Introducción

While one person’s weed might be another’s wildflower or pollinator host, for many farmers the word ‘weed’ means the wrong plant in the wrong place at the wrong time, regardless of its technical classification. For millennia, from the earliest cultivated crops to the present day, weeds have been identified as plants capable of stealing water and nutrients, harboring pests and diseases, and reducing both the yield and quality of the crop being grown.

Nowhere is this more starkly illustrated than in field crops, where uniform varieties of wheat and other small grains, corn, soybeans, cotton, oilseeds, pulses, and other major crops are grown on large acreages, where producers see any other plant species as a direct threat to the crop until proven otherwise – for the reasons given above.

While weeds have always been with us and have adapted to changes in weed control methods over the centuries, the most destructive and aggressive weeds in the US today have become a problem because of their biological resistance to herbicides.

How Big is the Problem?

Many organizations are tracking the pervasiveness of herbicide resistant weeds. There are many ways to interpret ‘unique resistant weeds,’ but across multiple sites the estimates are high. Cornell states that there are 131 cases unique to the US.

WeedScience.org is the online home of the International Survey of Herbicide Resistant Weeds, a global collaborative project that tracks and documents cases of weeds that have developed resistance to herbicides. The database is constantly updated and is currently registering 138 resistant weeds unique to the US as of this writing.

The Northeastern IPM Center estimates even higher numbers: “In the United States alone, over 160 herbicide-resistant weed populations have been confirmed. In the Northeast, particularly in New York State, herbicide resistance among weed populations is becoming a serious and growing concern.”

Many production systems now use a combination of approaches for weed control, including cultural, biological, and mechanical management practices within an Integrated Weed Management (IWM) program. A forthcoming ATTRA publication will discuss these practices in greater detail, examining the principles of sustainable weed management across all types of production systems

Principal Weeds

Many weeds, such as Canada Thistle, Common Lambs quarters, Wild Buckwheat, Wild Mustard, Field Bindweed, and grasses like Wild Oats and Johnsongrass display resistance to select chemical groups. While the weeds described below tend to make up the top five of most lists, this list is not comprehensive, and many more weeds express different levels of herbicide resistance.

Most of the currently predominant and damaging weeds across the US have become a problem because of chemical resistance, not because of attempted control through cultural, biological or mechanical control methods. The problem is escalating, in part due to monoculture systems, but also due to GMOs and overuse of non-selective herbicides like Glyphosate and Diquat. Superweeds (those now resistant to multiple chemical classes) include:

Palmer Amaranth (Soybeans, Corn, Cotton, Peanuts). This weed thrives across the Southern US, the Great Plains, and is heavily encroaching into the Midwest. It is noted for its extreme growth rate and prolific seed production. https://blogs.cornell.edu/weedid/field-crop-weeds/

Palmer amaranth in soybean field. Photo courtesy of United Soybean Board.

Common Water hemp (Corn, Soybeans). Predominant in the Midwest (especially in Iowa, Illinois, and Indiana). It is infamous for its ability to emerge late in the season and quickly outgrow crops. https://growiwm.org/farmers-crown-waterhemp-worst-weed-at-commodity-classic/

Waterhemp: Amaranthus tuberculatus.

Kochia (Wheat, Field peas, Corn, Soybeans). Prevalent across the Northern Great Plains and Western US (Montana, Wyoming, the Dakotas, and Washington state). It is an early-emerging tumbleweed species well-adapted to semi-arid environments. https://smallgrains.wsu.edu/weeders-of-the-west/2023/03/30/tumbleweeds-tumbling-in-the-columbia-basin/

Kochia: Bassia scoparia. Photo credit: Matt Lavin.

Horseweed / Marestail (Soybeans, Corn, Winter cereals, Fruit and Nut orchards). Common in no-till systems and prevalent in the Mid-Atlantic, Northeast, and Eastern Corn Belt. https://blogs.cornell.edu/weedid/field-crop-weeds/

Horseweed/marestail: Erigeron canadensis. Photo credit: Charles T. Bryson, USDA Agricultural Research Service, Bugwood.org

Common/Giant Ragweed (Corn, Soybeans, Small grains). Mostly found in the Midwestern, Southern, and Eastern US, particularly flourishing in rich soils within corn and soybean rotations. https://www.uaex.uada.edu/yard-garden/resource-library/plant-week/common-ragweed.aspx

Ragweed. Photo credit: James H. Miller & Ted Bodner, Southern Weed Science Society, Bugwood.org

The Economic Cost of Weeds

Weeds are an expensive problem. According to an article from Colorado State University, “Difficult weeds cost the US agricultural industry $33 billion annually in losses. The fast-growing chemical herbicide resistant Palmer amaranth is a key example of the problem. It has been shown to decrease crop yields by up to 91% in corn and 79% in soybeans.”

There are many vested interests in keeping high production levels of corn and soybeans, for obvious reasons. These crops have multiple markets for human consumption, ethanol production, other biofuel production, livestock feed, and high-profile export markets. Federal insurance and the generous federal subsidies on key crop insurance policies help to maintain this production acreage (and for many other field crops, too) through years of drought, flood, hail, and other climatic events. Available policies protect the grower from rising input costs, crop failures, yield or revenue protection, and quality protection (such as with the Malting Barley Endorsement policy).

However, we should not assume that even with this level of support, crop insurance guarantees a safety net. With many county average yields dropping so low now, mainly due to prolonged droughts, some crops are not triggering insurance payments even with low yields.

Weed control remains one of the major priorities in preventing yield loss and quality control issues in crop production, which will help build more sustainable systems that are not so reliant on support mechanisms such as crop insurance.

One weed management approach some producers are taking is to use chemical herbicides, just more precisely. Precision herbicide application technology continues to develop to target weeds via drones or as targeted application systems via conventional spraying such as “See and Spray” (John Deere), “One Smart Spray” (BASF/Bosch), Green Eye Technology, and the Weed-It system. All are designed to try to reduce weed resistance to herbicides (and reduce overall herbicide use) by delivering the correct dose of chemical to the correct plant only.

An additional option to prevent herbicide resistant weeds for conventional producers is to add more complex crop rotations where possible, therefore allowing for a more diverse chemistry for weed control. But the question remains: how do we chemically control weeds that are already biologically resistant now?

If you are struggling with herbicide resistant weeds, or if you’re curious about what sustainable weed control practices would work best in your context and you don’t want to wait for the forthcoming publication, you can always reach out to us directly for one-on-one technical assistance through our ATTRA line at 800-346-9140, via askanag@ncat.org, or through our website’s chat function. Our agriculture specialists, myself included, will be happy to talk through your challenges and work toward possible solutions for your farm.

 

Este blog ha sido elaborado por el Centro Nacional de Tecnología Apropiada (NCAT) a través del programa de Agricultura Sostenible ATTRA, en el marco de un acuerdo de colaboración con el Departamento de Desarrollo Rural del Departamento de Agricultura de los Estados Unidos (USDA). NCAT.ORG/ATTRA.