Selenium intake must be balanced because excessive intake of it is toxic. Therefore, I would suggest you to visit a vet to know and feed according to his body and nutrition requirement.
_________________ Selenium Speciation
When I lived in OR and worked a couple different barns all the horses got North West Supplement. It was pelleted feed formulated for horses living in the NorthWest because of the selenium deficiency up there. Don't know if they make it anymore, this was in the early 90s.
I also calved heifers up there and we gave the calves a Vit. E/Selenium shot as soon as they hit the ground. Selenium deficiency in calves causes growth retardation, muscle atrophy, inability to gain and even collapse and death.
They do still have a NW Supplement CowChick! I don't use it solely because my horses are all on Dynamite and so it would be an unbalanced thing to add that in as well.
It's been a few months now on the selenium and there has been a marked difference in the muscle tone, balance/movement and weight of my horse. I'm pretty confident the selenium deficiency was the cause.