The effect of thyroxine on wool growth and lamb production

Abstract

Summary In a series of trials over the period 1954-57 inclusive some 650 sheep of the Corriedale breed have been implanted with 1-thyroxine and the following observations made: — i. The rate of wool growth is stimulated if the implantations are made during the late summer and autumn period (January-April) but implantations made in the winter, spring, and early summer (June-December) are ineffective. ii. The optimum dose level appears to be in the region of 60–90 mg as a single implantation in sheep of 120–130 lb body weight. iii. 90 mg implants cause an 8 lb loss in body weight over the first month, which is slowly but not fully regained in subsequent months. iv. 90 mg thyroxine implanted at the optimum time gives on average a fleece weight increase of 0.6 lb (or 6%) the increase being mainly due to additional staple length. Fleece grade is not significantly improved, but the percentage of cotted fleeces is reduced. v. Late-summer implantations made before mating for maximum wool response have caused a sl...