Until the 9th month of pregnancy, the diet should not change from what you gave her before her pregnancy.
Then , you can start to rise it progressively to double it because her needs in proteins (especially lysine) will increase from 80 to 85%.
The amino acids are essential and yo can find it in lucerne or soy (that you had to the ration).
You can use hemp oil if your mare is loosing her appetite, it will help her getting omega 3 and 6 that will fix vitamins better. This is very good for the end of her pregnancy but you need to reduce a little her rations.
The measure of the rations depends of your mare and her weight, don't hesitate to see a vet to know if she's overweight or underweight.