Weed Control Considerations in Corn

A large percentage of the corn crop has now emerged. Fields that did not receive a pre-emergent herbicide application should be scouted in order to identify the spectrum of weeds and prioritize the order in which those fields should be managed.

Weed Emergence Patterns in Selected Fields

Figures 1 and Figure 2 illustrate the weed emergence patterns in 2 different fields during the week of May 15, 2006. Note that in both field scenarios the weeds have emerged around the same time as the corn crop. Weeds that emerge with the crop will have the greatest affect on yield if left uncontrolled. Therefore fields where weeds are emerging with the crop at high densities should be managed first.

Perennial Weeds - Management Considerations

The presence of perennial weeds can complicate herbicide application timings since perennial weeds will generally emerge later (e.g. field bindweed). If significant populations of annual weeds have emerged with the crop and it is known that historically perennial weeds are present, then it may be best if weed control is initiated in 2 phases (1 application for the early emerging weeds and 1 application later for the perennial weeds). If annual weed pressure isn't heavy or emergence of weeds has occurred after crop emergence then herbicide applications could be delayed to target emergence of the perennial weeds.

Grass Weed Stage and Herbicide Applications

A number of fields have grassy weeds that have emerged (See Figure 3). Grassy weeds that are beyond the 2 leaf stage will not be controlled by several herbicides that can be applied to emerged corn (See Table 1).

Figure 1 - The emergence of Lamb's-quarters and spiny
annual sow-thistle with corn crop at 1 field location.

Figure 2 - The emergence of Lamb's-quarters, green foxtail
and spiny annual sow-thistle with corn crop at 1 field location.

Figure 3 - Grassy weeds at the 2 leaf stage at one field location.

Table 1 - The Maximum leaf stage of grassy weeds
whereby they will be controlled by postemergence herbicides.

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