Effect of alfalfa forage preservation method and particle length on performance of dairy cows fed corn silage-based diets and tallow.

A study was conducted to evaluate the effect of including alfalfa preserved either as silage or long-stem or chopped hay on DMI and milk fat production of dairy cows fed corn silage-based diets with supplemental tallow (T). Fifteen Holstein cows that averaged 117 DIM were used in a replicated 5 x 5 Latin square design with 21-d periods. Treatments (DM basis) were: 1) 50% corn silage:50% concentrate without T (CS); 2) 50% corn silage:50% concentrate with 2% T (CST); 3) 25% corn silage:25% short-cut alfalfa hay:50% concentrate with 2% T (SAHT); 4) 25% corn silage:25% long-cut alfalfa hay:50% concentrate with 2% T (LAHT); and 5) 25% corn silage:25% alfalfa silage:50% concentrate with 2% T (AST). Cows were allowed ad libitum consumption of a TMR fed 4 times daily. Diets averaged 16.4% CP and 30.3% NDF. Including 2% T in diets with corn silage as the sole forage source decreased DMI and milk fat percentage and yield. Replacing part of corn silage with alfalfa in diets with 2% T increased milk fat percentage and yield. The milk fat of cows fed CST was higher in trans-10 C18:1 than that of cows fed diets with alfalfa. No effect of alfalfa preservation method or hay particle length was observed on DMI and milk production. The milk fat percentage and yield were lower, and the proportion of trans-10 C18:1 in milk fat was higher for cows fed LAHT than for cows fed SAHT. Alfalfa preservation method had no effect on milk fat yield. Ruminal pH was higher for cows fed alfalfa in the diets, and it was higher for cows fed LAHT than SAHT. Feeding alfalfa silage or chopped hay appears to be more beneficial than long hay in sustaining milk fat production when 2% T is fed with diets high in corn silage. These results support the role of trans fatty acids in milk fat depression.