Influence of infusion of prostaglandin F2 alpha (PGF 2 alpha) and weaning on surface and histologic populations of ovarian follicles in early postpartum beef cows.

Twenty-four beef cows were infused continuously for 11 d (d 2 to 13 after parturition) into the descending aorta with either prostaglandin F2 alpha-Tham salt (PGF2 alpha; 33.5 mg/d; n = 12) or with .9% NaCl (saline; n = 12) vehicle. Cows from each infusion treatment then were assigned to three slaughter groups (G). Cows in G1 and G2 were suckled until slaughter on d 15 and 35, respectively, whereas calves in G3 were weaned on d 31 and cows were slaughtered on d 35 after parturition. Nonatretic and atretic (greater than 4 pycnosis) antral follicles (greater than .15 mm) on ovaries ipsilateral to the previously gravid uterine horn were evaluated by histological techniques. Compared with saline infusion, PGF2 alpha increased mean diameters of the largest (F1) follicles on the ipsilateral (d 15 and 35; P less than .07) and contralateral (d 15; P less than .07) ovaries (surface evaluation) as well as mean diameters of the largest (F1, d 35; P less than .002), second- and third-largest (F2, F3 d 15 and 35; P less than .01) nonatretic follicles (histologic evaluation) in suckled cows. Compared with suckled cows slaughtered on d 35, weaning on d 31 increased the number of medium follicles (3 to 5 mm), diameters of the three largest nonatretic follicles, percentage of large nonatretic follicles (greater than 3.67 mm) and mitotic index of class 3 (.68 to 1.57 mm) follicles in saline-infused cows. In contrast, in PGF2 alpha-infused cows, weaning decreased (P less than .05 to P less than .001) all of these responses. Surface and histologic evaluations of follicular activity indicated that postpartum infusion of PGF2 alpha stimulated development of large follicles. In response to weaning, PGF2 alpha infusion favored development of those large follicles toward ovulation or atresia on nonovulated follicles.