Due to the wet conditions, weeds have likely emerged in no-tillage fields. This may be the case even when an early preplant herbicide application was made. It is critical to scout the field prior to preemergence herbicide application to determine whether or not weed populations are developing. Many early spring weeds, such as horseweed, are extremely small and may germinate beneath existing plant residue. Thus it is important to observe the fields closely. If weed populations have begun to develop, a nonselective burndown herbicide treatment may be necessary.
There are a number of options available for corn; selection should be based on weed type and size. If only broadleaf weeds are present, treatments that include a triazine herbicide, 2,4-D or dicamba will provide acceptable control. While triazines have some activity on annual grasses, control of grass weeds larger than 1.5 inches is marginal and largely dependent on growing conditions. The inclusion of a surfactant, crop oil concentrate, and using liquid nitrogen may improve the activity of triazines.
When grasses are larger or the predominant vegetation in the field, more consistent control will be obtained with a nonselective herbicide such as Gramoxone Extra or Roundup. These herbicides can be tank-mixed with residual herbicides to improve control.
Nonselective treatments are typically used in no-tillage soybean production systems and due to the later planting dates, weeds may be larger this year than normal. This increases the importance of proper rate for Roundup. Control can be improved with the addition of 2,4-D ester or Sencor. Sencor plus 2,4-D ester will provide burndown activity and Gramoxone Extra also is an effective nonselective burndown option. Application timing is an important consideration. Typically, burndown treatments are most effective when applied several days prior to planting. The colder the conditions, the longer it will take for herbicide activity to occur. Timing burndown applications immediately prior to, or after planting may result in poor performance due to the planting operation. Weeds that are disturbed by the planter may not properly or completely take the burndown herbicide into the plant and thus are not effectively controlled.
The key to effective burndown weed control is field scouting prior to application to determine the type and size of weeds. Only with proper scouting can the proper treatments be determined.