Periparturient endocrine changes of conceptus and maternal units in Jersey cows bred for milk yield.

Control cows, sired by and bred to bulls with assumed zero estimated breeding values, and selected cows, sired by and bred to bulls of high predicted difference for milk yield, were used. Blood samples were collected via jugular venipuncture on alternate days from 35 to 14 days prepartum, from 14 to 28 days postpartum, and daily from 14 days prepartum to 14 days postpartum. We examined blood hematocrit, and concentrations in plasma of protein, estrone, estradiol, estrone sulfate, progestins, glucocorticoids, luteinizing hormone, and prolactin. Total plasma volume was measured on days -21, -7, and +11. Weekly body weights, prepartum and postpartum, were recorded. All data were analyzed by least squares analysis of variance with day and hematocrit as continuous independent variables. Hematocrit was higher for selected cows throughout the sampling period. Prepartum concentrations of progesterone were higher in selected cows, but concentrations of estrone, luteinizing hormone, and prolactin were lower than in control cows. Hormonal and physiological responses indicated that selection for milk yield influenced both the conceptus and maternal units as measured by prepartum endocrine function.