The effect of thyroxine on the mean and variance of wool production of Merino sheep on natural pasture

Abstract

Administration of sodium-1-thyroxine to 14-15 months old medium Peppin Merino wethers resulted in a significant increase in both greasy and clean wool production (P<0.05 in each case) when the wethers were grazing on natural pasture in south-western Queensland. This was demonstrated in an analysis of wool production during a total period of 24 months. Within this period, the wethers treated with thyroxine produced seven per cent more greasy wool per head than did the untreated control sheep (P<0.01) and seven per cent more clean wool (P<0.05) during the 15 weeks following implantation. During the subsequent 17 weeks the treated wethers produced three per cent more greasy wool than the controls (P<0.05), but there was no significant difference in clean wool production. There were no significant differences in either greasy or clean wool production in either of the last two periods of the 24-month experiment. There were no significant differences in wool production among the three groups of wethers whose dose rates at a single implantation were 30 mg, 60 mg, and 90 mg respectively. Administration of thyroxine resulted in a significant increase in the variance of greasy wool production during the 15 weeks following implantation (P<0.05) hut had no effect on the variance of clean wool production. The rate of gain in body weight displayed by all groups that received thyroxine was retarded, but groups had returned to their pre-treatment body weight ranking five months after implantation. Mortality was one per cent per annum among implanted animals over the two years following implantation. It is suggested that the response to thyroxine administered in mid-summer may have resulted partly from making good a seasonal deficiency in thyroid secretion. The tactical use of thyroxine to increase wool production per head in a region of considerable within-year or between-year variation of rainfall characterized by unpredictable flushes of pasture growth is discussed. The limited influence of thyroxine on the variance of wool production does not encourage its use as an aid to selection for clean wool production per head.