CORN OR ALFALFA AS THE FORAGE SOURCE IN PREDOMINANTLY SILAGE DIETS FOR LATE-LACTATION DAIRY-COWS

Twelve multiparous and three primiparous cows were used in a 10-wk lactation study with two repeated measures to evaluate diets containing 90% timothy-alfalfa silage (TA), 84% com silage (CS) or an equal mixture of each (TA-CS). Each diet was supplemented with minerals to meet requirements; barley was used as a carrier. Diets CS and TA-CS were also supplemented with urea, casein and soybean meal, making them similar to TA for undegraded intake protein (UIP) and degraded intake protein (DIP) levels. Two of the cows on each treatment were fitted with a ruminal cannula. There were no treatment effects on intake (27 g DM kg-1 BW), body-weight change or body-condition score. Milk yield averaged 15.3 kg d-1 and was not treatment influenced (P > 0.05); however, yield of fat and protein and percentage of lactose in milk showed a linear response to increased corn silage in the diet (P < 0.05). Rumen pH was least and rumen ammonia concentration greatest in the mixed silage diet (P < 0.05). Rumen pool sizes of organic matter and neutral detergent fiber were not treatment influenced (P > 0.05). All treatments supported modest levels of production for cows in late lactation, with increasing dietary energy density from com silage increasing energy output in milk.