Vitamin A is the only really one that comes to mind, but it is such general knowledge and most sources (for people and animals) are in the form of betacarotene. When the body needs more A, it will break down the betacarotene. You will turn bright orange before you overdose on betacarotene, it's a very safe way of getting A.
Thanks, Flash! So where would you see the orange on a horse if they were getting too much? On the sclera of the eye, perhaps? You can see jaundice there...maybe it works the same way.
A, D, E, & K are all fat soluble vitamins....so the ones possible to overdose. Not that too much of the water soluble vitamins can't cause problems too, but its less of a problem. And we hear a lot of cautions about getting "too much" selenium....so if anyone knows how much is too much of any of the above, I'd appreciate the info.
"The maximal tolerable level of selenium in horses is estimated at 2 mg/kg of diet (NRC, 1980), and the LD50 for orally administered selenium is considered to be approximately 3.3 mg of selenium (as sodium selenite)/kg of body weight (Miller and Williams, 1940). Copper pretreatment can increase the LD50 markedly (Stowe, 1980)."
And, according to the chart on this page, Vitamin A is toxic at 200,000 IU, D is toxic at 18,000 IU and it's unlikely that you could give enough E, B1 or B2 to be toxic.
Found this info in a 5 minute google search on equine vitamin A toxicity.
Even though the "toxic" levels of Vitamin A are listed as 200,000 IU/day and Vitamin D are listed as 18,000 IU/day, remember that when doing horse rations it is recommended not to exceed 100,000 IU/day of A and 10,000 IU/day of D.
Vitamin A above those levels can interfere with calcium absorption and create a calcium deficiency, which then leads to the horse pulling calcium from the bone for basic metabolic processes. This can then lead to osteoporosis in our horses.
Vitamin D in excesses over 10,000 IU/day can lead to soft tissue calcification which then leads to arthritis in our horses. Just a little more info on the subject!
Ditto on the selenium. While 2 ppm of the total diet is the "maximum tolterable" (by which I gather they mean something along the lines of "there ain't a horse alive that can tolerate it"), the recommended range is MUCH lower: .1 to .3 ppm of diet. According to Karen Briggs, in Canada it is mandatory that feeds containing supplemental Selenium print the warning: "Do not permit intake of supplemental selenium to exceed 0.3 in the total ration (hay or forage plus grain)." According to a study cited in the NRC, supplementation of 1 ppm/day for horses 1 to 6 years of age produced Se blood levels in some of the horses "considered well above the concentration associated with myodegeneration." (p. 17).