Sorry, only just spotted the new posts on this thread.
Selenium and vitamin E are both anti-oxidants, but they work in different ways, so both are needed.
Selenium is the one which can have the most powerful effect, even if vitamin E is deficient, but both are needed.
Vitamin E can be used by the body exactly as it is, without having to metabolise it at all. It works to protect the muscle cell membrane from damage.
Selenium has to be metabolised into selenoproteins like the enzyme glutathione peroxidase before it can act as an antioxidant, so it takes time to work. Glutathione peroxidase (shortened to GSHPX)is found within the muscle fibres, and it works to neutralise the damaging products of oxidative stress.
So in other words, vitamin E is a preventative. Selenium works once the damage has been done.
Vitamin E is found in good amounts in fresh grass, so as long as an animal is eating plenty of grass, there should be no need to supplement vitamin E. However, vit E does not last well in stored forages, so if hay/haylage forms a large part of the horse's diet, it is necessary to supplement vitamin E. Vit E is available in different forms, and is it the D-alpha tocopherol which is the most bioavailable form.
Selenium yeast is produced by growing yeast in an environment containing inorganic selenium. So as the yeast grows, it incorporates the selenium into its structure. The selenium thus becomes organic, and in particular it becomes selenomethionine (which is the form found in grass, and so is the "natural" source of selenium for grazing animals).
The amount of organic selenium contained in selenium yeast varies. More reputable producers will have around 70% organic selenium in selenium yeast, with only about 30% remaining as inorganic, unbound sodium selenite. However, one study I read found one source of selenium yeast in which it was literally just a mix of inorganic selenium and yeast. As the consumer, we can't tell whether it is what it says on the label. Therefore, I'd always stick with a reputable producer. Alltech is the key company, and they produce Selplex. I'd be happy with that.
If you buy selenium yeast to feed, you will need to accurately weight out very very small amounts to feed. That's what I'm doing at the moment as I'm topping up what my balancer provides (only doing this because of their muscle problems). I am weighing out tiny amounts - 0.3g a day for one pony; 0.7g for another. It's a fraction of a teaspoonful; more like a sprinkling of salt. An overdose can cause serious problems. I use a gram scale to measure it out accurately each day. It's a faff!
However, it's also a lot cheaper to weight it out than feed a bulked up selenium supplement. I would never ask anyone else to weigh out the selenium yeast though. I'd worry about someone giving a spoonful by mistake, and making the ponies seriously ill.
So the selenium yeast works out very cheap, but it does require accuracy in weighing. Bulked up supplements are easier/quicker to feed, and less likely to results in an accidental overdose.
I choose to supplement vitamin E and selenium separately to each other. That way I can tailor the amount of each.
For example, I feed up to around 5,00iu/day of vitamin E to my worst affected pony. The NRC "safe" limit is 10,000iu/day and some people go up to that amount, but that limit is based on other species, and I have no wish to go anywhere near an upper safe limit. It's more usual to provide around 500-1,000iu/day.
With selenium, there's no way you'd want to add 10X the recommended minimum amount. It would be way too much, so you couldn't use a mixed supplement to up the vitamin E without taking the selenium too high.
I've waffled on a bit, but hope some of it makes sense.
Organic selenium is the way to go, though! Just not worth bothering with inorganic (ie sodium selenite) going by the results with my ponies over the last couple of years.... as well as what's known about the higher bioavailability of selenium yeast.
Sarah
