Do you like horse racing?
I love horse racing, when it is done ethically. I used to work at a feed store where we made deliveries to some of the biggest equestrian facilities in FL 
Do horses work well in humid hot air?
They can, but they have to be monitored. Humidity and heat breed mold like no other, if they eat moldy feed they can die same day if they aren't cared for and there is much suffering involved
Wow, that doesn't sound very good for their health at all.
Yeah, the quality of hay is up-mostly important too, if it isn't dried right or it gets wet and they eat it they can colic. 
Can't they eat carrots aswell? what are their foods?
Equine nutrition
Equine nutrition is the feeding of horses, ponies, mules, donkeys, and other equines. Correct and balanced nutrition is a critical component of proper horse care.
Horses are non-ruminant herbivores of a type known as a "hindgut fermenter." Horses have only one stomach, as do humans. However, unlike humans, they also need to digest plant fiber (largely cellulose) that comes from grass or hay. Ruminants like cattle are foregut fermenters, and digest fiber in plant matter by use
by Smarty Jones' stable foreman, "Big Bill" Foster: "Number one in horse racing, you have to have the horse. Number two, you have to know what to do with the horse once you have him. A lot of people in this business have a lot of horses, and never made it because they broke them down. It happens a lot. Mismanaged. Mistrained. All because people want to be in the limelight. Which is where we're different. We don't want to be