Introduction

36
Weed Control
The use of herbicides to control weeds is often important in determining the success or failure of a crop. However, many
other practices can be implemented before and after a herbicide application to help reduce weed competition. The use of
these practices is termed Integrated Weed Management.
Integrated Weed Management
A farming system that uses an array of inter-dependent cultural, biological, mechanical and herbicidal weed control
practices is implementing Integrated Weed Management (IWM). It is essential that IWM involve a variety of tools
including the rotation and/or mixes of available herbicide groups, ensuring that weeds are exposed to a diverse range of
control mechanisms. The goal of IWM is to improve the health and vigour of crops so that they may out-compete weeds
emerging in the stand. This helps to reduce selection for resistance to any single control agent and to delay or prevent the
development of herbicide resistant weeds.
Practicing IWM does not mean abandoning chemical weed control, just relying on it less exclusively. For example:
• You may decide to choose a taller wheat variety or a tall, viny pea variety for a certain field. These crop selections will
compete strongly with weeds, possibly allowing you to skip a spray operation in more competitive crops.
• You could insert a short-term forage crop into your crop rotation. Studies show that short-term (3 year) alfalfa stands
can reduce wild oat and green foxtail populations by up to 80 percent the year after breaking.
• Early sown barley may give you enough of a “jump” on the weeds that you can avoid herbicide applications.
• Use of vigorous, high-quality seed, sown shallow, can give you better crop competition than poor-quality or deeply
sown crop seed.
• Banding nitrogen near the seed can give your crop an advantage over weeds.
For more information, refer to “Integrated Weed Management: Making it Work on Your Farm” factsheet, available from both
Manitoba Agriculture and Saskatchewan Ministry of Agriculture.
Making Spray De ci sions
Field Scout ing
Field scouting is an important tool for making informed spray decisions. Accurately assessing the type and number of
weeds in the field will help you determine if a spray operation is necessary. The scouting pattern diagram on this page
provides a guideline for scouting a field. The entire field should be walked to get a feel for the distribution and species of
the weeds present. A minimum of 20 weed counts should be taken across the field. A smaller number may be used, but be
aware that accuracy decreases as the number of counts gets smaller. Count the number of weeds in a 1 m 2 or a 0.25 m 2 area
and divide the total number of weeds by the number of counts taken to obtain an average for the field. If using 0.25 m 2
samples, make sure to multiply by four so your average is for a 1 m 2 area.
Some weeds are not distributed uniformly and may be found in patches
(for example, Canada thistle) or in low spots. As well, the type and
number of weeds found along the field edges may be very different
from those found inside the field. These areas should be considered
separate from the rest of the field. If possible, patches, low spots,
and field borders should be treated separately, as field wide spraying
may not be required. Look out for new invading weeds and patches
of herbicide-resistant weeds. Herbicide-resistant weeds and new
invaders should be removed (manually if necessary), regardless of
their number, to prevent them from spreading and becoming a serious
control problem. Mapping your field’s weed problems will allow you to
monitor the spread of weed patches over time and help you assess the
effectiveness of your control program.
Yield Losses Caused by Weeds
Knowing the amount of crop yield loss caused by a given weed density will help you decide if a spray operation is required.
The tables on the following pages give an indication of the yield loss caused by some of the important grassy weeds.