VARIOUS PERSISTENCY MEASURES AND RELATIONSHIPS WITH TOTAL, PARTIAL AND PEAK MILK YIELDS

Seven persistency measures and their relationships with total, partial and peak yields were studied. Heritabilities, genetic and phenotypic correlations were estimated using mixed model equations with multiple trait restricted maximum likelihood (REML). Data were from 31,482 firs t lactation Belgian Holstein-Friesian cows. Differences in direction a nd size of the correlations between measures of persistency and total yield were found. Two types of measures were used. Persistency measure s based on ratio showed positive correlations to total yield. Measures based on standard deviations of test, days or partial yields were cor related negatively with total yield. Correlations with partial yields also showed a similar pattern. Genetic correlations between ratio meth ods and peak yield were positive. Ratio methods not only describe pers istency, but are highly influenced by total yield. Methods using stand ard deviations of test day yields showed a similar problem, but the in fluence of the level of total production was negative. Heritabilities of several persistency measures were above 0.10. After considering tho se results, a newly defined method, based on the standard deviation of partial yields, emerged as the best choice to describe persistency an d for use in genetic evaluation of persistency.