Very pretty girl you have
I agree with everybody on the sweetfeed. Since you have that cowboy/nutritionist right there, you're all set.
Base for everything is hay. Everything above needs to be supplemented. The easiest way to do this is with a ration balancer. Your filly looks in good shape, so the RB is most likely all she needs.
Once you start working her harder, just keep watching her. Should should she lose condition it's time for adding some energy in one firm or another.
I'd like to borrow your cowboy every once in a while,tho