ESTIMATION OF INDIVIDUAL CROSS-BREEDING EFFECTS ON MILK-PRODUCTION TRAITS OF THE GERMAN-BLACK-PIED DAIRY-CATTLE USING DIFFERENT GENETIC MODELS

Milk performance data of cross-breds in the synthetic breed German Bla ck Pied Dairy Cattle (SMR), that had calved for the first time between 1970 and 1984, were used to estimate individual crossbreeding effects on milk production traits. Three million pure-bred and cross-bred cow s that were in their first lactation and had originated from conventio nal housing systems were involved. For 1111331 cows genetic groups acc ording to their respective ancestry could be formed. The additive bree d differences and heterosis effects estimated by the Dickerson model c orrespond to those effects determined by the majority of other authors . For milk yield, highly significant negative effects were obtained fo r the Holstein Friesian x Black Pied Cattle and Jersey, respectively. The estimates ranged between 4.8 % and 5.0 % of the mean of all cross- breeding groups. In cross-breds with Jersey the recombination loss for fat percentage was between 5.3 % and 6.0 %. These results support the theory that recombination loss occurs predominantly when breeds, whic h have been selected for a long period for certain traits, are crossed . The recombination loss for fat yield attains values between 1.9 % an d 6.8 %. Different genetic models for the estimation of cross-breeding effects were compared. In contrast to the Dickerson model, the modifi ed genetic models of JAKUBEC et al. (1991) and MATHER and JINKS (1971) (models A, B, C and D) and those of KINGHORN (1980, 1982, 1987) enabl ed estimation of the genetic components underlying heterosis. Model A accounts for additive, dominant and additive x additive effects and th is model can be transformed by linear functions into the DICKERSON mod el. As model A shows an exact separation of the heterosis and epistati c effects, this model is preferred to that of DICKERSON (1969, 1973). The estimated additive x dominance effects, obtained using model B, ar e significant and half as large as the additive x additive effects. Th e extent of dominance x dominance effects (models C and D) could not b e determined with the restrictions used. If these epistatic interactio ns are taken into consideration, the standard errors of the estimates increase and the accuracy of the estimates decreases.