Fertilizer and herbicide combinations are often applied prior to planting, at planting, or after crop emergence, a procedure sometimes called "weed and feed". The opportunities for utilizing this system are numerous with the major advantage being saving an extra trip across the field. In order for the system to work, it must ensure that both fertilizer and herbicide are present when needed. When both the crop plant and weedy plant are at growth stages which allow for compatible applications with a single treatment, this can be highly successful. Conversely, when these two plant systems are not at compatible growth stages, severe crop injury and/or a lack of weed control can result.
There are several methods of applying herbicides and fertilizers together, but regardless, always follow directions on the pesticide label for specific instructions and/or restrictions.
Witt, William W. and Rice, Harold B., "Weed and Feed" (1984). Soil Science News and Views. 174.