Effects of utilizing rumen protected niacin on core body temperature as well as milk production and composition in lactating dairy cows during heat stress

Niacin has been shown to increase resistance to thermal stress in cattle by increasing evaporative heat loss in vivo and cellular heat shock response by increasing gene expression of heat shock proteins 27 and 70 during thermal stress in vitro. To determine effects of feeding encapsulated niacin on vaginal temperature, milk yield and composition, a total of 198 primiparous and 229 multiparous lactating Holstein cows were randomly assigned in a crossover design to either control (no niacin additive, n = 213) or niacin (NIA; cows supplemented with 12 g/d/cow of an encapsulated niacin, n = 214). Treatments were balanced for days in milk (DIM), milk yield, and parity prior to the start of the study, which was conducted between August 7th and October 7th, 2007 on a commercial dairy in Arizona (USA). Cows remained on their original treatment for 30 d (period 1) and then switched to the other treatment on day 31 and continued until day 60 (period 2). Vaginal temperatures were recorded using temperature data loggers attached to a blank controlled internal drug release device and inserted into a random sub-sample of 16 cows from each pen with similar days in milk, milk yields and parity for 8 d. Effects of heat stress were more severe in period 1 due to a higher temperature humidity index compared with period 2. Dry matter intakes did not differ by treatment or period. Both period and NIA affected vaginal temperatures with NIA reducing vaginal temperatures and period 1 having higher vaginal temperatures compared with period 2. In addition, vaginal temperatures decreased (P<0.01) with NIA treatment during peak thermal load from 1300 to 1600 h for 4 of 8 d. A NIA by period interaction occurred for several milk production parameters. The NIA fed cows had higher milk yield in period 1 (higher heat stress) and was lower in period 2 (P<0.01). In contrast, NIA reduced milk fat yield in period 1 and increased it in period 2 (P<0.01). Both true protein and lactose yield was lower in period 1 with no effect in period 2. Supplementation of lactating cows with NIA reduced vaginal temperature, but had differing effects on milk production variables dependent on the period of the study which may be due to a more severe heat stress experienced in period 1 compared with period 2, and/or a more advanced stage of lactation in period 2.