Seasonal variation in plasma metabolite and hormone concentrations, amino acid utilization and responses to Thyrotropin-releasing hormone in fleeceweight-selected and control Romney rams

Abstract

Two studies were conducted to examine the effects of season on differences between the Massey University fleeceweight-selected and control lines of Romney sheep in hormone and metabolite concentrations, phenylalanine irreversible loss rate, urea and creatinine clearance and space, and response to an intravenous thyrotropin releasing hormone (TRH) challenge. Rams from the two lines were studied in autumn, winter and spring (5, 9 and 13 months of age) in Experiment 1 or winter and spring (9 and 14 months of age) in Experiment 2. Basal concentrations of urea and creatinine were lower in fleeceweight-selected (FW) than in control (C) rams in both experiments. A significant linexseason interaction, reflecting a greater difference between the lines in winter than in autumn or spring, was observed only in Experiment 1. This interaction could not be accounted for by a corresponding effect on distribution space or clearance of these metabolites. Phenylalanine irreversible loss rate (per unit metabolic body weight) was significantly lower in FW than in C rams, but the magnitude of this difference was not influenced by season (winter v. spring). No differences were observed between the lines in basal concentrations of thyroxine or triiodothyronine, or in the responses of these hormones to TRH. Prolactin response to TRH was lower in FW than in C rams during spring, but this reflected the higher pre-challenge concentrations of this hormone in FW rams. It is concluded that, compared with C rams, FW rams have lower basal concentrations of urea and lower irreversible loss rates (per unit metabolic body weight) of phenylalanine. However, these observations do not explain the less seasonal patterns of wool production in fleeceweight-selected compared with control sheep.