| Heat stress resulting in animal production losses costs the dairy industry hundreds of millions of dollars annually in the USA, especially in southern areas such as California. Niacin is a dietary supplement which typically induces flushing, or 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 feeding ruminally protected niacin (RPNi) is effective in alleviating heat stress in dairy cows and thereby increasing productive performance. Two 2x2 factorial experiments, each with 28 d periods, were conducted in the summer of 2010. In Expt. 1, 2 pens, each with ∼180 early lactation multiparity cows were used, and in Expt. 2, 2 pens of ∼180 mid-lactation mixed parity cows were used. The basal total mixed ration, of 171 g/kg dry matter (DM) CP, 332 g/kg aNDF and 147 g/kg starch, was the same for all cows with the exception of RPNi added to the RPNi diets. RPNi was added to treatment pen diets in order for each cow to consume ∼19 g RPNi/d, which was estimated to deliver ∼6 g of intestinally absorbable niacin/cow/d as determined by RPNi evaluation, including ruminal in sacco incubation. In Expt. 1, respiration rate (RR) and panting score (PS) were measured 4 times/d in subgroups of ∼35 cows/pen. RR were lower (P=0.02) at 09:00 h, but not impacted at other times, 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. There was no difference in DM intake or milk and milk component yields, but milk fat proportion was lower in RPNi cows (P<0.01). In addition, the fat:protein production ratio was lower in cows fed RPNi (P<0.01). In Expt. 2, where PS and RR were not measured, DM intake and milk yield did not differ between treatments, and there were no differences in milk yield, or milk protein or lactose proportions. However, fat proportion and the fat:protein production ratio was higher in RPNi cows (P<0.01 in both). While indicators of heat stress were improved for cows fed RPNi, this increase in cow comfort did not result in increased DM intake or productivity, due to the relatively mild heat stress expressed by the cows. Differences in milk fat proportion and yield responses in early lactation cows suggests lipid metabolism in the cows was affected by niacin, most likely by reducing plasma triglyceride (TG) production. A reduction of plasma TG production was also likely in the mid-lactation cows but, because they were producing less milk and consuming the same amount of DM, dietary fat intake was likely able to compensate for the reduction of de novo synthesis. Further research with RPNi is suggested, especially in transition fresh and mid-lactation cows, fed a higher level of RPNi in a hotter environment. |
