Originally Posted by riccil0ve
The pacific NW is notoriously low in selenium. Studies have shown it causes a lot of deaths by colic.
It is important to keep in mind though that the range from too little to too much is very slight.
I use this, it's made specifically for this area and requires very little. Horse Guard Posted via Mobile Device
Horse Guard is exactly what I was going to suggest. It's ~$.45/serving IIRC. Another option would be to feed a ration balancer- it may cost a little bit more (depending on which one you buy; I feed my horse Triple Crown 30% Supplement, which comes out to $.78/serving. There are others that are cheaper as well.) but offers more complete nutrition.
Most fortified grains, ration balancers and vit/min supplements have added selenium, so I'm guessing if he's deficient he's not getting one of those already.