Increasing conception rate by addition of a second prostaglandin injection in an Ovsynch and progesterone treatment programme for dairy cows not detected in oestrus before the start of the seasonal mating period.

To assess the effect on conception rate to first service, 3- and 6-week in-calf rates, final pregnancy rate and the hazard of pregnancy, of addition of a second prostaglandin (PG) injection 24 hours after the first, in a synchrony programme combining Ovsynch and progesterone in pasture-based dairy cows that had not been detected in oestrus before the start of the seasonal mating period. This prospective, negative-controlled study was conducted using anoestrous cows (n = 1,411) from nine spring-calving dairy herds in the Waikato region of New Zealand. Ovaries of cows not detected in oestrus were examined by transrectal ultrasonography and the presence of a corpus luteum determined. All cows were then treated with an intravaginal progesterone-releasing device and injected I/M with gonadorelin (Day -9). Cows were injected I/M with cloprostenol at device removal (Day -2) and on Day -1 were injected with either water (single PG; n = 720), or cloprostenol (double PG; n = 692). On Day 0 cows were injected again with gonadorelin, unless previously detected in oestrus, and were inseminated on Day 1. Based on dated pregnancy diagnosis by transrectal ultrasonography, cows were categorised as conceiving to first insemination, becoming pregnant in the first 3 or 6 weeks, or becoming pregnant by the end of the mating period. Cows in the double PG group had increased conception rate to first service (45.3 (95% CI = 45.1-45.4)% Addition of a second PG injection 24 hours after the first, increased conception rate to first service and the proportion of cows pregnant by 3 and 6 weeks after commencement of the mating period. Improved outcomes can be achieved when treating those cows not detected in oestrus by the start of the seasonal mating period by addition of a second PG injection to the currently recommended treatment protocol for these animals.