Differences between progeny of beef sires in susceptibility to fescue toxicosis.

Fifteen progeny of two Polled Hereford sires were fed endophyte-infected tall fescue seed (E+) to investigate variability in susceptibility to fescue toxicosis. One sire, bred in Missouri, was reputed to produce calves that were resistant to fescue toxicosis. The Control sire, from Virginia, had unknown merit for susceptibility. There were two phases in which E+ was included in the diet (2 and 4) and three in which it was not (1, 3, and 5). Voluntary feed intake, serum prolactin, cholesterol, and alkaline phosphatase concentrations; ability to maintain homeothermy (afternoon minus morning rectal temperature); and heat-transfer inefficiency (afternoon rectal minus afternoon surface temperatures) were monitored. Intake and prolactin concentration were decreased by E+, but sire groups did not differ in responses across phases. Cholesterol levels were lower during E+ phases, and the rate of increase in cholesterol concentration after withdrawal of E+ differed between groups. Alkaline phosphatase concentrations were lower during E+ phases; calves from the Control sire showed greater reduction than calves sired by the Missouri sire. Ability to maintain homeothermy was reduced by E+, but the reduction did not differ between sire groups. Heat-transfer inefficiences were not indicative of fescue toxicosis in this study. Evidence was not compelling to support the existence of differences between the sire groups for susceptibility to toxicosis. However, progeny of the Missouri bull had higher feed intake and lower rectal and body surface temperatures through nearly all phases of the trial. This may account for the reputation of the Missouri bull to sire calves with relatively high resistance to fescue toxicosis.