Their diets are in the initial post. Neither is on a whole grain. Annie is on a 10% pellet feed, and abby only gets alfalfa cubes to mix her supplements in, no grains at all. Annie has a mineral block, abby has just a salt block.
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Maybe you need to switch to a feed that gives a higher level of nutrition has all the vitamins and minerals already added if you feed to recommended amount but is low starch/sugar - something like Triple Crown Lite and add some non molassed sugar beet to it. You could then stop with the extra supplement you're giving them.
I would at least put a mineral block for Abby and see what she does with it.