Impacts of manipulating ration metabolizable lysine and methionine levels on the performance of lactating dairy cows: A systematic review of the literature

A group of 22 treatments from 6 experiments in which lactating dairy cows were fed diets with different ingredient compositions, and in which the amino acid (AA) profile of their duodenal protein was measured with the same techniques and methodologies, were assembled. Correlative statistics were used to assess relationships between proportions of various crude protein (CP) sources in the diet and the proportion of methionine and lysine in duodenal protein. Changes in the percentages of dietary CP coming from several classes of feeds had some impact on the lysine and methionine proportions of duodenal CP, but changes were generally modest either when predicted by simple (i.e., increased diet canola CP increased, and increased corn CP decreased, lysine in duodenal digesta CP, while increased diet canola and barley/wheat CP increased, and increased corn CP and grass/legume CP decreased, methionine in duodenal digesta) or multiple correlation analysis (i.e., increased legume/grass, canola, linseed and blood increased, and increased corn and canola decreased, lysine in duodenal digesta, while increased corn and barley/wheat decreased lysine in duodenal digesta and increased dietary CP from barley/wheat, canola, linseed and fish meal increased methionine in duodenal digesta). Feeding experiments that used lactating Holstein cows, from peer review publications since 1984, were then used to determine the ability to predict production responses of lactating dairy cows to feeding ruminally protected (RP) methionine, lysine or lysine/methionine combinations based upon the composition of their diets. The 54 published experiments included 54 comparisons of a control diet to one supplemented with RP methionine, 12 to RP lysine and 47 to a combination of both AA. Differences among the three treatments (i.e., RP methionine, RP lysine, RP lysine/methionine addition to the diets) in the characteristics of the cows and diets assigned to the control treatments, as well as their responsiveness to AA addition, were analyzed using a systematic review of the literature with a model that considered each experiment to be an observation. There were differences in the ingredient and chemical characteristics of the control diets utilized to evaluate responses to RP methionine, lysine and methionine/lysine combinations. RP methionine supplementation increased milk energy output as well as milk protein and fat percentages, while increasing the proportion of dietary N captured as milk N. RP lysine supplementation decreased dry matter (DM) intake and increased the milk/DM intake ratio. RP lysine and methionine combinations increased milk and milk energy yield, milk protein percentage as well as increased the proportion of dietary N captured as milk N and the milk/DM intake ratio. However these changes were judged to be small (i.e., the largest was a 3.9% increase in capture of dietary N as milk N for lysine and methionine combinations) and largely not predictable based upon characteristics of the control cows and diets among studies. This review provides strong evidence that levels of lysine and methionine in duodenal protein do change with source of dietary proteins, and that manipulation of the lysine and methionine proportions of duodenal protein by feeding RP methionine or lysine/methionine combinations does have modest impacts on dairy cow performance, although RP lysine alone has a negative impact. However these differences are small and relatively unpredictable in practice making their utility as a practical means to improve the productivity and/or reduce the environmental impact of dairy cows on commercial dairy farms very limited.