...From the pages of Agricultural Research magazine
ForumBanking on Weed Control
Weed management is a lot like health management: You have to choose
between a preventive and a reactive approach. Do you wait until you're
sick and then go to a doctor for medicine, or do you watch your diet
and lifestyle to keep from getting sick in the first place?
A holistic, preventive approach can be applied equally well to weed
management and to human health. It focuses on preventive practices and
natural processes to regulate weed populations. Emphasis is placed on
maximizing beneficial ecological processes within farming systems to
maintain weed populations at low, manageable levels. This requires research
to better understand weed biology and ecology. Several ARS
labs are currently engaged in such work.
To avoid weed problems, farmers can either prevent weeds from growing
and going to seed or create conditions that destroy weed seeds before
the next crop is grown. Both approaches will reduce the number of seeds
stored in the soilweed seed banks, as they're known. As the article
on page 14 explains, those seeds are the source of the weeds we see
every year. For weeds as a group, there are typically about 1,000 seeds
per square yard, although a farm field could have as many as 50,000
seeds per square yard from some species. Sometimes, a single weed plantof
pigweed, for examplecan produce 500,000 seeds.
Unlike crop seeds, weed seeds retain their natural dormancy, which
keeps them from sprouting all at once and risking total annihilation
from bad weather conditions, like droughts or floods. Only a small percentage
from each weed species sprouts each year. That percentage increases
when weather conditions are right. Each species has its own weather
trigger, and one bad day can trigger the buried weed seeds of one species
to stay dormant that year. For foxtail, this bad day could be one that
heats the soil to 90 °F. For pigweed, it could be a day when the
soil gets too dry. The same goes for common garden weeds like crabgrass
and common purslane. The seeds shed from just one weed plant can come
back to haunt farmers for years to come. ARS scientists are working
to genetically control weeds' dormancy and destroy them with little
or no herbicide. The weed seed collection discussed in the story on
page 12 is a key tool to help us understand dormancy enough to someday
Fortunately, many factors prevent weed seeds from lasting in the soil
very long. About half the seeds present at any one time come from the
previous year's weeds. The rest were produced over the previous 2 years
or more. So, if farmers manage to control weeds for 1 year, they will
automatically have about 50 percent fewer weeds the next. ARS scientists
at Morris, Minnesota, are developing and refining computer models that
incorporate seed-bank dynamics to predict seedling emergence for better
These banks of weed seeds in the soil are subject to continual "deposits"
and "withdrawals." To prevent deposits, farmers can take many
simple measures before intervening with herbicides. For example, they
can change conditions to keep weeds off balance. Since each weed species
is adapted to particular crops and practices, such as tillage methods,
farmers can plant a summer annual crop like corn or soybeans, followed
by a winter annual crop like wheat, or a perennial crop like alfalfa
grown for hay in rotation. This helps ensure that weeds can't find continuous
seasons to emerge when they won't face intense competition from crops
or be cut for hay. If they do emerge, chances are good they won't survive
long enough to go to seed.
Another technique is to keep the soil covered with dense vegetation
that will smother those weeds that do emerge. This can be accomplished
by planting crop seeds in denser patterns or by growing cover crops
during months when cash crops are not being grown.
Still another approach focuses on creating a favorable habitat for
seed-eating creatures, such as ground beetles, ants, and birds. This
can be done many ways, including with no-till, which leaves a mulch
cover of plant residue in which seed eaters can thrive. The tree borders
or vegetative buffer strips that are often planted around farm fields
to filter out sediment and pesticides in runoff may also provide habitat.
Increasing a soil's organic matter by adding compost or cover crops
also encourages seed eaters that live on the carbon in organic matter.
Fortunately, the practices of sustainable farmingsuch as improving
soil quality, maintaining high biodiversity, and continuously covering
soil with vegetationalso provide better habitat for weed-seed
Continued research on weed biology and ecology will improve our understanding of how weed seeds are regulated in agricultural soils and help producers develop strategies for eliminating them before they become a problem.
John R. Teasdale
"Forum" was published in the August 2003 issue of Agricultural Research magazine.