The milk production and survival of spring-calving carryover cows in New Zealand

The objective of this study was to compare breed proportions, genetic merit, milk production and survival of carryover and non-carryover dairy cows in New Zealand. Calving interval was used as a tool to identify cows that had an extended non-lactating period and then returned to a milking herd. A greater proportion of Holstein-Friesian was observed in the breed composition of the carryover cows. Estimated breeding values (EBVs) for milk traits were greater, but the EBVs for fertility were lower for carryover cows, than those for non-carryover cows. Carryover cow genetic merit values were greater than those for non-carryover cows in the year prior to their carryover period. Carryover cow milk production was significantly greater than that of non-carryover cows. The average survival days and probability of survival for second-lactation carryover cows was lower than that for first-lactation and lactation-matched non-carryover cows. These results are important for culling decisions made by pastoral dairy farmers.