Nitrogen and sulfur metabolism and plasma thyroid hormone concentrations in fleeceweight-selected and control Romney sheep at two ambient temperatures

Abstract

This study was conducted to determine the effect of ambient temperature on nitrogen and sulfur metabolism in 7-month-old rams from the Massey University fleeceweight-selected (FW) and control (C) lines of Romney sheep. Eight rams from each line were exposed to either 6�_C or 25�_C ambient temperature under a constant photoperiod. Detailed studies of sulfur metabolism were made on four of the rams from each line at each ambient temperature. Intake and faecal output of dry matter and nitrogen were lower at 25�_C than at 6�_C, but there was no effect of ambient temperature on digestibility of either dry matter or nitrogen. Nitrogen balance was lower at 25�_C than at 6�_C, but none of the parameters relating to nitrogen balance was influenced by selection line. There were also no effects of ambient temperature or selection line on clearance rates of urea or creatinine. Blood samples were collected from the animals at 2 h intervals over a 24 h period to determine plasma concentrations of urea, creatinine, thyroxine and triiodothyronine. Although the between-line differences in concentrations of these hormones and metabolites varied throughout the 24 h period, and with ambient temperature, there were no consistent interactions between the effects of selection line and ambient temperature. There were no consistent effects of selection line, or interactions of selection line with ambient temperature, on sulfur and sulfate balances, concentrations of leucine, cyst(e)ine and glutathione in whole blood or on irreversible loss rates of leucine and cyst(e)ine. However, when considered across lines and ambient temperatures, both the whole blood concentrations (P < 0.10) and the irreversible loss rates (P < 0.05) of leucine and cyst(e)ine were positively related to midside wool growth. The only significant (P < 0.05) interaction between selection line and ambient temperature was observed in the wool sulfur concentration which was lower in the FW rams than in the C rams at 6�_C, but not at 25�_C. It is concluded that none of the metabolic measurements made in this study explains previous inconsistencies observed in metabolic differences between sheep from lines divergently selected on the basis of fleeceweight. Wool sulfur content was again shown to be a potentially useful predictor of genetic merit for fleeceweight, but no clear metabolic basis for differences between the lines could be demonstrated even with detailed measurements of sulfur metabolism.