Genetic parameters across milking frequency for first lactation milk production traits in New Zealand

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 (rg) 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 rg 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 (