HETEROSIS LOSSES RESULTING FROM INCORRECT MATINGS IN A 3-BASED ROTATIONAL CROSSBREEDING SYSTEM IN PIGS

Losses in individual heterosis were estimated using performance record s from 11,700 pigs, 959 litters, and 377 pens of pigs from a three-bre ed rotational crossbreeding system. Three types of rotational crossbre d sows were produced using the Duroc, Landrace, and Yorkshire breeds. Twenty-nine Duroc, 27 Landrace, and 25 Yorkshire boars were mated to 2 75 sows to produce pigs with theoretical levels of breed heterozygosit y of 85.7, 71.4, or 42.9% depending on the genetic composition of the sows and boars. Orthogonal polynomials for unequally spaced levels wer e used to partition sum of squares for individual heterozygosity into linear and quadratic responses. There was a linear decline in 56-d lit ter weight (-.222 kg; P < .01) for each 1% decrease in heterozygosity. Reponses 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. L ikewise, postweaning average daily gain (-.0004 kg/d; P <.05) and feed conversion (-.0001; P <.01) responses were linear for each 1% decreas e 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 b irth to 105 kg (excluding stillborn pigs) (-.129%; P <.001) for every 1% decline in heterozygosity from 85.7 to 42.9%, respectively. Quadrat ic responses were not important for any of the traits examined (P >.05 ). Decreased productivity, resulting from decreased heterozygosity, ca n be expected when three-breed rotational crossbred sows are mated to boars from the wrong breed.