Heritability of digital dermatitis in Polish Holstein-Friesian cows

Introduction
Lameness is the one of the most important reasons for involuntary culling in Polish dairy farms. The importance of hoof health increased over the time because it results in economic losses due to decreasing milk production and negative consequences on fertility. Environmental factors have the largest impact on the occurrence of the diseases however susceptibility to diseases has a genetic component (Heringstad et al. 2018). Therefore hoof health related traits were included in breeding goals in many countries in recent years (Jamrozik et al., 2018; Perez-Cabal and Charfeddine, 2015). Long term selection for resistance to hoof lesions may reduce the prevalence of them, despite the low heritability. An increased accuracy of estimated breeding values requires high quality data therefore ICAR prepared guidelines with recommendations on recording claw trimming data. To include claw health traits in the national breeding goal, the Polish Federation of Cattle Breeders and Dairy Farmers developed the web-based application “CGen trimming” to record claw health data during trimming sessions. The aim of this study was to estimate heritability of digital dermatitis, which is the most frequently registered claw disease in Polish Holstein-Friesian cows. 

Materials and Methods
Data collected by hoof trimmers from October 2017 to September 2018 were used in the analyses. The data included 42 425 claw health records from 24 714 cows recorded at 1089 trimming sessions in 181 herds. In Polish recording system codes for all claw disorders were implemented from “ICAR Hoof Health Atlas”. Records with no valid animal ID or missing birth data were removed. To avoid problems with preselecting cows which were trimmed, data were restricted to herds with at least 50% of the cows trimmed (Perez-Cabal and Charfeddine, 2015) and 10 or 10% healthy cows reported (Ødegård et al. 2013). To ensure reliability of data claw health information recorded by trimmers with less than 1000 records were excluded. Only records from calving to 365 days in milk (DIM) in the first three parities were subject to analysis. The cow was defined as affected by digital dermatitis (DD) when the claw disorder was present at least once during lactation. Stage of lactation (months after calving) and season (calendar months) were defined based on date of the first occurrence of DD or date of the first trimming (for healthy cows) in lactation. Three classes of age at calving were created: below 23, 24-25 and over 26 months for cows in the first parity; below 36, 37-39 and over 40 months for cows in the second parity and below 49, 50-53 and over 54 months for cows in the third parity. Only records of the cows which were the progeny of sires with at least 5 daughters were included in the analysis. The final data set contained records from 13 014 cows, which were daughters of 722 sires and which calved in 109 herds. Single trait animal model was used for analyses. The fixed effects of herd, trimmer, stage of lactation, month of trimming and month of calving were included. Fixed effects and potential interaction effects were tested for significance in R (Anova). The variance components were estimated by REML, using AI-REMLF90 (Misztal et al. 2002).   […]