Although the NRC421 suggests that horses are about as sensitive to oral Se as cattle, sheep, and goats, our research indicates that species sensitivity is horses > cattle > sheep and goats. Experience at several regional diagnostic labs indicates horses may be poisoned while ruminants using the same forage and water remain unaffected.426,508,509 With the exception of one study in antelope, there is insufficient dose-response data upon which to base safety recommendations in large mammalian wildlife. That said, there are reports of elk and deer sharing pastures with horses, sheep, and cattle, where the horses developed alkali disease, without any measurable ill-effects in the elk and deer.510 Thus, water that is safe for horses should be safe for other livestock and ruminant wildlife species.
The effects of water-borne Se are, like many other elements, additive with feed content. The chemical form of Se in surface waters is predominately SeO32- or SeO42-, which is fortunate as these ions are the forms most thoroughly researched. In theory, relatively small concentrations of Se in water may be sufficient to push animals on moderately high Se forages over the edge of toxicity. Unfortunately, the Se content of forage and hay in this region varies from marginally deficient to downright toxic, and other dietary factors such as protein, vitamin E, or cyanogenic glycosides may modify the effects of a given concentration of dietary Se. For purposes of this report, we assumed a "typical" forage containing 1 ppm Se (mostly as selenomethionine), normal protein, vitamin E, and other trace element concentrations. The threshold for chronic poisoning in horses from the literature is 0.05-0.1 mg/kg BW/day. This agrees with unpublished observations from our laboratories. Thus, water that contains 0.25 mg Se/L, consumed at a rate of 10% BW, combined with "average" Se forage, would constitute a potentially hazardous dose. In extremely hot weather, working horses drinking 20% BW of 0.125 mg Se/L water (a very conservative assumption) would receive a hazardous dose.
In areas where forage Se concentrations are higher, or if horses are receiving dietary supplements that contain Se, safe water concentrations will have to be adjusted downward, but under normal conditions, 0.1 mg/L should not cause problems.