The oestrous and ovarian activity of booroola merino crossbred ewe hoggets

Abstract

The oestrous and ovarian activity of 342 crossbred Booroola Merino, Coopworth and Romney ewe hoggets was recorded from March 19, 1979 until all ovarian activity had ceased. Four Booroola crossbred groups (½ Booroola ½ Coopworth, ¼ Booroola ¾ Coopworth, ½ Booroola ½Romney, ¼ Booroola ¾ Romney) were compared with Coopworth, Romney and Coopworth x Romney groups. Vasectomised Coopworth rams equipped with 'Sire-Sine' crayon harnesses were employed to detect oestrus. Ovulation rates were measured by laparoscopy at fortnightly intervals until behavioural oestrus was exhibited and thereafter at each recorded or expected oestrus. Following the last recorded oestrus for each hogget, fortnightly laparoscopic examinations were again made until ovulation had ceased to occur and the hogget was considered to have entered into anoestrum. Two levels of nutrition (high and low) were imposed within each breed group from December 22 1978, which resulted in a difference in mean bodyweight between the treatments of 4.6kg on March 12 (high plane: 37.7 ± 0.3kg; low plane: 33.1 ± 0.3kg).

Behavioural oestrus was exhibited by 96.7% of the high plane and by 90.1% of the low plane flocks; 22.4% and 13.7% respectively of these hoggets did so without a preceding silent heat. The majority of the hoggets exhibited their first oestrus during the month of April, and by the end of the first week in May only 15 (8.6%) of the high plane and 26 (17.8%) of the low plane hoggets which ultimately attained puberty had yet to exhibit oestrus. These differences in attainment of puberty and in time of first oestrus between the nutritional treatments were significant (p < 0.05). Overt oestrous activity extended from March 10 until July 31. The number of silent heats recorded during the breeding season represented 7.1% of the total number of oestrous cycles exhibited and such heats were experienced by 24.4% of the hoggets. There were significant effects of both breed and level of nutrition on bodyweight at first oestrus, date of first oestrus, number of overt oestrous cycles exhibited, the duration of the breeding season and ovulation rate. These effects are discussed. There was considerable variation in the reproductive performance of individual Booroola crossbred hoggets which was attributed to sire effects.

Significant positive correlations were evident between date of birth and date of first oestrus, date of first oestrus and the duration of the breeding season, ovulation rate and the duration of the breeding season. Significant negative correlations between age at first oestrus and date of birth, and between date of first oestrus and weaning weight, daily liveweight gain from weaning to March 12 and bodyweight on March 12 were also noted.

In the high plane flock, ovulation rate was highest at the first overt oestrus and declined as the season progressed. The repeatability of ovulation rate between successive oestrous cycles was determined by linear regression and was highest in the Booroola crossbred groups, among which pooled mean values ranged from r = 0.56 to r = 0.70.

The right ovary was found to ovulate more frequently than the left particularly under the low plane nutritional treatment (p < 0.001). However multiple ovulations tended to occur most frequently in the left ovary.