Estradiol regulation of luteinizing hormone secretion in heifers of two breed types that reach puberty at different ages

The working hypothesis was that 17beta-estradiol (E-2) negative feedback on the hypothalamic-pituitary axis in regulation of LH secretion decreases during peripuberty in heifers of 2 different genotypes. We investigated whether Bos indicus heifers had a period postpuberty, as compared with prepuberty, of greater E-2 inhibition of LH secretion at a time when heifers of this genotype have been reported to have a period of anestrus. Prepubertal heifers 9 mo of age of 2 genotypes (B. indicus and B. taurus) were assigned to 3 groups (6 animals/group) to either remain intact (control), be ovariectomized, or be ovariectomized and implanted with E-2. Variables evaluated from 10 to 28 mo of age were circulating concentrations of progesterone (P-4), presence of corpora lutea, and pulsatile pattern of LH release. Results confirmed that B. taurus heifers attained puberty at younger ages (P<0.001) and at lower live weights (P=0.015) than did B. indicus heifers (507&PLUSMN;37 days of age vs. 678&PLUSMN;7 days of age; 259&PLUSMN;14 kg vs. 312&PLUSMN;11 kg; respectively). There was cessation of E-2 inhibition of LH pulses coincident with the onset of puberty in heifers of both breed types but at a much younger age in B. taurus heifers. There was no evidence of enhanced negative feedback of E-2 on LH secretion subsequent to puberty in B. indicus heifers nor was there cessation of estrous cycles in control heifers of either breed type after puberty.