Genetic evaluation of persistency in extended lactations
In April 2023, a new breeding value for milk yield persistency was introduced for German Holsteins, Jerseys, Red Dairy Cattle, German black-and-white dual purpose, and red-and-white dual-purpose breeds. The aim is to allow selection of animals that are genetically suited for voluntarily extended lactations. The new breeding value was derived from the Random-Regression-Test-Day-Model (3 yield traits: protein-kg, fat-kg, milk-kg; 3 lactations each; 2nd order legendre polynomials in the genetic effect) that is used for routine genetic evaluations. The new trait is defined as the slope of the genetic Legendre polynomial between DIM 150 and 305 for each yield trait in each lactation. EBVs for fat and protein persistency are then combined according to their economic impact, using the same weights as for the overall yield index RZM: fat-kg:protein-kg weighted 1:2. The newly defined persistency index has a cumulative heritability of 0.34. Correlations of genomic breeding values to the traits that are included in the total merit index RZG are close to zero, except for RZM (0.24) and longevity RZN (0.18). Reliabilities for youngest genotyped animals without performance observations are approx. 0.60, which is lower than for the RZM. We observe a small positive genetic trend in the newly defined persistency trait in German Holsteins, which is expected, given the small but positive correlation to RZG that stems from the correlations to RZM and RZN.
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