Does the amount of progesterone in intravaginal implants used to synchronise oestrus affect the reproductive performance of brahman heifers artificially inseminated at a fixed time

Abstract

The study tested the hypothesis that reduced intravaginal implant progesterone (P4) concentration to synchronise oestrus would increase pregnancy rates to fixed-time artificial insemination (FTAI) in Bos indicus heifers. Brahman heifers (n = 294; 2 year) were body condition scored (BCS), weighed and scanned for presence of a corpus luteum (CL). Only cyclic heifers were selected and allocated randomly within BCS and 25 kg bodyweight category to one of three P4 treatment groups. On day 10, heifers received a P4 implant (CueMate-1-pod, 0.78g P4; CueMate-2-pod, 1.56g P4; or CIDR-B, 1.9g P4), 2 mg oestradiol benzoate (ODB) intramuscularly (IM) and 250 ug cloprostenol IM. At day 2, the implant was removed, 250 ug cloprostenol was injected IM and tail paint applied. The heifers received 1 mg ODB 24 h later and were FTAI 48–54 h after implant removal (day 0). Ten randomly selected heifers per group were blood sampled and scanned at days 10, 2, 0 and 6 to define the P4 profiles pre- and post-FTAI. Heifers were heat-detected 18–20 days post-FTAI and oestrous heifers AI’d by the AM/PM rule. Bulls joined the heifers on day 27 post-FTAI. Transrectal ultrasonography estimated conception date on day 72. Statistical analysis examined the effects of treatment, technician, semen, ovarian status, BCS and liveweight, on pregnancy rate (PR) to FTAI. There was no significant difference (p = 0.362) in PR between treatment groups (CueMate 1-pod, 36.4%; CueMate 2-pod, 39.6%: CIDR-B, 28.3%), but PR was higher in those heifers with increased BCS between FTAI and pregnancy diagnosis (p = 0.005). Thirty-three per cent of monitor heifers had plasma P4 concentrations of <1 ng/ml on day 6 after FTAI; only 20% of these conceived vs 60% of heifers with P4 ≥ 1 ng/ml. In summary, no significant difference in PR was identified between treatments but good BCS and a rising plane of nutrition were critical to PR of these pure grade Brahman heifers in northern Australia.