What is he lacking?

[loki888]

Could be boredom, could be minerals, could be one of those weird horse habits lol. One of mine kept licking dirt around the water trough for months.

After checking the diet more closely and running through a nutrition review, I started using Omneity from Mad Barn and the behavior became a lot less frequent over time. Never figured out the exact cause though.

 

[Sook38143]

With spells that short, I'd leave it unless you notice other signs he's missing something. If you do want to try adding salt, mixing it in might work better than fighting him like you did with Robin.