NITROGEN AND SULFUR METABOLISM AND PLASMA THYROID-HORMONE CONCENTRATIONS IN FLEECEWEIGHT-SELECTED AND CONTROL ROMNEY SHEEP AT 2 AMBIENT-TEMPERATURES

This study wits conducted to determine the effect of ambient temperatu re on nitrogen and sulfur metabolism in 7-month-old rams from the Mass ey University fleeceweight-selected (FW) and control (C) lines of Romn ey sheep. Eight rams from each line were exposed to either 6 degrees C or 25 degrees C ambient temperature under a constant photoperiod. Det ailed 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 degrees C than at 6 degrees C, b ut there was no effect of ambient temperature on digestibility of eith er dry matter or nitrogen. Nitrogen balance was lower at 25 degrees C than at 6 degrees C, but none of the parameters relating to nitrogen b alance was influenced by selection line. There were also no effects of ambient temperature or selection line on clearance rates of urea or c reatinine. Blood samples were collected from the animals at 2h interva ls over a 24h period to determine plasma concentrations of urea, creat inine, thyroxine and triiodothyronine. Although the between-line diffe rences in concentrations of these hormones and metabolites varied thro ughout the 24h period, and with ambient temperature, there were no con sistent interactions between the effects of selection line and ambient temperature. There were no consistent effects of selection line, or i nteractions of selection line with ambient temperature, on sulfur and sulfate balances, concentrations of leucine, cyst(e)ine and glutathion e in whole brood or on irreversible loss rates of leucine and cyst(e)i ne. However, when considered across lines and ambient temperatures, bo th 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 betw een selection line and ambient temperature was observed in the wool su lfur concentration which was lower in the FW rams than in the C rams a t 6 degrees C, but not at 25 degrees C. It is concluded that none of t he metabolic measurements made in this study explains previous inconsi stencies observed in metabolic differences between sheep from lines di vergently selected on the basis of fleeceweight. Wool sulfur content w as again shown to be a potentially useful predictor of genetic merit f or fleeceweight, but no clear metabolic basis for differences between the lines could be demonstrated even with detailed measurements of sul fur metabolism.