Will weed and Feed fertilizer kill St. Augustine grass in East Texas. Thank you

First be sure that your lawn needs both fertizer and an herbicide. Apply only what it needs and not simply as a preventive measure. Mow your St. Augustine lawn no higher than 2 1/2 and no lower than 1 1/2 inch regularly. Only purchase a weed and feed product if it reads that it is safe for your grass. And always use it only as directed, no more or less. Follow this advice and you should not go wrong.