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Estradiol Regulation of Luteinizing Hormone Secretion in Heifers of Two Breed Types That Reach Puberty at Different Ages¹

^a Australian Institute of Tropical Veterinary and Animal Science ^b Department of Physiology and Pharmacology, James Cook University, Townsville 4811, Australia ^c Department of Animal Science, University of Nebraska, Lincoln, Nebraska 68583

The working hypothesis was that 17ß-estradiol (E₂) 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 E₂ 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 E₂. Variables evaluated from 10 to 28 mo of age were circulating concentrations of progesterone (P₄), 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 ± 37 days of age vs. 678 ± 7 days of age; 259 ± 14 kg vs. 312 ± 11 kg; respectively). There was cessation of E₂ 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 E₂ 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.

First decision: 17 December 1998.

¹ This work was supported by the Australian Meat Research Corporation.

² Correspondence. FAX: 61 7 47791526; lee.fitzpatrick{at}jcu.edu.au

³ Current address: Department of Animal Science, Ohio State University, Columbus, OH 43210

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