Performance of British Landrace pigs selected for high and low incidence of halothane sensitivity 2. Growth and carcass traits

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Abstract
The effects of halothane phenotype on growth were estimated from generations 1 to 3 of British Landrace positive and negative selection lines. A total of 241 full-sib pairs (castrated male and gilt) from 133 second litters were performance tested from 25 to 85 kg live weight on either ad libitum or scale (0.69 g/g ad libitum daily food intake) feeding. Compared with negative reactors over feeding regimes, positive pigs showed significant advantages in food conversion ratio (-0.07, s.e. 0.02), eye-muscle area (0.8, s.e. 0.3 cm2), proportion of lean in sample joints (16, s.e. 7 g/kg), and visual conformation scores, accompanied by disadvantages in meat colour (5.9, s.e. 2.1 EEL units), incidence of pale, soft, exudative (PSE) meat (0.36 v. 0.05, P < 0.001), post-weaning mortality (0.09 v. 0.01, P < 0.001) and ad libitum daily food consumption (-0.08, s.e. 0.02 kg). In addition, there were non-significant indications of an advantage in carcass lean proportion (11, s.e. 10 g/kg), with disadvantages in carcass length (-14, s.e. 7 mm) and proportion of lean in high-priced cuts (-6, s.e. 2 g/kg). Only daily food consumption showed significant selection line .times. feeding level interaction. Averaged over feeding regimes, an estimated extra return of .pnd.1.70 (s.e. 0.50) from lean growth would be offset by increased PSE and mortality to give a net loss of roughly .pnd.3.90 (s.e 1.00) per positive reactor. Taking these results as the minimum difference between homozygote, and if the gene is additive for lean proportion but recessive for stress susceptibility, the heterozygote would have an advantage of .pnd.0.90 over the normal homozygote. The effects of the gene on growth and carcass traits in British Landrace appear similar to those reported in other breeds, although the advantage of positive reactors in lean proportion may be slightly smaller.