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: #x2014;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#x2013;90 mg as a single implantation in sheep of 120#x2013;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 slight increase in barrenness and decrease in twinning, both statistically non-significant.vi. No adverse effects other than those mentioned in (iii) and (v) have been observed.vii. The practical implications of the use of thyroxine to stimulate wool production are briefly discussed.
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