Niacin delivery to the intestinal absorptive site impacts heat stress and productivity responses of high producing dairy cows during hot conditions

Heat stress resulting in animal production losses costs the dairy industry hundreds of millions of dollars annually, especially in tropical and semi-tropical areas. Niacin is a dietary supplement which typically induces flushing (i.e., increased blood flow to the skin), causing a decrease in core body temperature, which has been shown to reduce heat stress in dairy cows. Our objective was to determine if use of a rumen protection technology to deliver niacin (RPNi) to the intestinal absorptive site is effective in alleviating heat stress in dairy cows and increasing productive performance. Two 2 × 2 factorial experiments, each with 28 d periods, were conducted on a commercial dairy farm in central California (USA). In Experiment 1, 2 pens of ∼180 early lactation multiparity Holstein cows were used and, in Experiment 2, 2 pens of ∼180 mid-lactation mixed parity Holstein cows were used. The basal total mixed ration, of 171 g/kg crude protein, 332 g/kg neutral detergent fiber and 147 g/kg starch on a dry matter (DM) basis, was the same for all cows with the exception of RPNi added to the treatment total mixed ration. The RPNi was added to treatment pen diets at a consumption level of 19 g of RPNi/d, which was estimated to deliver ∼6 g of intestinally absorbable niacin/d as determined by evaluation of the RPNi. In Experiment 1, respiration rates (RR) and panting scores (PS; a 1-4.5 visual score of the extent of panting in a cow) were measured 4 times/d in subgroups of ∼35 cows/pen. The RR were lower (P=0.02) at 09:00 h, but not impacted at other times of the day, while PS were lower (P≤0.01) at 04:30, 09:00 and 20:30 h, but not impacted at 16:30 h, in cows fed RPNi compared to control. However, RR and PS were relatively low overall due to the cooler than normal summer weather. There was no difference in DM intakes or milk yields, but milk fat proportion was lower in RPNi cows (P<0.01). In Experiment 2, where PS and RR were not measured, DM intake and milk yield did not differ between treatments, with the exception of fat proportion, which was higher in RPNi cows (P<0.01). While indicators of heat stress were improved for cows fed RPNi, this slight increase in cow comfort did not result in increased DM intake or productivity. Differences in milk fat responses in our early lactation cows suggest lipid metabolism in the cows was affected by intestinal niacin delivery, most likely by reducing plasma triglyceride production. A reduction of plasma triglyceride production was also likely in the mid-lactation cows but, because they were producing less milk and consuming a similar amount of DM, dietary fat intake was likely able to compensate for the reduced de novo synthesis.