Influence of dietary protein concentration and degradability on performance of lactating cows during hot...

To test effects of protein concentrations and degradability, 60 lactating Holstein cows in midlactation were offered the following diets during three trials between May and October in Tucson, AZ: high protein (18.4%), high degradability; high protein (18.5%), medium degradability; medium protein (16.1%), high degradability; and medium protein (16.1%), medium degradability. Diets comprised 39% alfalfa hay, 12% cottonseed hulls, 10% whole cottonseed, and 39% concentrate (DM) and concentrates contained 60, 40, 57, and 40% degradability, respectively, as determined by ficin assay. Rectal and ambient temperatures suggested that cows were under moderate to intense heat stress, as did group water intakes, which were increased about 15% by high degradability. Milk yields (3.5% FCM) and persistencies were lower for the high protein, high degradability diet than for all others. Mean DM intakes across treatments were quite high but were lower on high than medium protein; whereas ruminal ammonia and blood serum urea were higher on high protein. Milk composition, ruminal VFA, serum glucose, thyroxine, triiodothyronine, and cortisol were not affected by treatment.