No. If you don't feed them, you will automatically buy the necessary seeds at the end of the day at whatever the market price is. Always best to keep an eye on it and buy low so you aren't forced into buying at an expensive time!
To add to the other comment, when you're forced to auto-buy seed, you can go into debt, negative money value. The only penalty to debt is that you can't buy things. You have to work off the debt until you're in the positive to begin buying things again.
