New Zealand Journal of Agricultural Research abstracts
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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