THE EFFECTS OF ENDOPHYTE-INFECTED TALL FESCUE CONSUMPTION ON DIET UTILIZATION AND THERMAL REGULATION IN CATTLE

Two experiments were conducted to determine the effects of endophyte fungus (Acremonium coenophialum) ingestion in tall fescue diets and environmental temperature on heat dissipation and diet utilization by cattle. In Exp. 1, 12 Angus heifers (average weight 244 kg) were allotted by weight to either an endophyte-free (E-) or endophyte-infected (E+; 381 ppb of ergovaline) diet. Environmental temperature varied between 22 and 32-degrees-C. Voluntary DM and water intakes were similar (P > .10) among treatments. Rectal temperatures and concentrations of prolactin in plasma were lower (P < .05) after ingestion of the E+ diet. Plasma triiodothyronine, thyroxine, and cortisol concentrations were not affected by diet. In Exp. 2, 24 Holstein steers (average weight 114 kg) were allotted by weight to either E- or E+ (285 ppb of ergovaline) and one of two environmental temperatures (22 or 32-degrees-C). At 32-degrees-C, feed intake was reduced by 22%, and water consumption was increased by 62% compared with steers housed at 22-degrees-C. Consumption of E+ reduced feed intake by 10% but did not influence water consumption. Plasma concentration of prolactin was decreased (P < .05) within 48 h after consumption of E+. Rectal temperatures increased in response to both environmental temperature (P < .05) and E+ consumption (P = .06). Digestibilities of DM and OM for E+ were 9% lower (P < .05) than for E-. An interaction between diet and temperature (P < .05) revealed that steers fed E- at 32-degrees-C had greater skin vaporization, but steers that consumed E+ did not increase vaporization; values were similar to steers housed at 22-degrees-C. These results suggest that consumption of E+ at elevated temperatures decreased the ability of cattle to remove excess body heat through surface vaporization.