Effects of annual thyroxine implantations on Romney ewes

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Abstract
The effects on reproduction, wool production, and liveweight have been studied in Romney ewes implanted with either 60 or 90 mg of l-thyroxine in March of 3 successive years. In the first year, when the ewes were 2-tooths, implantation was followed by a loss of liveweight, a delay in the rate of conception, fewer multiple births, and an increase in barrenness. In the second year these effects were less pronounced, and in the third year absent. No effects were found on the viability or the growth rate of the lambs. Greasy wool production was increased 3.6 per cent in 1956, while there was no appreciable effect on fleece weight in 1957 or 1958. In 1956 there was an increase in staple length, number of crimps per inch, and staple strength, and a slight decrease in fibre diameter. Both count and style grading of the fleeces were unaffected by the treatment. The lack of response in the second and third years was possibly due to a low rate of uptake of thyroxine from the pellets used in these years.

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