Genetics of leg weakness in performance-tested boars

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Abstract
Genetic relationships among leg and performance traits were estimated for 23 975 Large White and Landrace boars fed twice daily to appetite from 27 to 91 kg live weight at Meat and Livestock Commission testing stations between 1966 and 1972. For each boar, an overall leg score was derived as the sum of scores for 19 individual leg traits categorized as ‘absent’ (0), ‘slight’ (1) or ‘severe’ (2) at 91 kg. Heritabilities of leg score were 017 ± 0·03 in Large White and 0·19 ± 0·04 in Landrace. Genetic correlations with a visual ‘leg action’ score on a scale from 1 to 5 averaged 0·93 ± 0·02 over breeds. Genetic and phenotypic correlations between leg scores on the same boars at 27 and 91 kg averaged 0·50 ± 0·17 and 0·15 ± 0·01 respectively.Both breeds showed significant adverse genetic correlations ranging from 0·20 ± 0·10 to 0·40 ± 0·08 between leg score and boar ultrasonic backfat. From slaughtered littermates, leg scores showed favourable genetic correlations with eye-muscle area (0·30 ± 0·10) and killing-out proportion (0·35 ± 011) in Large White and unfavourable correlations with carcass length (0·31 ± 0·10), lean content (0·30 ± 0·14) and backfat (0·33 ± 0·11) in Landrace. There were no associations with growth rate, feed efficiency or the performance index on which boars are selected. The study indicates that leg condition and fatness are adversely genetically related, but that selection on the present national performance index would not be expected to cause a rapid decline in leg condition. Culling on leg score would be expected to reduce the frequency of leg weakness.