And don't believe everything you read on a website.
I didn't say that using flax will kill a horse form the cyanide but I'm just making her aware of it.
A direct quote from Equine wellness magazine July/Aug 09, From veterinarian Dr. Hannah Evergreen :
Q: Is it true that the levels of cyanide in flax can harm my horse?
A: It is true that flax contains cyanide. If fed in proper amounts, however, flax is not harmful to your horse. Studies have shown that high volumes of flax oil can cause diarrhea and colic signs in horses, but it is safe at the recommended levels of 1/3 to 1/2 cup ground flax seed per day.
Another side I found, Ask the Expert -- Supplements
"Q:I have fed flax seed in the past: cooked whole seeds, then ground raw flax meal. I have always gotten 25 -50 lb bags. The last time I picked up a bag, I noticed that the label stated, “Not to be fed to horses intended for food. Not for human consumption.” What is the story? Is it toxic raw or ground? Is it worth feeding?"
A: "I know many people who do swear by flax seed, however, I do not have any personal experience using it. The raw seeds contain cyanogenic glycoside compounds which, when the seeds are ground, and especially if moistened, will release cyanide! However, this really is not a concern, since the enzymes needed to cause the cyanide release are inactivated by boiling and gastric acids. So, as long as you don't add water before feeding the flax meal, it should not be a problem. If the meal is processed in any way it is also probably totally safe. Read the label again to see if there are other substances added. If not, the warning is probably due to the minuscule risk of cyanide poisoning. But it shouldn’t hurt in small enough quantities.
The presence of cyanogenic glycosides in the diet is significant only in relation to dose and the nutritional status of the consumer. Flax seed meal contains two cyanogenic glycosides, linustatin and neolinustatin. Many foods are slightly cyanogenic (e.g. wheat and barley!), probably as an evolutionary adaptation to discourage herbivory, and our body and our horses have a limited capacity to detoxify low concentrations of cyanide through addition of sulphur (from amino acids). Thus, if the dietary levels of sulphur-containing amino acids are high, the body can resist a low intake of cyanide, but if the diet is low in protein overall, then we see toxicity.
It has been determined that up to 50g high-alpha-linolenic acid flaxseed is palatable, safe and may be nutritionally beneficial in humans by raising n-3 fatty acids in plasma and erythrocytes and by decreasing post-prandial glucose responses. (Cunnane S, et al, (University of Toronto, Toronto, Canada) Br J Nutr, 69:443, 1993)"