| Four studies were conducted to evaluate the effects of supplemental cooling and niacin on heat stress (HS) responses in vivo and in vitro in lactating dairy cows. For experiment one, lactating dairy cows were fed four levels of dietary niacin, NIASHURERTM (0,4,8,12 g/d) for 21 d. Heat stress decreased niacin levels in whole blood, red blood cells and plasma as compared to thermoneutral. Water intake, plasma and milk niacin concentrations increased linearly with increasing dietary niacin in HS cattle. In thermoneutral, but not HS cows, niacin increased skin temperature compared to controls suggesting niacin increased skin blood flow and sensible heat loss. In experiment 2, lactating cows were used to evaluate the impact of feed--line soaking (FLS) combined with niacin supplementation. In evaporative cooled barns, FLS reduced body temperatures; however the addition of niacin did not improve heat status of these cows. For experiment 3, 200 lactating dairy cows were used to determine the effects of conductively cooled bedding (CC) compared to feed--line soaking with fans (FLSF). Conductively cooled bedding can reduce skin and vaginal temperatures in cows after nighttime rest. However, FLSF were more effective in decreasing body temperature, as cows had lower heat parameter indices, higher milk yield and longer rest time. For experiment 4, three cell types were used to evaluate niacin in vitro. Niacin induced heat shock proteins (HSP) that protected cells during HS in transformed bovine mammary epithelial cells but not in primary bovine mammary epithelial cells (BMEC) or bovine endometrial cells (BEND). Effect of niacin on HSP may depend on enzymes involved in DNA--binding activity of heat shock factor 1. These results suggest that niacin may be involved in whole body metabolism during heat stress and is cell dependent. We did not find dietary niacin to be commercially efficacious in treating HS in cows. Further research is warranted to improve efficacy of CC and FLSF under high temperature humidity index conditions. |
