Abstract

In New Zealand cows are usually milked twice a day (TAD), however milking cows once-a-day (OAD) for the entire lactation is steadily increasing. This study aimed to estimate genetic correlations (r g) for milk production traits across TAD and OAD milking frequency (MF) environments from first lactation cows. Lactation yields of milk (MY), fat (FY) and protein (PY), and somatic cell score (SCS) of progeny from 1126 sires that were herd-tested during 2008–2012 were analysed. The r g estimates of the same trait expressed in the two MF were lower than unity but greater than 0.80. Genetic correlation between MY and PY across MF environments was high (0.79), moderate between MY and PY with FY (0.43–0.51), and low when the estimation involved SCS (<0.15). The results show that genetic evaluations made in TAD should include the G×MF effects, and although these effects are not expected to cause significant reductions in the efficiency of selection programmes in OAD systems for milk production traits in New Zealand dairy cattle. These estimates presented in this study can be used to predict genetic progress for the dairy population milked OAD when sires proven with daughters milked TAD are used in OAD herds.