Effect of spring rotational grazing of toxic endophyte-infected tall fescue on ergot alkaloids kinetics, thermoregulatory and behavioral responses of beef steers.

Bovine fescue toxicosis (FT) results from ingesting tall fescue infected with the ergot alkaloid (EA)-producing endophyte Epichloë coenophiala. EA are pleiotropic and affect peripheral and core vasoconstriction, disrupting thermoregulation and nutrient absorption. This spring rotational grazing study evaluated: a) onset, recovery, and residual effects of FT clinical signs, b) environmental temperature thresholds and climatic conditions affecting thermoregulation, c) anatomical differences in heat dissipation, d) EA kinetics relative to clinical responses, e) selected plasma lipid profiles, and f) ruminal and fecal volatile fatty acids. Eighteen steers grazed novel (NE), toxic (E+), or endophyte-free (E-) pastures; after 14 days, E+ and E- groups switched pastures for additional 14 days. Body weight, respiration rate (RR), rectal temperature (RT), urine, blood, rumen fluid, saliva, feces, and plant tissue were collected. Wireless sensors recorded tail and ear skin surface temperatures (SST), behavioral activity, and environmental conditions. Total EA in E+ pastures exceeded 1075 ppb. EA concentrations (lysergic acid predominated; ergovaline only detected in rumen fluid) increased one week after E+ exposure and returned to baseline within the same time frame post-removal, aligning with the onset and recovery of clinical signs (RT, RR, and weight gain). In contrast, SST responses were site- and climate-dependent and exhibited residual impairments, particularly during rainy and cooler periods of warmer days. E+ grazing altered steers' behavior in an environment-dependent fashion and induced early pro-inflammatory lipid changes. Spring rotational fescue grazing is a feasible strategy, but lingering thermoregulatory vulnerability after E+ pasture exposure calls for caution.