Impact of the halothane gene on muscle quality and pre-slaughter deaths in Western Canadian pigs

The genotype with respect to the point mutation at position 1843 of the ryr-1 gene (halothane gene) and longissimus muscle quality of 1006 pigs in two Western Canadian packing plants was assessed to determine the frequency of this mutation within the commercial population and the relationship of genotype to the frequency of PSE (pale, soft, exudative) pork. The frequencies of nn (homozygous carrier), Nn (heterozygous carrier) and NN (homozygous normal) pigs within the commercial population were 0.3, 9.4 and 90.3%, respectively. Based on the subjective pork quality standards of Agriculture and Agri-Food Canada, the frequencies of paler-than-normal (colour score <3) and darker-than-normal pork (colour score >3) were 21.7 and 11.2%, respectively. The frequencies of soft, exudative pork (structure score <3) and firm dry pork (structure score >3) were 14.8 and 7.6%, respectively. The frequency of pale loin colour was 80 and 20% higher for Nn than for NN pigs in the two packing plants, whereas the frequency of dark pork was unaffected by genotype. Nn pigs exhibited a twofold higher incidence of soft, exudative pork than did NN pigs, but because of their very low frequency in the commercial kill, had a very small impact on the overall incidence of soft, exudative pork. Removing this mutation from the pig population would have very little impact on the frequency of pale or soft, exudative pork within the two packing plants involved in this study. Determination of the halothane genotype of 401 pigs, arriving dead or dying at the packing plants, indicated that pigs of the nn, Nn and NN genotypes were responsible for 27.7, 25.2 and 47.1% of deaths. The frequency of deaths within the nn, Nn and NN genotypes was estimated to be 9.2, 0.27 and 0.05%, respectively. Approximately 90% of the PSE condition in Western Canada is caused by factors other than the halothane gene, but this gene has a major negative influence on the frequency of pre-slaughter deaths.