Effects of low versus physiologic plasma progesterone concentrations on ovarian follicular development and fertility in beef cattle

Abstract 

The objective of this study was to determine the effects of low versus physiologic plasma progesterone concentrations during the ovulatory wave on fertility in cattle. Suckled beef cows (Bos taurus; n=129) and pubertal heifers (Bos taurus; n=150) at random stages of the estrous cycle were given a luteolytic dose of prostaglandin F_2α (500μg cloprostenol; PGF) twice, 11 d apart. Ten days after the second PGF treatment, cattle were given estradiol benzoate im (1.5 and 1.0mg for cows and heifers, respectively) and a progesterone-releasing intravaginal device (Cue-Mate) with a single pod containing 0.78g progesterone (Day 0). Cattle in the low-progesterone group (n = 148) received a luteolytic dose of PGF on Day 0, whereas those in the high-progesterone (i.e., physiologic plasma concentrations) group (n=131) were allowed to retain their corpora lutea. On Day 8, the Cue-Mate was removed, and PGF was given to both groups. Fifty-four hours to 56h later, cattle received 12.5mg of porcine LH (pLH) im and were concurrently artificially inseminated. The dominant follicle in the low-progesterone group was larger (P<0.001) than that in the high-progesterone group on the day of insemination (14.9±0.3mm vs. 12.7±0.3mm, mean±SEM). At 7 d after ovulation, the low-progesterone group had a larger corpus luteum (24.5±0.54mm vs. 21.9±0.64mm, P<0.01) and higher plasma progesterone concentration (4.0±0.3 vs. 3.1±0.2, P<0.01) than that of the high-progesterone group. However, pregnancy rates did not differ (79 of 148, 53.4%, and 70 of 131, 53.4%) for low- and high-progesterone groups, respectively). In summary, low circulating progesterone concentrations during the growing phase of the ovulatory follicle resulted in a larger dominant follicle and a larger CL that produced more progesterone, with no significant effect on pregnancy rate.

Keywords: Cattle, Corpus luteum, Ovarian follicular development, Progesterone, Synchronization of ovulation