Hi-I have St Augustine Grass in Florida, and my front lawn was looking great with just a few weeds through out, and my side yard had a some St Augustine, but a lot of weeds. A Lowes employee came over, said he used to run a landscaping business, and told me to get weed and feed. I got the bag, but my spreader was not working properly at all, so I would walk and sometimes some would come out, other times non would come out. Then I would open it up and then it would pour out, and then others nothing. So, of course, I am worried because I think I may have screwed up an otherwise healthy lawn. I have no idea which got some, none, or a lot. This was 5 hours ago. I have just started watering the lawn to dilute it, but I just want to see if that is the right thing to do. Sure would appreciate the advice.
Time for a new spreader and while drop spreaders like I think you are describing are good for some things, a whirlybird broadcast spreader is a better option for most yard chemical applications. Among other things you will never get lap marks with a broadcast spreader like you can with a drop spreader. Get a plastic one either handheld or on wheels if your yard is large. Rinse out after each use. Break up any clumps of material that might prohibit the proper flow of material through the spreader.
Nothing you can do now but water heavily and try to leach the over application through the root systems as fast as possible. Pray you did not burn it too badly. You know of course you should have stopped when the spreader was obviously not functioning properly.
Chalk it up to a lesson learned. By the way, this seems awfully late in the season for someone to have recommended a weed n feed product. You might consult a real nursery person the next time and not someone in an apron that had a lawn care business combined with a paper route three years before eligible for a driver's license?
Given you describe something that was not a widespread problem, I would have pointed you to a hose end or tank sprayer product like Weed-B-Gone I think but earlier in the year. I would have had you apply it only where needed.