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Reciprocal Recurrent Selection in Swine: Postweaning Growth¹

US Department of Agriculture, Beltsville, MD 20705

Abstract

Selection involving six cycles of reciprocal recurrent selection (RRS) was conducted over a period of 12 years, followed by a 4-year evaluation period. Selection in two unrelated selected strains (B_S and L_S) was based on an index of sow productivity for preweaning traits in the pure strains and on postweaning gain, carcass backfat thickness and yield of preferred cuts in their reciprocal cross-progeny (B_SL_S and L_SB_S). In addition, concurrent selection in two control strains (B_C and L_C) derived from the same base populations was based mainly on the dam's preweaning litter productivity. This study compared responses in postweaning growth traits in the B_S and L_S strains and in their cross-progeny with postweaning growth in the B_C and L_C strains. Standardized selection differentials among parents for growth traits were generally larger when based on performance of their crossprogeny rather than on their own performance and, as expected, were far short of potential maximum based on single-trait truncation selection. Responses to selection during the selection period were estimated by the regression on generations of deviations of performance of B_S and L_S strains and their cross-progeny from means for contemporary B_C and L_C strains. Results for postweaning gain showed a significant positive response to RRS among cross-progeny, but a significant negative response in the parental B_S and L_S strains. In the evaluation phase, comparisons of heterosis effects in the selected and control strains largely supported conclusions from the selection phase. A more definitive evaluation of the effectiveness of the selection practiced must await further planned studies.

¹ The authors acknowledge with appreciation the valuable contributions of William H. Peters, Animal Operations Unit, who supervised the breeding herd management and the collection and recording of performance data during the experiment.

² Nonruminant Animal Nutrition Laboratory, Animal Science Institute, ARS, NER, Beltsville, MD.

³ Beltsville Agricultural Research Center, Research Animal Geneticist; retired.