No abstract

Three experiments investigated ovarian follicular development in sows whose litters were weaned at 28 to 31 d of lactation. Unilateral ovariectomy near the time of weaning was used to assess early follicular characteristics and to identify those sows that would not return to estrus within 10 d after weaning. This allowed segregation of and exclusion from the study those sows that had a prolonged interval from weaning to first estrus. In Exp. 1, 82 and 72% of the large follicles that were marked at 48 or 72 h after weaning (10 sows per time point) were subsequently identified as corpora lutea. In Exp. 2, sows (seven to nine per time point) were unilaterally ovariectomized at 0, 6, 12, 18, 24, or 48 h after weaning, and follicular fluid was evaluated for changes in steroid concentrations. Progesterone concentrations in fluid from medium-sized (4 to 6 mm) follicles increased by 6 h after weaning and then declined through 24 h concomitant with increases in testosterone and estradiol. For Exp. 3, follicular fluid and granulosa cells from individual follicles were obtained from sows (seven to nine per time point) at 0, 6, and 24 h after weaning. In follicular fluid, insulin-like growth factor I (IGF-I) concentrations were not correlated (P greater than .05) with concentrations of progesterone, testosterone, or estradiol, or with granulosa cell production of estradiol during culture in androstenedione-supplemented medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Three studies were conducted to examine the role of progesterone in stimulating sexual receptivity in estrogen-treated, ovariectomized gilts. Progesterone was administered either before, simultaneously with, or 48 h after estrogen. In each study, gilts were treated with either a suboptimal or an optimal dosage of estradiol benzoate (EB). Progesterone treatment (600 micrograms/kg BW-1 X injection-1) on alternate days for a total of four injections produced serum concentrations of progesterone that were maximal at 9.4 ng/ml and remained greater than 1 ng/ml for 15 d. Estradiol benzoate was administered 22 d after the first of these progesterone injections. When progesterone was administered concurrently with or 48 h after EB, the dosage was 100 micrograms/kg BW and produced a maximal serum progesterone concentration of 1.8 ng/ml 4 h after treatment. Gilts were placed in an evaluation pen with a boar for 5 min on d 3 and 4 after EB treatment. Traits of interest were total number of mounts by the boar, mounts before the gilt showed the immobilization response, proportion of gilts that showed the immobilization response, and latency from entry of the gilt into the evaluation pen until the immobilization response. In none of the three studies did progesterone improve any of the traits of interest. In each study the immobilization response was observed in a higher proportion of gilts treated with the optimal than in those treated with the suboptimal dosage of EB. Latency from entry of gilts into the evaluation pen until the immobilization response was less on d 4 than on d 3 after EB in all studies.(ABSTRACT TRUNCATED AT 250 WORDS)