Grass hay and all grain are deficient in Calcium. Both green and cured grasses lack Calcium -- some worse than others. Cereal grains and grasses like winter wheat, rye grass and 'lush' grass in the spring can be so deficient in Calcium and Magnesium that you see fatal cases of Milk fever and grass tetany in cattle. Alfalfa 'usually' is not but can be. It all depends on how depleted the soil it that it is grown on.
Pica - eating wood, dirt, trees, tails, etc is the main symptom of a mineral deficiency and it is usually Calcium and frequently Magnesium. Colts, horses and calves that eat manes and tails are always deficient in minerals. Horses can chew wood if they are confined or have other problems like ulcers. But, when they devour wood and swallow it like they are hungry for it, they are missing something in their diets.
Magnesium is needed for the assimilation of Calcium, so Calcium is useless in a diet if Magnesium is missing. Vitamins A and D are also necessary to utilize Calcium, Phosphorus and Magnesium. I do not even take Calcium tablets myself that do not have Magnesium in them. [I take Citrical Plus.]
Since I have always lived where there was depleted soils, I have always had to feed grass hay that was very low in Calcium. And, Like I said before, all grain is deficient in Calcium, no matter where or how it is grown.
I use a loose mineral that is about 24 -25% Calcium and 2% Magnesium. It is 5% Phosphorus and 22% salt. We used to breed a lot of outside mares to our stallions and nearly all of them came in mineral starved. Some would stand around and eat mineral like it was grain. I had a lot of people tell me that when their horses got home, it was the only time they had not eaten trees and wood. It was simply the first time they were not starved for Calcium.
If you have a doubt that this is the problem, cut down some young green saplings -- willow, cottonwood and poplar are the best. Throw them in a pen with all of the leaves and twigs attached and see how long it takes your horses to eat them up.