New approaches to fertility evaluation in a seasonal calving system

This paper describes the investigation of possible new approaches for fertility genetic evaluation in the New Zealand seasonal calving system. An earlier study using data from the National Herds Fertility Study database indicated that redefining the 0/1 binary trait for calving rate in the first 42 days of calving (CR42) to a continuous trait expressed as calving day relative to the calving start day (CSD) could significantly increase the accuracy of fertility traits estimates. This paper investigates alternative fertility trait definitions (calving rate and percentage mated), as well as alternative definitions of planned start of mating and calving using a much larger industry dataset than used in the earlier study. Variance components were estimated for fertility traits, including heifer fertility, and genetic correlations estimated, including correlations with production and conformation traits. The results confirm the findings from the previous study - that is, redefinition of CR42 to CSD resulted in improved use of the phenotypic information for genetic prediction. Heifer calving season day was also found to be highly genetically correlatd with later cow calving traits, meaning it is a potentially valuable early predictor of daughter fertility in AI bulls. Modest genetic correlations between cow fertility traits and body condition score were found. Results also indicate that the current use of first lactation milk volume yield in the CR42 prediction might be better replaced with protein percentage in CSD. Further work is required using selection index theory to determine if a major revision to the national genetic evaluation for fertility is justified.