Responses to selection using a lean tissue growth index were estimated from performance data of the Lincoln University Dorset Down flock. Data were available for ewes (n=922) born in the years 1984 - 1991 and rams (n=816) born in 1984 - 1992. Data were from two closed lines, one control and the other selected for high lean tissue growth. Initially selection was on the basis of an index comprising measurements of live weight and fat depth. Subsequently muscle depth was incorporated into the index. Ewes and rams born in spring each year were run separately at pasture from weaning (late summer) onwards. Performance testing was in the following autumn for ewes and winter for rams. Measurements were live weight and in vivo ultrasonic fat and muscle depths. Responses were calculated from regression of the difference between the line means for breeding value against time. Breeding value estimates were derived by AIREML analyses. Both sexes exhibited desirable responses in all component traits: rams; +0.290kg/yr, +0.019mm/yr and -0.013mm/yr and ewes; +0.154kg/yr, +0.028mm/yr and -0.028mm/yr for live weight, muscle and fat depths respectively. Increases in muscle depth occurred after it was incorporated into the selection index.
Proceedings of the New Zealand Society of Animal Production, Volume 54, , 255-260, 1994
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