Maximizing genetic response in crossbreds using both purebred and crossbred information

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Abstract
A combined crossbred and purebred selection (CCPS) method, i.e. using crossbred and purebred information, was proposed to achieve genetic response in crossbred animals. Selection index theory was applied to establish a CCPS index. The CCPS was compared with pure-line selection (PLS) and crossbred selection (CS) methods. The genetic correlation between purebred and crossbred performance (rpc) and crossbred heritability (hc2) are crucial factors in the comparison. The CCPS is always better than PLS or CS when a fixed number of purebred progeny is tested. With a fixed total number of purebred and crossbred tested progeny, CCPS is only worse than PLS for very high values of rpc (>0·8). Superiority of CCPS over PLS increases and over CS decreases with decreasing rpc. The larger hc2is, relative to purebred heritability (hc2the more response CS and CCPS will achieve. The robustness of CCPS against inappropriate assumptions on rpcand hc2values was investigated. The expected response is always an overestimate, and the actual response is smaller than the optimal response when rpcis assumed one but the true rpcis smaller. The difference between actual and optimal response increases as rpcdecreases but it is small for large rpcvalues (e.g. 0·7). The expected response is smaller than the actual response when rpcis large and hc2> hp2Finally, the actual response to CCPS is larger than the optimal response to PLS for positive values for rpc. The main conclusions are: (1) CCPS method is optimal for obtaining genetic response in crossbreds; and (2) CCPS with inappropriate assumptions on rpcand hc2values (e.g. recognizing crossbreds as purebreds) achieves more genetic response than PLS for common values of rpcand crossbred heritability.