The use of postemergence herbicides allows weeds to grow with the crop from the time of emergence until application. This raises questions whether this short period of competition affects corn or soybean yields.

Several factors influence crop yield response to weed competition, including: weed species and population, environment, and duration of competition. Low-to-moderate infestations of weeds will not affect yields under most situations if the weeds are controlled within four to five weeks after planting. However, crop yields can be adversely affected during this time period if high weed populations are present or if dry soil conditions occur. Under these conditions, earlier postemergence applications may be required to protect crop yields. In fields known to have high weed populations, the use of preemergence herbicides or timely rotary hoeing can provide a wider application window for postemergence herbicides.

The data in the accompanying table illustrate the impact of weed population on the critical time period for competition. The primary weed species in the studies was giant foxtail. Corn yields at the site with low foxtail populations were not affected by competition when control strategies were implemented within five weeks of planting. However, at the location with high weed populations, delaying postemergence application three or more weeks after planting resulted in significant yield losses.

Effect of timing of weed control on corn yield losses
associated with early-season giant foxtail competition.

                                      Relative corn yield (%)
                                        Foxtail Population

Timing of control                    <10/sq. ft.   >40/sq. ft.
Preemergence                            100           100
2 WAP (weeks after planting)             95           109
3 WAP                                   112            85
4 WAP                                    98            81
5 WAP                                    97            74
Weedy control                            67            48
LSD 0.05                                 23            13

Owen, Lux and Franzenburg, ISU, 1991.