Twelve multiparous Holstein cows (145+/-9 d in milk) were randomly assigned to receive either 0 g/d of encapsulated niacin (control diet; C) or 12 g/d of encapsulated niacin (NI) and were exposed to thermoneutral (TN; 7 d) or heat stress (HS; 7 d) conditions in climate-controlled chambers. The temperature-humidity index during TN conditions never exceeded 72, whereas HS conditions consisted of a circadian temperature range in which the temperature-humidity index exceeded 72 for 12 h/d. Measures of thermal status obtained 4 times/d included respiration rate (RR); rectal temperature; surface temperature of both shaved and unshaved areas at the rump, shoulder, and tail head; vaginal temperature; and evaporative heat loss (EVHL) of the shoulder shaved and unshaved areas. Cows fed NI had increased free plasma niacin concentrations in both the TN and HS periods (1.70 vs. 1.47+/-0.17 microg/mL). Milk yield did not differ between dietary groups or periods. Dry matter intake was not affected by NI, but decreased (3%) for both C and NI treatments during HS. Water intake was increased during HS in both treatments (C: 40.4 vs. 57.7+/-0.8L/d for TN and HS, respectively; NI: 52.7 vs. 57.7+/-0.8 L/d for TN and HS, respectively). Average EVHL for shaved and unshaved skin for C and NI treatments was higher during HS (90.1 vs. 108.1 g/m(2) per hour) than TN (20.7 vs. 15.7+/-4.9 g/m(2) per hour). Between 1000 and 1600 h, mean EVHL for shaved and unshaved areas for NI fed cows was higher than for C fed cows (106.9 vs. 94.4+/-4.9 g/m(2) per hour). The NI fed cows had decreased rectal temperatures during HS compared with the C fed cows (38.17 vs. 38.34+/-0.07 degrees C) and had lower vaginal temperatures (38.0 vs. 38.4+/-0.02 degrees C). Calculated metabolic rate decreased during HS regardless of diet (50.25 and 49.70+/-0.48 kcal/kg of body weight per day for TN and HS, respectively). Feeding NI increased free plasma NI levels, increased EVHL during peak thermal load, and was associated with a small but detectable reduction in rectal and vaginal temperatures in lactating dairy cows experiencing a mild thermal load.
2010 American Dairy Science Association. Published by Elsevier Inc. All rights reserved.