I don't have the answer for why your horse is eating mud, but I experienced
the same thing with CBS Redman. Especially after a race, he would eagerly
eat mouthfuls of mud from a few favorite spots on our training loops. As
many of you have already found, many horses will drink from a warm mud puddle
better than a running stream. The mud and muddy water has something they
want or need. As a little experiment, I filled a jar with some of Redman's
favorite mud and brought it home for all the other horses to sample. All but
one of 10 or so Arabians ate it, some eagerly, others gingerly.
Redman always had the best of everything, including free choice minerals
available to him, so I just assumed he knew better than me and didn't worry
about the mud. This isn't an answer, but at least you know you're not alone.