SUPPLEMENTATION OF NICOTINIC-ACID FOR LACTATING HOLSTEIN COWS UNDER HEAT-STRESS CONDITIONS

Twenty-six lactating Holstein cows (90 d of lactation) were blocked ac cording to milk production, parity, and days of lactation for assignme nt to one of two dietary treatments. Diets included a control diet wit h no supplemental niacin and a diet supplemented with increasing conce ntrations of niacin (12, 24, or 36 g/d per cow over three consecutive 17-d periods. Cows were housed in a covered free-stall barn and were f ed and milked twice daily. Mean maximum air temperatures and temperatu re-humidity indexes were 28.5, 31.4, and 25.2 degrees C and 79.6, 85.1 , and 75, respectively, for the three periods. Rectal temperature was measured with a rectal probe, tail and rump temperatures by infrared t hermometry, and respiratory rate by visual observation. Measurements w ere made daily at 0800, 1600, and 2200 h. Rectal temperature was not a ffected by treatment. Comparison of skin temperatures for control cows and cows fed niacin showed higher temperatures at the tail (34.0 vs. 33.7 degrees C at 0800 h; 35.1 vs. 34.8 degrees C at 1600 h, respectiv ely) and rump (34.1 vs. 33.7 degrees C at 0800 h; 35.3 vs. 35.0 degree s C at 1600 h, respectively) for control cows during period 1. No diff erences in thermal responses were observed during period 3. Niacin did not significantly increase milk production but decreased skin tempera tures during periods of mild or severe heat stress.