Randomised controlled trial of the effect of concentration of progesterone before artificial insemination on fertility in ovulatory and anovulatory Bos indicus cattle.

OBJECTIVE: To investigate the effect of concentration of progesterone (P4) before artificial insemination (AI) on fertility in ovulatory or anovulatory Bos indicus cattle.

DESIGN: Randomised control study METHODS: The study included 162 heifers and 96 lactating cows. On days -10 to -12, animals were examined using transrectal ultrasound, administered PG and examined for a corpus luteum (CL). Those with a CL were allocated to Experiment 1. On day 0 they were administered an intravaginal progesterone-releasing device (IVD) containing progesterone (P4) (0.78 g), oestrodiol benzoate (ODB) and either saline or PG to induce high and low circulating P4 concentrations, respectively. Those without a CL were re-examined on day 0 and those without a CL at both examinations were allocated to Experiment 2. Cows and heifers were treated with an IVD containing P4 at 0.78 g or 1.56 g to induce low or high P4 concentrations, respectively. IVDs were removed on day 7 and PG and equine chorionic gonadotrophin (eCG) were administered. Females in oestrus on day 9 were inseminated; others were administered ODB and inseminated 22-26 h later.

RESULTS: Greater concentrations of circulating P4 increased the odds of pregnancy to AI in anovulatory females (P = 0.008), but decreased the odds of pregnancy in one year but not another in ovulatory animals (P × year, P = 0.019).

CONCLUSION: Manipulating P4 concentrations before AI has the potential to improve pregnancy outcomes to AI in B. indicus females, but treatment may need to vary between animals classified as anovulatory or ovulatory.