The California Dairy Quality Assurance Program (CDQAP) worked with leading academic experts at the University of California to develop the 40-page Dairy Welfare Evaluation Guide.
You can download CDQAP's"Animal Welfare Module: The Science of Proper Cattle Care & Welfare”.
Here's what the guide says on pages 10-11 about tail docking of dairy cows:
"Tail docking (removal of the lower portion of the tail including the switch) must not be routinely performed on the dairy herd. If performed due to tail injury, docking should be under the direction of a licensed veterinarian on an individual cow basis. There is no benefit to tail docking normal, healthy tails in dairy cattle based on peer-reviewed scientific studies and governmental-sponsored research. Commercial dairies may have high fly densities especially during the warm season. The tail serves as a "fly-swatter,” so tail docking is detrimental to welfare and comfort.
The available scientific data do not support claims that docking improves the dairy personnel's comfort during milking procedures or lowers their risk of leptospirosis. Additionally, while docked cows may be cleaner, their udders were not cleaner nor did cows have lower somatic cell counts or frequency of mastitis than undocked cows. Switch trimming may provide a compromise to milking personnel's comfort by trimming the switch in the winter when the tail is most likely to be dirty, and allowing the switch to grow back during the spring and summer when fly predation is greatest.”