Effects of supplementation of energy or ruminally undegraded protein to lactating cows fed alfalfa hay or silage

Alfalfa was harvested as silage or hay and fed in two 12-wk trials with a 4 x 4 Latin square design that used 12 (trial 1) or 24 (trial 2) multiparous lactating cows (4 ruminally cannulated cows per trial). Diets contained (dry matter basis) 75 or 50% alfalfa plus 24 or 40% high moisture corn (trial 1) or 50% alfalfa, 44 or 41% high moisture corn, with (3%) or without fish meal (trial 2). Experiments were conducted to evaluate the responses of cows fed alfalfa hay or alfalfa silage diets to an increase in protein supply from microbial protein synthesis (trial 1) or from the supplementation of ruminally undegraded protein (RUP) (trial 2). In trial 1, the increase in high moisture corn in the diet increased both milk protein and microbial crude protein yields (estimated from the excretion of purine derivatives) to a greater extent for the cows fed the alfalfa silage diets (170 and 337 g/d, respectively) than for the cows fed the alfalfa hay diets (100 and 100 g/d, respectively). In trial 2, RUP supplementation (as fish meal) increased milk protein yield 100 g/d for cows fed alfalfa silage diets and 20 g/d for cows fed alfalfa hay diets. These results indicated that protein status was poorer and, thus, more responsive to absorbable protein from microbial protein (trial 1) or RUP (trial 2) for cows that consumed alfalfa conserved as silage versus those that consumed alfalfa conserved as hay.