I’m still obsessing over the whole selenium thing, and have been reading a lot of research papers and scholarly articles. It does appear that performance horses, despite all the dire warnings about toxicity on feed labels etc., need substantially more selenium than idle horses. Just exactly how much extra is not clear though, and toxicity definitely is possible at not a whole lot more than the required amounts. Supplemental amounts depend on the selenium availability in local soil. Around here, in southern Ontario, selenium is pretty much zero. So we definitely need supplementary selenium. And most likely in somewhat more than conservative levels. Which means that you pretty much have to monitor selenium levels with regular blood tests (oh and there are two different tests… serum and whole blood – I’m still trying to figure out which is better under what circumstances and why).
Selenium bioavailability is also an issue. Organic selenium is better than sodium selenite, and this has been proven in multiple studies. I’ve only found a few supplements that are available locally which contain the organic type (selenomethionine, or selenium yeast) though . So far I’ve identified Sel-E-Nium (Herbs for Horses), Selenium (Riva’s Remedies), Elevate SE (Brooks Feeds). There is also a company out west that has one, but they don’t have dealers in Ontario. There are quite a number of organic selenium supplements available by mail order of course, mostly in the US.
Some of the best articles/studies I’ve found (or at least the most pertinent and understandable to my non-scientific brain!) have been from Kentucky Equine Research (who also produce the Elevate SE supplement sold by Brooks).
This article suggests that while 1mg per day is sufficient for mature, idle horses to prevent deficiency, 2.5 to 3.5 mg per day is a more reasonable dose for performance horses. Then later in the article she states that anything from 1 to 10 mg total from all feed sources per day is safe to feed, but to stay below 20mg which may be a toxic dose.
Selenium: How Important Is It?
Here is a study from Kentucky Equine Research that shows that after exercise, horses that are supplemented with Sodium Selenite excreted more selenium in their urine than they did during non-exercise days. Horses supplemented with organic Selenium (yeast based) did not excrete extra selenium after exercise.
Effect of Selenium Source on Selenium Digestibility and Retention in Exercised Thoroughbreds
In both selenium types, plasma and whole blood selenium levels went up immediately after exercise, and remained elevated after 4 hours post exercise. 24 hours after exercise, horses supplemented with Sodium Selenite dropped to lower blood/plasma levels. Horses supplemented with organic selenium remained elevated.
I’m going to quote a paragraph from the next study…
“Inorganic and organic forms of dietary Se supplements are metabolized differently (Surai, 2006). Selenomethionine is actively transported through intestinal membranes during the absorption process and non-specifically incorporated into tissue proteins in place of Met during protein synthesis (Schrauzer, 2003). In contrast, inorganic Se is absorbed via passive diffusion and little is retained in tissue reserves. Consequently, a large proportion of inorganic Se is excreted in the feces and urine (Wolffram, 1999). ”
Effect of Selenium Source and Dose on Selenium Status of Mature Horses
There is a whole lot more information in that study that I really don’t understand too clearly. But it does seem obvious that organic selenium (selenomethionine or selenium yeast) is processed more easily and will correct deficiencies more quickly than inorganic selenium (Sodium Selenite).