By Dwight Lingenfelter
Below are some post herbicide options for control of weeds in corn.
Reduced weed control from herbicides can be expected if preemergence herbicides did not receive at least 0.5 inch of rainfall within a week of application to properly activate the products or if reduced herbicide rates were used and high volumes (i.e., >3 inches) of rainfall occurred.
Postemergence herbicide will likely be necessary to clean up some of the escaped weeds. In most cases, weedy grasses and larger seeded broadleaf weeds (velvetleaf, ragweed, cocklebur, etc.) seem to be the mostly likely to break through in these situations. Herbicides that are effective for grass control include products such as, glyphosate, Resolve Q, Steadfast Q, Impact/Armezon, Capreno, and a few others. Typical products for broadleaf control include glyphosate, Liberty, dicamba (Clarity/Status/DiFlexx, etc.), Callisto, and others. There are numerous premixed and/or generic products that contain various combinations of herbicides that can be used.
However, keep in mind there are crop height restrictions on many of the post herbicides. Most products can be applied to corn that is up to 20 or 30 inches tall. Also, for best performance, spray weeds when they are not beyond their maximum height guidelines. Most annual grasses need to be sprayed before they reach 3 inches tall, but this can vary depending on what herbicide is used.
The typical height for broadleaf weeds is 4 to 6 inches tall but again it can vary by product. For a listing of additional herbicides and maximum corn heights and information on maximum weed sizes for these products please refer to Tables 2.2-14 and 2.2-15 in the Penn State Agronomy Guide and check the most recent herbicide label for specific use guidelines. (Or refer to the herbicide label for additional use information— Crop Data Management Systems (CDMS) or Greenbook).
On a final note, most post herbicides require adjuvants to be included in the spray mixture to optimize performance. Make sure to check the herbicide label for details on the use of surfactants, oils, and/or nitrogen fertilizer additives.Source : psu.edu