Abstract
Holstein calves were given daily intramuscular injections (5.0 ml) of either: a) .85% NaCl (control); b) 500 µg thyrotropin releasing hormone (TRH); or c) 500 µg 3-methyl-thyrotropin releasing hormone (3-MET) beginning at 45 days of age and continuing for 90 days. At 30 to 45 days after injections commenced, calves receiving TRH began to gain more weight and consume more feed than calves receiving either saline or 3-MET. Weight gain (WG) and feed intake (FI) for the 90-day experimental period was 10% greater in TRH-injected calves than in saline-injected controls, however, feed efficiency was not affected. Injection of 3-MET did not alter weight gain or feed efficiency relative to controls. Serum collected just prior to and after injection on days 1, 15 and 60 was assayed for growth hormone (GH), prolactin (PRL), thyroxine (T4) and insulin (INS). Both TRH and 3-MET, but not saline increased serum GH, PRL and thyroxine concentrations. Magnitude of increase in serum GH and PRL concentrations and persistency of response between days were greater for TRH than for 3-MET. The increase in serum thyroxine after TRH and 3-MET on day 15 and 60 was decreased 63 and 49%, and 51 and 77%, respectively, relative to comparable responses on day 1. Serum INS concentrations averaged for all days and times after injection were 1.5, 1.2 and 1.1 ng/ml (P<.10) for calves given TRH, 3-MET and saline, respectively. Serum INS concentration increased with age (P<.001) and time after feeding (P<.05). Average serum insulin concentration was increased by day 15 of treatment in calves receiving TRH and by day 60 of treatment in calves receiving 3-MET. Copyright © 1977. American Society of Animal Science . Copyright 1977 by American Society of Animal Science.