Originally Posted by equitate
Flax seed easily goes rancid if pre ground. It does have cyanide compounds, especially when consumed raw, but that seems to also make it effective for certain skin growths.
Just as a side note- you'll see some references on the internet to either soaking or boiling flaxseed to get rid of the cyanide compounds. While it's true that flaxseed does contain components of cyanide, they're present in small amounts and appear to be neutralized in stomach acid; I'm not aware of any reported cases of cyanide poisoning from feeding raw flaxseed.
Soaking is the most dangerous way to feed flaxseed as the two components are able to combine in water so you end up with a bunch of flaxseed sitting in cyanide water. They combine in boiling as well, but it gets boiled off so it's no longer in the seeds that you would feed to your horses. However, heating flaxseed also destroys the omega-3 fatty acids, which is the main reason most people are feeding it to begin with. Read more here: Flax Seed
On another note, chia seeds are also a great source of omega-3. They're about 10x more expensive in my area, but I've seen at least one other HF member say they're cheaper than flax in her area.