Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/6/1996
Publication Date: N/A
Citation: Interpretive Summary: High yields and quality make alfalfa the principal forage crop grown to feed dairy cattle in the U.S. Alfalfa also is a major protein source for dairy cows. However, extensive breakdown of alfalfa protein in the rumen, the first compartment of the cow's stomach, results in inefficient utilization of protein and depressed production of milk and milk protein. This is because the rumen microbes, which both form and degrade protein, reduce the value of alfalfa by forming less protein than they degrade. The protein in alfalfa silage is particularly subject to degradation in the rumen; nevertheless, most alfalfa is fed as silage to dairy cows in the U.S. because harvesting silage requires much less manual labor than harvesting hay. Two trials were conducted with lactating cows fed all their forage as either alfalfa silage or hay to see which was the better source of protein. In the first trial, different levels of corn (which increases protein formation by the rumen microbes) were fed with each forage source. Yield of milk and protein was similar on alfalfa silage and hay and was increased by adding more corn to the diet. The increase in protein yield from added corn was greater on silage than on hay diets. In the second trial, fish meal, a feed protein known to escape breakdown in the rumen, was added to some diets to determine the degree of protein shortage on either silage or hay. Yield of milk and protein was similar on alfalfa silage and hay, but yields were increased much more by adding fish meal to silage than to hay diets. These results indicated that protein supply limited milk production more on silage than hay when lactating dairy cows were fed alfalfa as their sole forage. Dairy cows will make more efficient use of alfalfa protein when the alfalfa is fed as hay rather than silage.
Technical Abstract: Alfalfa was harvested as silage or hay and fed in two 12-wk 4 x 4 Latin square trials, with 12 (trial 1) or 24 (trial 2) multiparous lactating cows (4 ruminally cannulated cows per trial). Diets contained (DM basis) 75 or 50% alfalfa plus 24 or 40% high moisture corn (trial 1) or 50% alfalfa, 44% high moisture corn, and 0 or 3% fish meal (trial 2). Experiments were conducted to evaluate the responses from alfalfa hay and silage diets to increased protein supply from microbial protein synthesis (trial 1) or RUP supplementation (trial 2). In trial 1, increasing dietary high moisture corn increased both milk protein and microbial CP yields (estimated from purine derivative excretion) to a greater extent on alfalfa silage (170 and 337 g/d, respectively) than on alfalfa hay (100 and 100 g/d, respectively) diets. Strong correlations (P <.001) of milk protein secretion with plasma allantoin concentration (r**2 = .42) as well as milk protein secretion with hmilk allantoin secretion (r**2 = .44), suggested that plasma and milk allantoin responded to increased microbial CP supply. In trial 2, RUP supplementation (as fish meal) increased milk protein yield 100 g/d on silage and 20 g/d on hay diets. These results indicate that protein status was poorer and, thus, more responsive to absorbable protein from microbial protein (trial 1) or RUP (trial 2), in cows consuming alfalfa conserved as silage versus hay.