Estimation of genetic parameters for milk traits in cows milked once- or twice-daily in New Zealand

The objective of the present study was to estimate genetic parameters for milk yields, average somatic cell score (SCS) and milk composition traits in dairy cows milked either once a day (OAD) or twice a day (TAD) in New Zealand. The data set comprised 124,620 and 194,631 lactation records from OAD and TAD populations, respectively, during the period 2008-2012. Overall, estimates of parameters were similar between milking frequencies (MF), although heritabilities of production traits tended to be greater in the TAD cows. Estimates of heritability in OAD and TAD were: 0.33 and 0.36 for milk yield; 0.21 and 0.26 for fat yield; 0.22 and 0.25 for protein yield; and 0.12 and 0.12 for SCS, respectively. Estimates of correlations were similar across MF, in particular the genetic correlation between milk yield and protein yield (0.84 for TAD and 0.85 for OAD). Estimates of genetic correlations between SCS and other traits tended to be close to zero in both populations. The results indicate that genetic progress can be lower in the OAD population due to lower phenotypic and genetic variances compared to the TAD population. However, a potential disadvantage is that evaluating both dairy populations together could lead to systematic inaccuracies and biases in the estimation of breeding values for the population milked OAD as future dams.