Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/3/2009
Publication Date: 6/1/2009
Citation: Broderick, G.A., Stevenson, M.J., Patton, R.A. 2009. Effect of Dietary Protein Concentration and Degradability in Response to Rumen-Protected Methionine in Lactating Dairy Cows. Journal of Dairy Science. 92:2719-2728.
Interpretive Summary: Dietary protein supplies lactating cows with amino acids, the building blocks required by the cow to make protein in milk and body tissues. About half of the amino acids in protein are essential. This means that they cannot be made in the body; they must be absorbed intact from digesta passing through the intestine. Feeding too much protein to dairy cows results in excessive nitrogen excretion, especially in the urine; this form of excretory nitrogen is the most polluting. Supplementation with the limiting essential amino acid is commonly done in non-ruminant nutrition to allow the feeding of lower protein diets that result in reduced urinary nitrogen excretion. This strategy can now be used in dairy cattle because methionine, an essential amino acid that is often limiting on typical rations fed in North America, is commercially available in a rumen-protected form (coated with materials that protect the methionine from breakdown by the microbes living in the rumen, thus allowing the amino acid to be absorbed at the intestine when fed in the diet). A feeding study was conducted to test whether adding rumen-protected methionine to the diet would allow feeding of less soybean meal, the major protein supplement fed to dairy cows in the U.S. Two different kinds of soybean meal were tested in this trial: solvent soybean meal (the usual kind fed to cows) and heat-treated soybean meal in which the protein largely escapes the rumen and is directly useful to the cow. If less soybean meal can be fed without losing milk production, then dairy farmers will make more profit while at the same time helping the environment by reducing urinary nitrogen excretion from their cows. Forty-eight Holstein dairy cows were fed 8 different diets with protein at 15.8 or17.1% of dietary dry matter; regular or heat-treated soybean meal, and 0 or 9 grams/day of rumen-protected methionine, were fed at each level of dietary protein. Production of milk, fat-corrected milk, milk fat, and milk protein all increased at the higher level of protein in the diet; however, the cows ate more, too, and excreted more nitrogen on the higher protein diets. Feeding the heat-treated soybean meal did not change production or protein efficiency unless the rumen-protected methionine was also included in the diet. Feeding the rumen-protected methionine improved milk, fat and protein production and reduced urinary nitrogen excretion. As a matter of fact, adding rumen-protected methionine to diets at 15.8% protein resulted in production of milk, fat and protein that was equal to production with diets at 17.1% protein without the added methionine; and there was an added benefit of a 13% reduction in nitrogen excretion in the urine. This research indicates that wastage of dietary protein on U.S. dairy farms can be reduced by proper supplementation with rumen-protected methionine. Moreover, this strategy is even more effective when the protected methionine is fed along with heat-treated soybean meal. These results mean that U.S. dairy farmers could use supplementation of rumen-protected methionine to reduce nitrogen pollution of the environment while improving profits.
Technical Abstract: An incomplete 8 x 8 Latin square trial (4-week periods; 12 weeks total) using 48 Holstein cows was conducted to assess the response to rumen-protected Met (RPM; fed as Mepron®). The 8 diets contained [dry matter (DM) basis] 21% alfalfa silage, 34% corn silage, 22-26% high moisture corn, 10 to 14% soybean meal, 4% soyhulls, 2% added fat, 1.3% minerals and vitamins, and 27-28% NDF. Treatments were a 2 x 2 x 2 arrangement of diets: 15.8 or 17.1% CP, with or without supplemental rumen-undegraded protein (RUP) from expeller soybean meal, and 0 or 9 g RPM/d. Contrasts indicated that higher dietary CP increased DM intake and yield of milk, 3.5% fat-corrected milk (FCM), fat and true protein, plus apparent N balance and DM and fiber digestibility. However, milk urea N (MUN), estimated urinary excretion of urea-N and total-N also increased, and apparent N efficiency (Milk-N/N-intake) fell from 33 to 30%, when cows consumed higher dietary CP. Positive effects of feeding more RUP were small increases in feed efficiency, greater milk fat content and higher NDF digestibility, but milk protein content was also lower and MUN elevated, suggesting that soybean meal RUP was ineffective without adequate Met. Supplementation with RPM increased DM intake, milk fat content and fat and FCM yield, and gave a trend for increased protein yield. Numerically, the response to RPM was greater when diets contained supplemental RUP, although interactions among the main dietary factors were not significant. The NRC model indicated that these diets were limiting in absorbed Met and had Lys: Met ratio of 3.6 without RPM, while RPM supplementation decreased the ratio to 3.1.