PSVI-22 Effects on physiological parameters and peripheral thermoregulation in beef steers rotational grazing toxic endophyte-infected tall fescue under fall conditions.

Abstract The negative effects on weight gain and physiological parameters related to thermoregulation in animals grazing tall fescue infected with the ergot alkaloid (EA)-producing endophyte Epichloë coenophiala, are well known. Through a rotational grazing study, we aimed to determine a) the onset and resolution of clinical signs after exposure/removal from toxic pastures, b) residual effects post pasture removal, and c) climate conditions that impair thermoregulation and environmental temperature (ET) threshold at which this occurs. Eighteen steers grazed nontoxic (NT), toxic (E+), or endophyte-free (E-) fescue pastures. After 14 days, groups switched (toxic to nontoxic and vice versa) or remained on nontoxic pasture. Body weight, respiration rate (RR), rectal temperature (RT), and urine were collected repeatedly. Tail skin surface temperature (SST,°C) and environmental conditions were continuously recorded. Forage and urinary total EA were measured. E+ forage EA exceeded 6000 ppb. Urinary EA spiked within a week, remained elevated, and dropped to control levels two days post E+ removal. Weight gain was unaffected in the first half but declined >65% (P≤0.01) in E+ steers in the second period, during which, steers previously on E+ gained more than those always on non-toxic pasture (P< 0.04). RT and RR were higher (P≤0.05) in E+ steers during both periods, increasing a week after E+ placement and normalizing two days post-removal. RT and RR were similar (P >0.2) in steers switched away from E+ and those never exposed, indicating no residual effect. During the first 14 days, E+ steers had lower SST (34.4±0.1) than E- (35.2±0.1) and NT (35.4±0.1; P≤0.001). On heat stress days (THI≥72), E+ SST (35.8±0.1) was lower than E- (36.4 ± 0.1) and NT (36.5 ± 0.1; P≤0.001); especially during cooler hours (morning/night, ET< 20°C). On non-heat stress warm days (THI< 72), E+ SST was ~1°C lower (P≤0.001) than other groups. In the second 14 days, steers previously on E+ maintained lower SST (P≤0.001). No group differences were seen on coldest days (ET -3.5 to 18), regardless of time-of-day. As ET increased (THI≥72), steers previously on E+ showed reduced SST (P≤0.004), especially during cooler hours; they also had low SST (27.3±0.4; P ≤ 0.001), compared to newly exposed (30.3±0.4) and unexposed steers (29.5±0.4) when raining. Switching from non-toxic to E+ lowered SST (35.5 to 35.0; P< 0.0001), while E+ to nontoxic switching had no effect. Our data suggest that 14 days on E+ induce clinical signs, with urinary EA aligning with onset and recovery. Most physiological parameters recovered, but SST remained affected, worsened by rain and mild heat, not dry cold. Animals struggled to regulate body temperature within their thermoneutral zone when ET dropped below 18–20°C; above this, heat dissipation appeared unaffected. Rotational fescue grazing shows promise, but thermoregulation/environmental conditions warrant consideration.