Loco

I spent part of Friday evening in the 5-degree cold watching my friend Susan feed her horses, and I noticed that she added to their feed a scoop of vitamin E and selenium powder.
She explained that both are key nutrients in a horse's diet, and said she had heard years ago that a selenium problem in horses had somehow played a role in the 1876 defeat of  Lt. Col. George Amstrong Custer at the Battle of Little Bighorn in Montana.
I checked this out online when I got home (and my fingers warmed up enough to type), and it appears that an excess of selenium may have been to blame.
According to Karen E. Davison, manager of equine technical services for Purina Mills:

Custer’s demise at the Battle of Little Bighorn has also been attributed to selenium toxicity by some accounts. Some believe that Custer would not have been defeated at the Battle of Little Bighorn if the troops of [Capt. Frederick] Benteen and [Major Marcus] Reno had arrived in time. One report indicated that the horses and mules in the pack train with Benteen and Reno were lame and acting crazy, which delayed their arrival. The lameness may have been caused by selenium toxicity but the odd behavior was more likely caused by swainsonine, which along with high selenium levels, is found in some types of locoweeds that grow in the region. So, Custer’s last stand might have been avoided if Benteen and Reno had known their animals were grazing on locoweeds.