A QUANTITATIVE STUDY OF GENETIC DIFFERENCES IN THE INCIDENCE OF OESTRUS, OVULATION AND PLASMA LUTEINIZING HORMONE CONCENTRATION IN THE SHEEP

Abstract

Seasonal variation in the ovulation rate and incidence of ovulation was examined in 15 Romanov and 15 Solognote ewes by endoscopy. The ovulation rate of the Romanovs was found to be related to the incidence of ovulation (r = 0·78) and to the incidence of oestrus as measured by daily teasing with aproned entire rams (r = 0·83). The ovulation rate of the Romanovs was greater than that of the Solognotes, and was 2·6 and 1·1 corpora lutea in the two breeds, respectively, in the autumn. The durations of the breeding season and of oestrus of the Romanovs exceeded those of the Solognotes. The characteristics of the preovulatory discharge of luteinizing hormone (LH) and the daily plasma LH levels were studied by radioimmunoassay in the Romanov and Solognote ewes and in ewes of the Berrichon and Berrichon × Romanov breeds. The interval between the onset of oestrus and the discharge of LH was greater in the Romanovs (17·6 h) than in the other breeds (mean = 7·5 h). The total release of LH, however, was not related to the number of corpora lutea formed.