Postemergence herbicides are applied over both crops and weeds to selectively remove weeds later than early postemergence but usually prior to canopy closure. Postemergence application is appropriate for all management systems providing that selective postemergence herbicides are available. Directed postemergence herbicides are applied over the weeds and under most of the crop leaves. Annual crops must be in rows is used and the weeds lower growing than the crop. Maximum crop safety is obtained with this technique.
Herbicides are major contributors to control of weeds in annual crops.
Where tillage is restricted they frequently provide the total input provided by the grower for control of weeds. Two or more herbicides are generally mixed or sequenced in order to provide broad spectrum, full season weed control. Not all herbicides labeled for commercial field corn production are labeled for hybrid corn for seed production, sweet corn and popcorn.