A weed and feed formula is a mixture of fertilizer and herbicides that kill off weeds. So, if you have a lawn or grass that does not have any pest plants, then applying only fertilizer to promote grass growth should be enough. However, if you have grass that is not doing so well due to tons of weed growth, then you certainly need to administer a weed and feed formula. This will not only kill off weeds, it will also inhibit further growth of pest plants. Simultaneously, it will also nourish the grass and make it thicker and stronger.