The Effect of Unilateral Ovariectomy on Plasma Gonadotropin Levels, Estrus and Ovulation Rate in Sheep

The effect of unilateral ovariectomy on plasma gonadotropin levels estrus and ovulation rate in the sheep was studied. 5-6 year old parous Border Leicester-Merino cross ewes were ovariectomized unilaterally on Days 14 and 16 of the estrous cycle or during anestrus. Luteinizing hormone and follicle stimulating hormone (FSH) were measured. The proportion of ewes in estrus and the length of the estrous cycle during the breeding season were unaffected by unilateral ovariectomy. During the breeding season FSH levels increased (p less than .05) 5 1/2-12 hours after unilateral ovariectomy performed on either day. Removal of 1 ovary on Day 14 allowed for full compensation of ovulation rates by the next estrus. When surgery was performed on Day 16 the ovulation rate was incomplete by the next estrus. 12-14 days after estrus following laparotomy ovarian weights had increased significantly (p less than .05) in both groups. When laparotomy was performed during the anestrous season ovarian weight and FSH levels were similar to control values. Ovulation rate can thus be determined within the 3 days prior to the onset of estrus. It is suggested that FSH levels particularly around Day 14 of estrus are important in determining the ovulation rate for the next estrus.