If it were a bit earlier in the year you could pick it yourself and save a fortune, it grows in abundance near us . Actualy saying that iv'e just looked out the window and i think there is some still growing. hang it up to dry somwere cool and dry first. Idont know the amounts to feed tho.
Comfrey is not poisonous to horses. I fed my big chap it when we fell over and he chipped his knee. Its brilliant stuff. The other name for it is knitbone. Because I also did some serious damage to my collar bone I used to put some in a mug poor boiling water on it then strain and drink it
A1fie, you are sort of right. Comfrey is fine to feed horses but it is only recommended to be fed for short periods, not long term when it can be harmfull. For just a few months or so following an injury it is fine.
Comfrey IS poisonous (damages the liver) if fed for prolonged periods. It was originally only used as an external poultice. You would probably get away withi it for shorter periods, but I wouldn't use it for chronic arthritis.