Dietary Protein Degradability Research Articles

The effects of quebracho tannins on rumen degradation and post-rumen digestion of pea and lupin seeds

The use of alternative protein sources in ruminant diets is of interest because of restrictions on the use of animal proteins, loss of nitrogen (N) compounds to the environment and efforts to reduce costs of systems. A problem of some protein-rich-ingredients is that they degrade rapidly in the rumen. The use of tannins has been proposed as a means of reducing the extensive dietary protein degradation in the rumen. However, condensed tannins have been reported to exert detrimental effects on gut microflora and on the gastrointestinal tract, and they can affect post-rumen digestion. The two experiments reported here aimed to study the influence of quebracho tannins (QTs) on ruminal degradation of peas (Pisum sativum) and sweet lupins (Lupinus luteus), and to investigate the effect of QTs on postrumen digestibility of these feeds using a poultry model.

Effect of dietary protein degradability on net fluxes of free and peptide amino acids across the portal-drained viscera of steers

Four steers, average body weight 260±15 kg, fitted with portal catheters were used in a 4×4 Latin Square design to evaluate the influence of dietary protein degradability in the rumen on peptide and amino acid fluxes across the gastrointestinal tract. Dietary protein degradability was regulated by using different protein sources and the diets were calculated to contain 130 g CP/kg and 9·62 MJ ME per kg DM. Plasma concentrations of amino acids were analysed before and after acid hydrolysis of samples first subjected to chemical deproteinization and physical ultrafiltration, and peptide amino acids (PAA) were calculated as the difference between total and free amino acids (FAA). Portal blood flow and arterial concentrations of FAA and PAA were not affected by protein degradability or by diet. Venoarterial concentration difference and net portal flux of FAA tended to increase (P < 0·10) with increase of degradable protein intake. Portal-arterial concentration difference (P < 0·05) and net portal flux (P < 0·10) of PAA increased linearly as dietary protein degradability increased. The proportion of PAA in total amino acid (FAA+PAA) net flux was not modified by dietary protein degradability or by diet, and the mean value as a proportion was 0·32. The major PAA absorbed were glutamate, leucine, aspartate and lysine for all diets, accounting in total for 0·50 of PAA flux. The results demonstrate that PAA may contribute significantly to AA flux across the portal-drained viscera (PDV) of steers, and both FAA and PAA net fluxes can be affected by degradable protein intake.

Fütterungs- und nicht fütterungsbedingte Einflüsse auf den Milchharnstoffgehalt von Milchkühen – 1. Mitteilung: Fütterungsbedingte Einflussfaktoren auf den Milchharnstoffgehalt von Milchkühen

Abstract. Title of the paper: The impact of nutrition and non nutrition factors on milk urea concentration. I. The impact of nutrition factors on milk urea concentration The object of this study was to determine the impact of dietary crude protein / energy balance, stage of lactation parity, milk yield or milk protein yield, milk protein content, somatic cell count, season and herd on milk ureä concentration under the conditions of large herds (50–1000 dairy cows) and routine milk yield recording. From this data a model was created to predict the milk urea concentration for a single dairy cow or a feeding group For the analysis of the data a mixed linear model (proc mixed; SAS®, 1997) was used. The fixed effects (ratio of crude protein / energy, degradability of dietary crude protein, ruminal N-balance, lactation period, parity) was computed by generalized least-squares-means. The season, herd and month were used as random effects Milk yield or milk protein yield, milk protein content, somatic cell count and day in milk were included as Covariables in the model. The effect of dietary crude protein / energy balance on milk urea was significant (p < 0.0001). For diets with an optimal ratio of crude protein / energy (according to GfE, 1997) 226 mg/1 was calculated using least-squares-means. For diets with excess of crude protein this figure rose to 252 mg/1 In this study a significant difference in least-squares-means calculated urea concentration did not existed for degradability of dietary crude protein. On the other hand, a significant difference in least-squares-means calculated urea concentration existed for ruminal N-balance between 0 and 2.

The effect of midwinter shearing on rumen digesta kinetics in highly prolific crossbred ewes

The effective rumen degradability (ERD) of dietary protein is used to quantify the amount of dietary protein entering the small intestine of ruminants. Midwinter shearing of prolific ewes increases rumen degradation rate (Kd) of dietary protein (Šebek and Everts, 1999). However, these differences in Kd may be compensated by a different rumen passage rate (Kp), since cold exposure is often associated with increases in Kp. Kp can be estimated by use of an undegradable marker. This method, although reliable for fluids seems less reliable for solids. The objective of the present experiment was to quantify the effect of midwinter shearing on ERD in highly prolific ewes. As an alternative to the use of Kp, ERD of crude protein (CP) and of protein-free-organic-matter (OM_CP) were estimated from periodical changes in rumen volume partitioned as rumen degradable (Xd) and rumen undegradable (Xu) components.

Effects of Genetic Merit and Varying Dietary Protein Degradability on Lactating Dairy Cows

Eighty two multiparous Holstein cows were blocked by genetic merit (high vs. low) and assigned to one of two treatments [high rumen-undegradable protein (RUP): rumen-degradable protein (RDP) vs. low RUP: RDP] from d 21 before to d 150 after calving to study the effects of these treatments on production and reproductive performance. Diets were isonitrogenous (dry cow 10.5% crude protein; lactating cow 19.3%), isoenergetic (dry cow 10.0 MJ of metabolizable energy (ME); lactating cow 10.9 MJ of ME) and fed as total mixed rations. Feeding more RUP significantly increased dry matter intake and milk yield, reduced body tissue mobilization, and lowered concentrations of serum nonesterified fatty acids (NEFA) and plasma urea. Expression of estrus at first ovulation was improved, first service conception rate was higher, and calving to conception interval was shorter for the high RUP group. Cows of high genetic merit produced more milk, mobilized more body tissue, and had higher concentrations of plasma growth hormone. The dry matter intake and concentrations of blood metabolites did not significantly differ with genetic merit. Expression of estrus at first ovulation was significantly lower for cows of high genetic merit. Serum NEFA concentrations were significantly higher, and estrus was not observed at first ovulation for cows of higher genetic merit fed the low RUP diet. The interaction between dietary RUP and genetic merit was not significant for other measures of performance or fertility. Feeding a low RUP: high RDP diet had negative effects on some aspects of production and reproductive performance. The effects of diet on NEFA concentrations and estrus display were greater in cows of high genetic merit, indicating that potential interactions should be evaluated in future reproductive studies involving protein and fertility.

Monensin: an overview of its application in lactating dairy cow diets (2000)

The efficiency of feedstuff utilization by ruminal microorganisms and the cow's genetic ability to convert feed nutrients into milk and milk components are major factors that influence the profitability of a dairy herd. Monensin's ability to modify the movement of ions across biological membranes leads to alterations in bacterial populations and subsequent changes in the proportion of volatile fatty acids produced during ruminal fermentation. Manipulating ruminal microbial populations with ionophores has the potential to improve performance by reducing ketosis, acidosis, and bloat and increasing digestive efficiency. Monensin improves fiber digestion by preventing suboptimal ruminal pH, enhances amino acid use by reducing the degradation of dietary protein, and improves the energy status of periparturient animals. Monensin is not approved for use in diets for lactating dairy cows at this time, but its status is currently under review by the U.S. Food and Drug Administration. If approved, monensin will provide another management tool to the dairy industry.; Dairy Day, 2000, Kansas State University, Manhattan, KS, 2000

The effect of dietary protein degradability and rumen inert fat supplementation on calf performance and veal production

Extracted from text ... The effect of dietary protein degradability and rumen inert fat supplementation oncalf performance and veal productionC. W. Cruywagen # , E. Lategan and L. C. HoffmanDept. of Animal Sciences, University of Stellenbosch, Stellenbosch, 7600, South AfricaE-mail: # cwc@maties.sun.ac.zaIntroductionAn estimated 176 000 Friesian bull calves are born in South Africa annually. The theoretical potential vealproduction would be 17 600 tonnes if these calves were to be slaughtered at around 190-200kg to produce a 100kgcarcass. Although there is a market for pink veal in South Africa, it is not utilised optimally, mainly because of alack of knowledge by producers on calf rearing, ..

Influence of Controlling Protozoa on the Degradation and Utilization of Dietary Fibre and Protein in the Rumen and Nitrogenous Flow Entering the Duodenum of Sheep

Influence of Controlling Protozoa on the Degradation and Utilization of Dietary Fibre and Protein in the Rumen and Nitrogenous Flow Entering the Duodenum of Sheep

Effects of dietary crude protein concentration and degradability on milk production responses of early, mid, and late lactation diary cows.

Three experiments were conducted to investigate the effect of crude protein (CP) concentration and ruminally undegraded protein (RUP) concentration on milk production and composition of dairy cows at three different stages of lactation. Experiments 1, 2, and 3 using 39, 40, and 39 Holstein cows were conducted for cows in early (wk 4 to 14 postpartum), mid (wk 19 to 29), and late (wk 34 to 44) lactation, respectively. Cows were assigned to one of four corn-based diets: high CP, medium RUP (control); low CP, low RUP; low CP, medium RUP; and low CP, high RUP. Percentages of CP in the high and low CP diets were, respectively, 17.4 and 15.2 for Experiment 1, 15.3 and 13.3 for Experiment 2, and 14.2 and 12.6 for Experiment 3. The RUP concentrations (percentages of CP) for low, medium, and high diets averaged 35.5, 41.4, and 46.5%, respectively. For Experiment 1, production of milk, 4% fat-corrected milk, milk fat, and milk protein was increased by the high protein diets versus the low protein diets. Production of milk and fat-corrected milk increased linearly as RUP in the diet increased. During Experiment 2, lactational responses were not affected by treatment. During Experiment 3, dry matter intake, body weight, and body weight change increased for cows fed the high protein diets versus those same measurements for cows fed the low protein diets. Milk fat and milk protein percentage decreased linearly as RUP in the diet increased. Because there was no effect of diet on milk production, decreasing CP in diets fed to cows in mid or late lactation can reduce the cost of the diet and waste N excreted from the cow.

飼料タンパク質の分解率の違いが泌乳牛の窒素およびエネルギー出納に及ぼす影響

飼料タンパク質の第一胃内分解率の違いが泌乳牛の窒素およびエネルギー出納に及ぼす影響を検討するために,主な飼料タンパク質源として魚粉とビール粕(LD区)あるいは大豆粕(HD区)で分解率を調製した飼料をホルスタイン種泌乳牛4頭に給与して窒素およびエネルギー出納試験を行った.環境条件は気温18°C,湿度60%とし,1期2週間(本試験期5日)2期とするクロスオーバー法で実施した.その結果,1) 乳量に差はなかったが,LD区の乳脂率は有意に低下し,乳タンパク質率,乳タンパク質生産量は有意に上昇,増加した.2) 窒素出納は,窒素摂取量に差はなかったが,LD区の糞への窒素排泄量,乳および体蓄積への窒素移行量が増加する傾向にあり,尿への排泄量は明らかに減少する傾向であった.3) エネルギー出納は,LD区の糞へのエネルギー損失量がやや多い傾向であったが,尿,メタンとしてのエネルギー損失量および代謝率に差はなかった.また,LD区のエネルギー蓄積量が有意に高くなった.4) 摂取代謝エネルギーの生産(乳ユネルギー+蓄積エネルギー)に対する利用効率に差は認められなかった.以上から,エネルギーおよびタンパク質摂取量が充足した条件下において,乳成分および窒素出納は飼料タンパク質の分解率の影響を受けたが,エネルギー利用効率はほとんどその影響を受けないことが明らかとなった.

Effect of the Synchronization of the Degradation of Dietary Crude Protein and Organic Matter and Feeding Frequency on Ruminal Fermentation and Flow of Digesta in the Abomasum of Dairy Cows

Four Holstein cows in midlactation were equipped with ruminal and abomasal cannulas and used to study the effect of synchronized degradation of crude protein (CP) and organic matter (OM) and feeding frequency on digestion and outflow of nutrients. A 4 × 4 Latin square design was used. Diets were arranged in a 2 × 2 factorial design; the four diets contained high ruminally degradable OM and high ruminally degradable CP, high ruminally degradable OM and low ruminally degradable CP, low ruminally degradable OM and high ruminally degradable CP, and low ruminally degradable OM and low ruminally degradable CP. In each period, cows were fed four times daily from d 1 to 14 and two times daily from d 15 to 28. Mean daily ruminal ammonia N concentration was reduced by high ruminally degradable OM, low ruminally degradable CP, and twice daily feeding. Fluctuation in ruminal ammonia N was lower when cows were fed four times daily than when cows were fed twice daily. Plasma urea N concentrations were lower for cows fed diets that were high in ruminally degradable CP. Higher CP flow in the abomasum was found for cows fed the diet containing high ruminally degradable OM and low ruminally degradable CP. Microbial dry matter and CP flow to the abomasum were higher for cows fed twice daily than for cows fed four times daily. Flow of OM in the abomasum was not altered by concentrations of ruminally degradable OM or CP. These results suggest that the available energy in the rumen (ruminally degradable OM) is the most limiting factor for ruminal N utilization under our experimental conditions. Use of these data may improve the prediction of plasma urea N.

Indications and implications for testing of milk urea in dairy cattle: A quantitative review. Part 1. Dietary protein sources and metabolism

Milk urea concentrations in dairy cattle. There has been increased use of milk urea concentration as an indicator of dietary protein intake and protein metabolism in dairy cattle over recent years. The value of milk urea content data in predicting dietary composition, particularly for pasture-fed cattle, has not been well described. Protein metabolism and urea synthesis. Many factors influence the degradation of dietary proteins in the rumen, post-ruminal protein metabolism and urea synthesis in cattle. Strong positive correlations between nitrogenous fertiliser use and the crude protein content of pastures were identified by use of meta-analysis. Similar strong positive correlations were noted between dietary protein intake, rumen ammonia and plasma urea concentrations. The costs of urea synthesis include energy losses, and importantly, the loss of endogenous amino acids, which are deaminated in the synthesis of urea. Milk urea as an indictor of protein metabolism. Urea concentrations in blood, plasma and milk are strongly correlated. Milk is an adequate indicator of blood and plasma urea content, but non-nutritional factors may significantly influence milk urea concentrations. Recommendations for dietary protein management based on milk urea concentrations must be undertaken with care.

Estimation of in vitro degradation of dietary proteins in ruminants by using enzymes

Protein degradation in the rumen determines the supply of dietary nitrogen (N) to microbes and undegraded amino acids for direct absorption. The current systems of feeding ruminants identify rumen degradable protein (RDP) from undegraded protein (UDP) which is digestible post ruminally (Agricultural and Food Research Council (AFRC), 1992; Huntington and Givens 1995). The in sacco method has been useful to estimate RDP, UDP and rate of degradation of proteins in ruminants. However, its use is constrained by the fact that it requires surgically modified animals, is laborious, inconsistent and does not differentiate between proteins that disappear from the bag or which are digested by microbes. It is therefore important to develop rapid, robust and reproducible alternatives (in vitro) to simulate ruminal digestion of proteins. Chaudhry (1998) has shown a potential in Streptomyces griseus protease (enzyme) to estimate ruminal digestion in vitro given its consistently high proteolytic activity with purified proteins. This study examined the validity of this enzyme to estimate degradability of food proteins.Buffer, enzyme and foods A potassium phosphate buffer (buffer) with a pH 6-8 was used to prepare protein suspensions and a solution of the protease (P5147, SIGMA). Samples of sunflower meal (SF), maize-gluten meal (MG), distillers’ dark grains (DG) and field beans (FB) were dried and ground through 1 or 4 mm sieves for in vitro enzyme and in sacco studies respectively.

Use of Ionophores in Grazing Ruminants

The use of ionophores in ruminant diets is one of the success stories of application of technology to livestock production. They have been fed for more than 20 years with positive and repeatable responses across a wide range of production systems. Ionophores are antibiotics that affect certain bacteria, protozoa and fungi in the rumen. Three polyether ionophores (ionophores with carboxylic terminal groups) available for ruminants today are monensin (Rumensin ®), lasalocid (Bovatec®) and laidlomycin (Cattlyst®). When added to the diet per manufacturer's recommendations, they change the profile of fermentation products and the degradation of dietary protein in the rumen, thereby improving the efficiency of nutrient use by ruminants. The fermentative processes that are affected by ionophores - production of ammonia and volatile fatty acids (VFA) in the rumen - are key processes because they affect directly nitrogen and energy metabolism in the host ruminant. This review will emphasize the results of studies with grazing cattle, with data and information from other reviews30,48,62,65 as well as data from recent publications.

Effects of Amount and Ruminal Degradability of Protein on Nutrient Digestibility and Production by Cows Fed Tallow

Five cows with ruminal cannulas were used in a 5 × 5 Latin square design to determine the effects of fat and amount and ruminal degradability of dietary crude protein (CP) on nutrient digestibility and production of milk and milk components. Treatments were 1) control; 2) 15% CP, soybean meal; 3) 15% CP, by-product proteins; 4) 18% CP, soybean meal; and 5) 18% CP, soybean meal and by-product proteins. Diets 2 through 5 contained 3.5% tallow. Diets consisted of 28% alfalfa haylage, 22% corn silage, and 50% concentrate on a dry matter (DM) basis. Fat did not affect dry matter intake or percentages and yields of fat and CP in milk but increased milk yield 2.5 kg/d. Fat did not affect N fractions in milk but decreased the percentages of short- and medium-chain fatty acids (C6:0 to C16:0) and increased the percentages of long-chain fatty acids (C18:0 and C18:1) in milk fat. Fat did not affect ruminal fermentation characteristics or the percentages of dietary DM, organic matter, CP, acid detergent fiber, neutral detergent fiber, starch, ether extract, and energy that were digested. An increase in dietary CP from 15 to 18% increased dry matter intake 1.7 kg/d; increased intake of gross energy 8 Mcal/d; increased the percentages and quantities of DM, organic matter, CP, and energy digested; increased the quantities of acid detergent fiber and neutral detergent fiber digested; decreased ruminal pH; increased concentrations of total volatile fatty acids; and increased NH3 N in ruminal fluid. However, the difference in dietary CP did not affect milk yield or composition. Replacement of soybean meal in the diet with a mixture of by-product proteins decreased NH3 N in ruminal fluid, tended to decrease concentrations of total volatile fatty acids and increase pH of ruminal fluid, but did not affect milk yield or percentages and yields of milk components.

Modifying duodenal flow of amino acids by manipulation of dietary protein sources

Four multiparous Holstein dairy cows in mid-lactation were utilized in a 4 × 4 Latin square design experiment to measure the response in intestinal amino acid profiles to progressive substitution of blood meal for corn gluten meal as the protein supplement. In addition, the influence of these protein sources on rumen fermentation and digestion as well as forestomach bacterial growth and escape were compared with a diet supplemented with soybean meal, a rapidly rumen-degraded protein source. Cows were offered a mixed silage ration of alfalfa silage (79.8% of DM) and corn silage (20.2% of DM) twice daily. Cows were also offered mixed concentrate at 123% (DM:DM) of the mixed silage ration in six equal meals per day to provide soybean meal, corn gluten meal, blood meal or a combination of the latter two sources as the primary supplemental protein source. Rumen soluble non-ammonia N concentrations were higher in cows fed soybean meal and declined linearly as blood meal substituted for corn gluten meal in the concentrate. Forestomach disappearance of N tended (P = 0.09) to be higher for the cows fed soybean meal vs. all other diets, and tended (P = 0.09) to be higher with the combined corn gluten meal and blood meal diet vs. the diets containing either alone. The latter is consistent with higher forestomach digestion of DM and OM in cows fed the combined diet. Rumen pool sizes of most DM components were lower when cows were fed soybean meal and, with the exception of N pools, increased linearly as blood meal substituted for corn gluten meal. Duodenal flow of amino acid protein was lower when cows were fed soybean meal, due mainly to reduced bacterial flow, and was lowest for the combined diet within the corn gluten meal and blood meal diets. The amino acid profile of duodenal protein differed for 12 of 17 amino acids examined when cows were fed the soybean meal diet vs. the other diets, and 12 of 17 amino acids either increased or decreased linearly as blood meal substituted for corn gluten meal. Data reported here and by previous researchers suggest an associative effect of addition of corn gluten meal and blood meal to the diet which increased rumen degradation of dietary protein. These data also show that manipulating the dietary source of supplemental protein can influence the amino acid profile of duodenally delivered amino acid protein as well as indicate that the rate of rumen degradation of individual amino acids differs among protein sources. In this study, methionine, cystine and histidine appeared to be more rapidly degraded in corn gluten meal than in blood meal, whereas glutamic acid, proline, isoleucine, threonine and lysine were more rapidly degraded in blood meal. Key words: Dairy cow, rumen bacteria, amino acid

Effect of Degradability of Dietary Protein and Fat on Ruminal, Blood, and Milk Components of Jersey and Holstein Cows

Twelve Holstein cows and 12 Jersey cows were used in six 4 × 4 Latin squares to investigate the effects of the degradability of dietary protein and supplemental dietary fat on milk components. Dietary dry matter contained 16% crude protein with two concentrations of ruminally undegradable protein (RUP) obtained by substituting blood meal for a portion of the soybean meal. Treatments were 1) 29% RUP, 0% added fat; 2) 29% RUP, 2.7% added fat (Ca soaps of fatty acids); 3) 41% RUP, 0% added fat; and 4) 41% RUP, 2.7% added fat. The dry matter of the total mixed ration fed at 1000 and 1400h consisted of 30% corn silage, 29% alfalfa haylage, and 41% concentrate. Supplemental dietary fat depressed dry matter intake by 6.2%. Plasma urea N was greater at 0700 and 1600h for Jerseys fed diets containing added fat and greater at 0700h for Holsteins fed diets containing 41% RUP than for Holsteins fed 0% added fat and 29% RUP. When averaged across both breeds, milk production increased 7.1%, and production of 4% fat-corrected milk by Jerseys increased 8.4%, in response to added dietary fat. Milk protein was reduced when Holstein diets contained 41% RUP. Milk protein content was reduced 7.1 and 3.9%, and milk urea N was increased 4.9 and 8.5%, by added fat and 41% RUP in both breeds, respectively. Added fat reduced the concentration, but not the yield, of milk components. Substitution of blood meal decreased the concentration and yield of milk protein and casein N.

Effect of Rumen Protozoa on Nitrogen Utilization by Ruminants

Results obtained during the past decade indicate clearly that protozoa are actively involved in the degradation of dietary and microbial proteins in the rumen. Because of the great ability of protozoa to ingest the particulate matter suspended in the rumen, protozoa are more active in degrading insoluble than soluble proteins. This indicates that studies carried out using lysed and sonicated protozoa are not appropriate for quantifying the actual contribution of protozoa to protein degradation in the rumen. In vivo trials have confirmed that duodenal flow of both undegraded dietary protein plus bacterial protein generally is increased by defaunation. The decrease in ruminal ammonia concentration consistently observed after defaunation accounts for the lower urinary nitrogen (N) excretion found in defaunated animals, whereas the increase in fecal N excretion in the same animals probably results from a shift of plant cell wall digestion from the rumen to the large intestine. Total N excretion is not altered significantly by defaunation. A summary of literature data indicates there are contradictory effects of defaunation on ruminant performance. This implies that animal response to defaunation may depend on the specific nutrient-limiting performance on the one hand and on the modifications of digestion and metabolism resulting from defaunation on the other. Different methods are proposed to either eliminate or decrease the numbers of ruminal protozoa or to alter their makeup. However, none of these approaches has been tested under practical feeding conditions.

Evaluation of Dietary Nitrogen Utilization in Dairy Cows Based on Urea Concentrations in Blood, Urine and Milk, and on Urinary Concentration of Purine Derivatives

Abstract The effects of level and degradability of dietary protein on urea in blood, urine and milk, and on the urinary purine derivatives and creatinine in dairy cows, were studied. Diurnal variation in urinary concentration of urea, allantoin and creatinine was also studied. A total of 24 multiparous lactating dairy cows were selected from a production experiment and divided into two 12-cow groups according to their lactation stage (LG1, 14th-16th week of lactation; and LG2, 26th-30th week of lactation). Within groups, cows were fed four different rations (three cows per treatment). Four concentrate mixtures with two levels of protein [19 and 14% of crude protein on dry matter (DM) basis] and two different rumen degradabilities (72 and 52%) of the dominating protein feed rapeseed meal, were fed to the cows together with pasture silage and hay. In both groups of cows the urea concentration in plasma, urine and milk increased (p<0.05) with increasing protein intake. Based on the pooled data from both lact...

Evaluation of Chemical and Physical Properties of Feeds That Affect Protein Metabolism In the Rumen

The goal of the NC-185 Cooperative Regional Research Project is to provide the information needed to improve the nutrition and feeding of dairy cattle, a major factor determining composition of milk and cost of milk yield. Emphasis is placed on understanding how energy and protein nutrition of lactating cows can be manipulated to increase the quantity and improve the profile of AA passing to the small intestine and to improve yield of milk and milk protein. To achieve this goal, one of the major objectives of this project has been to evaluate quantitatively the chemical and physical properties of protein and energy sources that determine AA availability to lactating cows. Reliable measurements of microbial protein synthesis and protein degradation in the rumen are critical in the evaluation process. Therefore, one of the ongoing areas of investigation of this research project has been to determine the most appropriate methods for estimating microbial protein synthesis and dietary protein degradation in the rumen. Other areas have been investigated, using continuous culture fermenters and ruminally and duodenally cannulated cows, including factors that alter microbial metabolism of N in the rumen and subsequently protein supply to the small intestine, such as sources of carbohydrate, protein, and fat and interrelationships of protein and carbohydrate. Findings of the NC-185 Cooperative Regional Research Project Committee and other investigators are summarized in this review.