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Some Effects of Unilateral Ovariectomy in the Postpartum Beef Cow^{1, 2,}

University of Wisconsin, Madison 53706

Abstract

THE effects of unilateral ovariectomy and suckling were studied in 48 postpartum cows. The experimental design was a 2 x 3 factorial with half of the animals suckled and calves weaned from the others at the time of surgery. Three ovarian conditions were produced surgically 3 to 5 days postpartum: sham operated controls and unilaterally ovariectomized "ipsilaterals" and "contralaterals" retaining the ovary adjacent or opposite to the previously pregnant uterine horn, respectively.

Suckling lengthened the postpartum interval to first ovulation (43 vs. 23 days, P<.05) and to the first postpartum estrus (65 vs. 38 days, P<.01). The incidence of quiet ovulations was higher in suckled cows (1.26 vs. .64, P<.05) and fertility at first estrus was higher in cows that were suckled (95% vs. 44%, P<.01). Unilateral ovariectomy shortened the interval to first ovulation in suckled cows (37 vs. 56 days, P<.05), but had no significant effect on interval in nonsuckled cows. Conversely removal of an ovary had no effect on fertility in suckled cows but reduced it greatly in the nonsuckled groups. No fertilized eggs were found in the nonsuckled cow with contralaterally retained ovaries and only 50% were fertilized in the nonsuckled cows with ipsilaterally retained ovaries.

The numbers of follicles of various sizes were affected by unilateral ovariectomy and side of ovarian removal. Follicles found in unilaterally ovariectomized animals appeared to be of a more uniform medium size, i.e., fewer large and small follicles than were found in the controls. Side of ovarian removal also influenced the number of small (4 to 6 ram) and large (13 mm) follicles, 2.89 vs. 0.49 in the cows with ipsilaterally retained ovaries and 0.91 vs. 0.62 in the groups with contralaterally retained ovaries. Corpora lutea weight and uterine horn weight adjusted for length of postpartum interval were not influenced by unilateral ovariectomy or by suckling.

Unilateral ovariectomy in nonsuckled cows shortened the interval to the first postpartum ovulation, caused follicles to be of a more uniform size and affected fertility adversely.

¹ Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison and by North Central Breeders Research Council (East Central Breeders Cooperative, Waupun, Wisconsin; Midwest Breeders Cooperative, Shawano, Wisconsin; Minnesota Valley Breeders Cooperative, New Prague, Minnesota and Tri-State Breeders Cooperative, Baraboo, Wisconsin) and the Ford Foundation, Grant No. 63-505.

² Paper No. 1526 from the Laboratory of Genetics and No. 609 from the Department of Meat and Animal Science.