Today we're going to talk about cyanide poisoning. I'm sure we've all heard or seen it before - those lists of "horse prohibited foods", and apple cores are right at the top of the list because their seeds contain CYANIDE!! That's the poisonous stuff from movies that will kill you if you so much as touch it, right?? Well, not quite.
Small doses of cyanide in the body are easily translated into different compounds and excreted. In fact, cyanide is actually produced by your body and expelled with every single breath! It's in higher doses (around 1mg of cyanide/kg of body weight) that we start to see actual toxicity. At these levels, cyanide can kill off cells by preventing them from using oxygen. So back to the dreaded apple cores. Your average supermarket apple contains about 8 seeds, which each weigh about .7g. This means there are about 5.6g of seeds in each apple. Each apple seed has a cyanide concentration of 100mg of cyanide/kg of seed weight. Thus, each apple contains ~.56mg of cyanide. Returning to our 1mg of cyanide/kg of body weight number above this means a horse would have to eat ~970 apples very rapidly to reach toxicity. Heck, even if we say they only eat 700 apples to play it safe, a horse isn't physically capable of investing that many apples quickly enough to induce cyanide toxicity from the seeds, and there are loads of other health consequences that would arise long before you reached 700.
The moral of the story is this: apples, including their cores, are perfectly fine to feed to your horse after lessons or as a reward. The only concern with them is choke when feeding whole, be sure to hold the apple when feeding loose or hold the apple firmly. Go out and treat your ponies!