Estimation of genetic parameters for production traits in Polish Simmental cattle using a random regression test-day model

Abstract

In Poland, routine genetic evaluation of Simmental cattle for production traits is based on a multi-trait animal model using total yields for each of the first three lactations. The purpose of the study was to select a reference population for the Simmental breed, which is the second largest population of cattle in Poland, and to estimate variance components for production traits, using a single-trait, multi-lactation random regression test-day model (RR-TDM). Analyses were performed for the first three lactations within the range of 5 to 305 days in milk (DIM). Random regression was fitted using Legendre polynomials, and variance components were estimated by Gibbs sampling. The collected production traits included: milk yield in kilograms; fat, protein, and lactose expressed in kilograms and percentages; and somatic cell score. Phenotypic data from 13,966 cows that were purebred Simmentals, comprising 256,341 test-day records from the first three lactations, were used to estimate variance components. The dataset was prepared without crossbred animals and records from herds with only one cow. Based on additive genetic, permanent environmental, and residual variance components for each trait across lactation, daily heritability was calculated. The average heritability estimates were: 0.20 for milk yield, 0.18 for fat yield, 0.25 for fat content, 0.19 for protein yield, 0.37 for protein content, 0.22 for lactose yield, 0.32 for lactose content, and 0.13 for somatic cell score. The highest average heritability was observed for protein percentage, whereas the lowest was observed for somatic cell score. Depending on the trait, minimum and maximum heritability occurred at different stages of lactation. For most traits, maximum heritability occurred in the late stage of lactation. The results confirm that the RR-TDM approach is suitable for modeling production traits and for describing changes in genetic variation during lactation.