GENOTYPIC AND PHENOTYPIC ASSOCIATIONS WITH DIGITAL CUSHION THICKNESS IN DAIRY CATTLE

Abstract

The bovine digital cushion is a compression pad between the distal phalanx and sole. Digital cushion thickness (DCT) is associated with lameness and claw horn disruption lesions (CHDL) and estimated to be moderately heritable. This dissertation focuses on understanding DCT phenotypically and genotypically to evaluate whether DCT has an opportunity to be utilized for indirect selection against lameness and CHDL. To investigate whether DCT varied between digits, across lactation within the cow, or could be a predictor of CHDL or lameness, we collected DCT measurements at 4 time points across lactation for 183 Holstein cows from one farm. Digital cushion thickness varied within primiparous and multiparous cows based on stage in lactation and digit. Parity group and early lactation DCT measurements were predictors of CHDL and lameness during the subsequent lactation. Applying our results, we sampled 502 Holstein cows from 5 farms at 2 time points corresponding to when DCT was thickest and thinnest. The phenotypic results indicated DCT varied by time point, sacral height, parity, claw, farm, body condition score group (BCSG), and wither height. Genome-wide association studies were conducted to explore associations of genetic markers with DCT. From the associated markers, MC4R and DLG2 were identified as putative genes related to fat deposition and bone growth. To characterize DCT across multiple breeds and sexes, we sampled 698 cows and 85 bulls (Holstein and Jersey) from 8 farms. The phenotypic results indicated DCT for cows varied by breed, age, and digit; DCT for bulls varied by breed, age, digit, and BCSG. Genome-wide association studies were conducted on 9 datasets either separating or combining breed and sex. From the associated markers, MC4R, SFRS18, and LRRFIP1 function in fat deposition, DLG2, AHR, BZW2, EFNA5, USP45, and VAV3 in bone remodeling, and SOSTDC1 in epidermal keratinocyte function. Breeding programs can incorporate the markers from this work for marker assisted selection to reduce CHDL and lameness. Producers can apply the phenotypic results to adjust management practices or monitor animals with higher odds of developing CHDL and lameness. The results from this work will hopefully lead to future studies to identify the causal variants of DCT.