Abstract

Abstract: Context: In New Zealand, cows are usually milked twice a day (TAD), but in ∼8% of herds, cows are milked once a day (OAD) for the entire lactation. If a genetic correlation (rg) of the same trait expressed in two environments (such as TAD and OAD) is substantially <1.0, then the genetic merit assessed from TAD herds may not be reliable for predicting genetic merit in OAD herds. Initial evaluation of sires has been undertaken from progeny test herds with TAD milking, and the best sires have then been made widely available for use in TAD or OAD herds. Aim: The study was designed to test formally whether sire re-rankings occur in widely used sires at different milking frequencies. Methods: Regression coefficients and rank correlations (rS) for estimated breeding values (EBVs) of 242 widely used sires (86 Holstein-Friesian, 96 Jersey and 60 crossbred) were calculated for yields of milk, fat and protein, and for somatic cell score (SCS). The rS values were contrasted with expected rank correlations (rE) between TAD and OAD EBVs assuming rg = 1.0 between true BVs expressed at the two milking frequencies and accounting for the fact that sires were highly selected. Key results: Within and across breeds, regression coefficients of OAD on TAD EBVs for the same traits were <1.0, but rS values between TAD and OAD EBVs of the same sires were strong (>0.75) for milk yield, moderate-strong for fat and protein yield (0.55-0.77), and moderate for SCS (0.41-0.65). Estimates of rS were higher than their critical rE values, indicating no significant sire re-ranking across milking frequencies. Conclusions: On the basis of the results, a separate selection program to generate sires for use in OAD herds is not justified. Implications: Farmers operating under OAD systems can rely on genetic evaluation of sires evaluated in TAD systems and used in the OAD population. However, producers should recognise that the realised productive and economic advantage of daughters of elite sires born in OAD herds is diminished relative to that expected in TAD herds.