Genetic parameters for milk components including lactose from test day records in the New Zealand dairy herd

There are currently few published estimates of genetic parameters for lactose yield or lactose percentage for dairy cows. Recent trends in milk standardisation for whole milk powder have resulted in whole milk being standardised with the ratio of protein to protein-plus-lactose of at least 0.39. Currently whole milk powder produced from New Zealand milk has a protein to protein-plus-lactose ratio of 0.43, thus requiring additional lactose to be imported to maximise the return from the current product portfolio. Estimates of genetic parameters were obtained using 15,366 test day records from 4378 first-lactation cows in the Livestock Improvement Corporation sire proving scheme in the 2011–12 dairy season, distributed across 70 herds. These data included milk, fat, protein and lactose yields; fat, protein and lactose percentages; somatic cell count; days in milk; and the protein to fat ratio and protein to protein-plus-lactose ratio. Mean milk yield was 13.8 L/day, containing 5.16% fat, 3.93% protein and 5.12% lactose. Heritability estimates were 0.22, 0.35, 0.32 and 0.25 for milk yield, fat, protein and lactose percentages, respectively, which were lower than those reported in the literature but enough to allow for selection of lactose percentage.