Principles
of Integrated Weed Management: Use of Chemicals to Control Weeds
Excerpt from 2010-11 Chapter 1, Publication
75, Guide to Weed Control, Order this
publication
Table
of Contents
- Introduction
- Time
of Herbicide Treatment in Crop Areas
- Conservation
Tillage
- Weed Population Shifts
- Applying
Herbicides
- Other topics in Integrated Weed Management
- Related
Links
Introduction
Chemicals
may be used to control weeds either selectively in crops or non-selectively on
non-crop areas, such as roadsides, fencerows and areas difficult to cultivate.
The treatments listed in this publication are based on extensive field trials
and observation on farms. Herbicides should not be used in cold frames or greenhouses
unless specifically recommended.
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 will vary depending on the crop involved and
herbicide used. Refer to the stage of development of the crop plants unless otherwise
specified.
- Preplanting Treatments
- Preplanting
(PP) treatments are applied before the crop is planted. Some herbicides used in
this way act on germinating seedlings, others may also kill weed seeds. When used
prior to planting, 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 listed.
- 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 killing weed seedlings and/or 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 stage 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 seeds.
- 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 crop The types of weeds that may be controlled
depend on the susceptibility of the weed, and 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 will
require less herbicide and will 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,
then a herbicide applied to the weed-free soil to control germinating weeds.
Conservation Tillage
All
herbicides listed for use in specific crops in the conventional tillage sections
of this publication can be used in conservation tillage systems provided that
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 would be to use 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 and 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 a 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 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, reduced weed control may occur.
Postemergent foliar-applied herbicides are more appropriate under dry conditions.
Weed
Population Shifts
Weed population shifts refers 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 less likely to be 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. They may
also be required in ridge-till when seeding is delayed and/or when winter annual,
biennial or perennial weeds are present. Burndown treatments are almost always
in no-till and strip-till fields. Vegetation can be killed by:
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. Consideration should be given to early 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 would be preferred
over a slower-acting treatment.
For more specific information see also the
individual crop sections such as Corn
(Field and Sweet) in Chapter 9 and Soybeans
in Chapter 11.
Applying Herbicides
An exterminator's
license may be required by a person applying herbicides on property other than
their own. 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 phenoxy herbicides (eg. 2,4-D)
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.
Related Links
