Low Light and Flash
Rodeos are usually inside events, which mean that you will be dealing with low light photography situations. Use a high ISO setting. Experiment with a few shots at different ISO settings. Start with 800, and go all the way up to 3200 until the shots look right.
Just remember, however, your photos are going to look different on your LCD screen than what they actually look like. Unless there are breaks between events, you’re not going to have time to download and transfer your photos and take a good look at them. You’re just going to have to make a judgment call at some point and go with one setting.
Also, use your flash. You’re going to need it. You probably won’t have space to set up any type of external flash unit, and you don’t want to block anyone’s view either. So, you’ll just have to do with the camera’s flash or on-board flash.
One thing before you begin using your flash, make sure that the arena allows flash. Horses can get spooked by the flash. So, even if the venue does allow flash photography, don’t use flash photography right in front of a horse. You could get more than you bargained for in the process.