Giving horses slices of bread as a treat is common where I live (Australia). For many people it's as common as giving horses carrots and apples. Having said that, a horse may get what I refer to as snack amounts, one or two slices or up to 6 slices as a treat and, usually, no more in a day. The amount of grain in that amount of bread is so small compared to the grain intake of horses being hand fed grain. I feed a couple of my horses 4 one litre scoops of grain per day (in separate meals). Put into that perspective, the grain in bread is negligible.
I feed my horses bread as a treat, maybe once or twice a week and do so without any harm to their health and know of many people who do the same thing and use bread to help catch their horses. AFAIK there is no health issues for feeding snack amounts of white, whole meal or multigrain bread for horses. It's certainly not something that should form the basis of a diet, and neither should apples or carrots either.
I've taken out some interesting snippets from this article.
Assuming that the bread is made with wheat flour, yeast, salt, and water, there is nothing in it that is toxic to the horse, and it can contribute enormously to caloric intake if fed in large quantities. There is no harm in occasionally feeding bread, but it is not the most nutritious feedstuff when used as the sole concentrate. While bread is chock full of calories, it provides few nutrients.
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The high starch content of bread may be a potential problem if fed in large amounts. .../...
It is not customary to feed wheat grain to horses because of the high gluten content. The problem with wheat gluten balling up in the stomach or intestinal tract and causing blockage (colic) is not a concern with feeding bread because the yeast and the heat of baking have altered the gluten starch.
The caution with bread, as with any horse feed, hay, chaff, etc, is mould. Ensure you don't feed horses mouldy bread, this can make a horse very ill and even be lethal.
