On-farm supplementation of rumen-protected niacin: A randomized clinical trial.

B vitamins, including niacin (vitamin B), are synthesized by rumen microbes, but supplementation may provide additional benefits for ruminant health and productivity. Supplementing rumen-protected niacin (RPN) during the transition period can reduce lipolysis after calving and, consequently, may improve health and fertility of dairy cattle later in lactation. Our objective was to determine if supplementing RPN during the first 21 DIM would improve the health of dairy cows on a commercial dairy farm. We hypothesized that RPN would reduce mastitis, improve fertility, and reduce risk of leaving the herd during lactation. Holstein cows were blocked by parity and projected calving date, with treatments randomly assigned to cows within each block. Cows received RPN (n = 481; 26 g/d) through a supplement dispenser in the automated milk systems (AMS) in addition to their robot pellet, or the robot pellet only (CON; n = 593). Treatments were applied for the first 21 DIM, and cows were followed for the rest of their lactation. Milk yield, milk components (wk 1-3 of lactation), pre- and postpartum BCS, health records, and reproductive records were collected. Blood was collected from a random subset of 99 cows at 3 DIM and 97 cows at 10 DIM to assess plasma concentrations of niacin, metabolic biomarkers, and biomarkers of inflammatory status. Culling, proportion of cows pregnant, and mastitis risk were analyzed using Cox proportional hazard models. Mastitis incidence was analyzed with a linear mixed model and conception risk was analyzed using a χ test. Supplementing RPN increased plasma nicotinamide concentration by 1,740 nM ± 410.0 nM (SE; 50% increase), but it did not affect plasma nicotinic acid concentrations. Supplementing RPN reduced plasma insulin concentrations at 3 and 10 DIM across all parities. Circulating BHB and free fatty acid concentrations were greater for cows receiving RPN; the effect was greatest in cows in third or greater parity. Plasma haptoglobin was not affected by treatment. Rumen-protected niacin increased milk yield for first- and second-parity cows by wk 9 and 13 of lactation, respectively, and increased milk yield in those groups was sustained for the rest of lactation, resulting in 658 ± 259.4 kg and 675 ± 308.9 kg more milk for RPN-supplemented first- and second-parity cows. The risk of leaving the herd, mastitis incidence, and probability of pregnancy were not affected by RPN. Inquiries into the effects of RPN supplementation and its timing on metabolism are necessary to understand optimal supplementation strategies for RPN in dairy cattle.