Keeping Watch On Resistant Weeds

Recently, farmers have seen broadleaf weeds such as Palmer amaranth and waterhemp develop resistance to previously effective herbicides. These superweeds pose a threat to production and yields. To help you stay on top of managing these yield-robbing threats, we’ll identify some of the worst offenders, the damage they can cause and how you can control them.

Palmer amaranth

After emerging as a new weed threat during the late 1980s, Palmer amaranth has developed increased resistance in just 30 years, adapting to five different herbicide sites of action. The first glyphosate-resistant case of Palmer amaranth was confirmed in Georgia in 2005. Changing conditions have led to additional instances being reported in the southwestern United States as well as the Midwest. 

Palmer amaranth is classified as a dioecious plant, which means male and female flowers grow on separate plants. This inherent genetic diversity elevates the spread of herbicide resistance and adaptive traits that boost Palmer amaranth’s survival in farm fields. 

Aggressive and strong, Palmer amaranth devastates crops. Over the course of a growing season, the presence of this weed at a rate of 2.5 plants per foot of row may reduce soybean yield by up to 79%.1 And with just one female Palmer amaranth plant being able to produce 600,000 seeds, it’s easy to understand why this weed is so problematic.


Glyphosate-resistant waterhemp was first identified in Missouri more than 16 years ago. Since its discovery, it has spread to more states and has exhibited a growing resistance to additional classes of chemicals.

Waterhemp can quickly infest an area with upwards of 20 plants per square foot. It’s known for rapid growth — typically 1 to 1.25 inches2 per day during the growing season — and this growth has been shown to reduce soybean yield by as much as 44%.3

While waterhemp can emerge at any time during the growing season, a higher number of plants emerge later in the season than is typical for most summer annual weeds. The delayed nature of its emergence helps it to avoid many pre-emergent weed applications and may have contributed to its resistance of postemergent herbicide applications, like glyphosate.

A prolific seed producer, waterhemp can produce as many as 250,000 seeds per plant. These seeds can then be easily transported and remain viable in the soil for up to several years.

Like Palmer amaranth, waterhemp is classified as a dioecious plant, which gives the plant greater genetic diversity and adaptability and increases the likelihood of the spread of herbicide-resistant species.

A system for controlling resistant weeds

Palmer amaranth and waterhemp are born fighters — but so is every farmer. Through chemical research and crop system innovations, farmers have been able to gain more control over these tough weeds.

The Roundup Ready® Xtend Crop System puts advanced weed control to work in your fields. Combining control of tough weeds, like Palmer amaranth and waterhemp, with proven yield performance gives soybean and cotton farmers the results they need. Most importantly, the Roundup Ready Xtend Crop System provides tolerance to both dicamba and glyphosate herbicides. And with the addition of XtendFlex® soybeans (expected soon), you’ll have more flexibility and choice for herbicide options with additional tolerance to glufosinate as well as dicamba and glyphosate.

Advanced weed control at work in your fields

Part of what makes the Roundup Ready Xtend Crop System so effective is its tolerance to dicamba. When you’re dealing with an herbicide-resistant superweed, like Palmer amaranth or waterhemp, effective ways to fight are by using a full-system approach. Additionally, use overlapping residuals and diversified sites of action in your pre-emergent and postemergent applications and include XtendiMax® herbicide with VaporGrip® Technology, a restricted use pesticide.

XtendiMax herbicide is specifically formulated to work in conjunction with the Roundup Ready Xtend Crop System to combat glyphosate-resistant weeds. XtendiMax herbicide provides up to 14 days of soil activity on certain small-seeded broadleaf weeds and has proprietary technology that provides a significant reduction in possible off-target movement potential compared to generic dicamba formulations.

Say goodbye to next season’s weeds

To learn more about the Roundup Ready Xtend Crop System and how XtendiMax herbicide can work for you, click here.

Already planning on applying XtendiMax herbicide and have questions on how to go about it? Get registered for training here.