Alex N. Hristov
Professor of Dairy Nutrition
Department of Animal Science
The Pennsylvania State University, University Park, PA
The ruminant animal does not require metabolizable protein (MP), it requires amino acids (AA) for vital physiological functions, growth, gestation, and productive purposes. Expressing requirements in terms of MP is convenient, but does not address the AA requirements of the cow. Accurate prediction of AA outflow from the rumen and post-ruminal AA utilization are critically important for estimating AA supply in ruminants. There is increasing evidence and understanding that the efficiency of AA utilization decreases with increasing AA supply. Penn State’s data with cows producing around 90 lb milk/d showed that the apparent efficiency of use of dietary AA for milk protein synthesis increased by 19 (for all essential AA) to 22 (Lys) and 33% (His) when dietary crude protein concentration (and MP supply) decreased from around 16 to 14% (dry matter basis). Progress in our ability to accurately predict ruminal AA outflow, however, has been slow. A recent attempt to update the current NRC (2001) AA model resulted in prediction error of AA flow of 25 to 33% (of the mean values). It is obvious that this kind of accuracy is not acceptable for evaluating diets or meeting the AA requirements of the cow. Updated milk protein yield (MPY) equations based on digestible energy intake, certain essential and non-essential AA supply, and concentration of some dietary nutrients resulted in prediction error of around 12%, which is acceptable accuracy for diet formulation purposes. The essential AA that ended up as predictors for MPY were Arg, Leu, Lys, Met, Thr, and Phe. The models did not include His, Ile, and Val. This analysis confirmed that Met and Lys are among the most important AA for maintaining high MPY in dairy cows and emphasized the point that, under normal dietary conditions, there probably isn’t a single AA limiting MPY. In situations where dietary AA supply limits production, it is likely that supplementation of a consortium of AA will yield a response in MPY. It is important to understand that the AA needs of the cow depend on diet AA composition and dry matter intake (DMI), which determine AA supply and cow productivity. Data have shown, for example, that rumen-protected (RP) Lys supplementation did not have an effect in a soy-based diet, but resulted in increased milk production and MPY in a corn-based diet. Nutritionists have to be also aware of large variability in bioavailability of AA from RPAA products on the market. A recent study at Penn State showed 2- to 4-fold differences in experimentally-determined vs. manufacturer-estimated bioavailability of commercial RPAA samples. In dairy nutrition, decreasing dietary protein concentration has advantages in term of feed cost and environment. This, however, has to be cautiously balanced with potential decreases in DMI, milk production, and MPY due to rumen-degraded protein or AA deficiencies, particularly when energy supply is compromised. Research at Penn State has clearly shown the environmental benefits, such as decreased urinary nitrogen excretion and ammonia emissions from manure and increased milk nitrogen efficiency, of feeding diets close to or slightly below NRC (2001) protein recommendations. When feed AA supply is decreased, microbial protein becomes an even more important source of AA for the cow. The AA makeup of microbial protein is remarkably close to that of milk casein with one notable exception, His. Penn State data and a recent meta-analysis have shown that microbial protein has about 20 to 27% lower His than Met (these 2 AA are metabolized similarly and are in similar concentrations in casein). Thus, low-protein (particularly, low-rumen-undegraded protein) diets may be His-deficient. This could explain the positive responses in DMI, milk yield, and MPY observed with RPHis supplementation of MP-deficient diets reported from a series of long-term, randomized block design studies conducted at Penn State. This was confirmed in a recent experiment where we observed a linear increase (from 1.3 to 2.2 lb/d; P = 0.01) in DMI and a corresponding trend for increased milk yield (2.4 to 3.5 lb/d; P = 0.07) in cows fed diets that provided digestible His at 1.8 to 3.0% of MP (or 47 to 85 g/d). Overall, Met, Lys, and His are the AA studied the most in dairy cattle nutrition and it is likely that, under most dietary conditions, these AA may become limiting milk production and MPY. However, it is recognized that other essential AA may also be limiting production and dietary energy-AA interactions are central to protein metabolism in dairy cows.