That depends on a lot of things.
How good is your pasture? I see you are in QLD, are you still in fairly drought conditions, or have you had a good spring like us down south so have more feed? What condition is your mare in? Is she an easy keeper, hard keeper, etc? Has she ever foundered? Tied up? Is she insulin resistant?
I do PC, mustering, shows etc. on my horse, and he is getting Speedi-Beet (dried beet-pulp), Copra (coconut meal), and oaten chaff. I'll soon start supplementing him with Equilibrium, Vit E and Selenium supplement and a salt lick.
I'm a big fab of not feeding grain. A lot of research is showing that it is healthier for a horse to NOT be fed grain, or commercially produced 'sweet feeds' as they are too high in 'NSC' or 'non-structural carbohydrates', which are now beleived to cause founder, tying up, insulin resistance, etc.