Hi A.,
Your daughter can tell, easily, the difference between the formula and the excellent breastmilk you have been feeding her. There is a connection with Mommy every time she gets breastmilk, even if it is from a bottle held by someone else, and she feels that. I know it's a challenge to think about trying to keep up your breastmilk if you are returning to work and she is going to be in daycare, but if you have been breastfeeding all this time (GOOD FOR YOU!) then I don't see a reason why you would not be able to pump successfully for your time away from her. She is old enough now that you could breastfeed her in the morning before you leave her and in the evening as soon as you are together again, then again when she goes to bed (or whatever your routine is), and not have to provide all her caloric needs through breastmilk while she is in daycare. She can eat some solid foods, and you can provide supplemental breastmilk through pumping. If you work with a lactation consultant (contact La Leche League in your area) you will get plenty of experienced help with figuring out all the in's and out's of pumping and keeping up your supply for her.
Finally, my own youngest child is 9 years old, and my eldest, at 18, was the only one who--18 years ago--saw a regular allopathic pediatrician for advice on how to raise her. (I later found a ped and a couple of naturopaths who were completely willing to work with my orientation towards breastfeeding and natural parenting). The comments I am reading regularly on Mamasource about the assumption that you should start with regular milk at one year old are somewhat disturbing to me. It apparently is the "latest" pediatric advice, given how many mom's here are accepting this as a given, but when my first daughter's pediatrician told me that her one year birthday was the time to stop breastfeeding, I found myself a different doctor! There is NO REASON to stop breastfeeding or to switch to regular milk at one year--the average age of breastfeeding worldwide is much longer than that, and my own daughters (along with the children of many of my friends) continued regular breastfeeding well into the second year as a regular source of nutrition. For comfort, continued ease of care (for me, too!), and ongoing nutritional and immune system support, we kept nursing until 3 or 4--though only at night or when they got hurt or something.
Moms, if you are being told by your peds that you "should" stop nursing (or bottle feeding) and switch to cow's milk at 1 year old, please question this and do a little more research. Mothering Magazine is an excellent resource for a little reality check. There is no reason to switch just because of a date passing.
Blessings,
Fiora