Contrary to popular belief, 'Weed-n-Feed' products are NOT conducive to healthy laws or the environment. Might as well set a torch to the lawn!
Weed-n-Feed products usually contain an over-abundance of nitrogen. As any 101 chemistry student should know, too much nitrogen destroys the soil. There is no doubt a fast greening effect, but it is not sustainable.
Another key ingredient of weed-n-feed is phosphorous. Dumping bags of this on the lawn leaches into the groundwater and creates problems. It is bad enough that South Florida water tables are already brackish.
Granular weed-n-feed tends to have both herbicide and fertilizers in one bag. Herbicides do not selectively kill and can stress your lawn over time.