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Reproductive Performance of Dorset X Western Ewes when Mated to Dorset or Blackface Rams¹

Oklahoma Agricultural Experiment Station, Stillwater 74074

Abstract

The mating and lambing records associated with 2,368 ewe conceptions between 1959 and 1970 involving Western (predominantly Rambouillet) and Dorset x Western females mated to Dorset or Blackface (Suffolk or Hampshire) males were utilized to evaluate the reproductive performance of two specific mating systems. These were the backcross mating system exhibited by Dorset rams mated to Dorset x Western ewes and the triplecross mating system exhibited by Blackface rams mated to the Dorset x Western ewes. The Western ewes were used as a control to adjust for ram breed effect in the comparison.

The 2,368 ewe mating records consisted of 567 backcross matings (D x DC), 588 triplecross matings (B x DC), 592 Dorset x Western matings (D x W) and 621 Blackface x Western matings (B x W). Traits studied were conception rate, lambing rate, lamb mortality, lambs reared, birth difficulty and lamb vigor.

After adjusting for ram breed effects using the control matings the following advantages were found: A nonsignificant 3.0% advantage in conception rate to first mating in favor of the backcross mating system. The triplecross matings produced an average of 5.0 more lambs born per 100 ewes lambing (P < .20), and there was 3.3% less death loss among the lambs (P < .075). These two traits combined resulted in an advantage of 9.3 more lambs reared to two weeks of age per 100 ewes lambing (P < .025) for the triplecross mating system. Triplecross matings also showed a 2.2% advantage (not significant) in lambs born unassisted and a 7.0% increase (P < .01) in the percentage of lambs classified as strong at birth.

Assuming equal ovulation rates the results showed an apparent advantage for these triplecross lambs in traits related to increased viability of the embryo and/or fetus and increased livability and vigor of the newborn lambs. In traits related to lamb vigor there was a consistent tendency for the advantage of triplecross lambs over backcross lambs to be greater during the spring than during the fall.

¹ Journal Article 2726 of the Agricultural Experiment Station, Oklahoma State University, Stillwater. This research was conducted in cooperation with the U.S.D.A., Agricultural Research Service, Southern Region.

² Present address: Box 270, Meeker, Colorado 81641.