An ultrasonographic method to study reproductive seasonality in ewes isolated from rams

In studies concerning the seasonality in sheep, the effect of the teaser-ram, which can by itself induce oestrus as well as the development of ovary function bodies, has not been considered. In this study, a method is developed to determine ovarian activity during the year while excluding a possible influence of the ram. The study was carried out on 10 German Blackhead Mutton ewes (GBM) and 10 German Mutton Merino ewes (GMM) and lasted for 14 months. The ewes were kept without contact with any ram, and once a week a transrectal ultrasonography was conducted to determine ovary function bodies (corpora lutea, follicles) and a blood sample was taken to analyse progesterone (P4) concentration. In both breeds, the number of ovulating ewes, the ovulation rate (OR), the size of the corpora lutea (CL) and P4-concentrations showed significant seasonal fluctuations. During the main breeding period (September-February), the OR was higher (2.0 +/- 0.4) than during the rest of the year (1.6 +/- 0.5). Outside the main breeding season (March-August), approximately 40% GBM ewes and only approximately 15% GMM ewes produced CL. The cycle and the related seasonal changes in the P4-concentration coincide with different ultrasonically diagnosed CL (developing or regressing, insufficient, no CL). Between the volume of fully developed CL and P4-concentration, correlations of 0.57 (GMM) and 0.45 (GBM), respectively, were found. On average, P4-concentration in September to February was by approximately 1.60-1.66 nmol/l in the GMM and by approximately 1.80-1.86 nmol/l in the GBM higher than in spring and summer. Considering the OR, the P4-values corresponded with the CL volumes, i.e. in the months from August to February the CL values were significantly larger than in the months March to August. The ultrasonic ovarian diagnosis is a useful method to determine the ram-uninfluenced seasonality of reproduction in sheep.