Effects of endophyte-infected tall fescue on performance of genotyped pregnant beef cows supplemented with rumen-protected niacin

•Fescue toxicosis is a detrimental condition that affects the overall performance of cattle consuming endophyte-infected tall fescue.•Genetic tests for identifying animal tolerance or susceptibility to fescue toxicosis is currently available in the market.•Niacin is known by its vasodilatory effect, and it could potentially dampen the vasoconstrictive effect of fescue toxicity. A total of 28 Angus × Simmental pregnant dams (11 cows and 17 heifers) were selected based on genetic resistance to fescue toxicosis based on a commercial genetic test (T-snip™, Ag. Botanica, Columbia, MO). Subsequently, dams were divided based on genetic resistance to fescue toxicosis and randomly assigned to dietary treatments: 1) Susceptible Control (SC; n = 7); 2) Susceptible Niacin (SN; n = 7); 3) Tolerant Control (TC; n = 7); and 4) Tolerant Niacin (TN; n = 7). All animals received 1.16 kg of endophyte-infected (E+) tall fescue seeds for a period of 30 days. Ergovaline concentration in fescue seeds was ∼5,000 ppb on a DM basis. Dams in SN and TN groups received 6 g/hd/day as-fed of top-dressed rumen-protected niacin, following manufacturer's maximum dose recommendation. Dams in the TN group experienced an accelerated reduction in BW as compared to the rest of the treatments. However, TN offspring did not show BW differences at birth or weaning as compared to those born to dams in other treatments. Milk production was estimated by the weigh-suckle-weigh method, and no difference was observed among treatments. All dams had a reduction in circulating prolactin concentration, indicating that all animals were experiencing fescue toxicosis. In addition, blood metabolites that indicate liver health status were markedly decreased at the end of the study. Overall, these results suggest that dams receiving E+ tall fescue seeds were effectively exposed to fescue toxicosis symptoms. Nevertheless, neither rumen-protected niacin supplementation nor genetic test for resistance to fescue toxicosis could be used as an efficient mechanism of dampening these symptoms.