Derivation of a breeding value for persistency in German dairy cattle focusing on performance in extended lactations

Persistency in production traits provides an opportunity to extend the lactation period. Enhanced lactation periods significantly over 305 d have potential benefits for fertility, animal health, farm management, and economic outcomes. However, the random regression test-day model, which is currently used in Germany, only covers d 5 to 305 of lactation. This study focuses on increasing the genetic capacity for persistency in Holstein breeding animals for use in extended lactations. The solutions of the second-order Legendre polynomial for the animal effect were obtained using the standard EBV routine in Germany. The breeding value estimation involved 19 million phenotypic lactation records resulting in EBVs for 17 million females and 200,000 males. In a next step, these regression coefficients were used to define genetic slopes for various lactation intervals between d 9 and 245 and 305 after calving (b_g;x,305). The resulting dataset was combined with a second dataset containing 955,345 Holstein cows with lactations extending to at least 400 DIM and having a calving interval of at least 550 d. For these cows, phenotypic slopes for each production trait and lactation between d 150 and 400 (b_p;150,400) were estimated. The combined dataset was subdivided into quartiles based on the genetic slopes (b_g;x,305) Finally, the DIM interval that best discriminated the phenotypic slopes (b_p;150,400) of cows according to top and bottom quartiles was used to determine the most appropriate breeding value for persistency. The interval between 150 and 305 DIM proved to be the most informative for predicting b_p;150,400 across all production traits. The final persistency index, called EBV persistency, combines the genetic slopes of protein and fat yield (kg) over the first 3 lactations with equal weighting, and it merges protein and fat persistency into a single index with a 2:1 weighting, reflecting the economic priorities of German breeding organizations. This EBV was officially introduced to the German Holstein breed in April 2023. The genetic basis of the EBV persistency, analyzed using the index theory, shows an estimated cumulative heritability of 0.34. Genomic EBV persistency shows negligible Pearson correlations with most traits in the total merit index. Only slight positive correlations are observed for the milk production index (0.24) and the longevity index (0.18). The new defined trait describes the genetic ability of animals for persistency in production traits, also in extended lactations.