Once I saw a woman living with all sorts of animals she had 'resued'. Many of them were dead or in advanced stages of starvation. What did she feed them? Bread. There was a mountain of bread rejected by bakeries and grocery stores in her kitchen.
Man cannot live by bread alone....and neither will your horse. Bread, as a treat, is not bad for your horse. But bread, as a steady diet will most likely kill him. The dietary needs of a horse are so much more vast than a peanut butter sandwich (although I wouldn't mind one myself right now). If giving your buddy a slice of bread after a ride makes you and him happy, then do it. But a loaf of bread or even 6 loaves will never come close to replacing a quarter bale of hay or several scoops of pelleted feed.
The biggest threat from bread is not the chemicals or preservatives but the fact that it provides your horse with nothing that he needs nutritionally. It's a treat, it is NOT food.