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Ovulation Rate in Sheep Selected for Weaning Weight or Litter Size

University of California Davis 95616^,5

Abstract

Ovulation rate was measured by laparoscopy at two consecutive cycles on 366 ewes 2 yr old and over and 85 yearling ewes of five lines of Targhees from the base base population; 53 yearling linecross ewes were also included. The lines were two unselected controls (HCl and DC), two selected for 21 yr for increased 120-d weight (HW and DH) and one selected for 19 yr for multiple births (T). Ewes were synchronized in late July or early August at the start of the normal breeding season with intravaginal pessaries impregnated with 60 mg methylacetoxyprogesterone and examined at first and second estrus. Ovulation hadoccurred in both cycles in 327 (89%) and 177 (85%) of the mature and yearling ewes, respectively. Overall mean numbers of corpora lutea at first and second estrus were 1.42 and 1.63, respectively for ewes 2 yr and over and 1.20 and 1.44 for yearlings, indicating an effect of synchronizing treatment, season, flushing, or a combination of these. Among mature ewes, ovulation rate was higher (P<.05) in DH (+.20), HW (+.19) and T (+.16) than in controls at first estrus, and in HW (+.29) and T (+.21) but not DH (–.04) at second estrus. Among yearlings, differences were significant only at second estrus (HW, +.40; T, +.35) and again not for DH (+.08). The failure of line DH to increase in ovulation rate from first to second estrus as did other lines was transmitted to linecross progeny. Body weight within line affected ovulation rate significantly, with a greater effect at second estrus, in both age groups. Adjustment for body weight removed the difference between HW and controls but not between T and controls. Repeatability of corpora lutea count was .27 and .25 for mature and yearling ewes, respectively.

¹ Permanent address; The Agricultural Institute, Grange, Dunsany, Co. Meath, Ireland.

² Dept. of Anim. Sci.

³ Address for reprints.

⁴ Present address: 7950 Sanel Drive, Ukiah, CA

⁵ The authors express their appreciation to John Hayes, Gil Dow, Deborah Nelson, Mauricio Elzo and Gonzalo Conzalez for assistance with the project, and to Upjohn Co. for the progestagen treated sponges. Partial funding support came from the NC-111 Regional Research Project and from the Title XII Small Ruminant Collaborative Research Support Program, Grant Number AID/DSAN/X11-G0049.