Effect of feed restriction on plasma concentration of hormones and metabolites in steers fed grass silage

Abstract

Plasma concentrations of hormones and metabolites were measured in growing steers to establish their relation with degree of feed restriction. Sixty medium frame beef steers (259 ± 29 kg) were divided in groups of five and fed during 3 mo either timothy (T) or bromegrass (B) harvested at stem elongation of the first cut or at boot stage of the aftermath. Forages were fed at one of three levels of intake: ad libitum (FF), 80% of FF, or 65% of FF. Plasma concentrations of growth hormone (GH), insulin, glucagon, insulin-like growth factor-1 (IGF-1), glucose and nonesterfied fatty acids (NEFA) were measured on day 1 and day 78 before the morning meal. The effect of a meal on GH, insulin, glucagon, IGF-1, glucose and NEFA concentrations was also determined in 24 steers sampled from 3 h before to 3 h after a meal. Severity of feed restriction resulted in a linear decrease (P < 0.05) of plasma glucagon (0.139, 0.130, 0.121 ± 0.005 ng mL–1) and IGF-1 (109, 98, 84 ± 7 ng mL–1). Insulin concentration tended to be affected quadratically (0.37, 0.35, 0.27 ± 0.04ng mL–1 for FF, 80% of FF and 65% of FF, respectively, P < 0.10). Ingestion of a meal resulted in a decrease (P < 0.05) of GH, NEFA and glucose, and an increase of insulin concentrations (P < 0.05). There was a positive correlation (r = 0.53, P < 0.05) between IGF-1 and average daily gain, and lipid percentage of carcasses suggesting that IGF-1 is related to growth rate and fat accumulation in growing steers. Key words: Feed restriction, growth hormone, insulin, IGF-I, metabolites, grass silage