Comparison of Single Trait and Index Selection in Sheep

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Abstract
Single trait and index selection were compared in Hampshire sheep. The traits selected were high 90-day weight and low 90-day fat probe at the seventh rib. The index was the standardized deviation in weight minus the standardized deviation in probe. Traits were mass selected in rams only within two replicates of each single trait and three replicates of each index selection group. Selection for about one-half of each group was done within single born rams and the other half within multiple born rams. Ram lambs were used for mating. Ewes were first mated at approximately 19 months of age. Each replicate had approximately 40 ewes and four rams. A repeat mated control was used. Selection was effective in all lines over a 5-year period. Weight selection increased 90-day weight by .62 ± .52 kg per year, with correlated increases of .035 ± .051 cm in fat probe and .049 ± .069 units in index. Probe selection decreased fat probe by .029 ± .052 cm per year with essentially no change in weight and an increase of .069 ± .070 units in index. Index selection increased 90-day weight by .33 ± .53 kg per year, decreased fat probe by .017 ± .051 cm per year and increased index by .095 ± .069 units per year. Paternal half-sib heritability estimates were .16 ± .06 for 90-day weight, .15 ± .06 for 90-day probe and . 19 ± .06 for index. Realized heritabilities were similar. The genetic correlation between weight and probe was .60 ± .41. After 6 years of selection, the weight and probe selected , groups were crossed and compared with crosses of the index selected groups. No firm conclusions could be drawn from the comparison. Copyright © 1979. American Society of Animal Science . Copyright 1979 by American Society of Animal Science.

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