Performance and genetic parameters for fertility of seasonal dairy cows in New Zealand

Reproductive performance is an important determinant of dairy production efficiency, which has not been extensively studied in pasture-based seasonal production systems. The objectives of this study were to investigate the suitability of various fertility traits for describing the reproductive performance of dairy cows under the seasonal production conditions in New Zealand and to estimate genetic parameters for selected fertility traits. Data were derived from the first and second lactation of 66294 animals born between 1984 and 1990. Since the primary breeding objective in a seasonal production system is to have each cow in a herd conceive as soon as possible after the date selected for the start of breeding (SE), many of the parameters must be derived using SE as a reference point. Based on SE, the average intervals to first service (SBFS) were 17.5 and 17.0 days and intervals to successful service (SBCO) were 31.7 and 30.8 days for the first and second lactation, respectively, resulting in the respective intervals from first service to successful service (FSCO) of 14.2 and 13.8 days. The percentages of cows conceiving within the first 21 or 42 days (PR21 and PR42) after SE were 48.5% and 74.7% for first lactation and 50.0% and 76.5% for second lactation. Breeds differed in reproductive performance with Jersey cows being superior to Friesian cows mainly due to a shorter SBFS interval for Jersey cows. Heritability estimates obtained using data for the first or second lactation ranged from 0.01 to 0.06 except for age at calving (AC) at 0.13 and 0.12, respectively. The genetic coefficients of variation were higher for SBFS, SBCO, FSCO, PR21 and PR42 than for other traits studied. Genetic correlations between calving interval and SBFS or SBCO were high (0.92 and 0.96), and between SBCO and PR21 or PR42 were both -1.00. Antagonistic genetic correlations existed between milk production and most fertility traits. Phenotypic correlations between milk production and fertility traits were all close to zero except for AC. It is concluded that fertility traits based on SE are appropriate for measuring reproductive performance of dairy cows under seasonal production systems. Fertility traits should be incorporated into selection programmes for dairy cows to counteract the antagonistic relationships between milk production and fertility. PR21 is a suitable selection criterion for improving reproductive performance of cows in New Zealand dairy herds. (C) 1997 Elsevier Science B.V.