Heterosis losses resulting from incorrect matings in a three-breed rotational crossbreeding system in pigs

Losses in individual heterosis were estimated using performance records from 11,700 pigs, 959 litters, and 377 pens of pigs from a three-breed rotational crossbreeding system. Three types of rotational crossbred sows were produced using the Duroc, Landrace, and Yorkshire breeds. Twenty-nine Duroc, 27 Landrace, and 25 Yorkshire boars were mated to 275 sows to produce pigs with theoretical levels of breed heterozygosity of 85.7, 71.4, or 42.9% depending on the genetic composition of the sows and boars. Orthogonal polynomials for unequally spaced levels were used to partition sum of squares for individual heterozygosity into linear and quadratic responses. There was a linear decline in 56-d litter weight (-.222 kg; P < .01) for each 1% decrease in heterozygosity. Responses for pig weight at 56 d (-.02 kg; P < .001) and age at 105 kg (.12 d; P < .001) were linear for each 1% decrease in heterozygosity. Likewise, post-weaning average daily gain ( -.0004 kg/d; P < .05) and feed conversion (-.0001; P < .01) responses were linear for each 1% decrease in heterozygosity. There were linear responses for survival rates of pigs born alive to 21 d (-.089%; P < .01), alive from 21 to 56 d (-.025%; P < .05), alive from 56 d and to 105 kg (-.042%; P < .05), and from birth to 105 kg (excluding stillborn pigs) (-.129%; P < .001) for every 1% decline in heterozygosity from 85.7 to 42.9%, respectively. Quadratic responses were not important for any of the traits examined (P > .05). Decreased productivity, resulting from decreased heterozygosity, can be expected when three-breed rotational crossbred sows are mated to boars from the wrong breed.