Soybeans: Weed Control

Excerpt from Agronomy Guide for Field Crops (Chapter 4)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Critical Period for Weed Control
2. Crop Losses Due to Weeds
   • Soybean Yield Loss Due to Weeds at Known Populations - Table 4-17
3. Herbicide Resistance
4. Integrated Weed Management
5. Updates on Soybeans: Weed Control
6. Related links...

Critical Period for Weed Control

The critical period to control weeds in soybeans is
from the 1st to 3rd trifoliate-leaf stage (V2-V3).

Yield loss caused by weeds can be reduced to less than 5% by maintaining good weed control during the critical period. Later-germinating weeds have a minimal impact on yields and have relatively low weed seed production.

The product label for postemergent herbicides indicates the growth stage of the weeds when optimum control can be obtained. The timing of weed control should optimize the critical period for the crop and the optimum growth stage of the weed.

Crop Losses Due to Weeds

Crop scouting is essential to determine the weed species present and their density and population in the field. Some weeds are more competitive than others. Table 4-17, Soybean Yield Loss Due to Weeds at Known Populations, shows the comparative yield losses caused by different weed species.

Weed competitiveness should be considered when deciding whether to treat escapes. The estimates in Table 4-17 are based on normal weather conditions and adequate soil moisture. They also assume that the weeds emerged with the crop. Yield losses may increase under drier soil conditions and may be variable under conditions of plant stress.

The effects of weed populations on crop quality and harvest procedures also need to be considered. For example, Eastern black nightshade is not a big threat to yield but can have a severe effect on crop quality.

Adapted from Herbicide Application Decision Support System 2001.

¹Eastern black nightshade losses are due to quality concerns.

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Herbicide Resistance

Herbicide-resistant weeds have been found in Ontario. Resistance to triazine herbicides was first found in 1974 and can now be found in several species of broadleaf weeds and grasses. In 1997, pigweed plants resistant to ALS (Group 2) herbicide were found. Most recently, in 2000, some Eastern black nightshade and common ragweed plants were found to be resistant to some ALS herbicides.

Herbicide resistance usually develops from repeated use of the same type of herbicide. Using principles of integrated weed management helps reduce the onset of resistance and manage resistance when it is found.

Integrated Weed Management

Weed control in the soybean crop is influenced by:

• crop rotation
• row width
• variety characteristics
• tillage practices

In general, the agronomic practices that produce a healthy, fast-growing crop will allow it to compete best with weeds.

Crop rotations can be effective in reducing certain perennial weeds. Rotating to soybeans can allow for different herbicides or tillage practices that are particularly effective on specific species. The use of burndown herbicides in no-till soybeans has been effective for controlling weeds such as quackgrass or thistle.

Narrow rows and higher plant populations can increase the crop canopy and reduce the growth of weeds. Take care to balance the benefits of weed control with other requirements of the crop. Early planting can help the crop establish a competitive canopy before the weeds. However, a key strategy for weed control in organic soybeans is the use of spring tillage to stimulate weed germination. This is followed by a tillage operation prior to planting to kill the first flush of weeds. This delays the date of planting and may therefore reduce yield. If inter-row cultivation is to be used, the row width must be wide enough to permit it.

Equipment can carry weed seeds from one field to another. Combines, tillage equipment and soil erosion can all transport weeds. Weeds can also infest a field from manure application or other soil amendments. Proper sanitation and cleaning of equipment along with maintenance of field border areas all affect long-term weed management in the field.

Please refer to OMAFRA Publication 75, Guide to Weed Control, for more information, including herbicide products and rates, or order this publication.

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Updates on Soybeans: Weed Control

No updates available at this time.

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Related links...

• Soybeans
• Weeds
• Publication 75, Guide to Weed Control
• Field Crop Protection Guide - Publication 812

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