Fibre is the most important part of your horses diet, so a good quality chaff from the Dengie range and a feed balancer should support your natives needs, but it would be worth phoning a feed company for individual advise.  Age, workload, and many other factors can influence your mares diet requirements. 

 I agree, my rising 4 section c mare has been lots quieter since i changed her from pasture mix to high fibre nuts, i do soak them and feed them as you would sugar beet with dengie or garlic chaff and have slowly removed the course mix, i think this has made a real change to her. 

Something along the lines of D&H Pasture Nuts would be good as they have no barley in them, you need plenty of fibre without too much cereal. It all depends on how much work you are doing, what are you doing with her on a day to day basis?