Correspondence: Kantas D.
TEI Larissa, Dep. Animal Production, Tel.: 2410 684360, 2410 660421, Mob.: 6973 385500
E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

The objectives of the present study were to investigate the feasibility to extent the breeding season after pharmacological thyroid gland suppression in Karagouniko ewes. During the first experimental phase, we studied the circannual variation in thyroxin and triiodiothyronin concentration in 10 ewes. During the next year, on the basis of the results obtained from the first phase, thiamazole was administered in 10 of 20 ewes. Sample analysis from the first experiment showed that T3 and T4 fluctuate throughout the year, but at the period of increasing daylight their concentrations were higher in comparison to those of the periods of decreasing daylight. A clear and abrupt increase in both T3 and T4 was observed during the end of spring. In the second experiment we attempted to eliminate this increase by thiamazole administration. Thiamazole suppressed T3 and T4 increase, but estrous cycle characteristics (ovarian structures, progesterone concentration) and mainly the duration of the breeding season did not differ between the two groups. We infer that stronger and possibly longer thyroid gland suppression is required in order an extension of the breeding season to be achieved.

Key words: thyroxine, progesterone, thiamazole, ewe

Volume 59 (No. 1 p. 40-45) / 2008