Can anyone tell me what the symptoms would be if you were feeding your horse too much selenium? Information on SE seems to be all over the map; from different sources I have been told 1-2mg per day, another source says 3-5mg per day for "athletes or larger breeds". My mare is a QH/Belgian cross, 16+H and weighs 1300#.
In acute poisoning cases, symptoms include blind staggers, ataxia, and death. Horses ingesting chronically high yet non-lethal levels of selenium over time often present with dull appearance, hair loss, intermittent lameness, and hoof problems.
Those maps are not the most accurate way to determine if you need to supplement selenium. One example from personal experience:
I worked at a breeding farm where we got a fancy, fancy young broodmare for a steal of a deal because of sub-acute selenium toxicity, and it was because her original owner supplemented based on a map. Mare was sold as a broodmare prospect because she lame on and off and the owner was told she'd never pass a vet check. Mare came to the breeding farm with almost no main/tail, "founder rings" on her hooves, and a HUGE tub of selenium. We never gave her any selenium and all her issues resolved in a matter of months. To this day I still feel almost guilty; the original owner basically gave away a VERY nice horse because of selenium.
Don't fall for a girl who fell for a horse just to be number two in her world... ~EFO
The ONLY way to know if a given horse needs more Se, or needs a Se-limited diet, is to test his blood. You can know in general if the area you live in is Se-deficient or -abundant, but the horse has the final say as to what his body is going with whatever Se is available. I live in a pretty deficient area, but my horses all test normal on the diet they have.
Toxicity issues are the same as deficiency issues, with the symptoms being worse the more to the extreme you are - they are as tex defined, with the addition of feet cracking at the coronet band.
The *general* recommendation is about 3mg/day, a bit less for small ponies, a bit more for really large horses.
But, worry far less about that than what the horse's blood says. Now, obviously, you should worry about that 3mg if you decide to feed all sorts of feeds and supplements that put you feeding 6mg, for example, and you have no idea what the horse is currently doing.
______________________________ The CoTH CYA - please consult w/your veterinarian under any and all circumstances. - ET