The effects of genetic composition (GC) and environmental factors on milk production (MP), lactation length (LL) and milk yield per day of calving intervals (P/I) (n=2,016) of 822 crossbred (1/2, 3/4 and 7/8) Holstein-Gyr cows, raised in the West of Minas Gerais State, Brazil, were evaluated using least squares methodology. The effect of predicted transmitting ability (PTA) of the Holstein sires on MP of their daughters was also evaluated. The effect of genetic composition on the three traits was an important source of variation. MP, LL and P/I increased with the increase of Holstein contribution. Season of calving was an important source of variation for MP of 3/4 cows and for LL of 3/4 and 1/2 cows. The highest MP and LL were observed for lactations starting at the end of the rainy season. The effect of age at calving on MP was linear for 1//2 and 3/4 cows and curvilinear for 7/8 cows. For LL the effect of age at calving was curvilinear for 1/2 and 3/4 cows and linear for 7/8 cows. On P/I was linear for 3/4 and 7/8 cows and curvilinear for 1/2 cows. The effect of PTA on MP was not different from zero. Estimated repeatability for MP were .72, .69 and .63, respectively, for the 1/2, 3/4 and 7/8 Holstein-Gyr cows.
dairy cow; crossbred; milk yield; lactation length; environmental factors
