Effect of the previously gravid uterine horn and postpartum interval on follicular diameter and conception rate in beef cows treated with estradiol benzoate and progesterone

An experiment was performed to evaluate the effect of the side of ovulation with respect to the previously gravid uterine horn on fertility of cows in seminated at one of two periods postpartum All cows were treated with an in travaginal progesterone insert for 7 d and received estradiol benzoate (2 m g, i.m.) at the time of device insertion, prostaglandin F-2 alpha (25 mg, i .m.) at the time of device removal, and estradiol benzoate (1 mg, i.m.) 30 h after device removal. All cows were inseminated 28 to 30 h after the seco nd treatment with estradiol benzoate, regardless of observed estrus. Cows t reated in Period 1 received inserts at 16 to 20 d postpartum and were insem inated at 25 to 29 d postpartum. Cows treated in Period 2 received inserts at 26 to 30 d postpartum and were inseminated at 35 to 39 d postpartum. Dia meter of the largest follicle at insert removal was greater in cows treated in Period 2 (10.1 +/- 0.3; mm +/- SEM) than in cows treated in Period 1 (9 .1 +/- 0.3; P < .05). Diameter did not differ with the side of ovulation in respect to the previously gravid uterine horn. Diameter was greater in cow s 5 to 9 (10.3 +/- 0.3) than in cows 3 to 4 (9.0 +/- 0.3) or 10 to 13 (9.4 +/- 0.6) yr of age (P <.01). The proportion of cows that ovulated from the ovary contralateral to the previously gravid uterine horn was greater (P <. 05) than of those that ovulated from the ipsilateral ovary, and the inciden ce of ovulation was reduced in cows 3 to 4 yr of age (P <.01). Conception r ate tended to be greater for ovulation from the ipsilateral compared with t he contralateral ovary, relative to the previously gravid uterine horn (P < .10) and for ovulation from the right than the left ovary (P <.06). Concept ion rate was less if cows ovulated a follicle that was < 9 mm than a follic le greater than or equal to 9 mm in diameter at insert removal (P <.01) and was greater in cows inseminated in June than in April or May (P <.05). In conclusion, in cows in which estrus was synchronized at 25 to 39 d postpart um, ovulation from either the ovary ipsilateral to the previously gravid ut erine horn, or the right ovary, tended to increase fertility.