What is a Weed?
This is an easy question to answer in a monoculture such as wheat
stands or canola, but it is more difficult in a pasture situation.
Most agree that weeds are plants that possess some undesirable traits
such as being prolific seed producers; the seeds shatter as soon as or
even before they fully ripen and they often have effective means of
spreading. In grain crops, weeds often reduce yields and therefore
profits. In pastures, the most undesirable traits a plant may have are
that it is poisonous or not consumed by animals. Some examples are
thistles and woody species. Other species we consider as weeds in
pastures are those that exhibit low productivity or are productive for
only a short time during the summer. Some examples like dandelions
and wild plantains are perennials and others are annual weeds like
foxtail and ragweed. Whatever the weed problem, sound production
practices are the key to controlling them.
Weed Control in New Seedings
A Clean Start
The time to begin your weed control strategy in a pasture is in
the establishment phase. If weeds are not controlled at the
outset, they may choke out a new seeding or allow weeds to
encroach into the established stand.
When seeding a new pasture, test the soil and apply needed
fertilizers to the proper levels for your seed species. Start with
a well-tilled seedbed. No-till may also be used to establish
pastures but this method requires special no-till drills and some
type of sod suppression prior to seeding. Seeding rates are also
important. Become familiar with the seeding rates for the species
you are planting, then calibrate your seeder so you are sure you
are planting the correct rate. Using rates that are too low will
invite weed encroachment.
When possible, select fields with low weed populations. Avoid
fields severely infested with perennial weeds like Canada thistle.
If perennial weeds exist, it is important to control them prior to
seeding a new pasture. If pre-seed herbicides are utilized watch
out for products with carry-over residues.
One non-chemical strategy for controlling weeds prior to
seeding a pasture is to till or mow every 14 to 21 days throughout
the summer, then follow with a late summer seeding in early to mid
August. This helps deplete the weeds carbohydrate reserves,
weakening and eventually killing them. Late summer seeding means
fewer annual weed problems as well. Do not use a companion crop or
graze these seedings until the following spring.
Annual weeds may likewise threaten a new pasture seeding. A
companion crop such as oats or barley will decrease annual weed
levels and control soil erosion. Seed these crops at 1.5 bu/acre.
A higher seeding rate may be too competitive for the underseeded
species. It is most desirable to chop these small grains at the
boot stage rather than harvesting for grain. Harvesting as silage
or hay provides a higher quality forage and allows more time for
the underseeded grass and legumes to establish. When harvesting
for grain, consider an early maturing short-strawed variety that
stands well to avoid lodging and smothering the pasture seeding.
Pastures may also be seeded using perennial ryegrass as a
companion crop as it establishes quickly. If used, add 2 lb/acre
of of a forage type of perennial ryegrass (not a turfgrass
variety) to the mixture. Even with a companion crop, annual weeds
are likely to grow. In these instances, it will be necessary to
remove the weeds to avoid too much competition. This may be
accomplished by green chopping or even baling the material off the
field. This should be done when the weeds are 10 to 12 inches
tall.
Weed Control in Established Pastures
Once pastures are established, it is important to keep weeds
from invading and reducing pasture condition.
Rotational grazing
Many pastures become severely infested with thistles and other
problem weeds due to continuous grazing. Cattle will over-graze
areas of young, succulent growth and under-graze more mature
areas. Overgrazing results in an open sod that allows light to
penetrate to weed seeds and seedlings. Under grazing can be
harmful as well, as excessive growth will smother new shoots,
inhibit tiller development and weaken the desirable species. This
will create open spots which allow weed encroachment.
A properly managed rotational grazing system avoids these
problems. Cattle are left in a paddock until the grass has been
grazed to the proper height, then are moved to another paddock and
so on. The original paddock is grazed again only when it has had
sufficient time to recover. This helps maintain a healthy,
vigorous pasture which can easily compete with weeds. The proper
rest period for a paddock depends on the species and time of year.
Rest periods of 15 to 20 days are common in early spring. In late
summer, when growth has slowed, rest periods may be as long as 35
days. Maintaining proper fertility and pH is also critical in
maintaining a healthy stand and decreasing weed encroachment.
Walk Your Pastures
Another key to weed control in established pastures is to walk
your fields often. This is the best way to catch weed problems
before they become serious. Weeds should be hoed, pulled, sprayed
or cut before they set seed and spread.
Know your weeds
Scouting for weed problems early is an effective tool for
identifying and controlling weed problems before they develop into
situations that cannot be easily managed. This requires a trained
eye and the ability to identify weeds in their early growth
stages.
Tillage
It is nearly impossible to destroy all weed roots with tillage.
Repeated tillage of these areas for one growing season often
eliminates the infestation arising from roots, but this is seldom
a practical approach in pastures. Areas where thistles are
controlled with repeated tillage must be watched carefully for new
plants that start from seeds. These should be removed when they
are small so that they do not form roots with buds.
Biennial thistles in Manitoba include bull, and musk (or
nodding) thistle. Bull thistle seldom forms dense stands but musk
thistles are quite invasive and can render large areas unsuitable
for grazing if not controlled. Biennial thistles form a rosette of
leaves the year seeds germinate and must undergo winter to shift
from vegetative to reproductive growth. Plants flower the
following summer and die at the end of summer or with the first
frost. Preventing seed production is the key to managing biennial
thistles.
Mowing
Mowing is an option for weed control in pastures. Mowing annual
weeds once will usually control them if the pasture is healthy and
exhibits rapid regrowth. Serious annual weed problems are rare in
pastures and are indicative of declining health of desirable
species. These paddocks may be candidates for improvement or
renovation.
Perennial weeds like Canada thistle, milkweed and goldenrod
require a different approach. Mowing once will not control these
weeds. Mowing on an interval that allows these plants to regrow to
8 to 12 inches between mowing will eventually kill these plants.
Keep in mind that mowing this often will likely weaken desirable
species. These areas may need to be improved by one of the methods
listed below.
Herbicides
There are a number of
herbicides registered for use in pasture situations. Not all are
intended for use on newly seeded pastures. It is important to
read product labels carefully. Producers should pay particular
attention to pasture species registered for application, stage of
pasture development registered for application, weeds species
controlled, and grazing intervals. A complete list of products
registered for use in pastures and hayfields and the common weeds
controlled in these situations are listed in Table 17. of the
Guide to Crop Protection 2004 Page 43.
Pasture Improvement
If annual weeds are a serious problem, or weeds are encroaching
due to thin stands, introducing new species or increasing density
of current species, will help reduce these problems. Pasture
improvement may be accomplished by the steps listed below:
-
Mow or graze closely in late fall, prior to snowfall. This
weakens the existing species, making them less competitive with
the new seeding, and improves seed to soil contact when seeded
next spring.
-
Frost seed the following spring by spreading seed of
desirable species on the soil surface. Do this after snow has
melted, during the spring freeze/thaw cycles.
--or--
-
Interseed into the existing stand with a no-till drill as
early in spring as possible.
--or--
-
Disk lightly and interseed into the existing stand with a
grain drill as early in spring as possible. Using press wheels
or otherwise packing the soil after planting will improve
success.
-
Graze these paddocks lightly until mid to late summer to
allow the new plants to establish.
These techniques will be most successful when attempting to
introduce new species into an open sod. They are much less
effective on dense grass sods.
Summary
Remember, these keys to success when managing pasture weeds.
-
Soil test and fertilize accordingly
-
Select adapted species combinations
-
Use appropriate seeding rates
-
Control perennial weeds prior to seeding
-
Practice rotational grazing
-
Walk pastures to catch emerging weed problems
-
Control developing weed problems
-
Consider renovating pastures that are beginning to develop weed
problems
The above information was in part based on an article written by
Dennis Cosgrove and Jerry Dole of the University of Wisconsin.
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