As you’ve likely discovered, many broadleaf weed killers also harm St. Augustine grass. There are a few herbicides available that are rated for St. Augustine, but most of them are weed control only, not weed and feed.
A couple of thoughts about weed-and-feed products:
Unless you live in a frost-free climate, you should stop using any fertilizers at least six weeks before your average first frost date. St. Augustine is a warm-season grass that goes dormant for the winter, and you don’t want to stimulate growth that can be killed by frost. Weed-and-feed products are best reserved for spring and summer in these types of lawns.
The most common herbicide for St. Augustine lawns is Atrazine. Simply put, Atrazine is nasty stuff. It’s known to cause cancer and birth defects, and it’s showing up at alarming levels in drinking water. One look at the stern warnings on the label should make you think twice about putting it on your lawn!
A healthy lawn is naturally weed-resistant. If you have a widespread weed problem, focus instead on getting your lawn healthy, or on replacing the unhealthy lawn with groundcover or planting beds.