First of all, you need to know that a mare being bred has bigger nutritional needs , especially during lactation (the 2nd and third months, you can double the portions). She will have bigger needs in protein (lysine and methionine), calcium, vitamin (A, D3 and E) and also minerals and trace elements.
The nutrition depends on the category of your mare:
-sportive or race mare: you can use fodder or condensed food such as lucerne or soya that are very rich in amino acid.
-draft horse or pleasure horse: grass is enough, you don't need to complement the alimentation if there are enough grass.
You can buy special additional food for breeding mares, salt stone and also hemp oil.
Hemp oil is very good for your mare, it will bring her omega 3 and 6 that will help her fixing vitamins.
You can start bringing it into her food at the end of her pregnancy and third first months of lactation.