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Selection for Weaning Weight in Targhee Sheep in Two Environments. I. Direct Response

University of California⁵, Davis 95616

Abstract

In 1961, selection for 120-d weight was initiated in two flocks from a common base population of grade Targhee sheep. At Davis, sheep were maintained on a good plane of nutrition, on irrigated pasture or in drylot. At Hopland, sheep grazed annual grassland range, with supplementary feeding only at mating and lambing. Selected (DW) and control (DC) lines were maintained at Davis from 1961 through 1977. A selected (HW) line, replicate control (HC1 and HC2) lines and a line (DH) mated to the Davis DW rams were maintained at Hopland from 1961 through 1980, with the exception that HC2 was terminated in 1977. Multiplicative factors were used to adjust weights for effects of age of dam, sex and type of birth and rearing. Response to selection was estimated as the difference between selected and control line linear regression coefficients of adjusted line means on year. The Hopland replicate controls did not differ signifiantly from each other (HC1 – HC2 = .004 ± .056 kg/yr), and the control line data were pooled (HC). The overall control line mean 120-d weights on a female, single, mature-dam basis were 33.2 and 30.4 kg at Davis and Hopland, respectively. Direct response was greater at Davis than at Hopland: DW – DC = .524 ± .073 kg/yr (P<.001); HW – HC = .151 ± .034 kg/yr (P<.001). Corresponding realized heritabilities were .17 and .06. Direct response for the DH line was DH – HC = .226 ± .036 (P<001); realized heritability was .08. Response in the DH line was greater (P<.05) than that in the HW line: HW – DH = -.075 ± .037 kg/yr. This indicates that: (1) genetic improvement made on a higher plane of nutrition was expressed, but to a lesser degree, under range conditions and (2) selection under better feed conditions resulted in at least as much improvement in growth rate in a range environment as did selection under range conditions.

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.

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