Spinach = Iron -> give them fresh young leaves.
Dandelion = Vitamin C -> same as spinach.
Carrots = Vitamin A -> chopped in small pieces.
Green Peas = Vitamin B1 -> whole
Barley = Vitamins group D -> whole
Brewer Yeast = Vitamins B6 and B12
Multivitamin mix = for the rest of them -> in water once or twice a week.
Minerals and supplements are also important and thy get them from mineral grit.