Investigation into the variation in follicular and endocrine responses of prepubertal gilts treated with exogenous gonadotropins

The gonadotropin compound, PG600, is used to induce estrus in prepubertal gilts, but responses can be variable. This study was conducted to evaluate PG600 effects on follicles, estrus, ovulation and estrogen production. Prepubertal gilts (n = 50) were treated with PG600. Gilts were evaluated for estrus while daily boar exposure was occurring. A sub-population of gilts (n = 12) were slaughtered on Day 3 to assess cytochrome P450 aromatase (CYP19) immunohistochemical staining in ovarian antral follicles. Ovaries of the remaining gilts (n = 38) were evaluated on Day 3 using ultrasonography and blood samples were collected for quantifying estradiol-17β. On Day 3 following administration of PG600, 94.0 % of gilts had large follicles, but only 76.3 % had expressed behavioral estrus by Day 6. Furthermore, 92.1 % of gilts had ovulations, with 16.6 corpora lutea/gilt. There was no association of number of large follicles on Day 0 or 3 with occurrence of estrus or ovulation (P > 0.05). Estradiol-17β concentrations on Day 3 did not differ (P > 0.05) in anestrus compared to estrual gilts and varied in gilts with large antral follicles. Immuno-detection of CYP19 on Day 3 was greater (P < 0.01) in large and medium compared to small follicles, (64.3 %, 34.2 % and 14.7 %, respectively). Results validate there is a dissociation of large follicle development with estrogen production on Day 3 in gonadotropin-treated gilts. These results indicate failure to express estrus may be due to follicle variation in estrogen production or response to estrogen feedback at the hypothalamus.