First lactation production records of 275188 Holstein-Friesian dairy cows, progeny of 708 sires, and the second to fifth lactation records of survivors were analysed in order to estimate the regression coefficients of milk, fat and protein yield in lactations 1 to 5 on heterosis and recombination loss. Coefficients were fairly consistent over lactations. For milk yield in lactations 1 to 5 respectively, the regression coefficients were 100 kg, 111 kg, 155 kg, 124 kg and 118 kg for heterosis and - 156 kg, - 227 kg, - 256 kg, - 304 kg and - 218 kg for recombination loss. There was negligible change in the proportion of Holstein over lactations. The regression coefficients of survival to complete lactations 2 to 5 on heterosis and recombination loss were mainly positive. After fitting first lactation yield in the model and compared to a pure Friesian, there is an increased chance of survival to third lactation of 3.6% for a pure Holstein, 3.8% for an F1 and an F2, and 2.4% for a Friesian backcross.
