National Academies Press: OpenBook

Ruminant Nitrogen Usage (1985)

Chapter: 7 Digestion and Absorption in the Small Intestine

« Previous: 6 Microbial Growth
Suggested Citation:"7 Digestion and Absorption in the Small Intestine." National Research Council. 1985. Ruminant Nitrogen Usage. Washington, DC: The National Academies Press. doi: 10.17226/615.
×
Page 46
Suggested Citation:"7 Digestion and Absorption in the Small Intestine." National Research Council. 1985. Ruminant Nitrogen Usage. Washington, DC: The National Academies Press. doi: 10.17226/615.
×
Page 47
Suggested Citation:"7 Digestion and Absorption in the Small Intestine." National Research Council. 1985. Ruminant Nitrogen Usage. Washington, DC: The National Academies Press. doi: 10.17226/615.
×
Page 48
Suggested Citation:"7 Digestion and Absorption in the Small Intestine." National Research Council. 1985. Ruminant Nitrogen Usage. Washington, DC: The National Academies Press. doi: 10.17226/615.
×
Page 49
Suggested Citation:"7 Digestion and Absorption in the Small Intestine." National Research Council. 1985. Ruminant Nitrogen Usage. Washington, DC: The National Academies Press. doi: 10.17226/615.
×
Page 50
Suggested Citation:"7 Digestion and Absorption in the Small Intestine." National Research Council. 1985. Ruminant Nitrogen Usage. Washington, DC: The National Academies Press. doi: 10.17226/615.
×
Page 51
Suggested Citation:"7 Digestion and Absorption in the Small Intestine." National Research Council. 1985. Ruminant Nitrogen Usage. Washington, DC: The National Academies Press. doi: 10.17226/615.
×
Page 52

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Digestion and Absorption in the Small Intestine NITRO GEN SUPPLY Because of N transactions in the rumen, intake of pro- tein (IP) is not an accurate indicator of N flow to the small intestine (Whitlow and Satter, 1979J. Examina- tion of data obtained with cows, steers, calves, sheep, and lambs showed that for both concentrate and forage diets, duodenal flow (SCP) ranged between 10.5 and 12.5 g NAN (nonammonia nitrogen) per Meal ME con- sumed (Oldham and Tamminga, 1980~. This illustrates that energy consumption is the major determinant of the amount of N entering the small intestine. As discusser] in other sections, factors that affect microbial protein pro- duction and ruminal degradation of dietary protein can be expected to modify N supply tSCP) to the small intes- tine. The N entering the duodenum is a combination of mi- crobial (BCP), undegraded intake (UIP) and endoge- nous protein. Differences in amino acid composition be- tween bacteria and protozoa (Chalupa, 1972), IP and BCP (Smith, 1975; Laughren and Young, 1979), and IP and UIP (Smith and Mohamed, 1977) imply that the quantities of BCP and UIP entering the intestine can in- fluence the supply of absorbable amino acids. Unfortu- nately, technical problems in partitioning BCP and UIP and in estimating endogenous secretions have made ac- curate quantitation difficult. The N entering the cluode- num from the stomach can range between 0.3 to 1.0 BCP and 0 to 0.70 UIP (Smith, 1975~. Endogenous in- fluxes can equal the supply from the stomach (Nolan, 1975) From a nutritional point of view, it is important to know the chemical composition of intestinal N. The pro- portionate distribution of N in duodenal contents of cows (SCP) was described by Oldham and Tamminga (198(~) as essential amino acids, 0.35; nonessential 46 amino acids, 0.30; amides, 0.04; nucleic acids, 0.11; ammonia, 0.06; and unknown, 0.14. Increases in the ratio of essential amino acids to nonessential amino ac- ids in duodenal digesta as a function of the concentra- tion of intake protein (IPDM) indicate that the amino acid composition of IP may influence the balance of amino acids available for absorption (Laughren and Young, 1979~. Because it is measured easily, NAN is a frequently used measure of N entering the small intes- tine. In data summarized by Stern and Satter (1982), amino acid N in the duodenum of lactating cows was 0.79 of NAN. In sheep, amino acid N was 0.80 of NAN (Hogan and Weston, 1970~. Considerable work is needed before it will be possible to predict the amounts of specific amino acids presented to the small intestine. DIGESTION SYSTEM Digestion of protein in the abomasum and small intes- tine appears to be the same for ruminants as in nonrumi- nants except for the slow neutralization of digesta in the small intestine and the abundance of pancreatic ribQnu- clease (Armstrong and Hutton, 1975; Bergen, 1978; Chalupa, 1978~. Slow neutralization of digesta in the upper small in- testine of ruminants appears to be related to the low bi- carbonate content of pancreatic juice (Taylor, 1962~. This extends the activity of abomasal pepsin but delays the onset of activities of intestinal enzymes. Thus, con- siderable proteolysis in the duodenum is due to the gas- tric protease, pepsin. Optimal activity for trypsin, chymotrypsin, and carboxypeptidase does not occur until the middle jejunum, and peak activity of exopep- tidases and dipeptidases is found in the mid ileum (Ben- Ghedalia et al., 1974~. Breakdown of nucleic acids is achieved by DNases,

Digestion and Absorption in the Small Intestine 47 RNases, phosphodiesterases, and phosphomonestera~ses (Bergen, 1978; Roth and Kirchgessner, 1980~. An im- portant role for abundant pancreatic RNase in the rumi- nant is release of nucleic acid phosphorus for recycling to the rumen via saliva (Barnard, 1969~. It appears that the products of nucleic acid digestion that are absorbed are nucleotides, nucleosides, and bases (Bergen, 1978; Smith, 1979). ABSORPTION MECHANISMS The mucosa of the small intestine contains uptake sys- tems for free amino acids, peptides, nucleotides, and nucleosides (Armstrong and Hutton, 1975; Bergen, 1978; Scharrer and Amann, 1980~. The most active site for amino acid absorption in sheep is the mid to lower ileum (Johns and Bergen, 1973; Phillips et al., 1976), but the highest rate of amino acid disappearance in situ from the digesta in the small intes- tine occurs in the mid jejunum (Ben-Ghedalia et al., 1974~. Johns and Bergen (1973), using jejunal strips, demonstrated that amino acid uptake in sheep occurs against a concentration gradient, exhibits saturation ki- netics, and depends upon metabolic energy. Km and Vm ~` values for glycine, methionine, and lysine trans- port in sheep jejunum were similar to values obtained with rat jejunum. The preferential disappearance of es- sential amino acids over nonessential amino acids from digesta flowing through the small intestine has been demonstrated in sheep (Johns and Bergen, 1973; Phillips et al., 1976) and in cattle (Van's Klooster and Boekholt, 1972~. Using exteriorized intestinal loops, Williams (1969) ranked amino acid absorption as follows: Ile>Arg-Val>Leu>Met>Phe>Lys>Try> Asp ~ Ser > Ala > Pro > His 2 Thr 2 Glu > Gly. The order with jejunal strips in vitro was Met > Lys > Gly (Johns and Bergen, 1973) and with everted sacs in vitro was Met > Val > Thr (Phillips et al., 1976~. The overall order of uptake by sheep gut is similar to that noted in man and rats. A depressing effect of leucine on lysine uptake has been shown both in vitro Johns ant} Bergen, 1973) and in viva (Hume et al., 1972~. It seems likely that as in the nonruminant (Matthews, 1972; Munck, 1976), absorption of peptides is quantita- tively important in the ruminant. Steps involved in- clude peptide uptake, peptide hydrolysis, and transport of amino acids. Removal of the end products of nucleic acid digestion from digesta flowing through the small intestine implies efficient absorption mechanisms (Bergen, 1978~. Nucle- osides are absorbed from the small intestine by a Na- dependent saturatable transport process (Scharrer and Amann, 1980~. EXTENT OF APPARENT ABSORPTION Measuring disappearance of N (SCP) or amino (STP) acids between the duodenum and ileum provides an es- timate of apparent absorption. Samples from cannulae inserted into the duodenum prior to the entry of bile and pancreatic secretions only includes endogenous N from gastric secretions, whereas samples from cannulae in- serted posterior to the entry of bile and pancreatic secre- tions also contain N from pancreatic secretions. Apparent absorption of NAN and amino acids from the small intestine of lactating cattle, nonIactating cat- tle and sheep fed a variety of diets is listed in Appendix Tables 11, 12, and 13. Table 11 summarizes the results. Overall, apparent absorption was 0.65 of NAN and 0.68 of amino acids entering the duodenum (Table 11~. Ap- parent absorption of NAN was similar in the groups summarized and was less than absorption of amino acids in lactating cattle and sheep but not in nonIactating cat- tle. In experiments reviewed by ARC (1980) in which absorption of both NAN and amino acids were mea- sured, the values did not differ markedly. However, Tamminga (1980) conclucled that apparent absorption of total N is usually 0.05 lower than that of amino acids. Based upon the foregoing, values suggested for appar- ent absorption of NAN and amino acids from the small intestine are 0.65 and 0.70 of amounts entering the duo- denum. Apparent absorption of essential amino acids is about 0.05 greater than nonessential amino acids (Armstrong et al., 1977, Tamminga, 1980~. Apparent absorption of essential amino acids, as summarized by Tamminga (19803 suggests that absorption of lysine and arginine is greater while absorption of threonine, valine, and phenylalanine is less than the absorption of total essen- tial amino acids (Table 12). Apparent absorption of TABLE 11 Summary of Apparent Absorption of Nonammonia Nitrogen and Amino Acids from the Small Intestine of Ruminantsa Measurement Calculation nb x . SD CV NAN Lactating cattle Nonlactating cattle Sheep All Amino acids Lactating cattle Nonlactating cattle Sheep All 12 17 29 58 21 11 22 54 0.65 0.66 0.64 0.65 0.69 0.62 0.70 0.68 0.04 0.04 0.06 0.05 0.05 0.06 0.06 0.06 0.07 0.06 0.09 0.08 0.08 0.10 0.09 0.10 a Based upon data in Appendix Tables 11, 12, 13. bNumber of diets.

48 Ruminant Nitrogen Usage TABLE 12 Proportionate Disappearance of Amino Acids from the Small Intestinea Animal and Experiment Sheep Cow Ib 0.77 0.77 0.70 0.77 0.79 0.82 0.68 0.70 0.72 0.72 0.75 0.75 0.73 0.76 0.74 0.78 0.69 0.68 0.73 + 0.75 + 0.008 0.000 8 Amino Acid Cow IIb Cow IIIb Lysine Histidine Arginine Threonine Valine Methionine Isoleucine Leucine Phenylalanine Total essential amino acids No. observations 0.74 0.73 0.79 0.71 0.72 0.61 0.72 0.73 0.71 0.75 0.76 0.79 0.71 0.72 0.66 0.72 0.73 0.72 0.72 + 0.74 + 0.012 0.005 13 72 a Data summarized by Tamminga (1980~. ''Cow I, II, and III are not individual cows but refer to experiments involving cows. methionine was quite variable in the cow experiments and may be a consequence of location of the duodenal cannula in that only absorption of methionine was lower in experiments where duodenal samples were col- lected beyond rather than prior to the pancreatic and biliary duct. In experiments reviewed by Armstrong et al. (1977), apparent absorption of methionine was 0.06 + 0.05 more than apparent absorption of total essential amino acids. Net disappearance of cystine was only 0.40 to 0.50. ENDOGENOUS LOSS Calculation of true absorption requires correction for the influx of enclogenous nitrogen that is not reabsorbed from the small intestine. Endogenous protein enters the small intestine in the form of enzymes, bile, mucus, serum albumin, lymph, epithelial cells, and oth~egradable products from the gastrointestinal lining (Swanson, 1982~. Summation of the endogenous input to the entire gastrointestinal tract is large (Phillipson, 1964; Swanson, 1982~. In nonIactat- ing cattle, it is more than twice the maintenance value (Swanson, 1982~. In studies where i5N was used to study N metabolism in sheep, inputs (g/~) of NAN to the small intestine were: UIP, 6.5; BCP, 10.3; and intestinal se- cretions, 17.0 (Nolan, 1975~. Thus, enclogenous N was equivalent to NAN from the stomach. The N in duode- nal contents from abomasal juice, pancreatic juice, bile, and epithelial cells was estimated at 0.004 dry matter consumed (Tamminga et al., 1979~. In lactating cattle, this was equivalent to 0.10 to 0.15 of N from the stom- ach, but not included in this estimate are endogenous inputs posterior to the entry of bile and pancreatic juice. Total influx of endogenous N is important for an un- derstanding of the dynamic involvement of intestinal tissue in N metabolism. However, as indicated previ- ously, calculation of the true intestinal absorption of N derived from the stomach requires correction for the en- dogenous input that is not reabsorbed. Endogenous losses, as well as true digestibility, can be estimated as Y = a + bX where Y is disappearance be- tween two points in the gastrointestinal tract (i.e., mouth and anus; proximal duodenum and terminal il- eum), X is the supply (amount or concentration) to a point in the gastrointestinal tract (i.e., mouth; proximal duodenum), a is a negative value and represents the en- dogenous loss, and b is true digestibility (Van Soest, 1982~. Applying the regression approach to data obtained with sheep, Hogan and Weston (1970) calculated the endogenous loss from the small intestine that appeared in feces as 0.0016 organic matter entering the duode- num. The endogenous loss appearing in feces from the entire gastrointestinal tract was 0.004 organic matter consumed. This prompted Hogan and Weston (1970) and Hogan (1975) to conclude that only about one-third of the N in the classic metabolic fecal fraction is of en- clogenous origin and the remaining two-thirds is of mi- crobial origin. Regression analysis of other data yielded the follow- ing estimates of endogenous losses from the small intes- tine (g/day): sheep, 2.2 g NAN; lactating cattle' 56 g NAN and 250 g amino acids; nonlactating cattle, 0.77 g NAN and 98 g amino acids (Table 13~. In sheep, Tas et al. (1981) estimated the mean endogenous loss of amino acids secreted into the small intestine to be 13 gig. Since NAN is 0.8 amino acid nitrogen, this is equivalent to 2.6 gNAN/~13 . (0.8 x 6.25~. The enclogenous losses in Table 13 for sheep and lac- tating cattle are 0.10 to 0.13 of the N supply to the proxi- mal duodenum. With nonlactating cattle, the endoge- nous loss of amino acids was equivalent to 0.16 of the supply from the stomach. The endogenous loss of NAN, however, was only 0.01 of duodenal NAN. As shown in Table 13, estimates for the two data sets were not in agreement (Zinn and Owens, 1982, 0.01; Sharma et al., 1974, 0.24~. EXTENT OF TRUE ABSORPTION NAN and Amino Acidsfrom the Stomach Regression analysis used to estimate endogenous losses from the small intestine also provided estimates of true absorption (Table 13~. Expressed as a proportion of the N supply to the proximal duodenum, values obtained were: sheep, 0.75 NAN; lactating cattle, 0.78 NAN and

£ C~ _ =; Ct X ° c~4- Ct .= C~ ° ~ ~ o Ct 0 oo0 o oo - ' a0 0 < ~O co ~ c0 0 0 0 0 00 C~ ~0 * a) c~ ~ ~ 0 ~ co oo ~0 O C~ O _ C~ ~ ~ O ~ ~ ~CO OO 3 ==+~ +i ~ +1 +1 ~ +1 ~ +1~ ~ .o* * * ** * ~ oo =4 ~ ~ ~ C<) ~CO C;)C~ ~ C~ o ~ o ~ ~ ~ o oo ~ ooo o O +! +t +1 +1 +1 +1+1 C ._=4 ~ ~ _ C~ ~ ~ _ ~ ~ ~O - Ct oC~ O CO O C~ O C~ O CC O CD O~ O S:~=O O O O O O O O O O O OO O +! +1 +1 +1 +1 +1+1 E ·~-co oo ~ c~ 0 ~ ~ o co ~o ~ ~ ~ ~ ~ o ~ ~ ~ ~ _ ~ ~ ~ ~ ~ ~ ~ 3 ~ ~- +1 ~ +1 ~ +i ~ +1 ~ +1 ~ +1 ~ +1 o ;^ _ O C~ oo O C~ C~ (D co CO o _ & ~ ~ ~0O _ C~ ~ ~ C ~ ~C~ U) Q~,= ~ ~:;o +1 ~ +i ~ +1 ~ +! O- ~ +1Co ~ C~ 0a,oo .... O000 o ~ ~CO o ~ ~ ~-o C~ C~- CS ~ &'>< =5 ~C] - C] 00 ~ t~ 00 t- 1~ ~C] O) C~ ~ & 0- ~ == ~ +1 0 +1 ~ +1 0 +1 ~ ~ _ 0 0 t0-0 ~ZZ Z Z ~ ~C ~Z ~0 ~C) ,~ Ct ~¢ Z ~ZZ Z o O t_C~) ~C ~ Z C~_ _ _ _ ~ ci 00 - _ U) ~ ~~ ~~ ~ co , _ ~ ~ X _ E,e , O w 0 O _ ~s: A S S 9 ~ i ~i ; ~ ~ S S in cr A. 49 _ 1 Ct ~ o ~ .m o~ CtS = b4- C) C~ S: 3 ~ =0 =~ =5 11 == & c,,o 11 ~ S: "S: X E ~ ~ ct a4 i,6 11 o ~:5 ;~- ~ &- sO~ ~ 11 0 ~ U: ~ (~ X ~ ~ ~ ~ ~= ~ 6 _0 ~ ~ C: O ._ V: O ~ U3'c~ O ·_ Ct, -0 ~o O s Q ~ ~ ~ O C~ Z Z C~ o ~ ~ . _ ~ O o. o Ov V ~ O O ~ ~ ~; C; O O ~ ~ ~_ 4- ~_ ._ ._ ~ ~* *

50 Ruminant Nitrogen Usage 0.82 amino acids, nor~lactating cattle, 0.67 NAN and 0.83 amino acids. When endogenous loss is expressed as a proportion of the N supply to the duodenum, true absorption is the sum of apparent absorption and endogenous loss (i.e., apparent absorption = 0.65; endogenous loss = 0.10; then true absorption = 0.75~. Thus, the low value for true absorption of duodenal NAN from the small intes- tir~e of nonlactating cattle is ~ consequence of the small correction for endoger~ous loss. Our estimates of true absorption obtained by regres- sion analysis are in agreement with the few available published reports (Table 14~. With 23 diets, true ab- sorption of NAN from the small intestine of sheep was 0.76; true absorption of essential amino acids was 0.80 (Hogan and Weston, 1970~. Tas et al. (1981) obtained a mean value in sheep of 0.86 for true absorption of amino acids. Using data from Nolan's (1975) model of N utili- zation in sheep, true absorption was 0.80 of the NAN supply to the duodenum. Values suggested for true ab TABLE 14 Summary of True Absorption From the Small Intestine sorption of NAN and amino acids from the small intes- tine are 0.75 and 0.80 of amounts entering the duode num. Microbial Protein Information on the true digestibility of microbial N (BCP) is summarized in Table 14. In experiments re- viewed by Bergen (1978), digestion of pure cultures of rumen bacteria in vitro ranged from 0.44 to 0.93. In data summarized by Chalupa (1972) and Zinn and Owens (1982~7 true absorption of rumen bacterial and protozoa! protein in rats was 0.66 and 0.88, respec- tively. Labeling ruminal bacteria with 35S yielded val- ues of 0.74 (Bird, 1972) and 0.85 (Salter and Smith, 1977~. Early studies with i5N-labelecl rumen bacteria (Smith et al., 1974) gave low and variable estimates of true absorption (0.41 to 0.70~. In later studies (Salter and Smith, 1977) a value of 0.79 was obtained. i5N can label compounds such as DAP to give variable and low Source of True Fraction Specie Method Data Absorption NAN Amino acids Microbial N (BCP) Microbial amino acids (BTP) Escape N (UIP) Escape amino acids Endogenous amino acids Sheep Sheep Sheep Lactating cattle Nonlactating cattle Sheep (EAA) Sheep Lactating cattle Nonlactating cattle Rat Rat Sheep Sheep Sheep Nonlactating cattle Regression Regression 15N Regression Regression Regression Regression Regression Regression Isolated bacteria Isolated protozoa 35S-bacteria 35S-bacteria i5N-bacteria Regression Sheep Regression Sheep t5N Leaf protein Sheep t4C Chloroplast protein Nonlactating cattle Regression Sheep Sheep Regression Regression Table 12 Hogan and Weston (1970) Nolan (1975) Table 12 Table 12 X SD CV(~) Hogan and Weston (1970) Tas et al. (1981) Table 12 Table 12 X SD CV Chalupa (1972) Zinn and Owens (1982) Bird (1972) Salter and Smith (1977) Salter and Smith (1977) Zinn and Owens (1982) X SD CV(~o) Tas et al. (1981) Salter and Smith (1977) Smith et al. (1974) Zinn and Owens (1982) X SC CV(%) Tas et al. (1981) Tas et al. (1981) 0.78 0.76 0.80 0.-8 0.67 0.76 0.05 0.06 0.80 0.86 0.82 0.83 0.83 0.03 0.03 0.66a 0.88a 0.74 0.85 0.79 0.73 0.78 0.05 0.06 0.87 0.85 0.73-0.82 0.68 0.77 0.09 0.11 0.82 0.7&-0.84 UAssuming equal biomasses of bacterial and protozoa! nitrogen.

estimates of true absorption. Multiple regression analy- sis yielder] estimates of 0.87 for microbial amino acids (Tas et al., 1981) and 0.73 for microbial N (Zinn and Owens, 1982~. However, in the data of Zinn and Owens (1982), estimates of endogenous losses were small ant] true absorption of NAN was almost identical to appar- ent absorption. As indicated in a previous section, microbial N is 0.10 to 0.20 nucleic acid N. Data summarized by Bergen (1978) and Smith (1975) indicate that digestion and ab- sorption of nucleic acids is an efficient process. In sheep and cattle, 0.75 to 0.90 of the nucleic acids that enter the proximal duodenum are removed prior to the terminal ileum. Undegraded Intake Protein Estimates of the true absorption of protein that es- capes ruminal degradation (UIP) have been obtained by isotonically labeling plant materials and by regression analysis (Table 14~. Absorption of leaf protein labeled with i5N was 0.85 (Salter and Smith, 1977), while 0.73 to 0.82 of i4C-labeled chloroplast protein was absorbed (Smith et al., 1974~. By regression analysis, the true ab sorption of escape amino acids in sheep was 0.82 (Tas et al., 1981~. The true absorption of escape NAN in non- lactating cattle was, however, only 0.68 (Zinn and Owens, 1982~. Endogenous Nitrogen Information on the true absorption of amino acids in the endogenous influx to the small intestine is scarce. Applying regression techniques to data from sheep (Tas et al., 1981) yielded values of 0.78 to 0.84 (Table 14~. NITROGEN METABOLISM IN INTESTINAL TISSUE A substantial part of most amino acids apparently ab- sorbed from the small intestine are metabolized in pro- cesses associated with absorption (Bergman and Heit- man, 1978; MacRae, 1978; Tamminga and Oldham, 1980). An estimate of amino acids metabolized by intestinal tissue can be obtained by comparing amino acids disap- pearing from the small intestine with those appearing in portal blood. In sheep fed 800 g/d of a high-protein diet (19.8 percent) or 650 g/d of a me~lium-protein (15.6 per- cent) diet, 0.67 to 0.71 percent and 0.55 to 0.57 percent, respectively, of the amino acids absorbed from the small intestine were metabolized in the intestinal wall (Tagari and Bergman, 1978~. No preference appeared for either essential or nonessential amino acids. Digestion and Absorption in the Small Intestine 51 Measurements of absorption based upon appearance of amino acids in blood (Hume et al., 1972, Sniffen and Jacobson, 1975), therefore, reflect the balance of re- moval from intestinal contents and metabolism in intes- tinal tissue. SYNOPSIS Summaries of apparent and true absorption of NAN and amino acids from the small intestine of sheep and cattle are in Tables 11 and 13. These data suggest that absorption from the small intestine does not vary greatly. True absorption of microbial N (BCP) and N from dietary protein (IUP) that escaped ruminal degradation are similar. This is expected on the basis of the constancy of apparent and true absorption of NAN and amino ac- ids. Bacterial cells might be less digestible because of mucopeptides in bacterial cell walls. However, even though DAP passes quantatively from the small intes- tine (Mason armful White, 1971), cell walls and cell con- tents of 35S-labelecl rumen bacteria are digested to the same extent (Bird, 1972~. Duodenal N in animals fed purified flints containing urea or casein is largely microbial (BCP), whereas it is a mixture of BCP and IUP with purified diets containing plant proteins and with diets consisting entirely of natu- ral feed ingredients. In some experiments apparent ab- sorbability of NAN from the small intestine was similar in animals fed natural diets or purified flints in which urea was the sole source of N (Salter and Smith, 1977; Zinn and Owens, 1982~. In studies summarized by Arm- strong et al. (1977~7 apparent disappearance of amino acid nitrogen was 0.70, 0.69, 0.64, and 0.76 for purified diets containing urea, casein, corn gluten meal, and field beans. These data imply that escape amino acid N from corn gluten meal is less digestible, whereas escape amino acid N from field beans is more digestible than microbial amino acid N. Level of feed intake could affect absorption of N from the small intestine by influencing passage rate of digesta or by adjusting proportionate amounts of microbial and undegraded feed N. Absorption of amino acid N in sheep fed chopped dried grass or pelleted ciried grass at 900 or 1400 g/d was 0.06 + 0.05 greater at the low level of feed intake (Armstrong et al., 1977~. On the other hand, Zinn and Owens (1981a,b, 1982) observer] that increasing intake of a high-grain diet increased both by- pass of dietary protein and apparent absorption of NAN *om the small intestine. Although there are no comparable data with cows, studies with rats and ewes suggest that absorption may be a more efficient process in lactation compared with pregnancy and in pregnancy compared with the non

52 Ruminant Nitrogen Usage pregnant, nonlactating state. This is a consequence of an enhancer] absorptive area of the gut and an increased partition of cardiac output to the gut (Oldham, 1981, 1984). The potential for protecting feed protein from degra- dation in the rumen was discussed in a previous section. In many early experiments, lack of improvements in an- imal performance was often a consequence of overpro- tection and decreased intestinal digestibility (Chalupa, 1975a, 1984~. In more recent experiments, treatment of a wide range of feedstuffs with formaldehyde substan tially increased the amounts of leucine, isoleucine, valine, histidine, arginine, and phenylalanine absorbed from the small intestine (Barry, 1976~. Absorption of ly- sine, threonine, and sulfur-containing amino acids was increased little or in some experiments decreased. Based upon data summarizer! in this chapter and by other investigators, the following values are suggested: apparent absorption of NAN, 0.65; true absorption of NAN, 0.75; apparent absorption of amino acids, 0.70; and true absorption of amino acids, 0.80.

Next: 8 Nitrogen and Absorption in the Large Intestine »
Ruminant Nitrogen Usage Get This Book
×
Buy Paperback | $45.00
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

This book brings together the latest research on protein absorption by ruminants and takes a look at the calculation of optimum nutrient requirements, including bacterial digestion, in the calculations. It also describes the parameters of nitrogen conversion in the ruminant and examines the different kinds of protein found in animal feedstuffs. "Animal Feed Science and Technology" calls it "essential for all scientists and teachers actively working in ruminant nutrition research and instruction."

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!