Phenotypic characterization and genome-wide association studies of digital cushion thickness in Holstein cows.

Digital cushion thickness (DCT) at the typical sole ulcer site beneath the distal phalanx is a strong predictor of lameness and the claw horn disruption lesions (CHDL’s) of sole ulcers and white line disease in dairy cows. These CHDL’s are the most prevalent claw diseases associated with lameness and pain. Previous studies are inconsistent as to the thickness which is optimum. Apparently thin digital cushions predispose animals to lameness and lesions later in lactation. However, DCT varies by digit and particular digits of certain thickness increase the odds of lameness or non-infectious lesions in lactation. It appears that selecting for healthier digital cushions could have a faster rate of genetic gain and greater effect on decreasing the occurrences of CHDL and lameness on farms than selecting against certain lesions separately, due to the higher heritability of DCT than CHDL and lameness determined by visual locomotion scores. To take this further, the objectives of the study by Dr C.R. Stambuk and colleagues were: (1) to examine influences of management and environment on DCT across multiple farms, and (2) to identify genomic regions associated with DCT and how they might differ by foot, digit, or stage of lactation. The results of the study were published in the Journal of Dairy Science, Volume 103 of 2020, page 3289 to 3303, and the title of the paper was: Phenotypic characterization and genome-wide association studies of digital cushion thickness in Holstein cows. 

In a study with 502 Holstein cows from five farms in New York State, DCT and body condition score (BCS) data were collected twice, at  less than 137 days pre-calving and from 86 to 127 days in milk, corresponding to periods when the digital cushion is thickest and thinnest, respectively. The digital cushion of the cows was sonographically examined at the typical sole ulcer site for the right front and hind foot. The fixed effects included in the statistical analysis of DCT were time point, digit, wither height, sacral height, BCS group, and multiple farm system variables and the random effects were cows per farm, repeated measures, and multiple measurements from each cow.                                                                                                                                          

The results showed that DCT varied by sample time point, sacral height, parity, digit, BCS group, and wither height. For the genotypic study, 431 samples and 579,449 markers were used. A genome-wide association assessment was conducted for DCT inheritance and genetic variation of digit, foot, time point, and average thickness. Ten candidate genes were identified, with two genes on Bos taurus autosomes 24 and 29 involved in biological functions related to the digital cushion: MC4R and DLG2 were related to fat deposition and bone growth, respectively.

The genetic markers discovered in this study can be used in breeding programs using genomic selection to select against claw horn disruption lesions and lameness due to associations between the markers and DCT. Future studies on the biologically plausible candidate genes may identify causative genetic variants and how they relate to DCT through gene regulation, expression, structure, or copy number variation.