Effect of Methallibure on Estrous response of Behaviorally Anestrous Gilts Reared in Confinement

One-hundred-five Hampshire, Yorkshire and Duroc gilts reared in confinement were checked daily for estrus from 8 to 10 months of age. At 10 months, 22% of the gilts were classified as behaviorally anestrus on the basis of the absence of estrous behavior but the presence of corpora lutea, preovulatory follicles or corpora albicantia at laparotomy. Twenty-three of these anestrous gilts were randomly assigned to either Group I (11 gilts) or Group II (12 gilts). Fifteen cyclic gilts were assigned to Group III as controls. Gilts in Group I were fed a basal diet containing methallibure (100 mg/day/gilt) for 20 days and ovariectomized 2 days later. Gilts in Groups II and III were fed the basal diet without methallibure and were ovariectomized the same day as gilts in Group I. On Days 2, 21 and 42 after ovariectomy, all gilts were given an intramuscular injection of estradiol benzoate (10 μg/kg) and then were checked twice daily for estrus. The estradiol benzoate-induced estrous behavior response at 2 days after ovariectomy was greater (P<.01) for the anestrous methallibure-treated (Group I;90.9%) and cyclic-control (Group III; 93.3%) gilts than for anestrous gilts given no methallibure (Group II; 8.3%). Methallibure effectively reversed the inhibitory factors that prevented anestrous gilts from showing estrus in response to estradiol benzoate.