DIFFERENCES BETWEEN PROGENY OF BEEF SIRES IN SUSCEPTIBILITY TO FESCUETOXICOSIS

Fifteen progeny of two Polled Hereford sires were fed endophyte-infect ed tall fescue seed (E+) to investigate variability in susceptibility to fescue toxicosis. One sire, bred in Missouri, was reputed to produc e calves that were resistant to fescue toxicosis. The Control sire, fr om Virginia, had unknown merit for susceptibility. There were two phas es in which E+ was included in the diet (2 and 4) and three in which i t was not (1, 3, and 5). Voluntary feed intake, serum prolactin, chole sterol, and alkaline phosphatase concentrations; ability to maintain h omeothermy (afternoon minus morning rectal temperature); and heat-tran sfer inefficiency (afternoon rectal minus afternoon surface temperatur es) were monitored. Intake and prolactin concentration were decreased by E+, but sire groups did not differ in responses across phases. Chol esterol levels were lower during E+ phases, and the rate of increase i n cholesterol concentration after withdrawal of E+ differed between gr oups. Alkaline phosphatase concentrations were lower during E+ phases; calves from the Control sire showed greater reduction than calves sir ed by the Missouri sire. Ability to maintain homeothermy was reduced b y E+, but the reduction did not differ between sire groups. Heat-trans fer inefficiencies were not indicative of fescue toxicosis in this stu dy. Evidence was not compelling to support the existence of difference s between the sire groups for susceptibility to toxicosis. However, pr ogeny of the Missouri bull had higher feed intake and lower rectal and body surface temperatures through nearly all phases of the trial. Thi s may account for the reputation of the Missouri bull to sire calves w ith relatively high resistance to fescue toxicosis.