Variation among Angus cows in response to endophyte-infected fescue seed in the diet, as related to their past calf production.

Production of 18, 12-yr-old Angus cows was summarized as the averaged weaning weight deviations of each cow's calves from their like-aged, like-sexed and similarly managed contemporaries. These cows had spent a large part of their productive lives on pastures dominated by endophyte-infected tall fescue, so differences among them in calf production might have been induced partly by differences in susceptibility to fescue toxicosis. Cows were divided randomly into two groups for a 31-d summer feeding trial. In a crossover design, cows were fed 0 or .9 kg per cow per day of endophyte-infected fescue seed. Various traits were monitored to quantify differences among cows in response to the endophyte-infected fescue seed. Baseline serum prolactin concentration was depressed by fescue seed feeding, but differences among cows in the amount of depression were not related to past calf production. Prolactin release in response to thyrotropin-releasing hormone tended to be depressed by fescue, but the response also was sensitive to factors such as ambient temperature. Serum cholesterol and body weight change did not respond significantly to the fescue challenge. The experiment was not successful in unambiguously differentiating among cows in susceptibility to fescue toxicosis or in relating differential susceptibility to past calf production.