Comparison of variance components for test-day electrical conductivity of milk and test-day somatic cell score for first lactation cows in an experimental herd

In recent decades, electrical conductivity (EC) has been introduced as an indicator of mastitis, and genetic selection based on this trait may be possible. In this study, genetic parameters for test-day EC and test-day somatic cell score (SCS) were compared. Data were collected from a Danish experimental herd, including daily records of EC and SCS from 265 first lactation cows. Different genetic models were tested, and a random regression animal model with a 4th order Legendre polynomial for the permanent environmental effect for both traits, a 1st order Legendre polynomial for the additive genetic effect of EC and a 2nd order Legendre polynomial for the additive genetic effect of SCS, gave the best fit. The patterns of the curves were similar for both permanent environmental and additive genetic variance for the two traits. Heritability estimates ranged from 0.05 to 0.12, and from 0.01 to 0.09, for EC and SCS, respectively. The estimate of genetic correlation between the traits was high, and ranged from 0.86 to 0.98. Based on these results, EC could be a potential indicator trait in a breeding programme where selection for increased mastitis resistance is included.