An experiment has been carried out at Bahia Blanca to estimate the trend in ovulation rate throughout the year shown by Corriedale ewes of similar live weight in the south-west district of the Argentinian pampa.
Five groups of 16 animals were each subjected to two 20-day observation periods and the 10 observation periods in the experiment were evenly spaced within a year.
The proportion of ewes showing oestrus and ovulation (OO), the frequency of twin ovulations (TO) and, hence, ovulation rate (OR) were estimated for each period by using vasectomized rams and performing laparatomies.
A significant and periodic relationship was found between each trait and the number of days elapsed from the longest day (×). In turn, changes in the values of ×, arising from the progress of the experiment, were to some extent associated with changes in nutrition and surgical interference. However, it was concluded that these factors could not have affected the similar and remarkable seasonality shown by each trait in this study, and that this seasonality was mainly associated with differences in the natural light regime to which the ewes were exposed before and during their corresponding observation periods.
The predicted maximum values for each parameter were reached 1 month before the shortest day. Prior to and after this date both OO and TO declined, so that the predicted OR values became zero for a period of approximately 2 months before the longest day