Effects of treatment with human chorionic gonadotrophin or intravaginal progesterone-releasing device after AI on circulating progesterone concentrations in lactating dairy cows.

Adequate circulating progesterone (P4) is important for pregnancy. Lactating dairy cattle have lower circulating P4, particularly when smaller follicles are ovulated during timed AI protocols. The aim of the present study was to determine the supplementation strategy that resulted in P4 concentrations in lactating dairy cattle similar to those in heifers. Lactating Holstein cows (n=61) were synchronised using the Double-Ovsynch method and, on Day 5, were randomly assigned to receive no treatment (control), controlled internal drug release (CIDR), human chorionic gonadotrophin (hCG; 3300 IU) or CIDR+hCG. Heifers after normal oestrus were followed as controls (n=10). Profiles of circulating P4 concentrations were compared using repeated-measures ANOVA. Heifers had greater P4 concentrations than control cows at all times after Day 5 (P<0.0001). Cows receiving CIDR had lower P4 concentrations than heifers (P=0.0037) on Days 8-16. Treatment with hCG generally caused ovulation and resulted in circulating P4 concentrations greater than those in control lactating cows by 3 days after treatment (Day 8 after AI), but the treatment×time interaction (P=0.01) showed that cows treated with hCG generally had lower P4 concentrations than heifers. Supplementation with CIDR+hCG resulted in P4 concentration profiles similar to those in heifers. Thus, the use of CIDR and the production of an accessory corpus luteum with hCG elevates P4 concentrations in lactating cows to those seen in heifers. This information may be useful for designing future trials into P4 supplementation and fertility.