Old Weeds Return as Herbicide Resistance Raises New Risks for U.S. Crops
Cocklebur and ragweed are returning to U.S. fields as Palmer amaranth, waterhemp and marestail increase pressure on corn and soybean weed-control programs.
U.S. corn, soybean, small-grain and forage growers are ending the 2026 growing season with an increasingly complex weed problem, as familiar species such as cocklebur and common ragweed return to fields while giant ragweed, Palmer amaranth, waterhemp and marestail add new resistance challenges. Reports compiled this season by Penn State weed specialist Dwight Lingenfelter point to failures associated with herbicide timing, resistant populations, weather and residual products losing effectiveness before weed emergence ends. The issue matters beyond individual fields: USDA Agricultural Research Service describes herbicide-resistant weeds as a national challenge that is reducing chemical control options and increasing crop yield losses, making integrated weed management increasingly important.
Marestail remains one of the most persistent examples. Plants that survive spring burndown become progressively harder to control as they grow, particularly in soybean systems where resistance can eliminate important postemergence options. Penn State's July 2026 recommendations note that glyphosate-resistant weeds including marestail, Palmer amaranth and waterhemp cannot be controlled by glyphosate alone, while ALS-resistant marestail and pigweeds are also common. That combination makes early scouting, effective burndown programs and residual chemistry increasingly important. Growers also must match herbicide programs to soybean traits and crop growth stages rather than relying on rescue applications after weeds become established.
Weeds competing with crops can reduce yields and complicate harvest, especially when herbicide resistance limits growers' control options.
Annual and Italian ryegrass added another layer of difficulty during spring termination, particularly where cool, wet and cloudy conditions reduced the consistency of burndown programs. Application timing, cover-crop height, rates, adjuvants and tank-mix interactions can all influence performance. The larger lesson for growers is that herbicide performance cannot be separated from weather, weed size and application quality. Penn State Extension notes that weather can influence burndown performance and that insufficient rainfall can prevent soil-applied herbicides from activating properly. That becomes especially important when farms are trying to coordinate planting, crop insurance deadlines, machinery availability, input costs and increasingly narrow windows for effective weed control.
Old weeds return while resistant species raise the stakes
Cocklebur and common ragweed, weeds that many producers may associate with earlier generations of crop production, created renewed problems during 2026. One factor can be reduced reliance on atrazine in some corn programs, while repeated exposure can also influence herbicide performance in individual weed populations. Common ragweed populations may carry resistance to multiple herbicide groups, making longer-lasting residual programs and timely postemergence treatments increasingly valuable. The challenge is biological as much as chemical: many residual herbicides eventually lose activity, while species such as cocklebur and ragweed can continue emerging later into the growing season, leaving fields exposed after the original protection window has closed.
Giant ragweed may become an even bigger concern. Like its common-ragweed relative, it can be difficult to manage where populations develop resistance to multiple herbicide families. At the same time, Palmer amaranth and waterhemp continue to demand aggressive scouting. Penn State warns that these pigweeds can emerge well into the season and become very difficult to control after reaching roughly four inches in height. Its 2026 guidance emphasizes combinations of effective foliar treatments with overlapping residual herbicides rather than depending on a single chemistry. That approach reflects the broader resistance-management principle of diversifying control tactics and preventing surviving plants from replenishing the weed seedbank.
The yield implications explain why Palmer amaranth remains one of the most closely watched weeds in U.S. agriculture. Penn State has cited research in which severe Palmer amaranth infestations reduced soybean yields by as much as 78%, while high densities have been associated with corn yield losses reaching 91% under research conditions. Those figures should not be interpreted as typical farm losses, but they demonstrate the weed's competitive potential when populations escape control. Palmer and waterhemp can also produce very large numbers of seeds and continue emerging through the growing season, creating a long-term supply chain of weed pressure inside the field if seed production is not prevented.
Weed control is shifting from a spray decision to a full-season strategy
Cover crops are becoming another component of that strategy. Dense cereal rye or other competitive covers can suppress small-seeded weeds and reduce early-season growth, but they are not equally effective against every species. Penn State identifies cover crops as both a physical weed-suppression tool and part of herbicide-resistance management, while emphasizing broader integrated approaches that combine crop competition, rotation, cultivation and other practices. No single tactic is likely to provide durable control across the expanding mix of resistant weeds, particularly when large-seeded species or established perennial weeds can penetrate heavy crop residue.
The challenge extends beyond corn and soybeans. Hayfields and pastures continue to face horsenettle, hemp dogbane, Canada thistle and other persistent weeds, while annual grasses can reduce forage quality and compete with desirable species. Small-grain growers also face chickweed, speedwell, field pansy and problematic grasses, with fall management often offering a better control window than delayed spring treatments. Penn State notes that weed identification, life cycle and timing are fundamental to management decisions and that some winter annual species are easier to control in fall. For livestock operations, weed management therefore connects directly with forage yield, feed quality and the economics of producing homegrown feed.
For U.S. producers planning the 2027 crop, the message from the 2026 season is increasingly clear: weed control can no longer be viewed simply as selecting a herbicide and making a pass across the field. Scouting, accurate species identification, crop rotation, multiple effective sites of action, overlapping residuals, cover crops and preventing seed production all have roles in protecting future yields. USDA-ARS research is now explicitly focused on developing precision integrated weed-management systems for corn, soybean and cotton, combining new technology with nonchemical practices. As resistance expands and familiar weeds return, protecting yields will increasingly depend on managing the entire system rather than searching for a single rescue treatment.
