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Protein and Amino Acids Dietary protein generally refers to crude protein (CP), which is defined for foodstuffs as the nitrogen (N) content x 6.25. The definition is based on the assumption that the average N content of foodstuffs is 16 g per 100 g of protein. The calculated CP content includes both protein and nonprotein N (NPN). Feedstuffs vary widely in their relative proportions of protein and NPN, in the rate and extent of ruminal degradation of protein, and in the intesti- nal digestibility and amino acid (AA) composition of rumi- nally undegraded feed protein. The NPN in feed and sup- plements such as urea and ammonium salts are considered to be degraded completely in the rumen. IMPORTANCE AND GOALS OF PROTEIN AND AMINO ACID NUTRITION Ruminally synthesized microbial CP (MCP), ruminally undegraded feed CP (RUP), and to a much lesser extent, endogenous CP (ECP) contribute to passage of metaboliz- able protein (MP) to the small intestine. Metabolizable protein is defined as the true protein that is digested postru- minally and the component AA absorbed by the intestine. Amino acids, and not protein per se, are the required nutrients. Absorbed AA, used principally as building blocks for the synthesis of proteins, are vital to the maintenance, growth, reproduction, and lactation of dairy cattle. Presum- ably, an ideal pattern of absorbed AA exists for each of these physiologic functions. The Nutrient Requirements of Poultry (National Research Council, 1994) and the Nutri- ent Requirements of Swine (National Research Council, 1998) indicate that an optimum AA profile exists in MP for each physiologic state of the animal and this is assumed to be true for dairy animals. The goals of ruminant protein nutrition are to provide adequate amounts of rumen-degradable protein (RDP) for optimal ruminal efficiency and to obtain the desired animal productivity with a minimum amount of dietary CP. Opti- mizing the efficiency of use of dietary CP requires selection of complementary feed proteins and NPN supplements that will provide the types and amounts of RDP that will meet, but not exceed, the N needs of ruminal microorgan- isms for maximal synthesis of MCP, and the types and amounts of digestible RUP that will optimize, in so far as possible, the profile and amounts of absorbed AA. As discussed later, research indicates that the nutritive value of MP for dairy cattle is determined by its profile of essen- tial AA (EAA) and probably also by the contribution of total EAA to MP. Improving the efficiency of protein and N usage while striving for optimal productivity is a matter of practical concern. Incentives include reduced feed costs per unit of lean tissue gain or milk protein produced, a desire for greater and more efficient yields of milk protein, creation of space in the diet for other nutrients that will enhance production, and concerns of waste N disposal. Regarding milk protein production, research indicates that content (and thus yield) of milk protein can be increased by improving the profile of AA in MP, by reducing the amount of "surplus" protein in the diet, and by increasing the amount of fermentable carbohydrate in the diet. Major Differences from Previous Edition In 1985, the Subcommittee on Nitrogen Usage in Rumi- nants (National Research Council, 1985) expressed protein requirements in units of absorbed protein. Absorbed pro- tein was defined as the digestible true protein (i.e., digest- ible total AA) that is provided to the animal by ruminally synthesized MCP and feed protein that escaped ruminal degradation. This approach was adopted for the previous edition of this publication (National Research Council, 19891. The absorbed protein method introduced the con- cept of degraded intake CP (DIP) and undegraded intake CP (UIP). Mean values of ruminal undegradability for common feeds, derived from in viva and in situ studies using sheep and cattle, were reported. This factorial approach for estimating protein requirements recognized the three fates of dietary protein (fermentative digestion 43

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44 Nutrient Requirements of Dairy Cattle in the reticulo-rumen, hydrolytic/enzymatic digestion in the intestine, end passage of indigestible protein with feces) and separated the requirements of ruminal microorganisms from those of the host animal. However, a fixed intestinal digestibility of 80 percent for UIP was used, no consider- ation was given to the contribution of endogenous CP to MP, and no consideration was given to the AA composition of UIP or of absorbed protein. Some differences exist in terminology. To be consistent with the current edition of Nutrient Requirements of Beef Cattle (National Research Council, 1996), and to avoid implications that proteins are absorbed, the term MP replaces absorbed protein. To be consistent with the ~our- nal of Dairy Science, the terms DIP and UIP are replaced with RDP and RUP, respectively. The primary differences between the protein system of this publication and that used in the previous edition relate to predicting nutrient supply. Microbial CP flows are pre- dicted from intake of total tract digestible organic matter (OM) instead of net energy intake. The regression equation considers the variability in efficiency of MCP production associated with apparent adequacy of RDP. A mechanistic system developed from in situ data is used for calculating the RUP content of feedstuffs. Insofar as regression equa- tions allow, the system considers some ofthe factors (DMI, percentage of concentrate feeds in diet DM, and percent- age NDF in diet DM) that affect rates of passage of undi- gested feed and thus the RUP content of a feedstuff. The system is considered to be applicable to all dairy animals with body weights greater than 100 kg and that are fed for early rumen development. To increase the accuracy of estimating the contribution of the RUP fraction of individ- ual foodstuffs to MP, estimates of intestinal digestibility have been assigned to the RUP fraction of each foodstuff (range = 50 to 1001. Endogenous protein and NPN also are considered to contribute to passage of CP to the small intestine. Endogenous CP flows are calculated from intake of DM. And finally, regression equations are included that predict directly the content of each EAA in total EAA of duodenal protein and flows of total EAA. Flows of digest- ible EAA and their contribution to MP are calculated. Dose-response curves that relate measured milk protein content and yield responses to changes of predicted per- centages of digestible Lys and Met in MP are presented. The dose-response relationships provide estimates of model-determined amounts of Lys and Met required in MP for optimal utilization of absorbed AA for milk protein production. The inclusion of equations for predicting pas- sage of EAA to the small intestine along with assignment of RUP digestibility values that are unique to individual foodstuffs brings awareness to differences in nutritive value of RUP from different foodstuffs and should improve the prediction of animal responses to substitution of protein sources. P RO TE I N Chemistry of Feed Crude Protein Feedstuffs contain numerous different proteins and sev- eral types of NPN compounds. Proteins are large molecules that differ in size, shape, function, solubility, and AA com- position. Proteins have been classified on the basis of their 3-dimensional structure and solubility characteristics. Examples of classifications based on solubility would include globular proteins Falbumins (soluble in water and alkali solutions and insoluble in salt and alcohol), globulins (soluble in salt and alkali solutions and sparingly soluble or insoluble in water and insoluble in alcohol), glutelins (soluble only in alkali), prolamines (soluble in 70 to 80 percent ethanol and alkali and insoluble in water, salt, and absolute alcohol), histories (soluble in water and salt solutions and insoluble in ammonium hydroxide)] and fibrous proteins Le.g., collagens, elastins, and keratins (insoluble in water or salt solutions and resistant to diges- tive enzymes)] (Orten and Neuhaus, 1975; Rodwell, 1985; Van Soest, 19941. Globular proteins are common to all foodstuffs whereas fibrous proteins are limited to feeds of animal and marine origin. Albumins and globular proteins are low molecular weight proteins. Prolamines and glutel- ins are higher molecular weight proteins and contain more disulfide bonds. Generally, feeds of plant origin contain all of the globular proteins but in differing amounts. For example, cereal grains and by-product feeds derived from cereal grains contain more glutelins and prolamines whereas leaves and stems are rich in albumins (Blethen et al., 1990; Sniffen, 1974; Van Soest, 19941. A sequential extraction of 38 different feeds with water, dilute salt (0.5 percent NaCl), aqueous alcohol (80 percent ethanol), and dilute alkali (0.2 percent NaOH) indicated that the classic protein fractions (albumins, globulins, prolamines, and glu- telins) plus NPN accounted for an average of 65 percent of total N (Blethen et al., 19901. The unaccounted for, insoluble N would include protein bound in intact aleurone granules of cereal grains, most of the cell-wall associated proteins, and some of the chloroplasmic and heat-dena- tured proteins that are associated with NDF (Van Soest, 19941. Among the feeds that were evaluated, those with the highest percentage of insoluble protein (> 40 percent of CP) were forages, beet pulp, soy hulls, sorghum, dried brewers grains, dried distillers grains, fish meal, and meat and bone meal (Blethen et al., 19901. Feedstuffs also contain variable amounts of low molecu- lar weight NPN compounds. These compounds include peptides, free AA, nucleic acids, amides, amines, and ammonia. Nonprotein N compounds generally are deter- mined as the N remaining in the filtrate after precipitation of the true protein with either tungstic or trichloroacetic acid (Licitra et al., 19961. Grasses and legume forages contain the highest and most variable concentrations of

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Protein and Amino Acids 45 NPN. Most of the reported concentrations of NPN in CP of grasses and legume forages are within the following ranges: fresh material (lOB15%), hay (15B25%), and silage (30B65%) (Fairbairn et al., 1988; Garcia et al., 1989; Grum et al., 1991; Hughes, 1970; Krishnamoorthy et al., 1982; Messman et al., 1994; Van Soest, 1994; Xu et al., 19961. Hays and especially silages contain higher amounts of NPN than the same feed when fresh because of the proteolysis that occurs during wilting and fermentation. The proteoly- sis that occurs in forages during wilting and ensiling is a result of plant and microbial proteases and peptidases. Plant proteases and peptidases are active in cut forage and are considered to be the principal enzymes responsible for the conversion of true protein to NPN in hays and ensiled feeds (Fairbairn et al., 1988; Van Soest, 19941. Rapid wilt- ing of cut forages and conditions that promote rapid reduc- tions in pH of ensiled feeds slow proteolysis and reduce the conversion of true protein to NPN (Garcia et al., 1989; Van Soest, 19941. The NPN content of fresh forage is composed largely of peptides, free AA, and nitrates (Van Soest, 19941. Fermented forages have a different composi- tion of NPN than fresh forages. Fermented forages have higher proportional concentrations of free AA, ammonia, and amines and lower concentrations of peptides and nitrate (Fairbairn et al., 1988; Van Soest, 19941. The NPN content of most non-forage feeds is 12 percent or less of CP (Krishnamoorthy et al., 1982; Licitra et al., 1996; Van Soest, 1994; Xu et al., 19961. Mechanism of Ruminal Protein Degradation The potentially fermentable pool of protein includes feed proteins plus the endogenous proteins of saliva, sloughed epithelial cells, and the remains of lysed ruminal microorganisms. The mechanism of ruminal degradation has been reviewed (Broderick et al.,1991; Broderick, 1998; Cotta and Hespell, 1984; ;[ouany, 1996; ;[ouany and Ushida, 1999; Wallace, 1996; Wallace et al., 19991. In brief, all of the enzymatic activity of ruminal protein degradation is of microbial origin. Many strains and species of bacteria, protozoa, and anaerobic fungi participate by elaborating a variety of proteases, peptidases, and deaminases (Wallace, 19961. The liberated peptides, AA, and ammonia are nutri- ents for the growth of ruminal microorganisms. Peptide breakdown to AA must occur before AA are incorporated into microbial protein (Wallace, 19961. When protein deg- radation exceeds the rate of AA and ammonia assimilation into microbial protein, peptide and AA catabolism leads to excessive ruminal ammonia concentrations. Some of the peptides and AA not incorporated into microbial protein may escape ruminal degradation to ammonia and become sources of absorbed AA to the host animal. Bacteria are the principal microorganisms involved in protein degradation. Bacteria are the most abundant micro- organisms in the rumen (10~-~/ml) and 40 percent or more of isolated species exhibit proteolytic activity (Broderick et al., 1991; Cotta and Hespell, 1984; Wallace, 19961. Most bacterial proteases are associated with the cell surface (Kopecny and Wallace, 19821; only about 10 percent of the total proteolytic activity is cell free (Broderick, 19981. Therefore, the initial step in protein degradation by rumi- nal bacteria is adsorption of soluble proteins to bacteria (Nugent and Mangan, 1981; Wallace, 1985) or adsorption of bacteria to insoluble proteins (Broderick et al., 19911. Extracellular proteolysis gives rise to oligopeptides which are degraded further to small peptides and some free AA. Following bacterial uptake of small peptides and free AA, there are f~ve distinct intracellular events: (1) cleavage of peptides to free AA, (2) utilization of free AA for protein synthesis, (3) catabolism of free AA to ammonia and carbon skeletons (i.e., deamination), (4) utilization of ammonia for resynthesis of AA, and (5) diffusion of ammonia out of the cell (Broderick, 19981. The bacterial population that is responsible for AA deamination has been of considerable interest. Amino acid catabolism and ammonia production in excess of bacterial need wastes dietary CP and reduces efficiency of use of RDP for ruminant production. For many years it was assumed that deamination was limited to the large number of species of bacteria that had been identif~ed to produce ammonia from protein or protein hydrolyzates (Wallace, 1996~. However, this assumption was challenged by Russell and co-workers (Chen and Russell, 1988,1989; Russell et al., 1988) who concluded that the deaminative activity of these bacteria was too low to account for rates of ammonia production usually observed in vivo or in vitro with mixed cultures. Their efforts led to the eventual isolation of a small group of bacteria that had exceptionally high deaminative activity and that used AA as their main source of carbon and energy (Russell et al., 1988; Paster et al., 1993~. As a result of these and other studies, it is now accepted that AA deamination by bacteria is carried out by a combination of numerous bacteria with low deaminative activity and a much smaller number of bacteria with high activity (Wal- lace, 1996~. Of particular interest has been the observation that the growth of some of these bacteria with high deami- nating activity is suppressed by the ionophore, monensin (Chen and Russell, 1988,1989; Russell et al., 1988~. Protozoa also are active and significant participants in ruminal protein degradation. Protozoa are less numerous than bacteria in ruminal contents (105-6/ml) but because of their large size, they comprise a signif~cant portion of the total microbial biomass in the rumen (generally less than 10 percent but sometimes as high as 50 percent) (;Jouany, 1996;;Jouany and Ushida, 19991. Several differ- ences exist between protozoa and bacteria in their metabo- lism of protein. First, they differ in feeding behavior. Instead of forming a complex with feeds, protozoa ingest

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46 Nutrient Requirements of Dairy CattIe particulate matter (bacteria, fungi, and small feed parti- cles). Bacteria are their principal source of ingested protein (;[ouany and Ushida, 19991. As a result of this feeding behavior (i.e., ingestion of food), protozoa are more active in degrading insoluble feed proteins (e.g., soybean meal or fish meal) than more soluble feed proteins (e.g., casein) (Hino and Russell, 1987; ;[ouany, 1996; ;[ouany and Ushida, 19991. Ingested proteins are degraded within the cell to yield a mixture of peptides and free AA; the AA are incorpo- rated into protozoa! protein. Proteolytic specific activity of protozoa is higher than that of bacteria (Nolan, 19931. A second difference between protozoa and bacteria is that while both actively deaminate AA, protozoa are not able to synthesize AA from ammonia (;[ouany and Ushida, 19991. Thus, protozoa are net exporters of ammonia and because of this, defaunation decreases ruminal ammonia concentra- tions (;[ouany and Ushida, 19991. And lastly, protozoa release large amounts of peptides and AA as well as pepti- dases into ruminal fluid. This is the result of significant secretory processes and significant autolysis and death (Coleman, 1985; DiJkstra, 19941. ;Jouany and Ushida (1999) suggest that excreted small peptides and AA can represent 50 percent of total protein ingested by protozoa. Other studies indicate that 65 percent or more of protozoa! pro- tein recycles within the rumen (Ffoulkes and Leng, 1988; Punia et al., 19921. Much less is known about the involvement of fungi in ruminal protein catabolism. Currently, anaerobic fungi are considered to have negligible effects on ruminal protein digestion because of their low concentrations in ruminal digesta (103-4/ml) (;[ouany and Ushida, 1999; Wallace and Monroe, 19861. Kinetics of Ruminal Protein Degradation Ruminal degradation of dietary feed CP is an important factor influencing ruminal fermentation and AA supply to dairy cattle. RDP and RUP are two components of dietary feed CP that have separate and distinct functions. Rumina- lly degraded feed CP provides a mixture of peptides, free AA, and ammonia for microbial growth and synthesis of microbial protein. Ruminally synthesized microbial protein typically supplies most of the AA passing to the small intes- tine. Ruminally undegraded protein is the second most important source of absorbable AA to the animal. Knowl- edge of the kinetics of ruminal degradation of feed proteins is fundamental to formulating diets for adequate amounts of RDP for rumen microorganisms and adequate amounts of RUP for the host animal. Ruminal protein degradation is described most often by first order mass action models. An important feature of these models is that they consider that the CP fraction of foodstuffs consists of multiple fractions that differ widely in rates of degradation, and that ruminal disappearance of protein is the result of two simultaneous activities, degrada- tion and passage. One of the more complex of these models is the Cornell Net Carbohydrate Protein System (CNCPS) (Sniffer et al., 19921. In this model, feed CP is divided into five fractions (A, Be, B2, B3, and C) which sum to unity. The five fractions have different rates of ruminal degradation. Fraction A (NPN) is the percentage of CP that is instantaneously solubilized at time zero, which is assumed to have a degradation rate (k,) of infinity; it is determined chemically as that proportion of CP that is soluble in borate-phosphate buffer but not precipitated with the protein denaturant, trichloroacetic acetic (TCA) (Figure 5-11. Fraction C is determined chemically as the percentage of total CP recovered with ADF (i.e., ADIN) and is considered to be undegradable. Fraction C contains proteins associated with lignin and tannins and heat-dam- aged proteins such as the Maillard reaction products (Snif- fen et al., 19921. The remaining B fractions represent potentially degradable true protein. The amounts of each of these 3 fractions that are degraded in the rumen are determined by their fractional rates of degradation Ski ~ and passage (kp); a single kp value is used for all fractions. Fraction Be is that percentage of total CP that is soluble in borate-phosphate buffer and precipitated with TCA. Fraction B3 is calculated as the difference between the portions of total CP recovered with NDF (i.e., NDIN) and ADF (i.e., fraction C). Fraction B2 is the remaining CP and is calculated as total CP minus the sum of fractions A, Be, B3, and C. Reported ranges for the fractional rates of degradation for the three B fractions are: BE (120-400 %/h), B2 (3-16 %/h), and B3 (0.06-0.55 %/h). The RDP and RUP values (percent of CP) for a foodstuff using this model are computed using the equations RDP = A + BE Ek, BE / (k, BE + kp)] + B2 Ek,B2 / (k,B2 + kp)] + B3 Ek,B3 / (k,B3 + kp)] and RUP = BE Ekp / (k, BE + kp)] +B2Lkp/(k,B2+kp)] +B3Lkp/(k,B3+kp)]+C. This model is used in Level II of the Nutrient Requirements of Beef Cattle (National Research Council, 1996) report. The most used model to describe in situ ruminal protein degradation divides feed CP into three fractions (A, B. and C). Fraction A is the percentage of total CP that is NPN (i.e., assumed to be instantly degraded) and a small amount of true protein that rapidly escapes from the in situ bag because of high solubility or very small particle size. Frac- tion C is the percentage of CP that is completely undegrad- able; this fraction generally is determined as the feed CP remaining in the bag at a defined end-point of degradation. Fraction B is the rest of the CP and includes the proteins

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Protein and Amino Acids 47 TOTAL 1 -1 1 | BORATE l l NEUTRAL l | BUFFER | | DETERGENT | 1 A INSOL B2 B3 . C 1 A _ SZ . I NSOL B3 C ACID DETERGENT A . INSOL B2 B3 FIGURE 5-1 Analyses of crude protein fractions using borate- phosphate buffer and acid detergent and neutral detergent solu- tions (Roe et al., 1990; Sniffen et al, 1992~. that are potentially degradable. Only the B fraction is con- sidered to be affected by relative rates of passage; all of fraction A is considered to be degraded and all of fraction C is considered to pass to the small intestine. The amount of fraction B that is degraded in the rumen is determined by the fractional rate of degradation that is determined in the study for fraction B and an estimate of fractional rates of passage. The RDP and RUP values for a feedstuff (per- cent of CP) using this model are computed using the equa- tions RDP = A + B Ek, / (k, + kp)] and RUP = B Ekp / Ski + kp)] + C. This simple model has been the most widely used model for describing degradation and ruminal escape of feed proteins (e.g., AFRC, 1984; National Research Council, 1985; 0rskov and McDonald, 19791. It is noted that data obtained from in situ, in vitro, and enzy- matic digestions generally fit a model that divides feed CP into these fractions (Broderick et al., 1991) and that most of the in situ data used to validate results obtained with cell-free proteases have been obtained using this model (Broderick, 19981. As discussed later, it is this model in conjunction with in situ derived data that is used for pre- dicting ruminal protein degradability in this edition. Numerous factors affect the amount of CP in feeds that will be degraded in the rumen. The chemistry of feed CP is the single most important factor. The two most important considerations of feed CP chemistry are: (1) the propor- tional concentrations of NPN and true protein, and (2) the physical and chemical characteristics of the proteins that comprise the true protein fraction ofthe feedstuff. Nonpro- tein N compounds are degraded so quickly in the rumen (~300%/h) that degradation is assumed to be 100 percent (Sniffer et al., 19921. However, this is not an entirely correct assumption because degradability is truly related to rate of passage. For example, assuming a kp of 2.0%/h andak~ of 300%/h,then degradation = 3.00/~3.00 + 0.02) = 0.993 or 99.3 percent, and not 1.00 or 100 percent. Feedstuffs that contain high concentrations of NPN in CP contribute little RUP to the host animal. When dairy cattle are fed all-forage diets, measurements of passage of non- ammonia, non-microbial N (i.e., RUP-N plus endogenous N) often are less than 30 percent of N intake (Beever et al., 1976, 1987; Holden et al., 1994a; Van Vuuren et al., 19921. In contrast to NPN, which is assumed to be com- pletely degraded, the rates of degradation of proteins are highly variable and result in variable amounts of protein being degraded in the rumen. For example, the range in k~ given in Tables 15-2a,b are 1.4 for Menhaden f~sh meal to 29.2 for sunflower meal. Assuming a kp for each feed of 7.0 percent, the range in degradabilites of the B fraction would be 16.7 to 80.7 percent. Some characteristics of proteins shown to contribute to differences in rates of degradation are differences in 3-dimensional structure, dif- ferences in intra- and inter-molecular bonding, inert barri- ers such as cell walls, and antinutritional factors. Differences in 3-dimensional structure and chemical bonding (i.e., cross-links) that occur both within and between protein molecules and between proteins and car- bohydrates are functions of source as well as processing. These aspects of structure affect microbial access to the proteins, which apparently is the most important factor affecting the rate and extent of degradation of proteins in the rumen. Proteins that possess extensive cross-linking, such as the disulfide bonding in albumins and immunoglob- ulins or cross-links caused by chemical or heat treatment, are less accessible to proteolytic enzymes and are degraded more slowly (Ferguson, 1975; Hurrell and Finot, 1985; Mahadevan et al., 1980; Mangan, 1972; Nugent and Man- gan, 1978; Nugent et al., 1983; Wallace, 19831. Proteins in feathers and hair are extensively cross-linked with disulf~de bonds and largely for that reason, a considerable amount of the protein in feather meal is in fraction C (Tables 15- 2a,b). Similarly, a considerable portion of the protein in meat meal and meat and bone meal is in fraction C. Proteins in meat meal and meat and bone meal may contain considerable amounts of collagen that has both intramolecular and inter- molecular cross-links (Orten and Neuhaus, 19751. In contrast, a majority of the protein in menhaden fish meal is in fraction B but the fractional rate of degradation of fraction B is slower than in other protein supplements (Tables 15-2a,b). Heat used in the drying of f~sh protein was shown to induce the formation of disulf~de bonds (Opstvedt et al., 19841. Heat processing also coagulates protein in meat products which makes it insoluble (Bendall, 1964; Boehme, 1982), and cool- ing of the products causes a random relinkage of chemical bonds which shrinks the protein molecules (Bendall, 19641. Collectively, these effects of heating and cooling of proteins decrease microbial access and make the proteins more resis- tant to ruminal degradation. Other factors affecting the ruminal degradability of feed protein include ruminal retention time of the protein, microbial proteolytic activity, and ruminal pH. The effect

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48 Nutrient Requirements of Dairy CattIe of these factors on the kinetics of ruminal protein degrada- tion have been reviewed (Broderick et al., 1991; National Research Council, 19851. Nitrogen Solubility vs. Protein Degradation Several commercial feed testing laboratories in the United States provide at least one measurement of N solu- bility for feedstuffs. Although recognized that N solubility in a single solvent is not synonymous with CP degradation in the rumen, the general absence of alternatives other than using "book values" for RUP (e.g., National Research Council, 1985) left little else to help nutritionists ensure that adequate but not excessive amounts of RDP were fed. Solubility measurements have been useful for ranking feeds of similar types for ruminal CP degradability. This is because of the positive relationship that exists between N solubility and degradation within similar foodstuffs (e.g., Beever et al., 1976; Laycock and Miller, 1981; Madsen and Hvelplund, 1990; Stutts et al., 19881. Many studies have indicated that changing N solubility by adding or removing NPN supplements, by changing method of forage preserva- tion, or processing conditions of protein supplements affects animal response (e.g., Aitchison et al., 1976; Crish et al., 1986; Lundquist et al., 19861. Several different solvents have been used. At present, the most common procedure is incubation in borate-phosphate buffer (Roe et al., 19901. This method has gained in popularity because it is used for determining the A and Be nitrogen fractions in the CNCPS (Sniffer et al., 19921. Although a high correlation exists between N solubility in a single solvent and protein degradability for similar feedstuffs, the same does not exist across classes of feed- stuffs. For example, Stern and Satter (1984) reported a correlation of 0.26 between N solubility and in viva protein degradation in the rumen of 34 diets that contained a variety of N sources. Madsen and Hvelplund (1990) also reported a poor relationship between N solubility and in viva degradation of CP when used over a range of feed- stuffs. There appear to be several reasons for these poor relationships. First, as indicated in the section "Chemistry of Feed Crude Protein", the proteins that are extracted by a solvent depend not only on the chemistry of the proteins but also on the composition of the solvent. For that reason, different solvents provide different estimates of CP solubility (Cherney et al., 1992; Crawford et al., 1978; Crooker et al., 1978; Lundquist et al., 1986; Stutts et al., 19881. Second, soluble proteins are not equally sus- ceptible to degradation by rumen enzymes. Among the pure soluble proteins, casein is degraded rapidly whereas serum albumin, ovalbumin, and ribonuclease A are degraded much slower (Annison, 1956; Mahadevan et al., 1980; Mangan, 19721. Mahadevan et al. (1980) also observed that soluble proteins from soybean meal, rape- seed meal, and fish meal were degraded at different rates with rates of degradation for all three supplements being intermediate between those for albumins and casein. Therefore, structure as well as solubility determines degra- dability. Third, as indicated in the section "Mechanism of Ruminal Protein Degradation", solubility is not a prerequi- site to degradation. As an example, Mahadevan et al. (1980) observed that soluble and insoluble proteins of soybean meal were hydrolyzed in vitro at almost identical rates. Because bacteria attach to insoluble proteins and because protozoa engulf feed particles, insoluble proteins need not enter the soluble protein pool before attack by microbial proteases. And last, soluble proteins that are not yet degraded may leave the rumen faster than insoluble pro- teins. This is because of a more likely association of soluble protein with the liquid fraction of ruminal contents. For example, Hristov and Broderick (1996) observed that although feed NAN in the liquid phase of ruminal contents was only 12 percent of total ruminal feed NAN, 30 percent of the feed NAN that escaped the rumen flowed with the liquids. This indicates a disproportional escape of solu- ble proteins. In conclusion, a change in N solubility in a single solvent appears to be a more useful indicator of a change in protein degradation when applied to different samples of the same foodstuff than when used to compare different foodstuffs that differ in chemical and physical properties. Clearly, the relationship between solubility and degradability is the highest when most of the soluble N is NPN (Sniffer et al., 19921. Microbial Requirements for N Substrates Peptides, AA, and ammonia are nutrients for the growth of ruminal bacteria; protozoa cannot use ammonia. Esti- mates of the contribution of ammonia versus preformed AA to microbial protein synthesis by the mixed rumen population have been highly variable (Wallace, 19971. Studies using Nit ammonia or urea infused into the rumen or added as a single dose demonstrated that values for microbial N derived from ammonia ranged from 18 to 100 percent (Salter et al., 19791. The Nit studies of Nolan (1975) and Leng and Nolan (1984) indicated that 50 per- cent or more of the microbial N was derived from ammonia and the rest from peptides and AA. The mixed ruminal microbial population has essentially no absolute require- ment for AA (Virtanen, 1966) as cross-feeding among bac- teria can meet individual requirements. However, researchers have observed improved microbial growth or efficiency when peptides or AA replaced ammonia or urea as the sole or major source of N (Cotta and Russell, 1982; Russell and Sniffen, 1984; Griswold et al., 19961. Maeng and Baldwin (1976) reported increased microbial yield and growth rate on 75% urea + 25% AA-N as compared to

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Protein and Amino Acids 49 100% urea. Microbial requirements for N substrates of ammonia-N, AA, and peptides can also be affected by the basal diet and may explain some of the variability in the above experiments. There is evidence that AA and especially peptides are stimulatory in terms of both growth rate and growth yield for ruminal microorganisms growing on rapidly degraded energy sources (Argyle and Baldwin, 1989; Chen et al., 1987; Cruz Soto et al., 1994; Russell et al., 19831. However, when energy substrates are fermented slowly, stimulation by peptides and AA does not always occur. Chikunya et al. (1996) demonstrated that when peptides were supplied with rapidly or slowly degraded fiber, microbial growth was enhanced only if the fiber was degraded rapidly. Russell et al. (1992) indicated that microorganisms fermenting struc- tural carbohydrates require only ammonia as their N source while species degrading nonstructural carbohydrate sources will benefit from preformed AA. Recent experiments (Wallace, 1997) have confirmed the earlier results of Salter et al. (1979) showing that the pro- portion of microbial N derived from ammonia varies according to the availability of N sources. The minimum contribution to microbial N from ammonia was 26 percent when high concentrations of peptides and AA were present, with a potential maximum of 100 percent when ammonia was the sole N source. Griswold et al. (1996) examined the effect of isolated soy protein, soy peptides, individual AA blended to profile soy protein, and urea on growth of microorganisms in continuous culture. Griswold et al. (1996) demonstrated that N forms other than ammonia are needed not only for maximum microbial growth but also as NPN for adequate ruminal fiber digestion. Many reports of the uptake of Ci4-AA and peptides have indicated that mixed microbial populations preferentially took up peptides rather than free AA (Cooper and Ling, 1985; Prins et al., 19791. However, Ling and Armstead (1995) found that free AA were the preferred form of AA incorporated by S. Louis, Selenomonas ruminantium, Fibrobacter succinogenes and Anaerovibrio lipolytica, whereas peptides were preferred only by P. ruminicola. P. ruminicola can comprise greater than 60 percent of the total flora in sheep fed grass silage (Van Gylswyk, 19901. In other studies where an AA preference was exhibited, the preference may have been the result of specific dietary conditions where P. ruminicola numbers were lower. Wal- lace (1996) demonstrated that AA deamination is carried out by two distinct bacterial populations, one with low activity and high numbers and the other with high activity and low numbers. P. ruminicola occurs in high numbers but has low deaminase activity. Jones et al. (1998) investigated the effects of peptide concentrations in microbial metabolism in continuous cul- ture fermenters. The basal diet contained 17.S percent CP, 46.2 percent NSC, and 32.9 percent NDF. Peptides replaced urea as a N source at levels of O. TO, 20 and 30 percent of total N. a urea-molasses mixture represented 8.6, 7.O, 4.9, and 2.9 percent of DM with increasing peptide and glucose replacement. Digestion of DM and CP and microbial CP production were affected quadratically by peptide addition; the highest values for each variable occur- red at 10 percent peptide addition. Fiber digestion decreased linearly with increasing peptide addition. Reduced ammonia-N concentrations appeared to be the cause of reduced microbial CP production and reduced fiber digestion at levels of peptides greater than 10 percent of total N. The efficiency of conversion of peptide N to microbial CP increased with increasing peptides; however, there was no change in grams of microbial N produced per kilogram of OM digested. [ones et al. (1998) suggested that with diets containing high levels of NSC, excessive peptide concentrations relative to that of ammonia can depress protein digestion and ammonia concentrations, limit the growth of fiber-digesting microorganisms, and reduce ruminal fiber digestion and microbial protein pro- duction. Microorganisms that ferment NSC produce and utilize peptides at the expense of ammonia production from protein and other N sources (Russell et al., 19921. It should be noted that in continuous culture systems, proto- zoa can be washed out in the first few days of operation. Animal Responses to CP, RDP, and RUP EACTATION RESPONSES Crude protein. A data set of 393 means from 82 protein studies was used to evaluate the milk and milk protein yield responses to changes in the concentration of dietary CP (Table 5-11. The descriptive statistics for the data set are presented in Table 5-2. When CP content of diets change, the relative contribution of protein from different sources also change so this evaluation is confounded with source of protein and concentrations of RDP and RUP. Overall, milk yield increased quadratically as diet CP con- centrations increased. The regression equation obtained was: Milk yield = 0.8 x DMI + 2.3 x CP 0.05 x cp2 - 9.8 (r2 = 0.29) where milk yield and dry matter intake (DMI) are kilo- grams/d and CP is percent of diet DM. Dry matter intake was included in the regression to account indirectly for some of the differences among stud- ies such as basal milk production and BW. Dry matter intake accounted for about 60 percent and CP about 40 percent of non-random variation. Assuming a fixed DMI (there was no correlation between intake and CP percent in this data set), the maximum milk production was obtained at 23 percent CP. The marginal response to

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50 Nutrient Requirements of Dairy Cattle TABLE 5-1 Studies Used to Evaluate Milk and Milk Protein Yield Responses to Changes in the Concentration of Dietary Crude Protein Annexstad et al. (1987) Aharoni et al. (1993) Armentano et al. (1993) Atwal et al. (1995) Baker et al. (1995) Bertrand et al. (1998) Blauw~ekel and Kinca~d (1986) Blauw~ekel et al. (1990) Bowman et al. (1988) Broder~ck (1992) Broder~ck et al. (1990) Bruckental et al. (1989) Canf~eld et al. (1990) Casper et al. (1990) Chen et al. (1993) Christensen et al. (1993a, b) Crawley and Kilmer (1995) Cunningham et al. (1996) De Gracia et al. (1989) DePeters and Bath (1986) Dhiman and Satter (1993) Garcia-Bojalil et al. (1998a) Grant and Haddad (1998) Grinds et al. (1991) Grinds et al. (1992a) Grummer et al. (1996) Hadsell and Sommerfeldt (1988) Henderson et al. (1985) Henson et al. (1997) Higginbotham et al. (1989) Hoffman and Armentano (1988) Hoffman et al. (1991) Holter et al. (1992) Hongerholt and Muller (1998) Howard et al. (1987) Huyler et al. (1999) Jaquette et al. (1986) Jaquette et al. (1987) Maim et al. (1983) Maim et al. (1987) Kalscheur et al. (1999a,b) Kerry and Amos (1993) Khorasani et al. (1996a) Kim et al. (1991) King et al. (1990) Klusmeyer et al. (1990) Komaragir~ and Erdman (1997) Lees et al. (1990) Leonard and Block (1988) Lundquist et al. (1986) Macleod and Cahill (1987) Manson and Leaver (1988) Mantysaari et al. (1989) McCarthy et al. (1989) McCormick et al. (1999) McGuffey et al. (1990) Nakamura et al. (1992) Owen and Larson (1991) Palmquist and Weiss (1994) Palmquist et al. (1993) Polan et al. (1997) Polan et al. (1985) Powers et al. (1995) Robinson and Kennelly (1988b) Robinson et al. (199lb) Roseler et al. (1993) Santos et al. (1998a,b) Sloan et al. (1988) Spain et al. (1995) Voss et al. (1988) Wattiaux et al. (1994) Weigel et al. (1997) Wheeler et al. (1995) Windschitl (1991) Wohlt et al. (1991) Wright (1996) Wu et al. (1997) Wu and Satter (2000) Zimmerman et al. (1992) Zimmerman et al. (1991) TABLE 5-2 Descriptive Statistics for Data Set Used to Evaluate Animal Responses to CP and RDP Variable N Mean Std. Dev. Milk, kg/d Milk protein yield, g/d Day matter intake, kg/d CP, % of day matter RDP, % of day matter RUP, % of day matter 393 360 393 393 172 172 31.4 972 20.2 17.1 10.7 6.2 6.1 153 3.4 2.6 1.8 1.4 increased dietary CP (first derivative ofthe CP components of the regression equation) is: 2.3 - 0.1 x CP. Therefore, increasing dietary CP one percentage unit from 15 to 16 percent would be expected to increase milk yield an aver- age of 0.75 kg/d and increasing CP one percentage unit from 19 to 20 percent would be expected to increase milk yield by 0.35 kg/d. Although milk production may be increased by feeding diets with extremely high concentra- tions of CP, the economic and environmental costs must be compared with lower CP diets. The marginal response obtained from this data set was similar to that obtained by Roffler et al. (19861. With their equation, increasing dietary CP from 14 to 18 percent would result in an increase of 2.1 kg/d of milk and with the equation above the expected increase is 2.8 kg/d. Dietary CP was not correlated (P>0.25) with milk pro- tein percent, but was correlated weakly (r = 0.14; P<0.01) with milk protein yield (because of the relationship of dietary CP with milk yield). The regression equation was: milk protein yield (g/d) = 17.7 x DMI + 55.6 x CP 1.26 x cp2 + 31.8 (r2 = 0.19) where DMI is kilograms/ day and CP is percent of diet DM. Maximum yield of milk protein was obtained at 22 percent CP (essentially the same as for milk yield) and the marginal response is equal to 55.63 - 2.52 x CP where CP is a percent of diet DM. Rumen degradable and undegradable protein. A regres- sion approach also was used to evaluate lactation responses to concentrations of RDP and RUP in the dietary DM. To evaluate lactation responses to RDP in diet DM, 38 studies with 206 treatment means were selected in which diets varied in content of RDP (Table 5-31. All diets were entered into this edition's model for predicted concentra- tions of RDP and RUP in diet DM. As expected, concentra- tions of RDP and RUP (as percentages of diet DM) were correlated with concentrations of dietary CP (RDP; r = 0.78, P<0.001; RUP, r = 0.53, P<0.001), therefore it is not possible to separate effects of total CP from those of RDP or RUP. A regression equation for milk yield with RDP and RUP (both as percent of DM) was derived to overcome the problems associated with the correlation between CP and RDP and RUP (the correlation between RDP and RUP was not significant (r = - 0.11, P>0.051. Dietary RDP and RUP were calculated using the model described in this publication based on values in the data set described above. The regression equation also included DMI for the reasons explained above. The regression equa- tion (Figure 5-2) was: Milk = - 55.61 + 1.15 x DMI + 8.79 x RDP0.36 x RDP2 + 1.85 x RUP (r2 = 0.52)

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Protein and Amino Acids 51 TABLE 5-3 Studies Used to Evaluate Milk Yield Responses to Changes in the Concentration of Dietary Ruminally Degraded Protein Annexstad et al. (1987) Armentano et al. (1993) Baker et al. (1995) Barney et al. (1981) Bertrand et al. (1998) Blauwiekel et al. (1990) Casper et al. (1990) Christensen et al. (1993a,b) Cunningham et al. (1996) Dhiman and Satter (1993) Garcia-Bojalil et al. (1998a) Grant and Haddad (1998) Grings et al. (1991) 45 ~ , 4o Grings et al. (1992) Grummer et al. (1996) Ha and Kennelly (1984) Harris et al. (1992) Henson et al. (1997) Higginbotham et al. (1989) Hoffman et al. (1991) Holter et al. (1992) Hongerholt and Muller (1998) Kalscheur et al. (1999a) Khorasani et al. (1996b) Kim et al. (1991) lJP~ % aft 8 ~ 16 g92 ~ FIGURE 5-2 Response surface for data set described in "Ani- mal Responses to CP, RDP, and RUP" section. Maximum milk yield occurred at 12.2 percent RDP (percent of diet DM). Dry matter intake was held constant at 20.6 kg/day. where DMI and milk are kilograms/day, and RDP and RUP are percent of diet DM. Based on that equation, maximum milk yield occurred (DMI and RUP held con- stant) when RDP equaled 12.2 percent of diet DM, and the marginal change in milk to increasing RDP was 8.79 0.72 x RDP. The quadratic term for RUP was not significant and was removed from the model. Milk yield increase linearly to RUP at the rate of 1.85 kg for each percentage unit increase in RUP. In comparison this edition's model estimates an average RDP requirement of 10.2 percent for this data set. Pre- dicted milk yield (using the above regression equation) at 10.2 percent RDP (DMI and RUP held constant mean values of the data set of 20.6 kg/d DMI and 6.2 percent, respectively) is 31.7 kg/d and 33.2 kg/d when RDP is 12.2 percent. A portion of the discrepancy between model pre- dicted requirement for RDP and regression predicted max- imal milk production may be caused by the positive correla- tion between RDP and DM intake (DMI = 14.4 + 0.58 King et al. (1990) Komaragiri and Erdman (1997) Leonard and Block (1988) Mantysaari et al. (1989) McGuffey et al. (1990) Palmquist and Weiss (1994) Roseler et al. (1993) Santos et al. (1998a,b) Wattiaux et al. (1994) Weigel et al. (1997) Windschitl (1991) Wu and Satter (2000) x RDP; r = 0.35, P<0.0011. Based on that regression, an increase in 2 percentage units of RDP (i.e., 10.2 to 12.2 percent) would increase DMI by about 1.1 kg/d. Based on this edition's requirements (assumed 72 percent TDN), an increase of about 2 kg/d of milk is expected from that change in DMI. Increasing dietary RDP above model pre- dicted requirements may result in increased DM intake. A similar shaped function (data not shown) was obtained when milk protein yield was regressed on dietary RDP and RUP: Milk protein = - 1.57 + 0.0275 x DMI + 0.223 x RDP 0.0091 x RDP2 + 0.041 x RUP (r2 = 0.51) where milk protein and DMI are kilograms per day and RDP and RUP are percentages of dietary DM. Maximum milk protein yield occurred at 12.2 percent RDP (the same as milk yield). Milk protein yield increased linearly with increasing dietary RUP. Santos et al. (1998b) published a comprehensive review of the effects of replacing soybean meal with various sources of RUP on protein metabolism (29 published com- parisons) and production (127 published comparisons). Santos et al. (1998b) reported that in 76 percent of the metabolism studies, higher RUP decreased MCP flows to the small intestine. Supplementation with RUP usually did not affect flow of total EAA, and RUP supplementation usually did not increase or actually decreased flow of lysine to the duodenum. Supplementation of RUP increased milk production in only 17 percent of the studies and heat- treated or chemically-treated soybean meal or fish meal were the most likely RUP supplements to cause increased milk production (Santos et al., 1998b). When studies were combined, cows fed diets with treated soybean meal (P<0.03) or fish meal (P<0.01) produced statistically more milk than cows fed soybean meal. Cows fed other animal proteins (blood, feather, meat meals) or corn gluten meal produced similar or numerically less milk than cows fed soybean meal (Santos et al., 1998b). See additional discus- sion in Chapter 16.

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52 Nutrient Requirements of Dairy Cattle The regression equations derived above for milk and milk protein yield responses to dietary CP, RDP, and RUP should be interpreted and used cautiously in view of low r2 values. A more sophisticated statistical analysis (e.g., controlling for trial effects, adjusting for variances within trials, etc.) would probably yield different and more accu- rate coefficients. EFFECTS ON REPRODUCTION Protein in excess of lactation requirements has been shown to have negative effects on reproduction. Several workers have reported that feeding diets containing 19 percent or more CP in diet DM lowered conception rates (Bruckental et al., 1989; Canf~eld et al., 1990; Jordan and Swanson, 1979; McCormick et al., 19991. Others have observed that cows fed 20-23 percent CP diets (as com- pared to 12-15 percent CP) had decreased uterine pH, increased blood urea, and altered uterine fluid composition (Jordan et al., 1983; Elrod and Butler, 19931. In a majority of the studies reviewed by Butler (1998), plasma progester- one concentrations in early lactation cows were lower when diets contained 19-20 percent CP vs. lower concentrations of CP. In a review of protein effects on reproduction, Butler (1998) concluded that excessive amounts of either RDP or RUP could be responsible for lowered reproductive performance. However, intakes of"digestible" RUP in amounts required to adversely affect reproduction without a coinciding surplus of RDP would be uncommon. In most of the studies reviewed by Butler (1998), excessive RDP rather than excessive RUP was associated with decreased conception rates. Canf~eld et al. (1990) showed that feeding diets containing RUP to meet requirements while feeding RDP in excess of requirements resulted in decreased con- ception rates. Garcia-Bojalil et al. (1998b) reported that RDP fed in excess (15.7 percent of DM) of recommenda- tions decreased the amount of luteal tissue in ovaries of early lactation cows. Although most studies have indicated an adverse effect on reproductive performance of feeding high CP diets, others indicate no effect of diet CP on reproduction. Car- roll et al. (1988) observed no differences in pregnancy rate or first service conception rates of dairy cows fed 20 percent CP and 13 percent CP diets. Howard et al. (1987) reported no difference in fertility between cows in second and greater lactation fed 15 percent CP or 20 percent CP diets. There are many theories as to why excess dietary CP decreases reproductive performance (Barton, 1996a, 1996b; Butler, 1998; Ferguson and Chalupa, 19891. The first theory relates to the energy costs associated with meta- bolic disposal of excess N. To the extent that additional energy may be required for this purpose, this energy may be taken from body reserves in early lactation to support milk production. Delayed ovulation (e.g., Beam and Butler, 1997; Staples et al., 1990) and reduced fertility (Butler, 1998) have been associated with negative energy status. Another effect of negative energy status is decreased plasma progesterone concentrations (Butler, 19981. Another theory is that excessive blood urea N (BUN) concentrations could have a toxic effect on sperm, ova, or embryos, resulting in a decrease in fertility (Canf~eld et al., 19901. High BUN concentrations have also been shown to decrease uterine pH and prostaglandin production (But- ler, 19981. High BUN may also reduce the binding of leutinizing hormone to ovarian receptors, leading to decreases in serum progesterone concentration and fertil- ity (Barton, 1996a). Ferguson and Chalupa (1989) reported that by-products of N metabolism may alter the function of the hypophysealpituitary-ovarian axis, therefore decreasing reproductive performance. And last, high levels of circulat- ing ammonia may depress the immune system and, there- fore, may result in a decline in reproductive performance (Anderson and Barton, 19881. Milk urea nitrogen (MUN) and blood urea nitrogen (BUN) are both indicators of urea production by the liver. Milk urea N concentrations greater than 19 mg/dl have been associated with decreased fertility (Butler et al., 19951. Likewise, BUN concentrations greater than 20 ma/ dl have been linked with reduced conception rates in lactat- ing cows (Ferguson et al., 19881. Bruckental et al. (1989) found that BUN levels increased when diet CP was increased from 17 to 21.6 percent and pregnancy rate decreased by 13 percentage units. In a case study, Ferguson et al. (1988) observed that cows with BUN levels higher than 20 mg/dl were three times less likely to conceive than cows with lower BUN concentrations. Although high BUN concentrations have been associated with decreased repro- ductive performance, others have reported no adverse effects on pregnancy rate, services per conception, or days open with BUN levels above 20 m~/dl (Oldick and Fir- kins, 19961. Studies by Carroll et al. (1987) and Howard et al. (1987) indicate that maintaining a strict reproductive management protocol can reduce the negative effects of excess protein intake on reproduction. Barton (1996a) demonstrated that an intense reproductive program could be used to reach reproductive success regardless of diet CP level or plasma urea N concentrations. These studies highlight the idea that dietary protein is just one of many things that have an effect on reproductive performance. Protein intake, along with other factors such as reproductive management, energy status, milk yield, and health status all have an effect on reproductive performance in dairy cattle. in. Synchronizing Ruminal Protein and Carbohydrate Digestion: Effects on Microbial Protein Synthesis Microbial protein synthesis in the rumen depends largely on the availability of carbohydrates and N in the rumen.

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Protein and Amino Acids 53 Bacteria are capable generally of capturing the majority of ammonia that is released in the rumen from AA deamina- tion and the hydrolysis of NPN compounds. However, dietary conditions often occur in which the rate of ammonia release in the rumen exceeds the rate of uptake by ruminal bacteria. Examples of such conditions would include a surplus of RDP or a lack of available energy (Maeng et al., 19971. This asynchronous release of ammonia and energy in the rumen results in inefficient utilization of fermentable substrates and reduced synthesis of MCP. A variety of studies have focused on increasing the efficiency of micro- bial protein synthesis by manipulating dietary components (Aldrich et al., 1993a; Hoover and Stokes, 1991; Herrera- Saldana et al., 1990; Maeng et al., 19761. Excellent reviews describe the relationship between ruminal protein and car- bohydrate availability and its impact on MCP synthesis in the rumen (Hoover and Stokes, 1991; Clark et al., 1992; Stern et al., 1994; Dewhurst et al., 20001. Several studies indicate that synchronizing for rapid fer- mentation with fast degradable starch and protein sources stimulates greater synthesis or efficiency of synthesis of MCP. Herrera-Saldana et al. (1990) reported that MCP passage to the duodenum of lactating cows was highest (3.00 kg/d) when starch and protein degradability were synchronized for fast rates of digestion (barley and cotton- seed meal). Flows of MCP were lower when the primary fermentable carbohydrate and protein sources were either synchronized for slow degradability (milo and brewer's dried grains; 2.14 kg/d) or asynchronized (barley and brew- er's dried grains or milo and cottonseed meal; 2.64 and 2.36 kg/d, respectively). Efficiency of MCP synthesis (MCP/kg of truly digested OM) followed similar trends as MCP passage to the duodenum. Aldrich et al. (1993b) formulated diets to contain high and low concentrations of rumen- available nonstructural carbohydrates (HRANSC and LRANSC) and high and low concentrations of rumen- available protein (HRAP and LRAP) using high moisture shelled corn vs. coarse ground, dry ear corn and canola meal vs. blood meal, respectively. Flow of MCP to the duodenum was highest (1.64 kg/d) with HRANSC/HRAP and lowest (1.34 kg/d) with HRANSC/LRAP, flows were intermediate (1.46 and 1.48 kg/d) for the two LRANSC diets. Similar to the findings of Herrera-Saldana et al. (1990), efficiencies of synthesis of MCP were highest with the HRANSC/HRAP diet. Stokes et al. (1991a) reported that diets formulated to contain 31 or 39 percent NSC and 11.8 or 13.7 percent RDP in diet DM supported greater MCP synthesis than a diet containing 25 percent NSC and 9 percent RDP. Diets formulated to be synchronous vs. asynchronous in ruminal digestion rates of carbohydrate and protein have also increased flows and efficiency of synthesis of MCP in sheep (Sinclair et al., 1993, 19951. In the study by Sinclair et al. (1995), diets were similar in carbohydrate source (barley) and were either synchronous with rapeseed meal (diet A) or asynchronous with urea (diet B). The efficiency of MCP synthesis was 11-20 per- cent greater in sheep given diet A vs. diet B. Numerous other studies have reported higher MCP pas- sage (in viva or in continuous culture) when either the NSC level was increased or more degradable carbohydrates were substituted for those less degradable (McCarthy et al., 1989; Spicer et al., 1986; Stokes et al., 1991a; Stern et al., 1978) or when RDP in diet DM was increased (Cecava et al., 1991; Hussein et al., 1991; McCarthy et al., 1989; Stokes et al., l991b). A review of 16 studies indicated that MCP flow to the duodenum was increased by an average of 10 percent when slowly degradable sources of starch (e.g., corn grain) were replaced by more rapidly degraded starch (e.g., barley) (Sauvant and van Milgen, 19951. How- ever, there was no effect of differences in rate of starch degradation on the efficiency of conversion of ruminally digested OM to MCP. LyLos et al. (1997) evaluated diets formulated to have similar rates of RDP with three rates (6.04,6.98, and 7.94%/h) of NSC degradation in the rumen. Concentrations of RDP and NSC in diet DM were held constant across treatments. Rates of NSC degradation were achieved primarily by replacing cracked corn with ground high moisture corn. Flow of MCP to the duodenum tended to be the highest with the highest rate of NSC degradation. Efficiency of conversion of ruminally digested OM to MCP was increased as ruminal NSC availability increased, dem- onstrating the importance of timing of available energy to the ruminal microorganisms. Studies evaluating the importance of providing a gradual or even supply (vs. an uneven supply) of energy and N substrates to ruminal microorganisms are limited. Henning et al. (1993) investigated this issue in cannulated sheep fed both at maintenance and at a higher level of nutrition. Treatments consisted of a soluble carbohydrate mixture (maltose, dextrose and maltotriose) and a soluble N mixture (urea and sodium caseinate). Providing an even supply of energy increased passage of MCP and efficiency of MCP synthesis when the maintenance diet was fed but only tended to increase efficiency of MCP synthesis when the more adequate diet was fed. In contrast, the even supply of N increased passage of MCP only when the more ade- quate diet was fed. The results indicate that merely improv- ing the degree of synchronization between energy and N release rates in the rumen does not necessarily increase microbial cell yield and that a gradual or even release of energy and possibly N as well are also important. Synchronizing rates of ruminal degradation of carbohy- drates and protein may have a more pronounced effect in animals having high rates of ruminal passage (e.g., high DMI). Newbold and Rust (1992) observed in batch culture that a temporary restriction of supplies of either N or carbohydrate reduced subsequent bacterial growth rate. However, given the same total supply of nutrients, bacterial

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94 Nutrient Requirements of Dairy Cattle Kopecny, J., and R. J. Wallace.1982. Cellular location and some properties of proteolytic enzymes of rumen bacteria. Appl. Environ. Microbiol. 43:1026-1033. Kopecny, J., O. Tomankova, and P. Homolka.1998. Comparison of protein digestibility of rumen undegraded protein estimated by an enzymatic and mobile bag method: feeds for ruminants and anaerobic fungus. Anim. Feed Sci. Technol. 71:109-116. Koster, H. H., R. C. Cochran, E. C. Titgemeyer, E. S. Vanzant, T. G. Nagaraja, K. K. Kreikemeier, and G. St. Jean. 1997. Effect of increasing proportion of supplemental nitrogen from urea on intake and utilization of low-quality, tallgrass-prairie forage by beef steers. J. Anim. Sci. 75:1393-1399. Kowalski, Z. M., P. M. Pisulewski, and M. Spanghero. 1999. Effects of calcium soaps of rapeseed fatty acids and protected methionine on milk yield and composition in dairy cows. J. Dairy Res. 66:475-487. Kowalski, Z. M., A. Marszaek, and C. R. Mills. 1997. The use of Ca salts of rape seed fatty acids to protect protein against degradation in the rumen. Anim. Feed Sci. Technol. 65:265-274. Krishnamoorthy, U., T. V. Muscato, C. J. Sniffen, and P. J. Van Soest.1982. Nitrogen fractions in selected feedstuffs. J. Dairy Sci., 65: 217-225. Kung, L., JR., J. T. Huber, and L. D. Satter.1983. Influence of nonprotein nitrogen and protein of low rumen degradability on nitrogen flow and utilization in lactating dairy cows. J. Dairy Sci. 66:1863-1872. Lardy, G. P., G. E. Catlett, M. S. Kerley, and J. A. Paterson. 1993. Determination of the ruminal excape value and duodenal amino acid flow of rapeseed meal. J. Anim. Sci. 71:3096-3104. Laycock, K. A., and E. L. Miller. 1981. Nitrogen solubility and protein degradability of commercially and laboratory prepared rapeseed and soya-bean meals. Proc. Nutr. Soc. 40:103A. Lees, J. A., J. D. Oldham, W. Haresign, and P. C. Garnsworthy. 1990. The effects of patterns of rumen fermentation on the response by dairy cows to dietary protein concentration. Brit. J. Nutr. 63:177-186. Lehman, K. B., E. K. Okine, G. W. Mathison, and J. Helm. 1995. In situ degradabilities of barley grain cultivars. Can. J. Anim. Sci. 75:485-487. Leng, R. A., and J. V. Nolan. 1984. Nitrogen metabolism in the rumen. J. Dairy Sci. 67:1072-1089. Leng, R. A., D. Dellow, and G. Waghorn. 1986. Dynamics of large ciliate protozoa in the rumen of cattle fed on diets of freshly cut grass. Br. J. Nutr. 56:455-462. Leonard, M., and E. Block. 1988. Effect of ration protein content and solubility on milk production of primiparous Holstein heifers. J. Dairy Sci. 71:2709-2722. Licitra, G., T. M. Hernandez, and P. J. Van Soest. 1996. Standardization of procedures for nitrogen fractionation of ruminant feeds. Anim. Feed Sci. Technol. 57:347-358. Lindberg, J. E. 1985. 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