M.B.
calcium is found in so many other foods. remember, our bodies were made for human milk and cows milk was made for baby cows. don't doubt that your milk is less nutritious. plus, cows milk is fortified with vitamins.
it is just the US alone that breastfeeds their children for the short amount of time that we do. all other countries breastfeed for so much longer. and you know what? look at our incidence of child obesity, diabetes and heart disease, just to name a few. the INTERNATIONAL breastfeeding weaning age AVERAGE is about 5 years old. and this could mean getting breastmilk with a bowl of cereal.
as long as your child has a balanced diet with a variety of fruits, veggies, cheese, yogurt etc. along with your breastmilk, she should be fine.
here's a site from pediatrician Dr. Sears about helping kids start out and stay healthy.