Effect of human chorionic gonadotropin administered at specific times following breeding on milk progesterone and pregnancy in cows.

Two trials were conducted to investigate the efficiency of human chorionic gonadotropin (hCG) following breeding to increase progesterone (P(4)) secretion and pregnancy rates in cows. In Trial 1, 79 lactating Holstein cows were randomly allocated to four groups to receive hCG either at breeding (Day 0, n=20), Day 7 (n=20) or Day 14 (n=20), or to receive no hCG treatment (control n=19). Whole milk samples were collected every other day from breeding until Day 21 and, thereafter, at weekly intervals until Day 42 or until the return to estrus for determination of P(4) concentrations. Similar treatments were employed in Trial 2, and 121 lactating Holstein cows were randomly assigned to treatment at Day 0 (n=29), Day 7 (n=32), or Day 14 (n=29), or to receive no treatment and serve as a control group (n=31). Milk samples were obtained at weekly intervals from breeding until Day 42, or the return to estrus for determination of P(4) concentrations. Pregnancy diagnosis was made by palpation per rectum at approximately 60 days post breeding. Significant increases in P(4) concentrations were observed in Day-7 and Day-14 treated cows from Days 18 to 42 after breeding compared with the Day 0 or the control cows. A slight decrease in P(4) concentration throughout the sampling period was observed in the Day-0 treated cows. Significant increases in pregnancy rates were observed in hCG-treated cows compared with that of the controls, with the highest rate observed in the Day-7 treated group. The overall pregnancy rates were 47, 62, 55 and 40% for the Day 0, 7 and 14 groups and for the control groups, respectively. In nonpregnant cows the mean (+/- SEM) numbers of days to basal P(4) concentrations were 21.6 +/- 1.3, 24.1 +/- 1.6, 24.6 +/- 1.3 and 23.2 +/- 1.3 for cows treated on Days 0, 7 and 14 and for the control group, respectively. It is concluded that the administration of hCG at Day 7 or Day 14 after insemination could be used as a management tool to improve pregnancy rates in postpartum cows.