that both essential and nonessential amino acids be supplied to the mammary gland (Clark et al., 1978; Mepham, 1982~. Uptake 231 of free amino acids from the blood by the mammary gland can occur via several trans- port systems (Baumrucker, 1985~. Mepham (1982) has classifier] essential and nonessen- tial amino acids into three groups according to uptake by the mammary gland. Group I essential amino acids (methionine, histi- dine, phenylalanine, tyrosine, and trypto- phan) are taken up in amounts just sufficient to meet milk protein synthesis needs. Group II essential amino acids (valine, leucine, isoleucine, arginine, lysine, and threonine) are taken up in excess. However, some data (Thomas, 1983) suggest that lysine and pos- sibly leucine, isoleucine, and threonine should also be included in group I. Group III is the nonessential amino acicls. The amounts taken up vary with animal, time, and availability. In addition to free amino acid uptake from blood, there is evidence that rec] blood cells and the recycling of amino acids also contribute to the cellular amino acid pool (Baumrucker, 1985~.13reak- down of red blood cell glutathionine can make a significant contribution to the amount of cysteine, glycine, and glutamic acic! avail- able in the cell. Recycling of casein proteins is reporter! to account for at least 7 percent of the protein synthetic capacity in the mammary gland. Factors Affecting Milk Protein Content Bree~lGenetics Breeds differ in total milk protein per- centage and type of milk protein produced. Jersey and Guernsey cattle have the highest percentages of total protein, casein, and whey. Variability of the major protein frac- tions within breeds has also been reporter] (Roller) et al., 1956), with Holstein milk containing less of the major caseins and more gamma-casein than milk from other breecis. Genetic variants have been dem- onstrated for the milk protein groups, and breed differences have been found for the frequency of occurrence of these variants (Gaunt, 19801.
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232 Genetic selection would increase the per- centage of protein in milk 0.075 percentage units but decrease milk yield 231 pounds. loins selection for milk yield, protein, and fat is recommencled if the desired result is increased yield of protein and fat (Gaunt, 1980; Van VIeck, 1978; Wilcox, 1978~. Gaunt (1980) estimates] that it would take about 11 generations for milk protein percentages to equal milk fat percentages if protein yield with no change in fat percentage were used as the selection criterion. EnvironmentlManagement ness, 1985; Ng-Kwai-Hang et al., 1982; Rogers ant] Stewart, 19821. Milk protein percentage declines in cows older than 3 years, with a 0.4 percentage unit drop being reported over five lactations (Rogers and Stewart, 1982~. This decline appears to be primarily in the casein fraction; however, changes in whey protein fractions have also been reported (Kroeker et al., 1985~. Sug- gestec! reasons for the change are deterio- ration of unpiler tissue, selective culling for high production, and increased incidences of mastitis. The increase in immunoglobu- lins with advancing age reported by Kroeker et al. (1985) supports the latter suggestion. Stage of lactation has a considerable in- fluence on milk protein concentration (Dav- ies et al., 1983; Ng-Kwai-Hang et al., 1982, 1985; Rogers and Stewart, 1982~. At the beginning of lactation, colostrum is excep- tionally rich in protein containing large quantities of immunoglobulins and about twice the levels of casein, beta-lactoglobu- lin, ancl alpha-lactalbumin fount] in micI- lactation milk. Total protein amounts fall rapidly during the first few days of transition from colostrum to normal milk ant! reach a minimum about 5 to 10 weeks into lactation, corresponding inversely to maximum milk yield. Thereafter, the amount of protein tends either to increase gradually as lacta APPENDlX lion progresses or to rise sharply when the cow becomes pregnant. Milk protein percentage (Ng-Kwai-Hang et al., 1982) and yield (Keown et al., 1986) are higher during fall and winter than spring and summer. However, stage of lactation and feeding practices confound these ob servations as cows on spring pasture have elevated milk protein concentrations (Rog ers and Stewart, 1982~. Whey proteins have been found to have no definite seasonal variations (Kroeker et al., 1985~. High en vironmental temperatures, above 29°C, have been suggester! to depress milk protein percentage, but cows offered cold water Age has a significant e~ecton milk protein(10°C) cluring heat stress do not show in neroenta~f, an] nomno.sition in onw.s (~en-creased milk protein concentrations over cows offered 28°C water (Milam et al., 1986~. Variations in milking procedure or fre quency have a minor effect, if any, on milk protein percentage. Milk protein or SNF percentages clo not change during the milk ing process (lenness, 1985~. Extended milk ing intervals do not change milk protein or SNF percentages until intervals exceed 16 hours (Rogers and Stewart, 1982~. Increas ing milking frequency from twice to three times daily for more than 15,000 Holstein cows dill not change the percentage of SNF (Gist et al., 1986~. Similar results were reported by Amos et al. (1985) and DePeters et al. (1985~. HealthlPhysiology Mastitis has very little effect on total milk protein percentage; however, it drastically alters the composition of milk protein (Kitchen, 1981; Schultz, 1977~. The general effect of mastitis is to impair milk synthesis and loosen the connections between cells, thereby increasing permeability of blood! constituents Jenness, 1985; Wheelock, 1980~. Milk proteins synthesized in the mammary gland (caseins, beta-lactogIobulin, and al pha-lactalbumin) decrease (Kitchen, 1981; Schultz, 1977), whereas blood serum pro teins (whey proteins) increase (Kitchen, 1981;
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COMPOSITION OF MILK FROM DAIRY COWS Kroeker et al., 1985; Poutrel et al., 1983; Schultz, 1977~. Grappin et al. (1981) re- ported a whey protein to total protein ratio increase of 2.08 percent and a casein to total protein ratio decrease of 1.85 percent for every 1 log unit increase in somatic cell count. The same change in somatic cell count was reporter! by Ng-Kwai-Hang et al. (1982) to decrease the ratio of casein to total protein by 2.79 percent. The hormone requirement for milk syn- thesis and secretion is prolactin, acirenocor- ticotrophic hormone, and estrogens and the relative absence of progesterone. Of partic- ular importance to milk protein synthesis is prolactin (Tucker, 1985~. Current studies (Bauman et al.. 1985; Peel et al., 1985) on administration of exogenous growth hor- mone have generally shown increases in milk yield without significant changes in composition. However, Eppard et al. (1985) observed a slight decrease in milk protein percentage and an increase in alpha-lactal- bumin as a percentage of total milk protein with increasing dosage levels (0 to 100 IU/ day) of bovine growth hormone. Nutrition Dietary crude protein affects milk yield and consequently milk protein yield more than milk protein percentage (Emery, 1978; Kaufman, 1980; Thomas, 1980, 1983~. A small effect of dietary crude protein con- centration on milk protein percentage was reported by Emery (1978~: a 0.02 percentage unit increase in milk protein with every 1 percentage unit increase in clietary crude protein between 9 and 17 percent. More recently, Cragle et al. (1986) reported an increase of 0.1273 Meal in milk protein energy content per 1 Mcal gross energy increase in feed protein. Neither of these studies, however, considered source of di- etary crude protein or change in milk pro- tein composition. Thus, the increases in milk protein observed may have been in milk NPN and not true milk protein. Ele 233 vate(1 milk protein concentrations from cows fed cliets high in rumen-clegradable protein or NPN most likely will be from increased milk urea or NP~ levels (Oltner et al., 1985; Thomas, 1980~. On the other hand, diets low in rurnen-degraciable protein or bal- anced for optimal microbial protein synthe- sis should increase supplies of amino acids available to the mammary gland for protein synthesis, and thus, more true milk protein should be proclucec! (Kaufman, 1980; Old- ham, 1984; Thomas, 1980~. However, the proportions between true milk proteins (cas- eins, beta-lactoglobulin, and alpha-lactal- bumin) (lo not appear to change with in- creases or decreases in milk protein synthesis (Thomas, 1983~. In experiments where protein (usually casein) has been abomasally infused to in- crease amino acid supplies to the tissue, increases in milk protein percentage along with milk yield have been reporter! (Clark, 1975; Clark et al., 1977~. Abomasal infusions of amino acid mixtures also increased milk protein percentage, with methionine and lysine accounting for more than 68 percent of the observed increase (Schwab et al., 1976~. Baser! on these responses, it could be concluded that increasing the intestinal supply of amino acids through increaser! rumen protein synthesis or low rumen- degradable protein sources would increase milk protein percentage and probably milk yield. However, on a practical feeding basis, milk protein responses to dietary proteins with clifferent rum en ciegradabilities have been quite variable but generally of no effect. A number of studies (Crawford and Hoover, 1984; Crooker et al., 1983; Forester et al., 1983; Henderson et al., 1985; Holter et al., 1985; Kung ant! Huber, 1983; Lund- quist et al., 1986) reported no increases in milk protein when protected proteins were fed. Madsen's (1982) study, however, re- ported significant increases. Again, none of the studies cited determined milk protein composition except that of LuncIquist et al. (1986), which showed no change in milk
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234 NPN content within equal dietary crude protein percentages due to feeding formal- dehyde-treated soybean meal compared with feeding an untreated soybean meal. Kaufman (1980) summarized the effects of dietary protein supply on milk protein concentration. Insufficient amounts of die- tary protein will reduce milk protein con- centrations, but the reduction is minimized when low rumen-degradable protein sup- plements are fed. Increasing dietary crude protein supply has little effect on milk protein percentage. The amount of energy consumed, ctens~ty of energy in the flier, and the source of energy in the diet all influence milk protein percentage and yield. Cragle et al. (1986) compared 59 percent versus 49 percent concentrate feeding and found that cows fed rations containing 59 percent concentrate procluced an average of 11 percent more milk, 13 percent more protein, 3 percent more fat, and 11 percent more lactose than cows fed 49 percent concentrate rations. Of the increase in milk protein, 85 percent was attributed to increased yield and only 15 percent to increased percentage in the milk. Emery (1978) reported that milk protein percentage increases 0.015 percent for each Meal of additional net energy fed from 9 to 40 Mcal/ciay and that the increaser! protein percentage was usually accompanied by an increased milk yield. Mild energy malnu- trition has been reported to slightly reduce milk protein percentage; however, uncler severe energy malnutrition, milk protein percentage is unaltered but yields (decrease drastically (Thomas, 1980, 1983~. Rogers and Stewart (1982) reviewed the effects of various forage sources in the diet on milk composition. Cows grazing early spring pastures were reported to have in- creasec3 milk protein percentages. How- ever, the confounding of energy, protein, and condition of the cow in most forage studies where milk composition is reported prohibits the (lrawing of definite conclu- s~ons. APPENDIX Thomas (1980, 1983) discussed the notion that increasing propionic acid in the rumen through increased concentrate feeding or reduced forage particle size affects milk protein percentage. To summarize, there is a strong positive correlation between rumen production of propionic acid and milk pro- tein; however, the exact mechanism is un- known. One suggestion is that propionate increases glutamic acid availability to the mammary gland and, through its role in amino acid transamination, enhances syn- thesis of nonessential amino acids. A second hypothesis is that propionate through in- sulin couIcl enhance plasma concentrations of glutamine and alanine. Propionate could also enhance glutamate output from the liver by increasing its synthesis or reducing usage in gluconeogenesis. Intake of energy can also be increased through inclusion of fats or oils in the diet. Beetling protected lipids, vegetable fats, or vegetable oils to lactating cows depressed milk protein percentage, whereas animal fats had no effect or minimal effect on milk protein percentages (Linn, 1983; Palmquist and Jenkins, 1980~. Dunkley et al. (1977) indicated that the depressing effect was on the casein fraction. Although the exact de- pressing mechanism is unknown, it may be through altered glucose metabolism (Palmquist and Jenkins, 1980), changes in rumen metabolism Jenkins and Palmquist, 1984), or both. Thus, the source of increased dietary energy (carbohydrate versus lipid) fed to lactating cows has a significant eject on milk protein percentage changes. CARBOHYDRATES The predominant carbohydrate in milk is the disaccharide lactose. It is composed of one molecule of glucose and one molecule of galactose joined in a 1-4 carbon linkage as beta-galactoside. The principal biological function of lactose in milk is the regulation of water content and, thus, the regulation
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COMPOSITION OF MILK FROM DAIRY COWS of osmotic content (Davies et al., 1983; lenness, 1985~. Because of this function, lactose is the most constant constituent in milk, averaging 4.6 percent. Carbohydrates other than lactose that are found in milk are monosaccharides, sugar phosphates, nucleoticle sugars, free neutral and acid oligosaccharides, ant] glycosyl groups of peptizes and proteins (lenness, 19854. Free glucose and galactose and the sugar alcohol myo-inositol are also present in milk. However, the amounts of these carbohy- drate fractions are minor compared with that of lactose. Biosynthesis Glucose is the primary substrate for lac- tose synthesis, with 85 percent of the carbon secreted in lactose derived from blood glu- cose (Thomas and Chamberlain, 1984~. Lac- tose synthesis is initiated in the Golgi ap- paratus and continues in the vesicles with an influx of water and ionic constituents that causes the vesicles to swell as they pass toward the cell surface. Glucose and uridine cliphosphate (UDP)-galactose, derived from glucose, combine to form lactose under the action of the enzyme lactose synthetase. The milk protein beta-lactalbumin must be present for glucose and UDP-galactose to combine. Thus, beta-lactalbumin appears to be a prime regulator of lactose synthesis (Kuhn, 1983; Larson, 1985~. Entry of water into the vesicle is linked with lactose syn- thesis to maintain osmotic equilibrium with surrounding fluids. Thus, the rate of lactose synthesis regulates water secretion and con- sequently milk yield. MINERALS Factors Affecting Milk Mineral Content The mineral content of milk is derived from minerals fount] in circulating body fluids. The factors influencing mineral con- tent of milk are discusser] below. 235 BreedlGenetics Cerbulis and Farrel (1976) reported the ash, calcium, phosphorus, and magnesium contents of milk from different breeds of dairy cattle. The average ash content varied from 0.74 percent for Holsteins to 0.83 percent for Jerseys. The highest calcium and phosphorus contents in milk were re- ported for lerseys. EnvironmentlManagement It is well documented that the mineral composition of colostrum is higher than that of milk. Calcium, phosphorus, potassium, and chloride concentrations follow the same lactation curves as fat and protein-that is, high in colostrum, lowest at peak milk yield, and then gradually increasing as lactation progresses (Iyengar, 1982; Jenness, 1985~. Milk inorganic phosphorus levels were shown to be higher in first lactation cows than in multiparous cows, and milk phosphate lev- els were lowest during the summer (Forar et al., 1982). HealthlPhysiology Mastitis increases the percentages of so- dium and chloride in milk and decreases the percentage of potassium (Kitchen, 1981; Peaker and Faulkner, 1983; Schultz, 1977~. Bacterial infection of the udder results in damage to the ductal and secretory epithe- lium and increases the permeability of blood capillaries. Thus, sodium-and chloride, which are higher in blood, pour into the lumen of the alveolus, and in order to maintain os- molarity, potassium is decrease propor- tionally. Fernando et al. (1985) reported the decline in lactose and potassium and in- crease in sodium and chloride in mastitic milk was most prominent in strippings of milk after milking. The percentages of calcium and phospho- rus in milk decline with mastitis infections (Kitchen, 1981; Schultz, 1977~. Most likely
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236 this reflects lower casein levels, since both ions are complexed with casein micelles. Contradictory evidence exists regarding the effect of mastitic infections on levels of magnesium. Trace elements may increase slightly in mastitic milk (TalIamy and Ran- dolph, 19704. Administration of exogenous growth hor- mone has relatively little effect on the per- centages of minerals in milk, but yields of minerals increased with increasing milk pro- cluction (Epparc] et al., 1985~. Nutrition Normal dietary regimes have little influ- ence on the mineral composition of milk, especially the macromineral constituents. Forar et al. (1982) fed two levels of phos- phorus (0.31 and 0.54 percent) and two levels of calcium (1.0 and 1.8 percent) in four diets to lactating cows and found no differences in milk inorganic phosphorus percentages. Diets depressing milk fat per- centage have been shown to lower the percentage of citrate and soluble calcium in milk (Davies et al., 1983~. Changes in milk phosphorus en cl calcium percentages woulc! not be expected, since very few of these ions are in the free form in milk. Dietary factors affecting citrate ant! casein contents of milk would be expected to correspond with small changes in calcium, since calcium is complexed and secreted with these sub stances. Fettman et al. (1984) observed decreases in milk chloride percentage when cows were fed chloricle-deficient rations during early lactation. Milk potassium percentage de- clinec] along with chloride levels, reflecting altered mineral metabolism in chloricle-de- ficient cows. A recent report (Schneider et al., 1986) evaluating dietary sodium and potassium effects on heat-stressed cows found no change in milk potassium percentages based on quantity of potassium fed or source of sodium fed. However, cows offered shacle had higher milk potassium percentages than APPENDIX cows given no shade. Percentages of sodium in milk were lowered significantly by feed- ing cows sodium bicarbonate and only slightly by feeding salt or high levels of potassium, as compared with results for control cows. The lower milk sodium percentages corre- spondec3 with lower plasma sodium levels in cows fee} sodium bicarbonate. Milk iodine levels have been shown to increase with increased beetling of iodine. Franke et al. (1983) observed progressive increases in milk iodine concentrations dur- ing lactation when as little as 4 ppm of organic iodine were a(lded to the (lies. Larson et al. (1983) found cow age, season of calving, milk production, and health sta- tus to have no effect on the concentration of iodine in milk. Iyengar (1982) reported that iocline, man- ganese, molybdenum, selenium, zinc, and cobalt concentrations in milk could be al- tered by dietary means. However, very limited research has been directed toward! this end. Most of the changes that have been observed are the result of marked dietary changes. The effects of slightly un- clerfeeding or overfeeding a required dietary mineral or the effects of mineral interactions on the mineral composition of milk are not well known. OTHER MILK SOLUBLES There are many other components in milk in adclition to those already discussed. They can be categorized as either natural or contaminant. They appear in milk both from leakage during the normal secretory process and by actual secretion. Whatever the mode of entry, their concentrations can vary con- siderably, but their significance and purpose remain largely unknown. The natural compounds that have been detected in milk are gases, alcohol, alde- hydes, ketones, carboxylic acicI, sulfur-con- taining compounds, nucleotide material, hormones, phosphate esters, glucose, ace- tate, and citrate. Many of these are products
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COMPOSITION OF MILK FROM DAIRY COWS of intermediary metabolism ofthe mammary gland (Davies et al., 1983; lenness, 1985; Peaker and Faulkner, 1983~. The reasons for changes in the concentrations of these compounds in milk are unknown. An exception to the above is the com- pounc] citrate. The concentration of citrate in milk is moclifiable and is important from a milk-processing standpoint. Alteration of citrate concentrations changes the amount of free calcium in the soluble phase of milk, which, in turn, affects the precipitation of milk proteins. Milk citrate concentrations are highly correlates] with fat percentage, ant] therefore diets that lower milk fat per- centage also decrease the citrate content of milk (Faulkner ant] Peaker, 1982~. Stage of lactation and season of the year also affect milk citrate levels. It appears that the source of milk citrate is from synthesis within secretory cells and that secretion into milk is similar to that of lactose and casein (Faulk- ner and Peaker, 1982~. The other category of components-con- taminants inclucles compounds that are not normally found in milk but that enter acciclentally or by design. Included here are chemicals, pesticides, herbicides, fungi- cides, heavy metals, and drugs. These items are mentioned as a reminder that milk can contain compounds other than those of nu- tritional importance to humans and that maintaining a nutritious, wholesome milk supply is of utmost importance. MANIPULATING MILK CONSTITUENT~SUMMARY AND CONCLUSIONS Variations in milk composition arise from differences in relative rates of synthesis ant] secretion of milk components by the mam- mary gland. The processes involved for lactose, protein, and fat synthesis and se- cretion are independent but regulated through nutrient or substrate availability ant! hormonal control of nutrient utilization. Thus, genetics, which mediates hormonal 237 effects, and diet, which regulates nutrient availability, are the major factors affecting milk composition. The most variable milk constituent is fat. Consiclerable variation exists between and within dairy cattle breeds. Genetic selection for fat percentage can change fat content of -milk but will also affect other constituents since there is a high correlation between the percentage of components in milk. Ge- netic selection for fat content would! alter the quantity of fat produced but not the composition of the fat. The best hope for altering composition is through diet. Changes in fat percentage and composition can be accomplished by altering the flier to produce changes in fermentation patterns or the composition of fat absorbe(1 from the diges- tive tract. Diets that increase the proportion of propionate in the rumen depress milk fat percentage, but changes in fat composition are minimal, including slight increases in Cog polyunsaturated fatty acids and slight decreases in C~60 and C~80 fatty acids. In- clusion of fats in the diet, particularly ru- men-protected fats, is the most effective way to alter milk fat composition. Significant increases in long-chain fatty acids can be achieved by inclucling-fat containing these acids in the diet. However, the amount and composition of fats in the diet Heel] to be controlled to avoid impairment of digestion of other (lietary constituents in the rumen. Unsaturated fatty acids are hydrogenated extensively in the rumen. Milk protein percentage and composition can be manipulates] through genetic selec- tion. Variations in the casein, beta-lacto- globulin, and alpha-lactalbumin fractions are known to exist. Heritability estimates of protein percentage range from 0.3 to 0.7. Increasing milk protein percentage through genetic selection is feasible; however, in- creasing milk protein yield! through selec- tion is more desirable. The percentage of true proteins in milk cannot be manipulated through feecling. Total protein percentage in milk can be lowered by including fats in
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238 the diet or raised in relation to milk fat percentage by feeding high-concentrate diets. Dietary protein percentage has a minimal eject on milk protein percentage when it is within practical feeding ranges. Levels of other nutritional components of milk lactose, vitamins, and minerals are rather constant and not subject to large changes through genetic or nutritional ma- nipulation. The manipulation of milk components through changes in dairy management prac- tices, breeding, feecling, health, environ- ment, and general management appears to be rather limited. Milk fat percentage and composition can be changed through feed- ing, whereas milk protein percentage is best changed through genetics. Any changes will be slow in coming and minor compared to those achieved through processing and man- ufacturing. The goal of milk producers should be to modify composition as much as pos- sible to meet market demand but to em- phasize maximum yield of components in high-quality, wholesome milk. REFERENCES Allen, D. B., E. J. DePeters, and R. C. Laben. 1986. Three times a day milking: Effects on milk produc- tion, reproductive efficiency and udder health. J. Dairy Sci. 69:1441. Amos, H. E., T. Kiser, and M. Loewenstein. 1985. Influence of milking frequency on reproductive and productive efficiencies of cows. J. Dairy Sci. 68:732. ~ J Banks, W., J. L. Clapperton, and W. Steele. 1983. Dietary manipulation of the content and fatty acid composition of milk fat. Proc. Nutr. Soc. 42:399. Bauman, D. E., and J. M. Elliot. 1983. Control of nutrient partitioning in lactating ruminants. Ch. 14 in Biochemistry of Lactation, T. B. Mepham, ed. Amsterdam: Elsevier. Bauman, D. E., P. J. Eppard, M. J. DeGeeter, and G. M. Lanza. 1985. Responses of high-producing dairy cows to long term treatment with pituitary somatotropin and recombinant somatotropin. J. Dairy Sci. 68:1352. Baumrucker, C. R. 1985. Amino acid transport systems in bovine mammary tissue. J. Dairy Sci. 68:2436. Bragg, D. St. A., M. R. Murphy, and C. L. Davis. 1986. Effect of source of carbohydrate and frequency of feeding on rumen parameters in dairy steers. J. Dairy Sci. 69:392. APPENDIX Casper, D. P., and D. J. Schingoethe. 1986. Evaluation of urea and dried whey in diets of cows during early lactation. J. Dairy Sci. 69:1346. Cerbulis, J., and H. M. Farrell, Jr. 1976. Composition of the milk of dairy cattle. II. Ash, calcium, mag- nesium and phosphorus. J. Dairy Sci. 59:589. Chalupa, W., and P. L. Schneider. 1985. Buffers for dairy cattle. 20th Annual Proceedings of the 1985 Pacific Northwest Animal Nutrition Conference, sponsored by the Pacific Northwest Feed Manufac- turers. October 1985, Boise, Idaho. Christie, W. W. 1979. The effects of diet and other factors on the lipid composition of ruminant tissues and milk. Prog. Lipid Res. 17:245. Clark, J. H. 1975. Lactational responses to post-ruminal administration of proteins and amino acids. J. Dairy Sci. 58:1178. Clark, J. H., H. R. Spires, R. G. Derrig, and M. R. Bennik. 1977. Milk production, nitrogen utilization and glucose synthesis in lactating cows infused post- ruminally with sodium caseinate and glucose. J. Nutr. 107:631. Clark, J. H., H. R. Spires, and C. L. Davis. 1978. Uptake and metabolism of nitrogenous compounds by the lactating mammary gland. Fed. Proc. 37:1233. Coppock, C. E. 1985. Energy nutrition and metabolism of the lactating cow. J. Dairy Sci. 68:3403. Cragle, R. G., M. R. Murphy, S. W. Williams, and J. H. Clark. 1986. Effects of altering milk production and composition by feeding on multiple component milk pricing system. J. Dairy Sci. 69:282. Crawford, R. J., Jr., and W. H. Hoover. 1984. Effects of particle size and formaldehyde treatment of soy- bean meal on milk production and composition for dairy cows. J. Dairy Sci. 67:1945. Crooker, B. A., J. H. Clark, and R. D. Shanks. 1983. Effects of formaldehyde treated soybean meal on milk yield, milk composition and nutrient digesti- bility in the dairy cow. J. Dairy Sci. 66:492. Davies, D. T., C. Holt, and W. W. Christie. 1983. The composition of milk. Ch. 5 in Biochemistry of Lactation, T. B. Mepham, ed. Amsterdam: Elsevier. Davis, C. L. 1978. The use of buffers in the rations of lactating dairy cows. In Proceedings of the Begula- tion of Acid-Base Balance, sponsored by the Uni- versity of Arizona and Church and Dwight Co., Inc., W. lI. Hale and P. Meinhardt, eds. Tuscon: Uni- versity of Arizona. DePeters, E. J., and S. J. Taylor. 1985. Effects of feeding corn or barley on composition of milk and diet digestibility. J. Dairy Sci. 68:2027. DePeters, E. J., N. E. Smith, and J. Acedo-Rico. 1985. Three or two times daily milking of older cows and first lactation cows for entire lactations. J. Dairy Sci. 68:123. Dils, R. R. 1983. Milk fat synthesis. Ch. 5 in Bio- chemistry of Lactation, T. B. Mepham, ed. Amster- dam: Elsevier.
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COMPOSITION OF MILK FROM DAIRY COWS Dils, R. R. 1986. Comparative aspects of milk fat ~^~ synthesis. J. Dairy Sci. 69:904. Dunkley, W. L., N. E. Smith, and A. A. Franke. 1977. Effects of feeding protected tallow on com- position of milk and milk fat. J. Dairy Sci. 60:1863. Eigel, W. N., J. E. Butler, C. A. Ernstrom, H. M. Farrell, Jr., V. R. Harwalker, R. Jenness, and R. McL. Whitney. 1984. Nomenclature of proteins of cow's milk: Fifth revision. J. Dairy Sci. 67:1599. Emery, R. S. 1978. Feeding for increased milk protein. J. Dairy Sci. 61:825. Eppard, P. J., D. E. Bauman, J. Bitman, D. L. Wood, R. M. Akers, and W. A. House. 1985. Effect of dose of bovine growth hormone on milk composition: Alpha-lactalbumin, fatty acids, and mineral ele- ments. J. Dairy Sci. 68:3047. Faulkner, A., and M. Peaker. 1982. Secretion of citrate into milk. J. Dairy Res. 49:159. Fernando, R. S., S. L. Spahr, and E. H. faster. 1985. Comparison of electrical conductivity of milk with other indirect methods for detection of subclinical mastitis. J. Dairy Sci. 68:449. Fettman, M. J., L. E. Chase, J. Bentinck-Smith, C. E. Coppock, and S. A. Zinn. 1984. Nutritional chloride deficiency in early lactation Holstein cows. J. Dairy Sci. 67:2321. Fogerty, A. C., and A. R. Johnson. 1980. Influence of nutritional factors on the yield and content of milk fat: Protected polyunsaturated fat in the diet. Int. Dairy Fed. Bull. Doc. 125:96. Forar, F. L., R. L. Kincaid, R. L. Preston, and J. K. Hillers. 1982. Variation of inorganic phosphate in blood plasma and milk lactating cows. J. Dairy Sci. 65:760. Forester, R. J., D. G. Grieve, J. G. Buchanan-Smith, and G. K. MacLeod. 1983. Effect of dietary protein degradability on cows in early lactation. J. Dairy Sci. 66:1653. Franke, A. A., J. C. Bruhn, and R. B. Osland. 1983. Factors affecting iodine concentration of milk of individual cows. J. Dairy Sci. 66:997. Gaunt, S. N. 1973. Genetic and environmental changes in milk consumption. J. Dairy Sci. 56:270. Gaunt, S. N. 1980. Genetic variation in the yields and contents of milk constituents. Int. Dairy Fed. Bull. Doc. 125:73. Gisi, D. D., E. J. DePeters, and C. L. Pelissier. 1986. Three times daily milking of cows in California dairy herds. J. Dairy Sci. 69:863. Grappin, R., V. S. Packard, and R. E. Ginn. 1981. Variability and interrelationship of various herd milk components. J. Food Prot. 44:69. Hansen, W. P., D. E. Otterby, J. D. Donker, R. G. Lundquist, and J. G. Linn. 1984. Influence of grain concentrations, forage type, and methionine hydroxy analog on lactational performance of dairy cattle. J. Dairy Sci. 67(Suppl. 1):99 (Abstr. ). Henderson, S. J., H. E. Amos, and J. J. Evans. 1985. 239 Influence of dietary crude protein concentration and degradability on milk production, composition, and ruminal protein metabolism. J. Dairy Sci. 68:2227. Holter, J. B., W. E. Urban, Jr., H. H. Hayes, and H. A. Davis. 1977. Utilization of diet components fed blended or separately to lactating cows. J. Dairy Sci. 60:1288. Holter, J. B., W. E. Hylton, and C. K. Bozak. 1985. Varying protein content and nitrogen solubility for pluriparious, lactating Holstein cows: Lactation per- formance and profitability. J. Dairy Sci. 68:1984. Iyengar, G. V. 1982. Elemental Composition of Human and Animal Milk: A Review. International Atomic Energy Agency Technical Document 269. Vienna: International Atomic Energy Agency. Jenkins, T. C., and D. L. Palmquist. 1984. Effect of fatty acids of calcium soaps on rumen and total nutrient digestibility of dairy rations. J. Dairy Sci. 67:978. Jenness, R. 1985. Biochemical and nutritional aspects of milk and colostrum. Ch. 5 in Lactation, B. L. Larson, ed. Ames: Iowa State University Press. Kaufman, W. 1980. Protein degradation and synthesis within the reticulorumen in relation to milk protein synthesis. Intl. Dairy Fed. Bull. Doc. 125:152. Kawas, J. R., N. A. Jorgensen, A. R. Hardie, and J. L. Danelon. 1983. Change in feeding value of alfalfa hay with stage of maturity and concentrate level. J. Dairy Sci. 66:181 (Abstr. ). Keown, J. F., R. W. Everett, N. B. Empet, and L. H. Wadell. 1986. Lactation curves. J. Dairy Sci. 69:769. Kitchen, B. J. 1981. Bovine mastitis: Milk composi- tional changes and related diagnostic tests. J. Dairy Res. 48:167. Kroeker, E. M., K. F. Ng-Kwai-Hang, J. F. Hayes, and J. E. Mosley. 1985. Effect of beta-lactoglobulin variant and environmental factors on variation in the detailed composition of bovine milk serum proteins. J. Dairy Sci. 68:1637. Kuhn, N. J. 1983. The biosynthesis of lactose. Ch. 6 in Biochemistry of Lactation, T. B. Mepham, ed. Amsterdam: Elsevier. Kung, L., Jr., and J. T. Huber. 1983. Performance of high producing cows in early lactation fed protein of varying amounts, sources, and degradability. J. Dairy Sci. 66:227. Kuzdzal-Savole, S., W. Manson, and J. H. Moore. 1980. The constituents of cow's milk. Int. Dairy Fed. Bull. Doc. 125:4-13. Larson, B. L. 1979. Biosynthesis and secretion of milk protein: A review. J. Dairy Res. 46:161 Larson, B. L. 1985. Biosynthesis and cellular secretion of milk. Ch. 4 in Lactation, B. L. Larson, ed. Ames: Iowa State University Press. Larson, L. L., S. E. Wallen, F. G. Owen, and S. R. Lowry. 1983. Relation of age, season, production and health indices to iodine and beta-carotene con
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240 centrations in cow's milk. J. Dairy Sci. 66:2257. Linn, J. G. 1983. The addition of fats to diets of lactating dairy cows: A review. In Proceedings of a Feed Fat Seminar, sponsored by Central Bi-Prod- ucts. Redwood Falls, Minn. Linn, J. G., and D. E. Otterby. 1984. Feeding strategies in dairy nutrition. Pp. 1~22 in Proceed- ings of the 45th Minnesota Nutrition Conference. St. Paul, Minn.: University of Minnesota Press. Lundquist, R. L., J. G. Linn, and D. E. Otterby. 1983. Influence of dietary energy and protein on yield and composition of milk from cows fed methi- onine hydroxy analog. J. Dairy Sci. 66:475 Lundquist, R. L., D. E. Otterby, and J. G. Linn. 1986. Influence of formaldehyde-treated soybean meal on milk production. J. Dairy Sci. 69:1337. Madsen, J. 1982. The effect of formaldehyde-treated protein and urea on milk yield and composition in dairy cows. Acta Agric. Scand. 32:389. Marshall, S. P., and A. R. Voigt. 1975. Complete rations for dairy cattle. I. Methods of preparation and roughage-to-concentrate ratios of blended ra- tions with corn silage. J. Dairy Sci. 58:891. Mepham, T. B. 1982. Amino acid utilization by lac- tating mammary gland. J. Dairy Sci. 65:287. Mercier, J.-C., and P. Gaye. 1983. Milk protein synthesis. Ch. 7 in Biochemistry of Lactation, T. B. Mepham, ed. Amsterdam: Elsevier. Mertens, D. R. 1985. Effect of fiber on feed quality for dairy cows. Pp. 209-224 in Proceedings of the 46th Minnesota Nutrition Conference. St. Paul, Minn.: University of Minnesota Press. Milam, K. Z., C. E. Coppock, J. W. West, J. K. Lanham, D. H. Nave, J. M. Labore, R. A. Stermer, and C. F. Brasington. 1986. Effects of drinking water temperature on production responses in lactating Holstein cows in summer. J. Dairy Sci. 69:1013. Needs, E. C., and M. Anderson. 1984. Lipid com- position of milk from cows with experimentally induced mastitis. J. Dairy Res. 51:239. Ng-Kwai-Hang, K. F., J. F. Hayes, J. E. Moxley, and H. G. Monardes. 1982. Environmental influences on protein content and composition of bovine milk. J. Dairy Sci. 65:1993. Ng-Kwai-Hang, K. F., J. F. Hayes, J. E. Moxley, and H. G. Monardes. 1985. Percentages of protein and nonprotein nitrogen with varying fat and somatic cells in bovine milk. J. Dairy Sci. 68:1257. Oldham, J. D. 1984. Amino acid metabolism in rum- inants. Pp. 137-151 in Proceedings of the Cornell Nutrition Conference, sponsored by Cornell Uni- versity and American Feed Manufacturers Associa- tion. Ithaca, N.Y.: Cornell University. Oltner, R., M. Emanuelson, and H. Wiktorsson. 1985. Urea concentrations in milk in relation to milk yield, live weight, lactation number and amount and com- position of feed given to dairy cows. Livestock Prod. Sci. 12:47. APPENDIX Owen, J. B. 1981. Complete-diet feeding of dairy cows. In Recent Development in Ruminant Nutri- tion, W. Haresign and D. J. A. Cole, eds. London: Butterworth. Palmquist, D. L., and T. C. Jenkins. 1980. Fat in lactation rations: Review. J. Dairy Sci. 63:1 Peaker, M., and A. Faulkner. 1983. Soluble milk constituents. Proc. Nutr. Soc. 42:419. Peel, C. J., L. D. Sandles, K. J. Quelch, and A. C. Herington. 1985. The effects of long-term adminis- tration of bovine growth hormone on the lactational performance of identical-twin dairy cows. Anim. Prod. 41:135. Poutrel, B., J. P. Caffin, and P. Rainard. 1983. Phys- iological and pathological factors influencing bovine serum albumin content in milk. J. Dairy Sci. 66:535. Rogers, G. L., and J. A. Stewart. 1982. The effects of some nutritional and nonnutritional factors on milk protein concentration and yield. Aust. J. Dairy Technol. 37:26. Rolleri, G. D., B. L. Larson, and R. W. Touchberry. 1956. Protein production in the bovine. Breed and individual variations in the specific protein constit- uents of milk. J. Dairy Sci. 39:1683. Rook, J. A. F., and P. C. Thomas. 1980. Principles involved in manipulating the yields and concentra- tions of constituents in milk. Int. Dairy Fed. Bull. Doc. 125:66. Schingoethe, D. J. 1976. Whey utilization in animal feeding: A summary and evaluation. J. Dairy Sci. 59:556. Schneider, P. L., D. K. Beede, and C. J. Wilcox. 1986. Responses of lactating cows to dietary sodium source and quantity and potassium quantity during heat stress. J. Dairy Sci. 69:99 Schultz, L. H. 1977. Somatic cell in milk physiolog- ical aspects and relationship to amount and compo- sition of milk. J. Food Prot. 40:125. Schwab, C. G., L. D. Satter, and A. B. Clay. 1976. Response of lactating cows to abomasal infusions of amino acids. J. Dairy Sci. 59:1254. Shaver, R. D., A. J. Nytes, L. D. Satter, and N. A. Jorgensen. 1986. Influence of amount of feed intake and forage physical form on digestion and passage of prebloom alfalfa hay in dairy cows. J. Dairy Sci. 69:1545. Storry, J. E. 1980. Influence of nutritional factors on the yield and content of milk: Nonprotected fat in the diet. Int. Dairy Fed. Bull. Doc. 125:88. Storry, J. E., and P. E. Brumby. 1980. Influence of nutritional factors on yield and content of milk: Protected nonpolyunsaturated fat in the diet. Int. Dairy Fed. Bull. Doc. 125:105. Sutton, J.D. 1980. Influence of nutritional factors on the yield and content of milk fat: Dietary components other than fat. Int. Dairy Fed. Bull. Doc. 125:126. Sutton J. D. 1985. Digestion and absorption of energy substrates in the lactating cow. J. Dairy Sci. 68:3376.
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COMPOSITION OF MILK FROM DAIRY COWS Tallamy, P. T., and H. E. Randolph. 1970. Influence of mastitis on properties of milk. V. Total and free concentrations of major minerals in skim milk. J. Dairy Sci. 53:1386. Thomas, P. C. 1980. Influence of nutrition on the yield and content of protein in milk: Dietary protein and energy supply. Int. Dairy Fed. Bull. Doe. 125:142. Thomas, P. C. 1983. Milk protein. Proc. Nutr. Soc. 42:407. Thomas, P. C., and D. G. Chamberlain. 1984. Ma- nipulation of milk composition to meet market needs. Ch. 14 in Recent Advances in Animal Nutrition, W. Haresign and D. J. A. Cole, eds. London: Butter- worth. Tucker, H. A. 1985. Endocrine and neural control of the mammary gland. Ch. 2 in Lactation, B. L. Larson, ed. Ames: Iowa State University Press. Van Vleck, L. D. 1978. Breeding for increased milk protein. J. Dairy Sci. 61:815. Wheelock, J. V. 1980. Influence of physiological factors 241 on the yields and contents of milk constituents. Int. Dairy Fed. Bull. Doc. 125:83. Wilcox, C. J. 1978. Genetic considerations of economic importance: Milk yield, composition and quality. Ch. 2 in Large Dairy Herd Management, C. J. Wilcox, H. H. Van Horn, B. Harris, Jr., H. H. Head, S. P. Marshall, W. W. Thatcher, D. W. Webb, and J. M. Wing, eds. Gainesville: University Presses of Florida. WolEschoon-Pombo, A., and H. Klostermeyer. 1981. The NPN-fraction of cow milk. I. Amount and composition. Milchwissenschaft 36:598. Woodford, J. A., N. A. Jorgensen, and G. P. Barring- ton. 1986. Impact of dietary fiber and physical form on performance of lactating dairy cows. J. Dairy Sci. 69:1035. Young, C. W., J. K. Millers, and A. E. Freeman. 1986. Production, consumption, and pricing of milk and its components. J. Dairy Sci. 69:272.
Representative terms from entire chapter: