Follicular status, estrus behavior, and recombinant bovine somatotropin improve the rate of pregnancy per artificial insemination in beef cows subjected to a fixed timed-artificial insemination protocol

To evaluate the effect of rbST on insulin-like growth factor 1 (IGF-1) serum concentration, ovarian dynamics, and the reproductive response of beef cows subjected to an estradiol-based fixed-time artificial insemination (FTAI) protocol, 107 Bos indicus × Bos taurus multiparous, postpartum beef cows on two commercial herds were used. The animals were randomly divided into two experimental groups balanced by body condition score. Females in the control group did not receive any additional treatment, whereas cows in the treatment group received a subcutaneous injection of 500 mg of rbST. The ovarian dynamics and pregnancy were determined by transrectal ultrasonography. Ovarian dynamics were recorded at the beginning of the hormonal treatment (day 0) and when the intravaginal device was removed (day 8). IGF-1 serum concentrations were measured by enzyme-linked immunosorbent assay on days 0 and 8. The diameter of the dominant follicle and the IGF-1 serum concentrations on day 8 were higher (P < 0.05) in rbST-treated cows than in control cows. Logistic regression model indicated that cows with a dominant follicle on days 0 and 8 and in estrus had greater probability of pregnancy (P < 0.05), whereas use of rbST tended (P < 0.10) to increase the rate of pregnancy per artificial insemination (P/AI). The presence of corpus luteum on day 0 positively affected (P < 0.05) the P/AI rate. Our research indicated that beef cows subjected to the estradiol-based FTAI protocol with rbST injected on the first day had improved P/AI rate likely because the increased IGF-1 serum concentrations induced by rbST enlarge the diameter of the preovulatory follicle.