EFFECT OF GROWTH HORMONE-RELEASING FACTOR AND(OR) THYROTROPIN-RELEASING FACTOR ON HORMONE CONCENTRATIONS AND MILK PRODUCTION IN DAIRY COWS

Abstract

In a 2 × 2 factorial design, 32 cows (n = 8 per treatment) averaging 196 d of lactation received, for 10 consecutive days, daily subcutaneous injections of saline (group 1), human growth hormone-releasing factor (1-29)NH 2 (GRF, 10 μg kg −1 BW; group 2), thyrotropin-releasing factor (TRF, 1 μg kg −1 BW; group 3), or GRF (10 μg kg −1 BW) plus TRF (1 μg kg −1 BW; group 4). Blood was collected from 24 cows (n = 6 per treatment) on the first and last day of injection. The synergistic action of the two hypothalamic factors on growth hormone (GH) release was maintained throughout the experiment (P < 0.05). Prolactin (Prl) concentrations increased (P < 0.01) after TRF administration on both sampling days. There was no interaction between the two peptides on thyrotropin (TSH) and thyroxine (T 4 ) concentrations. TSH response to TRF (P < 0.01) observed on the first day was not detectable (P > 0.10) on the 10th day of injection and the TRF-induced T 4 response decreased (P < 0.05) from day 1 to day 10. GRF treatment increased (P = 0.06) insulin-like growth factor-I concentrations while TRF had no effect (P > 0.10). Milk production and composition and feed intake were recorded from 10 d before to 10 d after the injection period. Data were averaged for the last 5 d of each 10-d period. Milk production during the injection period averaged 18.8, 20.6, 20.7, and 22.3 kg d −1 for groups 1–4, respectively. Effects of GRF and TRF were additive. Treatment with TRF did not change milk composition but GRF treatment decreased milk protein content. Feed intake did not differ (P > 0.10) between treatments. In summary, the synergy between GRF and TRF on GH concentration was sustained through a 10-d treatment period. Despite this synergy, GRF and TRF had only an additive effect on milk production. Further investigation is needed to confirm whether this successful combination of these two hypothalamic factors can be used on a Song-term basis. Key words: GRF, TRF, milk production, hormone, insulin-like growth factor-1, cows (dairy)