Growth Rate and Secretion of Pituitary Hormones in Relation to Age and Chronic Treatment with Thyrotropin-Releasing Hormone in Prepubertal Dairy Heifers
Holstein heifers were treated with synthetic thyrotropin-releasing hormone (TRH) or saline twice daily from 1 wk through 6 mo. of age. Plasma concentrations of prolactin (PRL) and thyrotropin (TSH) were elevated (P < .01) within 30 min after the 1st TRH injection (1 wk of age). At 1 and 3 mo. of treatment, PRL and TSH increased in response to TRH, although the TSH response was reduced (P < .01) as compared to the 1st day of treatment. Although plasma growth hormone (GH) appeared to be elevated following the 1st TRH injection, this effect was not statistically significant (P > .05), nor was it significantly influenced by treatment following subsequent TRH injections. None of the 3 hormones, PRL, TSH or GH, was elevated following the final TRH injection at 6 mo. of age. Plasma concentrations of PRL and TSH were increased in a control heifer injected with TRH at 6 mo. Hormonal responsiveness to TRH stimulation apparently decreases with continued twice daily treatment at doses of TRH used in the present studies. Examination of weight gains indicated that chronic treatment with TRH was associated with increased growth rate through 6 mo. of age (10.6% increased average daily gains (P < .10), which was exhibited in a steeper slope (P < .05) of the growth curve in the TRH group. Feed intake was slightly greater in TRH heifers, although feed efficiency (kg feed/kg gain) was not different between the 2 groups. Plasma concentrations of PRL increased (P < .01) with age (r = +0.938) in control heifers, while plasma TSH and GH were not significantly related to age. This observation establishes a positive correlative relationship between PRL secretion and the approach of puberty in the dairy heifer. Elevation of PRL secretion by TRH treatment was associated with significant advancement of age at 1st observed estrus (9.4 vs. 10.5 mo.) suggesting that a functional relationship between PRL secretion and puberty may exist in dairy heifers.