Abstract
The objective of this study was to estimate genetic and phenotypic correlations and heritabilities for milk composition and fertility traits in cows milked either once-a-day (OAD) or twice-a-day (TAD) across 21 spring-calving dairy herds in New Zealand. Fertility traits evaluated were submission in the first 3 weeks of mating (SR21), the interval from the start of mating to conception (SMCO), in-calf in the first 3 weeks of mating (PR21), and conception to the first service (PRFS). Heritability estimates of fertility traits were low and varied from 0.02 to 0.07 across the population. Phenotypic correlations between milk production and fertility traits were close to zero and most of the genetic correlations were antagonistic in both milking systems. Protein to fat ratio had positive genetic correlations with SR21, PR21 and PRFS in both milking frequencies. Estimates of genetic correlations of P:F ratio with PR21 and PRFS were relatively greater in OAD than TAD indicating that cows milked OAD had a higher P:F ratio and were more likely to get in-calf early in the breeding season than cows milked TAD.
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