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Selection for Weaning Weight in Targhee Sheep in Two Environments. II. Correlated Effects

University of California⁵, Davis 95616

Abstract

Targhee sheep were selected for 120-d weightunder irrigated pasture-drylot conditions at Davis (DW) and under range conditions at Hopland (HW). Unselected control lines were maintained in both environments (DC, HC1 and HC2). At Hopland, a line (DH) was maintained in which ewes were mated to Davis (DW) rams. Selection for 120-d weight was successful in both environments, with more improvement made in the drylot environment. The genetic improvement made in the drylot environment was expressed, although to a lesser degree, under range conditions. Correlated responses were analyzed. Birth weight increased significantly in all three selected lines; the increase was less in line DH than in the other two lines. In all selected lines, weights of ewes of all ages at mating increased significantly compared with their respective controls. Proportion of ewes lambing decreased (P<.05) in line DH; the trend was negative but nonsignificant in line DW. Differences in Iitter size between lines within location were not significant. Lamb survival to weaning decreased in lines DW (P<.05) and DH (P<.01), compared with their respective controls; and the trend in HW was negative but nonsignificant. Fertility and survival data indicated that, under range conditions, the line selected under drylot conditions (DH) was less fit than the line selected under range conditions (HW). As a result of the decreases in lamb survival and fertility, none of the selected lines produced more total lamb weight weaned per ewe than the controls, in spite of the significant direct response to selection. Mature ewes of lines DH and DW produced less total lamb weight weaned per ewe (P<.001 and P<.05) than their respective controls. The results indicate that while single trait selection for growth rate to weaning results in heavier lambs, it does not increase and may decrease total lamb production per ewe.

¹ Present address: Dept. of Epidemiol. and Preventive Med., School of Veterinary Medicine, Univ. of California, Davis.

² Present address: 7950 Sanel Drive, Ukiah, CA.

³ Present address: Dept. of Anim. and Poul. Sci., Univ. of Guelph, Guelph, Ontario, Canada.

⁴ The authors express appreciation to Robert Finlay, Richard Rainoldi, John Hays and Gil Dow for care of the sheep used in this project, to A. H. Murphy, Superintendent of the Hopland Field Station throughout the study, and to Curt Finley, for computing assistance. Funds for computing were provided by Title XII Small Ruminants Collaborative Research Support Program, Grant No. AID/DSAN/XII-G-0049.

⁵ Dept. of Anim. Sci., address requests for reprints to Dr. G. E. Bradford.