Can There be Amino Acids Other than Methionine and Lysine Limiting Milk Production in U.S. Dairy Herds?

Traditionally, methionine (Met) and lysine (Lys) have been considered the first two amino acids (AA) limiting (1st and 2nd limiting, respectively) milk production and milk protein synthesis in typical North American dairy diets, i.e. diets primarily based on corn silage, alfalfa forage (silage, haylage, hay), corn grain, and soybean products. Indeed, a tremendous amount of work has been done with these two AA in research settings (either post-ruminal infusions of unprotected AA or feeding of ruminally-protected Met and Lys). Results, however, have been inconsistent. A couple of recent meta-analyses concluded that supplementing the diet with ruminally-protected Met can have a small but consistent positive effect on milk production and feed intake, particularly when AA balance was positive. In the vast majority of the studies summarized in these meta-analyses, however, cows were fed diets providing metabolizable protein (MP) in excess of their requirements. In these dietary situations, individual AA can still be limiting production, but it is more likely that the response to supplemental AA will be small or non-existent. 

The environmental advantages of feeding low-protein diets are well-known. Research at Penn State and elsewhere has shown that the most efficient way of increasing milk nitrogen (N) efficiency (MNE), i.e. milk protein N secretion as proportion of feed N intake, and reducing N losses is by feeding at or slightly below NRC (2001) requirements for MP. When such diets are fed, MNE can be as high as 40%, much higher than the 25 to 27% typical for most herds. These diets dramatically decrease urinary N losses (and also milk urea N, MUN). Fecal N excretion is actually not affected or may slightly increase. There is a large difference between urinary and fecal N, however, when it comes to N leaching, run-off, and gaseous emissions. Nitrogen in feces is primarily organic and does not contribute much to water and air pollution. Urinary N, on the other hand, can go through various conversions and is the primary source of nitrate, ammonia, and nitrous oxide emissions from manure during storage or following soil application. Thus, reducing dietary protein and feeding cows at or below their MP requirements affects mostly urinary N excretion and consequently has a marked impact on N losses from manure and MNE.

Feeding at or below MP requirements, however, may result in depressed feed intake, which would most likely lead to decreased milk production. Decreased milk protein content has also been observed with MP-deficient diets. Our experience at Penn State, for example, has been that a 14% crude protein diet (about 15% deficient in MP) could decrease feed intake (by about 2 lbs/d) and may decrease milk production by up to 6 to 7 lbs/d in cows milking around 85 to 90 lbs/d. The drop in milk production and milk protein yield result from a combination of decreased feed intake and deficiency of specific AA. Supplementing such diets with ruminally-protected Lys and Met can alleviate some of negative effects of MP-deficiency and increase milk production and milk protein yield to levels not statistically different from the control diet (which in our studies is about 16% crude protein and meeting NRC 2001 requirements for MP), but still not reach the level of milk production of the control cows.

Our studies with MP-deficient diets have clearly shown a potential histidine (His) deficiency based on drastic, up to 40%, decrease in blood plasma His levels in cows fed MP-deficient diets continuously for up to 70 d. It is important to point out that in parallel short-term Latin square design trials, plasma His concentrations in cows fed the same MP-deficient diets did not decrease statistically compared with the control diet. This must be kept in mind when evaluating data from long- vs. short-term, crossover type of experiments. The question we asked was, "Why His?" There is data from Europe with cows fed grass silage-based diets showing His being the 1st AA limiting milk production. These data, however, are from low-producing cows and the response to His supplementation was marginal. His has not been reported to be a limiting AA in cows fed diets based on corn silage, alfalfa forage, corn grain, and soybean products. These feeds are not His-deficient (they may be Lys-deficient) and the diet plus microbial protein synthesized in the rumen were considered sufficient to provide His for maintenance and milk protein synthesis. The piece of information that was missing, however, was the fact that microbial protein is deficient in His (rather than deficient in Met, as some reports have claimed). We analyzed rumen microbial protein samples from a number of experiments conducted at Penn State and found that indeed, microbial protein had about 27% lower concentration of His than Met. In comparison, milk protein has about the same concentration of Met and His. This fact helped build our hypothesis that in diets deficient in feed undegradable protein, which are also MP-deficient, microbial protein synthesized in the rumen becomes an even more important source of AA for the cow and, due to its low concentration in microbial protein, His may become limiting to milk production and milk protein synthesis.

To test this hypothesis, we conducted an experiment with 48 Holstein cows at Penn State’s experimental dairy farm. The experiment continued for 12 wk and included a 2-wk covariate period. Cows were blocked by days in milk and milk yield and randomly assigned to one of 4 diets: control, MP-adequate diet (AMP; average MP balance according to NRC 2001: +9 g/d); MP-deficient diet (DMP; MP balance: -317 g/d); DMP supplemented with ruminally-protected Lys and ruminally-protected Met (DMPLM); and DMPLM supplemented with an experimental ruminally-protected His product (DMPLMH). Crude protein content of the AMP and DMP diets was 15.7 and 13.5%, respectively. Compared with AMP, the DMP diets decreased apparent total tract digestibility of all measured nutrients (for example, fiber digestibility was decreased by about 10% - a clear indication of RDP deficiency). The DMP diet also decreased urinary-N losses by 36% and MUN by about 20% (10.3 vs. 13.0 mg/dL for the control diet). Milk N efficiency was greater for all DMP diets compared with AMP (34 vs. 29%, respectively). Compared with the control diet, dry matter intake tended to be lower for DMP, but was similar for DMPLM and DMPLMH (24.5, 23.0, 23.7, and 24.3 kg/d or 56.1, 50.6, 52.1, and 53.5 lbs/d, respectively). Milk yield was decreased by DMP (35.2 kg/d, or 77.4 lbs/d), but was similar to AMP (38.8 kg/d or 85.4 lbs/d) for DMPLM and DMPLMH (36.9 and 38.5 kg/d or 81.2 and 84.7 lbs/d, respectively), paralleling the trends in dry matter intake. Milk fat and true protein content did not differ among treatments, but milk protein yield was increased by DMPLM and DMPLMH compared with DMP and was not different from AMP. Plasma Lys and His were lower for DMP compared with AMP (Lys was about 16% lower and His about 47% lower). Supplementation of the DMP diets with ruminally-protected AA increased plasma Lys, Met, and His. Our conclusion from this trial was that diets that are about 15% deficient in MP (according to NRC, 2001), may decrease feed intake and consequently milk yield in dairy cows. Supplementation of such diets with ruminally-protected Lys and Met can diminish the difference in feed intake and milk yield compared with the control, and additional supplementation with ruminally-protected His can eliminate it. We also concluded that, similar to monogastric animals, AA may play a role in feed intake regulation in dairy cows. Our data implicated His as a limiting AA in high-producing dairy cows fed corn silage and alfalfa haylage-based diets, deficient in MP. This was primarily a result of the relatively low His concentration in microbial protein synthesized in the rumen. When formulating diets for high-producing dairy cows fed MP at or below NRC (2001) requirements, nutritionists should also balance for digestible His supply. Our recommendation is that digestible His requirements should be similar to digestible Met requirements, i.e. about 2.2% of MP.

This research is reported in J. Dairy Sci. (Lee, C., A. N. Hristov, K. S. Heyler, T. W. Cassidy, H. Lapierre, G. A. Varga, and C. Parys. 2012. Effects of metabolizable protein supply and amino acids supplementation on nitrogen utilization, production and ammonia emissions from manure in dairy cows. J. Dairy Sci. 95:5253–5268 and Lee, C., A. N. Hristov, T. W. Cassidy, K. S. Heyler, H. Lapierre, G. A. Varga, M. J. de Veth, R. A. Patton, and C. Parys. 2012. Rumen-protected lysine, methionine, and histidine increase milk protein yield in dairy cows fed metabolizable protein-deficient diet. J. Dairy Sci. 95:6042–6056).