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Phenotypic Response and Time Trends to Date of Birth Selection in Southdown Sheep^{1, 2,}

University of Kentucky, Lexington 40506

Abstract

Considerable research effort has been directed toward getting earlier lambs through estrous induction and synchronization of ewes with various hormone treatments (S.C.S., 1966). However, no published results are available where only early-born lambs have been selected as replacements for several generations. Thus, the present study was conducted to estimate phenotypic response and time trends to date of birth selection in Southdown sheep.

Data utilized for this study were collected over the 14-year period, 1956 through 1969, from a flock of purebred Southdown sheep which has been closed to outside breeding since 1957 and selected for date of birth. To initiate the selection experiment, 46 bred ewes and 15 open ewes were purchased during the fall seasons of 1955 and 1956, respectively, from eight different flocks. The ewes were bred to three rams during the fall of 1956 and to three different rams during the fall of 1957. These six rams served as foundation sires for the selection experiment.

¹ Published with approval of the Director of the Kentucky Agricultural Experiment Station as Journal Article 70-5-140.

² A publication of Southern Regional Sheep Breeding Project, S-29.

³ The authors acknowledge the assistance of Mr. T. P. Goerge in assembling a portion of the data.