Toesnails dry and brittle - what causes this?

Dry or brittle nails could be a sign of a lack of silica. Silica can
be taken in the form of Silicea, which comes in a pure form like a gel that you put into water and drink.
I was taking some of this, although not for nails, but for strengthening
connectivity tissue, due to recently having a hernia (which I had
repaired by surgery on 18 Dec 2007).

we give the horse zinc and biotin, for cracked hoofs.its a deficiency issue.maybe same for you