Influencing factors on the level of the ovulation rate in sheep during the main breeding season--an ultrasonographic study

The main breeding season for sheep in Germany is from the beginning of September until the end of November. The aim of this study was to determine the factors which influenced the level of the ovulation rate (o. r.) during this period. The examination was conducted over three years and included Booroola.Mutton Merino crosses heterozygous- (F+) carriers and non-carriers of the Booroola- (FecB) fecundity gene], German Mutton Merino (GMM) and German Blackheaded (GB). In total 1326 examinations were carried out. Between the 8th to 12th day after estrous detection an ovary diagnosis was performed by transrectal scanning with a 7.5 MHz linear probe. The number of the corpora lutea per ewe (o. r.) was documented. The weight and the body condition of the ewes was also determined. During the breeding season the breed of the ewe had the main effect on the level of the o. r. (p < 0.05). This is the reason that the statistical model can only be applied for each breed. The effects of age, year of examination and day of the main breeding season were tested. The age of the ewes and the date of the examination during the breeding season had the main effect on the o. r. In GMM and GB the age influenced the o.r. with an effect of 8.3% and 4.2%. The effect of the season was vice versa (Gmm 3.2%; GB 8.2%). Non-carriers of the Booroola- (FecB) fecundity gene behaved similar like the GMM. However, only the high fecundity carriers of the FecB-gene were strongly influenced by the season. For each breed there exists a typical threshold value for the body weight above which further weight has no effect on the o. r. (GMM 65-70 kg, GB 75-80 kg, non-carriers of the Booroola-FecB gene 55-60 kg and carriers of the FecB-gene 60-65 kg). This value lies at about 80% of the highest measured weight of each breed. The results demonstrated that a balanced age structure in combination with a optimum weight influenced the level of o. r. In addition the date of the season is especially important.