I ran across this article the other day and had some pretty intense "hmmm" moments. Especially when I read how little protein in hay is bioavailable. It explains so much!
Anyway, I was reluctant to share it because some folks may throw the whole article under the bus due to the reference to soybean meal as a dense protein supplement, but if you don't or won't feed soybean meal, overlook that and evaluate the take-home message. Chronic Protein Deficiency In Horses