Genomic breeding value for clinical mastitis in Czech Holstein cattle

Genomic breeding values for resistance to clinical mastitis (KM) (GEPH_KM) have been estimated using a multi-trait model including linear udder traits and somatic cell count. 13,794 Holstein cows were used, reaching 18,570 lactations with a lactation incidence of clinical mastitis for all lactations 16.95 %. All cows had the first lactation and possibly the second or third. They calved between 2017 and 2020. KM was defined as the incidence of yes or no during the lactation, the number of somatic cells as the average value for lactation. The linear type traits were udder depth, udder width, suspensory ligament and the subjective score for udder in %. Genetic correlations between traits and coefficients of heritability have been estimated. The heritability of KM was 0.04, a number of somatic cells expressed as score 0.11; udder width 0.16; suspensory ligament 0.19; udder depth 0.38 and udder 0.22. The genetic correlation between somatic cell score and KM was 0.93; the genetic correlations of KM and linear type traits ranged from 0.30 to 0.41. The single-step genomic method was used for the prediction of GEBV_KM. The pedigree included 67,078 individuals and 3,625 genomic bulls. The results showed that GEPH_KM distinguished between animals with high and low genetic backgrounds for KM with sufficient reliability. For genomic bulls (3,625 animals), reliability was on average 0.33; for cows with the health phenotype (13,794) 0.30. For genomic bulls, the lower quartile of GEPH_KM Q0.25 was represented by -0.009 and the upper quartile Q0.75 by 0.033; for cows with health phenotype, these values were -0.013 and 0.036. The average reliability corresponding to GEPH_KM up to Q0.25 or above the Q0.75 range was 0.35 and 0.30 for bulls, respectively. For cows with the health phenotype, the values for quartiles of reliabilities were 0.29 and 0.31, respectively. With the reliability of genomic breeding values from 0.40 above, Q0.25 GEPH_KM was -0.02, and Q0.75 0.03 for bulls; for cows with phenotypes, 0.01 and 0.06, respectively. Average reliability increases with the year of birth of the animals to reach the maximum for bulls in 2013 (0.39; 0.45 genomic bulls) and cows in 2016 (0.31). Information on the health phenotypes of cows is only available in recent years. Average GEPH_KM for both bulls and cows were higher than 0.01 between 1990 and 2000. Since 2000, a decrease in the average GEBV_KM of bulls was observed in our results.