Omasal Flow of Soluble Amino Acids in Dairy Cows

Protein degradation by ruminal microbes results in the formation of ammonia, and peptides and amino acids (AA) are intermediates in this process. Ammonia that accumulates in excess of microbial requirements is converted to urea in the liver and excreted in the urine, resulting in inefficient nitrogen (N) utilization by the animal. Mounting evidence indicates that peptides and free AA stimulate microbial fermentation and yield. Furthermore, di- and tripeptides may contribute to AA absorption from the intestines of ruminants and may have nutritional benefits for the host animal. Therefore, alteration of protein breakdown and peptide formation in the rumen by dietary manipulation may have nutritional benefits for both ruminal microbes and the host animal, improving utilization of dietary N for productive purposes.

Computer models for ration formulation rely on rate constants to estimate protein degradation in the rumen and passage to the small intestine. The in situ method has been the most commonly used technique to assess the kinetics of protein degradation in the rumen. However, this technique has theoretical limitations. Two major concerns are 1) that the rates of ruminal degradation of the soluble N fraction (fraction A) from all feeds incubated in situ are assumed to be infinite and (2) that the portion of dietary protein that is degraded in the rumen (fraction B) is assumed to be entirely used for microbial protein synthesis or production of ammonia and carbon skeletons or both. However, if a portion of protein fractions A and B escape ruminal degradation as soluble proteins, peptides, and free AA, then the validity of the estimates of the quantities of dietary protein and AA (rumen undegradable protein) passing to the small intestine may not be correct.

The objective of this experiment was to study the effect of feeding diets differing in concentration, ruminal degradability, and source of protein supplement on passage from the rumen of soluble proteins, peptides, and free amino acids at the omasal canal.


Materials and Methods

Three ruminally and duodenally cannulated lactating Holstein cows were assigned to an incomplete 4x4 Latin square design with four 14-day periods and were fed one of four diets differing in the proportion and source of the protein supplement. The total mixed ration contained (DM basis) 35% corn silage, 25% alfalfa silage, 2.7% vitamins and minerals, and 37% concentrate [crude protein supplement plus ground shelled corn (GSC)]. Diets differed in the proportion and source of crude protein (CP) supplement: diet A (urea; 34.8% GSC, 2.3% urea); diet B [32.1% GSC, 5.2% solvent extracted soybean meal (SSBM)]; diet C [32.7% GSC, 4.6% xylose-treated SBM (XSBM; Soypass, LignoTech USA, Inc., Rothschild, WI)]; or diet D [28.6% GSC, 8.5% corn gluten meal (CGM)]. Yields of milk, milk fat, and milk true protein averaged 31.2, 1.39, and 0.91 kg/day per cow across diets. Passage of nitrogen fractions at the omasal canal were measured. Soluble AA in omasal digesta were fractionated by ultrafiltration into soluble proteins greater than 10 kDa, oligopeptides between 3 and 10 kDa, peptides smaller than 3 kDa, and free AA.

Diets were formulated to be isoenergetic at 1.58 Mcal/kg of NEL. They contained two concentrations of CP: high (19.7 and 18.2% for diets with urea and CGM) and low (15.8 and 15.3% for diets with SSBM and XSBM). Within each CP level, diets differed in the form of N (urea vs. true protein), the predicted ruminal degradability of the soybean protein (SSBM x XSBM), and the source of the protein (soybean vs. corn). Diets differed substantially in the proportion of the total N in AA, with CGM being highest (82.3%), SSBM (68.8%) and XSBM (64.4%) intermediate, and urea the lowest (35.5%).

There was no difference in dry matter intake (DMI) for cows fed urea or CGM or for cows fed SSBM and XSBM (Table 1). Replacing XSBM with CGM decreased DMI. Cows fed diets supplemented with urea had smaller flows of total non-ammonia N (NAN) and non-ammonia non-microbial N (NANMN) at the omasal canal than cows fed CGM. Cows fed XSBM had a greater flow of NANMN but a similar flow of microbial NAN compared with cows fed SSBM, resulting in a trend for a greater flow of NAN. Replacing XSBM with CGM depressed the omasal flows of total NAN and NANMN, at least in part because of the associated effects on DMI.

The intakes of all AA, except lysine, were greater for cows fed CGM compared with urea, and intakes of individual AA and total AA were not different for cows fed SSBM and XSBM diets.

Although cows fed CGM consumed 4.5 kg less dry matter than cows fed XSBM the higher CP content and thus the higher total AA concentration of the CGM diet resulted in intakes of leucine, methionine, phenylalanine, alanine, cystine, glutamic acid, proline, and tyrosine that were higher than those for cows fed XSBM. Conversely, the higher lysine content in soybean meal and the greater DMI of cows fed XSBM increased lysine intake by cows fed XSBM compared with those fed CGM.

Except for arginine, histidine, lysine, methionine, threonine, valine, and glycine omasal flows of individual and total AA were greater for cows fed CGM compared with urea. Replacing SSBM with XSBM increased the flows of phenylalanine, glutamic acid, and serine and resulted in a trend for an increase in total AA flow. Flows of arginine, isoleucine, and lysine were reduced when XSBM was replaced with CGM in the diets. However, compared with XSBM, the higher intake of leucine by cows fed CGM resulted in a greater omasal flow of this AA.

There were no dietary effects on flows of individual AA of microbial origin or on flow of total microbial AA. However, flows of AA of dietary origin were affected by diets. Cows fed diets supplemented with urea had lower flows of leucine, phenylalanine, valine, alanine, cystine, glutamic acid, proline, serine, tyrosine, and total AA of dietary origin compared with cows fed CGM diets. The flow of leucine was greater for cows fed CGM, compared with those fed XSBM. Replacement of SSBM with XSBM did not affect the flows of individual AA of dietary origin.

Small peptides isolated by ultrafiltration contributed between 13 and 20% of the total AA in the soluble fraction and less than 3% of the total AA flowing at the omasal canal of cows fed diets differing in the concentration and source of supplemental protein. In contrast, AA in soluble 3-10K oligopeptides averaged 71%, whereas free AA accounted for less than 2% of total soluble AA flows across diets. Therefore, results suggest that hydrolysis of 3-10k oligopeptides was the rate-limiting step during microbial degradation of soluble proteins and that small peptides and free AA were rapidly utilized by ruminal microbes and did not accumulate in the rumen.

Flow of total soluble AA at the omasal canal ranged from 254 to 377 g/day per cow or 9 to 16% of total AA flow (Table 1). On average, 73% of total soluble AA and 10% of total AA flows were of dietary origin, indicating a substantial escape of dietary soluble AA from ruminal degradation. Therefore, these data suggest that the use of in situ estimations of protein degradation to predict the flow of rumen undegradable protein or the use of computer models that assume soluble dietary AA do not escape ruminal fermentation underestimate the passage of AA to the small intestine by 10 to 15%. Although the omasal flow of some individual AA associated with peptides and oligopeptides was affected by dietary treatments, these changes were relatively small compared with the total flow of AA in their soluble and insoluble forms.


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