There are a number of tropical pasture species that contain large quantities of calcium oxalate. And even lucerne (alfalfa) if fed as a large proportion of the diet can supply a lot of oxalates.
Too much oxalate creates symptoms that often look like calcium deficiency (shifting lameness, short stepping gaits and sometimes enlarged facial bones). The enlraged facial bones are unlikely to develop in older horses so look out for the other symptoms.
We have conducted extensive trials on horses in Australia and actually concluded that in reality it is a calcium regulation disease not a deficiency and we actually treat it by reducing the calcium in the diet and replacing it with a SMALL amount of chelated calcium.
Interestingly we even see this problem without tropical grasses and they occur with very high calcium soils. So, if you have poor mobility and a grumpy horse talk to us about this problem.