Association between digital dermatitis lesions and test-day milk yield of Holstein cows from 41 French dairy farms.

The objective of this study was to estimate the association between digital dermatitis (DD) lesions and test-day milk yield (TDY) in dairy cows, taking into account the severity of the lesions. Data were collected for 6 mo on 47 French dairy farms endemically affected by DD and involved in a clinical trial aiming to assess the effectiveness of collective treatments against DD. The hind feet of all lactating cows were scored for DD by 14 trained investigators on a monthly basis using a 4-point M-stage scoring system (M0 to M4, M standing for Mortellaro). The DD status was defined in 3 categories at the animal level: no DD [scores of M0 and (or) M4 on both feet], moderate case (score of M1 on 1 or both feet and no M2 score), and severe case (score of M2 on 1 or both feet). All monthly TDY in the lactation were collected. The final complete data set included 7,599 TDY of 1,782 Holstein cows from 41 herds. The effect of DD lesions on the following TDY (i.e., within 30 d after detection of a DD lesion) was analyzed separately for primiparous and multiparous cows, using mixed-models ANOVA, with TDY as repeated measures. During the trial, 38% of the primiparous and 41% of the multiparous cows were observed at least once with a DD lesion (moderate or severe case), the cows being observed with a DD lesion, on average, for 2 consecutive visits. Milk yield decreased significantly for cows diagnosed with a DD lesion. Primiparous cows produced, on average, 0.63 kg/d less when DD was moderate and 0.50 kg/d less when the disease was severe, compared with unaffected cows. Multiparous cows produced, on average, 0.50 kg/d less when DD was moderate and 0.75 kg/d less when the disease was severe, compared with unaffected cows. These results confirm that DD lesions have a significant effect on the milk yield of dairy cows, including when animals are rigorously treated. Milk yield losses, thus, should be considered when evaluating the costs and benefits of DD control programs.