Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/21/2006
Publication Date: 10/1/2006
Citation: Brito, A.F., Broderick, G.A., Reynal, S.M. 2006. Effect of varying dietary ratios of alfalfa silage to corn silage on omasal flow and microbial protein synthesis in dairy cows. Journal of Dairy Science. 89:3939-3953. Interpretive Summary: Alfalfa silage and corn silages are complementary feedstuffs because the first is rich in protein and the second is rich in energy. However, more purchased protein must be fed when lower protein corn silage replaces alfalfa silage in the diet. We found that feeding about 60% of dietary forage as alfalfa silage and 40% as corn silage gave the highest production but protein efficiency was best when about equal amounts of the two forages were fed. Dairy cows get part of their protein from protein formed by the microbes living in the rumen, the first compartment of the cow's stomach, plus dietary protein that escapes breakdown by the rumen microbes. Both sources of protein flow out of the rumen to get digested in the intestine. To help determine what effect different amounts of alfalfa silage and corn silage had on the protein supply, 8 Holstein cows were fitted with rumen cannulas (holes put into their rumens by veterinarians using surgery) so protein flowing out of the rumen could be measured. The cows were fed diets containing four different ratios of alfalfa silage to corn silage, 51:0, 37:13, 24:27, and 10:40. The rest of the diet came largely from soybean meal (added as alfalfa was decreased to maintain similar total protein) plus high moisture corn (a major feed ingredient fed to US dairy cows). Although the amount dietary protein escaping the rumen did not change, microbial protein declined as corn silage replaced alfalfa silage. This research showed that the most microbial protein would be produced when diets contained 38% alfalfa silage and 12% corn silage, but that protein quality and supply of amino acids (the protein building blocks needed by the cow) would be best with 32% alfalfa silage and 18% corn silage. This research also means that dairy farmers will get the most effective protein nutrition for their cows by optimizing the balance of alfalfa and corn silages in the diet.
Technical Abstract: Eight ruminally cannulated multiparous Holstein cows that were part of a larger production trial were used to study the effects of varying dietary ratios of alfalfa silage (AS) to corn silage (CS) on omasal flow of nutrients and microbial protein. Cows were blocked by days in milk and randomly assigned to 2 replicated 4 x 4 Latin squares (28 d periods) and fed diets that contained [dry matter (DM) basis]: A) 51% AS, 43% rolled high-moisture shelled corn (HMSC), and 3% solvent soybean mean (SSBM), B) 37% AS, 13% CS, 39% HMSC, and 7% SSBM), C) 24% AS, 27% CS, 35% HMSC, and 12% SSBM), or D) 10% AS, 40% CS, 31% HMSC, and 16% SSBM). Crude protein (CP) contents were 17.2, 16.9, 16.6, and 16.2% for diets A, B, C, and D. Microbial NAN flow was lower on diet D (423 g/d) compared to diets A (465 g/d), B (479 g/d), and C (460 g/d). A significant quadratic effect indicated that microbial protein synthesis was maximal at 38% AS. Supply of rumen-degraded protein decreased linearly from 3068 g/d (diet A) to 2469 g/d (diet D). Omasal rumen-undegraded protein (RUP) flow did not differ among diets and averaged 1528 g/d. However, when expressed as a percentage of DM, RUP increased linearly from 5.59% (diet A) to 6.13% (diet D), probably because CP from SSBM was more resistant to degradation than CP from AS. Generally, there was little effect of diet on omasal AA flow. However, Lys flow tended to be lower on diet D, which may explain its lower milk yield, and a quadratic effect was observed for omasal Met flow, with maximum at 32% AS. Positive interactions from feeding CS with AS improves ruminal out-flow of microbial protein and Met.