Progesterone responses to human chorionic gonadotropin in dairy cattle supplemented with beta-carotene

Twenty Holstein cows were assigned to one of four groups according to milk production and beta-carotene supplementation to evaluate effects of supplementation and milk production on reproductive performance and response of corpus luteum to human chorionic gonadotropin indicated by release of progesterone and secretion of luteinizing hormone in blood plasma. Cows were divided into groups of high and low production and divided further into beta-carotene supplemented and unsupplemented groups. Blood samples were taken 10 days postestrus at 10-min intervals from 1 h prior to injection of 5000 IU human chorionic gonadotropin until 5 h postinjection. Mean days to first detected estrus were 33, 37, 34, and 36 in supplemented, unsupplemented, high-production, and low-production cows. Treatment with beta-carotene and milk production did not affect days to first breeding (63, 67, 62, and 67), or services per conception (3.2, 5.1, 4.6, and 3.7). However, days open were reduced by supplementation (116 versus 186), but milk production had no effect (173 versus 129). Regressions of progesterone on sampling time within treatment and within milk production were significant. Regression of luteinizing hormone on time within production group was also significant. Neither beta-carotene nor milk production affected most reproductive measures, but differences were subtle in corpus luteum response to human chorionic gonadotropin, which may alter endocrine function.