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Weaning Trait Comparisons among Inbred Lines and Selected Noninbred and Randomly Bred Control Groups of Rambouillet, Targhee and Columbia Sheep¹

US Department of Agriculture², Dubois, ID 83423

Abstract

A program of inbreeding with concurrent selection was followed in the development of 26 Rambouillet, 20 Targhee and 10 Columbia inbred lines for subsequent linecross and top-cross performance evaluation. Inbreeding coefficients averaged 30.4, 28.5 and 29.8% for the three breeds, respectively, at the end of the study. Selected noninbred control and unselected randomly bred control groups were maintained for each breed. Weaning trait data were evaluated for 1,332 Rambouillet, 1,557 Targhee and 1,049 Columbia inbred line and control group lambs during the final 2 years of inbreeding. Results showed a slight response to selection within inbred lines for the more highly heritable traits and some degree of genetic divergence among lines at the current levels of inbreeding. In general, the traits most sensitive to inbreeding depression were those more closely associated with fitness and those of low to moderate heritability. Even though a fairly large number of lines was developed, only a few inbred lines were superior (P<.05) to their respective selected control groups — and usually for only one trait. Significantly, not a single Rambouillet, Targhee or Columbia inbred line was superior (P<.05) in overall merit to its appropriate selected control group. Unselected control group means tended to be slightly superior to means for most inbred lines for traits of low heritability (weaning weight, body type and body condition) and inferior for more highly heritable traits (staple length, face cover, and neck folds). Notably, only six of a total of 56 inbred lines were superior (P<.05) in overall merit to their appropriate unselected control groups.

¹ It is most appropriate to acknowledge the contribution of Dr. C. E. Terrill, who was primarily responsible for initiating and developing the inbred lines up through 1955.

² SEA, US Sheep Exp. Sta., Dubois, ID 83423, in cooperation with the Univ. of Idaho, Moscow.

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