Depends on what kind of feed you feed them. If it's just hay and grain, no. Moldy, dusty hay isn't good for horses, but it won't kill them, it'll just make them a bit sick.
Grass and hay.
Several animals eat jaggery. It can be given raw as a treat or used to sweeten feed. A couple animals that eat it are cows and horses.
Hay is used to feed animals when grass is not available.
So they dont die. Why do you eat every day?
Yes but i wouldn't feed them to them because they could get sick and probably die
Race horses are feed a diet of high energy foods such as oat, however feeding a horse before they work is not a good idea is it can cause colic so they are not feed before they race as they could die.
Don't feed horses rotten apples
I have never heard of 'Cow' Vetch, but i have heard of Crown Vetch and it is toxic to horses. It is best to remove the plant from your property, and feeding alfalfa can help to 'cure' the poisoning. However there is another type of Vetch that is perfectly fine to feed to horses, but is a legume like alfalfa hay and should be fed sparingly.
Feed or corm
Feed them alfalfa
Horses in Minecraft eat hay. This can be made using wheat. You can also feed them apples.
Humans don't digest all plant material very well. If you can get nutrition out of it, it's more efficient if you can eat the plant material directly. If you have to feed it to a cow, and then eat the cow, a lot of the nutrition will be lost in the transition. But if you can't get nutrition out of it, and the cow can, then feeding it to a cow would be sensible.
Not as a main feed source, no. But they may eat it nontheless.