How's your diet and weight? I suspect that if your daughter is getting most of her nutrition from food, you may be running a nutritional deficit that could be remedied by an adjustment, perhaps just in calories and/or fat. I have heard it takes 4 years for a woman's body to recover from pregnancy.
My daughter was still breast-feeding at your daughter's age, and I know she wasn't getting much milk out of it.
Lots of women do conceive while breast-feeding. My sister and I are only 13 months apart, and my mother was always overweight.
But to answer your question, if your daughter is getting most of her nutrition from a variety of healthy food, she can get all of it from that food. But it would be a shame to wean your daughter and unnecessarily break the special bond that nursing gives you.