Index selection for improved growth and carcass characteristics in a population of Large White pigs

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Abstract
An experiment was carried out over 11 years to investigate selection for economy of production and carcass lean content under ad libitum feeding in Large White pigs. Two lines, a selection (S) and a control (C) line, were involved in the study. The S line comprised 80 females and 10 males and was based at two centres. Boars were performance tested centrally at one of the centres and gilts were on-farm tested. Testing was carried out in groups of two or three full-sibs over the live-weight range 27 o t 87 kg. Selection was based on an index (I) incorporating individual daily live-weight gain (DLWG) and ultrasonically measured backfat thickness (USBF) and a group food conversion ratio (FCR) and generations were overlapping. The C line (32 females and 16 males) was maintained at one centre and males were performance tested alongside S boars to monitor genetic progress. Cumulative realized selection differentials over years 1 to 10 were equivalent to 5·5, 51, 7·0 and 9·4 phenotypic standard deviations for DLWG, USBF, FCR and I respectively and generation intervals averaged 17·0 months. There was little genetic change in DLWG, however, USBF, FCR and I showed substantial improvements with cumulative responses in year 11 of —12·3 mm, —0·22 kg/kg and +45·2 points respectively. The reduction in USBF occurred in the first half of the study with no further improvements being achieved after year 6. This study illustrates the effectiveness of index selection for a limited number of economically important traits but highlights limitations to this approach.