Periparturient and Postpartum Endocrine Changes of Conceptus and Maternal Units in Jersey Cows Bred for Milk Yield

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Abstract
Control cows, sired and bred by bulls of zero estimated breeding value, and selected cows, sired by and bred to bulls of high predicted differences for milk yield, were used to evaluate maternal endocrine changes from 14 days prepartum to 28 days postpartum. Examined were concentrations in plasma of 13, 14 dihydro-15 keto-prostaglandin F2.alpha., progesterone estrone sulfate, estrone, luteinizing hormone and prolactin. Ability of cows to release prolactin and luteinizing hormone on day 10 postpartum was evaluated after a simultaneous injection of thyrotropin releasing hormone (100 .mu.g) and gonadotropin releasing hormone (100 .mu.g). Changes in progesterone and estrogens prepartum lead to peak concentrations of prolactin and prostaglandin at parturition and 3 days postpartum, respectively. Higher basal concentrations of prolactin for control cows prepartum were associated with a higher prolactin release by thyrotropin releasing hormone at 10 days postpartum. Although release of luteinizing hormone in response to gonadotropin releasing hormone did not differ between groups on day 10 postpartum, a subsequent increase in progesterone to above 1 ng/ml was earlier and more precisely synchronized among control cows (16 .+-. 0.43 vs. 23 .+-. 2.33 days). Within cow concentrations of F2.alpha.13,14 dihydro-15-keto-prostaglandin F2.alpha. were correlated with size of previous gravid uterine horn (0.67) and milk yield (-0.39). Selection for milk yield influenced postpartum endocrine function.