Check your feed because I have written to feed companies asking if they have an oat based feed or a feed without soy. I could only find whole oats and everything else has soy, soy hulls, soy meal and soy oil. It is the new bulking ingredient. It is difficult to find a pelleted feed without it.
My mare is a maiden, 6 years old, never foaled.
I wondered if anyone ever had a problem they resolved by eliminating soy in the diet?