Principles of Integrated Weed Management: Critical Period of Weed Control

Excerpt from 2008 Chapter 1, Publication 75, Guide to Weed Control, Order this publication

Table of Contents

1. Introduction
2. Critical Periods for Weed Control in Field Crops
• Table A: Critical Periods for Weed Control in Field Crops
3. Critical Periods for Weed Control in Horticultural Crops
• Table B: Critical Periods for Weed Control in Horticultural Crops
4. Economic thresholds
• Table C: Percent Yield loss in corn due to known populations of annual grass and broadleaf weeds that have emerged with the crop and been left uncontrolled.
• Table D: Percent Yield loss in soybean due to known populations of annual grass and broadleaf weeds that have emerged with the crop and been left uncontrolled.
5. Other topics in Integrated Weed Management
• Introduction
• Non-Chemical Weed Control
• Managing Weeds Resistant to Herbicides
• Use of Chemicals to Control Weeds
6. Related links

Introduction

The critical period of weed control is an important concept in Integrated Weed Management (IWM) systems. This period has been defined as an interval in the life cycle of the crop when it must be kept weed-free to prevent yield loss.

This concept helps in determining the most effective time for non-residual postemergent herbicide applications, and reduces the practice of season-long residual herbicides, and unnecessary late applications of herbicides. The critical periods are defined relative to a crop growth stage to account for soil, weather and seasonal variations. The critical period is based on a yield loss of less than 5% due to weed interference. In other words, the crop has to be weed-free during these stages to prevent a yield loss of more than 5%.

If weeds have been controlled throughout the critical period, the weeds that emerge later will not affect yield and can be controlled prior to harvest with a harvest aid to burn down the weeds and desiccate the crop.

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Critical Periods for Weed Control in Field Crops

The critical period can vary depending on weather and growing conditions. Some research with corn and soybeans has indicated that the critical period may vary with soil type and tillage system and where the end of the critical period may be extended slightly later on clay soils or when using no-till tillage systems.

Corn - The critical period extends from 3-8-leaf stage (4-10-leaf tip showing in the whorl, or V2-V6). Excellent weed control must be maintained throughout this critical period. Corn kept weed-free for approximately 34 days after planting (DAP) or until 6-8-leaf stage (8-10-leaf tips) had a yield loss of 0-5% under conditions of heavy weed pressure.

Soybeans - The critical period extends from the 1st to the 3rd trifoliate stage of soybean growth (V2-V3). Soybeans kept weed-free for approximately 15 days after crop emergence had a yield loss of 0-5%.

White beans - The critical period extends from the 2nd trifoliate to the 1st flower (V3-R1) stage of whitebean (approximately 45 days after planting).

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Critical Weed-free Period for Horticultural Crops

Horticultural crops are very sensitive to weed competition, and need to be kept weed free from planting, emergence or budbreak, until the end of their Critical Weed-free Period. This differs slightly from the Critical Period for Weed Control, but is useful for crops where soil applied preemergent herbicides or cultivation are the main option for controlling weeds. If the crop is kept weed free for the Critical Weed-free Period, generally no yield reduction should result. Again, weeds emerging after the critical weed-free period will not affect yield, but control efforts after this time may make harvest more efficient, or reduce weed problems in subsequent years in perennial crops.

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Economic thresholds

Weed economic thresholds help in determining if weed density and interference is sufficient to justify control measures, i.e., if the yield loss avoided is greater than the cost of weed control. The time at which weeds emerge relative to the crop is a major determinant of yield loss. Early weeds that emerge at the same time as the crop cause more yield loss than weeds emerging after the crop is established. For example, economic threshold studies conducted with redroot pigweed, ragweed and barnyard-grass have found that the time of weed emergence in relation to the crop stage is more important than weed density when evaluating weed control options. Crop yield loss depends on the relative time of weed seedling emergence. Tables C and D estimate the amount of yield loss associated with certain weeds emerging in corn and soybeans.

*Adapted from the Herbicide Application Decision Support System Software, 2002.

*Adapted from the Herbicide Application Decision Support System Software, 2002.

Critical periods and economic thresholds however do not apply in the same way to weeds, such as black nightshade, which may reduce the quality of the crop. In such cases the economic threshold may be zero.

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Related Links

• Weeds in Ontario
• Pesticide Application and Safety
• Pesticide Companies and Agents
• OMAFRA Sprayer Calibration Calculator
• PMRA website
• OMAFRA Publication 505: Ontario Weeds
• OMAFRA Publication 75: Guide To Weed Control

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