The aim of this study was to evaluate ovarian activity and the size of oocytes in ovarian follicles in sexually mature Landrace-Yorkshire gilts in relation to the individual seasons of the year. The study was carried out on 240 gilts slaughtered at an abattoir during the four yearly seasons. The size and weight of the ovaries, the number of follicles and corpora lutea (CL) according to individual size categories were evaluated. The oocytes were aspirated from follicles and their sizes were measured. Our evaluation of the size of the ovaries showed that they were the largest in autumn, when their mean length reached 25.8 ± 3.4 mm, while in winter their mean length was 24.2 ± 2.9 mm. The smallest weight of the ovaries was determined in autumn (mean 5.7 ± 1.4 g) and the highest in spring (mean 6.2 ± 2.2 g). The largest number of follicles in the ovaries of the gilts was recorded during the autumn months, with a predominance of follicles up to 3 mm (mean number 17.9 ± 7.5). The largest number of corpora lutea was observed in spring (mean number 12.1 ± 2.6) and the smallest in winter (mean number 6.1 ± 1.1). The oocytes from follicles of up to 3 mm size, were the smallest in spring (mean size 16.99 × 103 ± 3.42 × 103 µm2) and the largest in winter (mean size 18.90 × 103 ± 2.99 × 103 µm2). In total, the largest oocytes were aspirated from 4—6 mm follicles in autumn (mean size 19.60 × 103 ± 5.37 × 103 ± µm2). The values recorded indicated that the seasons affected the ovarian activity and the growth of oocytes in gilts.
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