Seasonal changes in oestrus activity and ovulation rate in Rasa Aragonesa ewes maintained at two different body condition levels

Abstract

Onset and duration of seasonal anoestrus and seasonal changes in ovulation rate were studied for two consecutive years in two groups of Spanish Rasa Aragonesa ewes with body condition (BC) scores ⩽ 2.5 (low BC group) or ⩾ 2.75 (high BC group). Animals were kept penned throughout the experiments (November 1987–September 1988 and February–September 1989) (latitude 41° 40′N). Mean liveweights and BC were similar at the beginning and end of both study periods. Oestrus was tested daily using aproned males. Ovulation rate was measured by laparoscopy 6 d after identification of oestrus. Animals were blood sampled for progesterone determination every 6 d during the 1989 experiment. The high BC group of ewes showed a shorter seasonal anoestrus than the low BC group in both study years (64 vs. 113 and 76 vs. 92 d in 1988 and 1989) ( P < 0.05). Differences in percentage of detected oestrus between groups were found in the transition periods between breeding seasons and seasonal anoestrus, and vice versa; and were higher in 1988 (74 vs. 34% in April, and 92 vs. 50% in July) ( P < 0.01) than in 1989 (50 vs. 21% in April) ( P < 0.05). Plasma progesterone concentrations showed a great number of ovulations were unaccompanied by oestrus, specially in the anoestrus period, ranging between 10.8–46.4% in May/June cycles. There was a marked seasonal change in ovulation rate. For the low BC group, the ovulation rates were consistently lower than those of the high BC group in both years, especially in September (1.25 vs. 1.67 and 1.09 vs. 1.67 corpora lutea in 1988 and 1989) ( P < 0.001). Ewes with lower body fat reserves less effectively reactivated their ovulation potential in the first cycles of each new breeding season.