I've never heard of 'hollow tail' as a disease. However, the symptoms as described (which do not fit the loss of this tail) do fit with fescue toxicosis which is caused by alkaloids produced by the endophyte specific to KY31 tall fescue. The dry gangrene can result in withered body parts that potentially can be hollow from withering/tissue shrinkage after tissue death. Normally tail switches (not the whole tail) top half of ears, and feet. 'hollow tail' is an apt description for this condition, and often low/no weight gain or weight loss is associated with the condition as well, and a low grade fever. A change of feed will solve many of the symptoms. Won't make the body parts grow back though. Put salt in the tail, OUCH! unless the tail is dead already.