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Reciprocal Recurrent Selection in Swine: Carcass Traits¹

US Department of Agriculture, Beltsville, MD 20705^,3

Abstract

Selection involving six cycles of reciprocal recurrent selection (RRS) was conducted over a period of 12 yr, followed by a 4-yr evaluation phase. Selection in two unrelated selected strains (B_S and L_S) was based on an index of sow productivity for preweaning litter traits in the parental strains and on postweaning gain, carcass backfat thickness (BF) and yield of five preferred cuts (percentage PC) in their reciprocal strain-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 limited to the dam's preweaning litter productivity. Standardized selection differentials among parents when based on their cross-progeny averaged .28 for percentage PC and –.09 for BF. These values were far short, as expected, of 1.02 standard deviations, the theoretical maximum selection differential based on single-trait truncation selection and the average percentage of animals selected to produce the next generation among those tested. Responses to selection during the selection phase were estimated by the regression on generations of deviations of carcass traits in B_S and L_C strains and in their cross-progeny from means for contemporary B_C and L_C strains. Results showed significant negative trends in deviations for both percentage PC and BF among cross-progeny and negative but nonsignificant regressions among the parental strains. The main divergence in the results from expected was in the sharply negative (P<.01) response of percentage PC among cross-progeny. In the evaluation phase, heterosis effects in the selected strain crosses and in the control strain crosses were essentially zero for percentage PC and BF, indicating minimal nonadditive genetic effects for these traits. Differences between parental selected and control strains were mostly nonsignificant but were largely significant between selected and control strain crosses, suggesting a positive response to RRS.

¹ USDA, Agricultural Research Service, Beltsville Agricultural Research Center, Animal Science Institute, Nonruminant Animal Nutrition Laboratory, Beltsville, MD 20705.

² USDA, Agricultural Research Service, Beltsville Agricultural Research Center, Beltsville, MD 20705, retired.

³ Mention of a trade does not constitute a guarantee or warranty by the USDA and does not imply its approval to the exclusion of other products that may be suitable.