Total milk yield per lactation (PL), milk yield in 305 days (PL305) and lactation length (DL) were estimated based on records of the Brazilian Association of Girolando Breeders (3,574 lactations). Milk production was recorded monthly and 10.8% of the herds were raised under an extensive feeding system, 67.9% under a semi-intensive feeding system and 21.3% under an intensive feeding system. Data were analyzed through the least square method by GLM procedure (SAS, 1996). Preliminary analysis indicated a high interaction between genetic group and feeding system. Thus, the performance of genetic groups was evaluated within each feeding system. Genetic effects of additive difference between (g) Holstein and Gir breeds, dominance (d) and additive x additive epistatic interactions (gg) were also studied. It was observed that, in some cases, the effects of epistatic interactions should be taken into account. Moreover, increasing the proportion of Holstein-Friesian genes did not have any significant effect on milk yield when herds were raised under poor management conditions. However, as such conditions improved, crosses with higher proportion of Holstein tended to show better performance.
crossbreeding; epistatic interactions; heterosis; lactation length; milk yield
