New Zealand Journal of Agricultural Research abstracts

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Suppression of thyroid gland function and its effects on the breeding season of Coopworth ewes

S. Maurenbrecher
G. K. Barrell*

Animal and Food Sciences Division
P.O. Box 84
Lincoln University
Canterbury, New Zealand

*Author for correspondence.

Abstract  To investigate the effectiveness of different depths of suppression of thyroid gland function on their breeding season, Coopworth ewes (N = 30, n = 6) were either thyroidectomised or were euthyroid and treated daily with 0, 1.25, 2.50, or 5.00 µg/kg of tri-iodothyronine (T₃) for 19 weeks. They were grazed on pasture and the onset of seasonal anoestrus was determined from twice-weekly plasma progesterone measurements. Effectiveness of the treatment with T₃ for suppression of thyroid function was examined by measuring plasma concentrations of total thyroxine (T₄) and total T₃. Euthyroid ewes entered anoestrus in early-to-mid August (mean, 9 August ± 1.6 days) and T₃-treated ewes were not significantly later (means, 11–13 August). In contrast, thyroidectomised ewes had not entered anoestrus at 19 September. Unlike the case with thyroidectomy, the profound suppression of thyroid function achieved by treatment with T₃ was not effective for preventing the onset of anoestrus in these sheep. sheep; breeding season; thyroid hormones; thyroidectomy

Keywords  sheep; breeding season; thyroid hormones; thyroidectomy

A02041; Received 4 July 2002; accepted 26 November 2002; published 26 March 2003
New Zealand Journal of Agricultural Research, 2003, Vol. 46: 1–7
0028–8233/03/4601–0001 $7.00/0 © The Royal Society of New Zealand 2003

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