Due to the wet conditions, weeds will likely have emerged prior to herbicide application. Early preplant applications will not be effective unless a burndown herbicide is included. Preemergence applications will also need a burndown herbicide in most instances.
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 Banvel will provide acceptable control. Triazines also have some activity on small grass. If grass size is larger than 1.5", control may not be satisfactory. The inclusion of a surfactant, crop oil concentrate and using liquid nitrogen will improve the activity of cyanazine and atrazine.
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 soybeans. Due to the later planting dates, weeds are often larger than found in corn production. This increases the importance of herbicide rate for Roundup. Control can be improved with the addition of 2,4-D ester or Sencor. Sencor plus 2,4-D ester also will provide burndown activity. Gramoxone Extra is also an effective burndown option in soybeans.
The key to effective burndown weed control is to scout the field prior to application to determine the type and size of weeds. The proper treatment and rate can be determined with timely scouting.
Grazing restriction chart is available on request through the Extension Entomology office at Iowa State University.