Responses to restricted index selection and genetic parameters for fat androstenone level and sexual maturity status of young boars

A restricted index selection experiment was conducted over four generations in order to investigate whether it is possible to reduce fat androstenone (AND) level with no adverse effect on sexual maturity status in young boars. Fat AND level was measured on a biopsy sample of backfat taken at 118 kg live weight. Sexual maturity status was assessed by bulbo-urethral gland (BUG) thickness measured by echotomography at 99 kg live weight. The experimental design included a control and a select line having a Large White–Landrace genetic background, and both lines were intended to comprise five sires at each generation. A total of 949 boars were recorded for both index traits throughout the experiment. The pattern of direct responses to this ‘antagonistic’ selection differed from expectation, and consisted of no response in fat AND level and a significant positive genetic trend in BUG development. The reasons for this discrepancy are discussed in terms of selection differential being achieved for each index trait (‘index in retrospect’) and deviations of the estimated (by REML) genetic parameters from the expected ones. Heritability estimates were close to 0.50 for fat AND level and 0.60 for BUG development whereas the genetic correlation among them amounted to about 0.65. Moreover, a significant genetic correlation of 0.30–0.40 was found between the sexual maturation of boars and that of related gilts.  2000 Elsevier Science B.V. All rights reserved.