Effects of infusion of GnRH pulses and level of body condition on ovarian function in postpartum beef cows

Endocrine and behavioral observations during transition of non-breeding into breeding season in female American bison (Bison bison)

This study provides endocrine data in relation to behavioral events during the transition of the non-breeding into the breeding season in American bison (Bison bison). Fecal progesterone metabolite patterns (20-oxo-P) were obtained in 13 adult female American bison and hormonal data were correlated with behavioral observations; i.e. copulation, male tending, female tail-up behavior and gestation length. Based on fecal progesterone metabolite patterns, the breeding season started between the middle of July and early August. Predictable short cycles reflected the transition from non-breeding to the breeding season; the luteal phase of these cycles was 4.10+/-0.86 days. Copulations and female tail-up behavior were reliably associated with the hormonally detected ovulation. Male tending behavior was more loosely associated with hormonally detected ovulation. The observed hormonal pattern in the study females indicated that 9 of 10 pregnant cows conceived during the second ovulatory period in the breeding season. One other cow conceived during her third ovulatory period, and one cow did not conceive until later in the breeding season by beginning of October. Gestation duration was on average 266.30+/-1.00 days. In summary, this study confirmed that the bison is a seasonally polyestrous species; the transition from the non-breeding into the breeding season was characterized by short cycles with low progesterone metabolite values.

Ovarian follicular wave synchronization and induction of ovulation in postpartum beef cows

An experiment was designed to evaluate a) the effect of a progesterone-estradiol combined treatment on ovarian follicular dynamics in postpartum beef cows, and b) ovulation and the subsequent luteal activity after short-term calf removal and GnRH agonist treatment. Multiparous Angus cows (25 to 40 d after calving) were assigned to the following treatments: untreated (Control, n = 9); short term calf removal (CR, n = 8); progesterone (CIDR, n = 9) and progesterone plus estradiol-17 beta (CIDR + E-17 beta, n = 9). Progesterone treatment (CIDR) lasted 8 d and the day of device insertion was considered as Day 0. Cows in the CIDR + E-17 beta group also received an i.m. injection of 5 mg of E-17 beta on Day 1. On Day 8, calves were removed for 48 h (CR, CIDR and CIDR + E-17 beta groups) and 6 h before the end of calf removal these cows also received an i.m. injection of 8 micrograms of Busereline (GnRH). Anestrus was confirmed in all cows by the absence of luteal tissue and progesterone concentrations below 1 ng ml-1 at the beginning of the experiment. Although mean (+/- SEM) interval from the beginning of the experiment (Day 0) to wave emergence did not differ (P > 0.05) among treatment groups (Control, 1.9 +/- 1.0, range -2 to 7 d; CR, 3.9 +/- 0.7, range 0 to 6 d; CIDR, 2.8 +/- 0.5, range 0 to 4 d and CIDR + E-17 beta, 4.1 +/- 0.2, range 3 to 5), the variability was less (P < 0.05) in the CIDR + E-17 beta group. The proportion of cows ovulating 24 to 48 h after GnRH administration tended (P = 0.08) to be higher in cows from CIDR + E-17 beta group (8/9) than in those of CR (5/8) or CIDR (6/9) groups, respectively and was associated with a higher proportion (P < 0.05) of CIDR + E-17 beta treated cows (9/9) that had a dominant follicle in the growing/early static phase at the time of GnRH treatment compared to the other GnRH treated groups (5/8, and 4/9 for CR and CIDR groups, respectively). Two CR cows ovulated 0-24 h after GnRH and only one Control cow ovulated the day before the time of GnRH administration. Cows pretreated with progesterone had longer (P < 0.05) luteal lifespan (CIDR, 14.5 +/- 0.7, CIDR + E-17 beta, 13.9 +/- 0.6 d) than those not treated with CIDR (Control, 5, CR, 4.0 +/- 0.4). We conclude that progesterone plus estradiol treatment results in tightly synchronized wave emergence and high GnRH-induced ovulation rate with normal luteal activity in postpartum beef cattle.