I would discuss this with your vet. If you want, you can have them draw blood and do a test to sew what your horses Selenium Levels are.
I wouldn't give Selenium without having them tested for it. As you know, it can be toxic.
The colic, more than likely was caused due to other reasons, not due to Selenium. Horses digestive systems are designed and require consistant access to roughage. The roughage aids in keeping their systems regular and regulated.
Horses have delicate systems that were designed for constant pasture grazing, but modern horsekeeping can throw your horse's gastrointestinal tract off balance. High-grain diets combined with the stress of training and competition may leave your horse prone to ulcers, digestive problems and poor overall health. But if you build your horse's diet based on his individual needs, you can help bring him back into balance.
Throw hay, dump grain, turn the horses out. Do stalls, ride, bring the horses in and feed. Sound familiar? If you're like a lot of horse owners, it probobly does. What is doesn't sound like is the way our horses were designed to live. In fact, modern horsekeeping is just about the opposite of what Mother Nature had in mind.
In their natural habitat, horses spend up to 20 hours a day roaming and grazing on a variety of forages. Their digestive systems have evolved to rely on a slow, steady intake of complex carbohydrates, like grasses. As a result, their stomachs constantly secrete gastric acid, whether they're eating or not.
As horses chew, they automatically produce saliva, which contains bicarbonates that act as a natural buffer against the acids in their stomach. Feeding infrequent meals leaves your horse's stomach empty for long periods of time. Some barns spread meals out by adding a "lunch". This additional serving definitely helps, but it's important to remember what is happening in your horse's stomach between meals.
With the constant secretion of gastric acid and no saliva to buffer it, an empty stomach is at high right for gastric ulcers. This painful condition can affect your horse's appetite and digestive function, leading to weight loss, and unthrifty appearance, decreased performance, a poor attitude and even colic.
Fianally, the equine hindgut was designed for continuous fermentation throughout the day. Fasting and then feasting can cause digestive upset, disrupting the delicate balance in the large intestine, which can result in a number of problems, including painful build-up of excess gas. The modern diet was not designed for your horse's optimum health.
The majority of every horse's diet should consist of roughage, like hay or fresh pasture. In fact, your horse should be eating 1-2% of his body weight in hay or other forage every day. For a 1,000lb horse, that is 10-20lbs daily! Ideally, your horse would have free choice, but if that's not possible, try to spread out his meals throughout the day.