Principles of Integrated Weed Management: Use of Chemicals to Control Weeds

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

Table of Contents

1. Introduction
2. Time of Herbicide Treatment in Crop Areas
3. Chemical Weed Control in Non-Crop Areas
4. Conservation Tillage
5. Weed Population Shifts
6. Applying Herbicides
7. Other topics in Integrated Weed Management
   • Introduction
   • Non-Chemical Weed Control
   • Critical Period of Weed Control
   • Managing Weeds Resistant to Herbicides
8. Related Links

Introduction

Chemicals may be used to control weeds, either selectively in crops, or non-selectively for the control of perennial weeds, or on land not in crops, such as roadsides, fencerows and areas difficult to cultivate. The treatments listed in this publication are based on extensive field and observation on farms. Herbicides should not be used in cold frames or greenhouses unless specifically recommended.

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Time of Herbicide Treatment in Crop Areas

The susceptibility of both crops and weeds to herbicides is related to the time of application. Therefore, it is important to use the chemical at a time when the crop is at its maximum resistance and the weeds are at their maximum susceptibility. This time varies depending on the crop involved and herbicide used. Terms describing the times at which herbicides may be applied, unless specified differently, refer to the stage of development of the crop plants.

• Preplanting Treatments
  • Preplanting (PP) treatments are applied before the crop area is sown or planted. Some herbicides used in this way act on germinating seedlings, others may also kill weed seeds. When used preplanting, some herbicides must be thoroughly incorporated (PPI) with the soil soon after application. Directions for incorporation are given throughout the publication where these herbicides are recommended.
• Preemergence Treatments
  • Chemicals used in preemergence (PRE) treatments are applied after seeding but before the specified weed or crop emerges. The chemicals may control weeds by (1) killing weed seedlings and/or (2) establishing a toxic layer of chemical on or near the soil surface in which germinating seeds and young seedlings cannot survive.
  • For successful preemergence treatments the crops must either be tolerant of the chemical at the seedling state or the toxicity must have disappeared before the crop emerges.
  • A smooth, well-prepared seedbed that is free of clods is necessary for best results. The surface soil should also be moist and the temperature favourable for the rapid germination of weed seed.
• Postemergence Treatments
  • Postemergence (POST) treatments are applied after the crop and weeds have emerged. A selective chemical is used and the weeds are killed with little damage to the desirable plants.
    The types of weeds that may be controlled depend on:
    1. the susceptibility of the weed, and
    2. the tolerance of the crop to the chemical. Treatment at the correct stage of crop development is important. Since most weeds are more susceptible to chemicals when young, early treatments require less herbicide and result in less damage to crops from weed competition and from spray equipment.
  • Sometimes a herbicide may be applied postemergence to the crop but preemergence to the weeds. For example, a crop may be cultivated, and then a herbicide applied to the weed-free soil to control germinating weeds.
    

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Chemical Weed Control in Non-Crop Areas

• Treatment with "Soil Sterilants"
  • Perennial weeds may be killed and the growth of annual weeds prevented by using soil-active chemicals that are toxic to most plants. These "soil sterilants" may be applied at any time. Best results are obtained if application is followed by a period of rainfall.
• Treatment with Translocated Chemicals
  • In eliminating perennial weeds, apply heavier dosages of 2,4-D and of other translocated chemicals than normally used. Mixtures of 2,4-D with other chemicals may be used to keep areas free of vegetation.
• Treatment with Contact Chemicals
  • These chemicals are not selective but kill all foliage they contact. While this kills annual weeds, regrowth usually develops from the roots or crowns of perennial plants.

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Conservation Tillage

All herbicides recommended for use in specific crops in the conventional tillage sections of this publication can be used in conservation tillage systems provided they are used according to the labeled uses and the labeled rates.

• Minimum Tillage involves very little use of tillage equipment for seedbed preparation. The soil, however, is tilled prior to seeding, resulting in somewhat rough planting conditions by conventional standards. There is very little living vegetation and a crop residue cover of more than 20% after seeding. Weed control in this cropping system is similar to weed control in conventional tillage. Preplant incorporated herbicide treatments may not be practical if crop residues are heavy. Good uniform soil mixing is required to produce satisfactory results with incorporated herbicides. If this condition cannot be achieved without extra tillage, then the preferred method to use is preemergent or postemergent herbicides. Use inter-row cultivation with herbicides. Burndown treatments are not normally required.
• Ridge Tillage involves seeding the crop directly into the ridge formed by aggressive cultivation the previous year. The ridge planter skims off 2–5 cm of soil, crop residue, weeds and weed seeds from the top of the ridge and deposits these in the hollows between the rows. Preplant incorporated treatments are impractical with this system. This system is well suited to broadcast and/or banded applications of preemergent or postemergent herbicides. Excellent results have been obtained by band-applying preemergent herbicides at planting time to fresh, moist soil directly behind the seeding unit on the planter. Also, timely cultivation between crop rows while, at the same time, spraying the crop row with postemergent herbicides can provide excellent weed control. The ridging procedure that follows provides additional mechanical weed control. Proper adjustment of the ridge planter and early cultivation after seeding may eliminate the need for a burndown treatment. (A burndown treatment may be required prior to crop emergence if cultivation is delayed after seeding.)
• Strip Tillage suggests that only narrow bands of soil in the seeding area are tilled, by either power-driven rotovators or fluted coulters. The area between rows is left untilled and the crop residue is undisturbed. Success with preplant incorporated treatments depends on the amount and type of residue, the nature of the specific herbicide and the tilling device used. Row middles are not usually cultivated but could be if appropriate equipment is available for inter-row cultivation. Usually, broadcast application of preemergent or postemergent herbicides is used. Band applications could be used if inter-row cultivation is an integral part of the production system. If vegetation is present at seeding time, a burndown treatment is necessary.
• No-Till cropping systems are the most demanding with regards to weed control. The crop is seeded directly into untilled soil with no follow-up cultivation. Weed control depends entirely on herbicides. Burndown treatments are almost always required. Preemergent and postemergent herbicides are used to control weeds throughout the entire growing season. Crop residues may intercept a portion of preemergent herbicide. If rainfall is sufficient to wash the herbicide from the crop residue into the soil, weed control results will be satisfactory. However, if dry conditions follow application, results could be very disappointing. Postemergent foliar-applied herbicides are more appropriate under dry conditions.

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Weed Population Shifts

Weed population shifts refer to the changes in the kinds of troublesome weeds that occur with a change in tillage systems. When tillage is reduced the environment of weed seeds and underground parts of perennial weeds is altered. These environmental alterations affect weed species differently. Deep burial of weed seeds under conventional tillage systems enforces seed dormancy and increases the life of seeds, perpetuating weed problems. Reducing tillage keeps more weed seeds at the soil surface where they are subject to the ravages of nature. Greater percentages of seed at the soil surface are destroyed before they germinate (by winterkill, birds, insects or rodents). Most seeds that do survive germinate sooner and, if they are controlled, they do not return seeds to the soil. If good weed control is obtained in the early years of reduced tillage, eventually the weed seeds in the soil become fewer, reducing the potential of serious weed problems that originate from seed. Pressure from annual weeds tends to decline after a few years in reduced tillage.

On the other hand, reduced tillage favours weeds that reproduce from underground roots and stems; these underground parts are not mechanically injured or destroyed in reduced tillage systems. Perennial and biennial weeds tend to be more troublesome in ridge-till, strip-till and no-till fields. In reduced-tillage fields, these weeds emerge earlier and do not spread as much as they do in conventionally tilled fields. This allows more appropriate timing for spray application and more spot treatments. The most effective herbicides are foliar-applied systemic types that translocate well to underground plant parts before the above-ground parts are killed. When non-selective herbicides are used, perennial and biennial weeds must be treated between crops or the crop must be sacrificed in treated areas.

Burndown treatments are meant to kill existing vegetation before crops emerge. Burndown treatments may be required in minimum tillage, especially when cover crops are used; in ridge-till when seeding is delayed and/or when winter annual, biennial or perennial weeds are present; and almost always in no-till and strip-till fields. Vegetation can be killed by:

• contact herbicides like paraquat
• translocated systemic herbicides like glyphosate, dicamba or 2,4-D
• residual herbicides that have postemergence foliar activity such as atrazine, linuron or metribuzin plus surfactants or oils and
• a contact or systemic herbicide tank-mixed with a residual herbicide such as paraquat + atrazine or glyphosate + dicamba.

All of these treatments are most effective on young, actively growing plants. Coverage is extremely important with contact treatments. Burndown treatments do not necessarily provide residual weed control.

When dense, vigorous growing vegetation is present just before or at planting time, large amounts of soil moisture are removed by that vegetation. This can be an advantage during springs when there is an over-abundance of soil moisture, but is a definite disadvantage during springs when dry conditions prevail. As a result of this, consider applying application of burndown herbicides early, perhaps even the previous fall when perennial cover crops or sod fields are involved. Under dry conditions, a rapid-acting burndown is preferred over a slower-acting treatment.

For more specific information see also the individual crop sections such as Corn (Field and Sweet) and Soybeans.

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Applying Herbicides

An exterminator's license may be required by a person applying herbicides on property other than their domestic property. An operator's licencse is required by a person operating an extermination business. For further information, contact any Ministry of the Environment Regional Office or the Ministry of the Environment Pesticides Section, 135 St. Clair Avenue West, Toronto, M4V 1P5

Successful chemical weed control requires the application of the correct quantity of herbicide uniformly over the area. With selective weed control in field crops, this application becomes a precision operation.

The chemical is applied as an emulsion, solution or suspension. The volume of the mixture used per hectare depends on the herbicide and the crop being treated. Generally, low volumes (50-200 L/ha) are used when 2,4-D chemicals are sprayed on emerged weeds, and higher volumes (150-300 L/ha) when preemergence herbicides are applied.

Small areas may be treated with a garden sprayer, or even with a watering can, if care is taken to wet all foliage.

Power equipment is necessary for larger areas. Equipment used for applying 2,4-D-like chemicals should not be used later for applying other solutions to 2,4-D susceptible crops or garden plants.

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Related Links

• Weeds in Ontario
• Engineering topics
• Problem Weed Control in Field Crops
• Integrated Pest Management (IPM)
• Pesticide Application and Safety
• Pesticide Companies and Agents
• OMAFRA Sprayer Calibration Calculator
• Search the PMRA website
• OMAFRA Publication 505: Ontario Weeds
• OMAFRA Publication 75: Guide To Weed Control

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