Effect of a pharmacological dose of testosterone on growth rate, feed intake and energy metabolism of steers gaining or losing weight on roughage diets

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Abstract
Summary: The effect of a pharmacological dose of testosterone propionate, administered by intramuscular injection, on feed intake, live-weight change and fasting metabolism of steers was determined in two experiments. The first consisted of three growth phases: normal weight-gain on a long-chopped lucerne (Medicago sativa) hay dietad libitum(8 weeks), live-weight loss on a low-quality pasture (Dichanthium aristatum) hay dietad libitum(12 weeks) and recovery gain on a lucerne hay dietad libitum(8 weeks). During normal weight-gain, the testosterone-treated steers had significantly (P< 0·01) higher weight gains than untreated steers (1·48v. 0·95 kg/day, respectively). Feed intakes and fasting metabolic rates of treated and control steers were not significantly different, being 28·2 and 29·1 g dry matter/kg live weight and 85·6 and 91·0 KJ/kg per day, respectively. The efficiency of feed conversion was significantly (P< 0·01) better in the treated steers (6·7v. 9·6 feed/kg weight-gain). There was no significant effect of treatment on any of these characters during live-weight loss or recovery gain. The weight changes during these periods wereca.–0·7 kg/day and 1·5 kg/day, respectively. After 12 weeks of severe weight loss, metabolic rate per unit live weight had decreased by 25% compared with that at the start of the weight-loss period.The second experiment examined the effect of the same dose of intramuscular testosterone propionate on another group of steers fed the low-quality roughage diet during a period of weight loss. Again there was no significant effect of treatment on weight loss, feed intake or fasting metabolism.It was concluded that the growth response to pharmacological doses of testosterone in well nourished steers was greater than that usually observed in comparisons between bulls and steers, and that testosterone was ineffective when administered to steers losing weight or in recovery gain after severe weight loss.