Weed resistance has continued to expand over the years with resistant weed species now found in all major field crops, including corn, soybeans, cotton and wheat. The number of herbicide resistant weed species has continued to grow and some species exhibit resistance to PPO, glycine (glyphosate) and most recently HPPD mode-of-actions. In some cases, individual weed species have developed resistance to multiple modes-of-action. Glycine resistance is particularly concerning due to the widespread utility of the product and the aggressive nature of some of the weeds that have developed resistance to glyphosate.
Resistant weeds that are known to be very aggressive, such as waterhemp, Palmer amaranth, marestail, lambsquarters and common and giant ragweed, are often referred to as ‘driver weeds’ since they alone can make some fields unfit for growing corn, soybeans and cotton. In wheat, tough-to-manage and resistant weeds include kochia, Russian thistle, wild oat, green foxtail, prickly lettuce, Palmer amaranth and other common pigweed species.
Severity of Waterhemp and Palmer Amaranth Resistance
Farmers in the Midsouth and Southeast have recognized for several years that resistant Palmer amaranth can be devastating to fields. Resistant Palmer amaranth continues its spread into the Midwest, Southwest and even the Far West. Waterhemp is also in the firefight as the first broadleaf weed to become resistant to multiple herbicide classes: ALS inhibitors, PPO inhibitors, glyphosate and HPPD inhibitors. Alternative herbicide-tolerant seed traits and compatible herbicides will offer other options for corn, soybean and cotton growers.
The growing prevalence of weed resistance coincides with the fact that no new herbicide modes of action have been introduced in the past 30 years. Therefore, it is imperative to use best management practices to sustain the viability of existing chemistries and help protect crop yields.
Dr. Jason Norsworthy, University of Arkansas professor and weed scientist, points to his research and experience in following the development of Palmer amaranth and waterhemp weed resistance. “Growers need to take a year-round approach to weed control. Zero tolerance to prevent weeds from going to seed offers the best way to manage resistance and keep herbicides and fields sustainable,” he says.
“Growers need to use diversity in weed management practices to keep weeds confused and fight resistance,” Norsworthy says.
In a four-year research trial near Fayetteville, Ark., Norsworthy conducted a trial in which resistant Palmer amaranth weed seeds were introduced into four fields. In year one, only a streak of weeds escaped herbicide treatments. By year three, all four fields were overtaken by resistant weeds, resulting in complete crop failure.
Norsworthy also cites the example of one Arkansas farmer’s diversified weed control practices, which he began in 2006. The farmer was diligent in field scouting and using herbicide with multiple modes of action. Any weed escapes from emergence through harvest were removed by hand. The farmer was an early adopter of the zero-tolerance mindset, allowing him to keep herbicide costs in check, while his neighbors sometimes bore triple the herbicides costs to manage resistant weeds.
“Growers need to use diversity in weed management practices to keep weeds confused and fight resistance,” Norsworthy says. “One weed escape is too many. Rotate different classes of chemistry with different modes and sites of action, and use other cultural practices such as tillage, crop rotation and hand removal of weeds.”
Crop Science Solutions
Crop Science has a broad portfolio to combat tough-to-control and resistant weeds. A well-thought-out herbicide program, using multiple modes of action, should be implemented to sustainably manage weeds. Before applying any herbicide, please read the entire label for the best possible results and to confirm that the product is effective on the weeds you wish to control. Not every product is suitable for every situation, and use of the correct application technique will ensure the best results.
The following Crop Science solutions are valuable tools to consider for your program.
To learn more about using herbicides with effective modes of action, visit the Crop Science website at bayercropscience.us/products/weed-management/resources. You can also find information about Respect the Rotation™, the Crop Science resistance management program, at bayercropscience.us/learning-center/articles/herbicides-respect-the-rotation.