Rates of Inbreeding in Yorkshire, Landrace, Duroc and Hampshire Performance Tested Pigs in Ontario

Coefficients of inbreeding accumulated over a 10-yr period were computed for 140 376 Yorkshire, 89 673 Landrace, 29 776 Duroc and 21 873 Hampshire pigs performance tested on the Canadian Swine Improvement Program in Ontario between 1977 and 1987. All tested pigs were identified as to sire and dam. When all pigs were considered, both inbred and non-inbred, average inbreeding levels were less than 1%. Among inbred pigs, average levels were 4.5–8.8%, depending upon breed. The maximum inbreeding coefficient was 37.5%. Although average inbreeding levels were low, some herds had average inbreeding levels as high as 14%. Linear rates of increase in inbreeding coefficient ranged from 0.10 to 0.13% per year. However, rates of increase were non linear in all breeds, and by 1987 all breeds had average inbreeding coefficients of only slightly more than 0.8%. By 1987, 29, 19, 17 and 13% of Yorkshire, Landrace, Duroc and Hampshire pigs, respectively, were inbred to some degree. The rate of increase in proportion of inbred pigs was generally linear. Implications with respect to genetic evaluation and selection are discussed.