Abstract
Sheep are seasonal breeders, but a lower seasonality has been reported for sheep breeds from the Mediterranean countries enabling spring matings. To further substantiate this, the present study compared seasonal variation in oestrus behaviour and ovulation in two Greek breeds (Chios and Serres) maintained in the same environment. In 27 adult ewes of each breed, oestrus behaviour, the occurrence of ovulation and ovulation rate were monitored during 2 successive years by daily inspection by rams, weekly progesterone assay in circulating blood plasma and monthly ovarian laparoscopy. The periods of anoestrus (109 +/- 8 (wk) vs 178 +/- 5 d) and anovulation (63 +/- 8 vs 149 +/- 6 d) were significantly shorter (P < 0.001 and P < 0.001) and more variable in Chios than in Serres ewes. Furthermore, the percentage of ewes exhibiting oestrus at least once monthly was significantly higher from March to July in Chios than in Serres ewes. Continuous ovulation throughout the year was noted in 5 and 2 Chios ewes during year 1 and 2, respectively. Ovulation rate (3.25 +/- 0.15 vs 1.45 +/- 0.04) was higher in Chios than in Serres ewes (P < 0.001) while duration of oestrus was also longer in Chios ewes (1.8 +/- 0.05 vs 1.5 +/- 0.06 d; P = 0.002). There were significant correlations between the duration of anovulatory periods of the 2 successive years in Chios (r = 0.43) but not Serres ewes. Ovulation rate was positively related to the onset of the sexual season in Serres but not in Chios ewes. The present data suggest that: i) the efficiency of Serres but not in Chios ewes. The present data suggest that: i) the efficiency of spring mating in these breeds is suboptimal since it is a period when the percentage of ewes exhibiting oestrus and ovulation as well as ovulation rate are limited, ii) owing to the large variation in seasonality within Chios ewes and to the good between year correlation in features of seasonality, selection against seasonality in Chios ewes could be worthwhile.
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