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Across Ohio, weeds are attempting to make their home in soybean farmer’s fields. Weeds that were once easily managed by herbicides such as glyphosate are showing resistance, calling for farmers to implement more diverse and stringent control methods.

Even if farmers have not experienced this problem in their fields yet, proper management of marestail, giant ragweed, common ragweed, waterhemp, Palmer amaranth and other herbicide-resistant species is important to avoiding these challenges.

According to a recent checkoff-funded survey, 53 percent of farmers felt that herbicide resistance was a moderate or severe problem on neighboring farms, while only 35 percent thought of it as a problem on their own farm. Weed seeds however, do not know boundaries, and what once was a neighbor’s problem, can soon spread. Herbicide-resistance is rapidly moving across the entire U.S. soybean-producing region.

Mark Loux, Ph.D., horticulture and crop science professor at The Ohio State University, recommends the following practices to effectively manage for weeds:

• Recognize the different biology of weeds. Not all weeds fit into one type of management program.
• Ensure that weeds don’t limit your yield potential by implementing control early in the growing season.
• Add diversity to herbicide applications, including using different herbicides and sites-of-action.
• Focus on the amount of seed returned to the soil. In some cases, weeds left standing can produce enough seeds to create thousands of additional plants.

According to Loux, diversification in weed management is the key. “We are wearing out the effective tools and any new tools coming can also lose effectiveness,” he says.

Implementing a weed-management plan can help farmers combat weeds. For help developing a weed-management plan specific to your farm, contact your local extension agent or crop advisor to discuss recommendations.

In collaboration with a team of extension weed scientists from 15 universities, including The Ohio State University, the soybean checkoff is leading the Take Action effort to help farmers in Ohio and across the country manage herbicide-resistant weeds. Take Action aims to increase farmers’ awareness of the effects that these weeds can have on yields, as well as recommend management strategies. Take Action is supported by BASF, Bayer, DuPont, Dow, Monsanto and Syngenta.

 Facts about Herbicide-Resistant Weeds in Ohio:

Marestail (Horseweed)

• Seeds can germinate as soon as they fall off of a mature plant.
• Plants emerge from late March through June and late summer into fall.
• Competes with soybeans throughout the growing season and reduces crop yield.
• Glyphosate-resistant populations in Ohio were first reported in areas where the following production practices are common:
  • soybeans grown in the same field for consecutive years
  • use of only glyphosate for weed control
  • little or no tillage

Giant Ragweed

• A single plant can produce up to 5,100 seeds, which germinate from late March through July. Giant ragweed is one of the first summer annuals to emerge in the spring and is often present before crops are planted.
• Plant height often reaches one to five feet taller than the crop with which it is competing, but can reach up to 17 feet tall.

• Soybean yield losses as high as to 50 percent can occur with season-long interference.
• Giant ragweed biotypes in Ohio have developed resistance to glyphosate, while some have been identified with resistance to ALS inhibitor as well. Glyphosate resistance has developed in continuous Roundup Ready soybean fields in response to repeated exclusive use of the herbicide. Resistant plants are not completely immune to glyphosate, but resistance appears to be the cause of poor control in some fields.

Palmer Amaranth

• Has the capability to produce at least 100,000 seeds when in competition with a crop, and upward of a half-million seeds with no competition. The small seeds are easily transported through grain, seed, or by equipment.
• Grows aggressively at a rate of two to three inches per day in ideal conditions.
• Creates yield losses of 17-79 percent in soybeans when allowed to compete throughout the growing season.
• Resistant to multiple herbicide modes of action, including ALS inhibitors, triazines, HPPD-inhibitors, dinitroanilines and glyphosate, with a large majority of southern populations being glyphosate- and ALS-inhibitor-resistant
• Waterhemp in west central Ohio and common ragweed in northwest Ohio have also been identified with resistance to glyphosate and ALS inhibitors.

Mark’s presentation on Herbicide Mode of Action

Herbicide Classification poster

Site of Action presentation

Agronomic Crops Network Weeds link

Source: The Ohio State University