Estradiol Treatment and Luteinizing Hormone (LH) Response of Prepubertal Holstein Heifers1

Abstract

Twenty-one prepubertal Holstein heifers 29 ± 1 weeks of age (188 ± 9 kg) were given estradiol (E₂) at various dosages, in different forms and by different routes of administration. Response was measured by changes in blood serum LH. Estrous behavior was not induced in any of the heifers treated with E₂. Priming with progesterone had no effect on the LH response in heifers given 4 mg 17β-estradiol intramuscularly. An LH surge was observed at all dosages of E₂ (62.5-4000 µg) but not in all heifers treated. The failure of heifers to respond was not related to dosage of E₂, to pretreatment serum progesterone levels or to the size or age of the heifers. However, pretreatment serum LH levels tended to be higher (1.7 ± 0.2 vs 0.9 ± 0.1 ng/ml; P=0.07) m heifers which failed to respond. Over all experiments, a slight relationship (b=0.002, P=0.06) was observed between the dosage of E₂ and the quantity of LH released. Peak serum LH, time to peak serum LH and total serum LH released were equally effective in measuring the responsiveness of the heifers to E₂. An LH release was observed in 80% of the heifers at 18 ± 0.8 h following E₂ treatment. Peak serum LH levels averaged 42.5 ± 8.8 ng/ml and were highest following i.m. injection of 17β-estradiol-3-benzoate (74 ± 22 ng/ml). A 24 h i.v. infusion of E₂ (2.6-41.7 µg/h) also induced an LH release. We conclude that the prepubertal heifer, similar to the mature cow, has a functional hypothalamic-pituitary-gonadal feedback system.