Managing Glyphosate-Resistant Waterhemp in NY Corn and Soybean

Waterhemp (Amaranthus tuberculatus) is a summer annual broadleaf weed native to the United States. It is one of the most problematic pigweed species in the Midwest and it is spreading throughout the eastern and southern United States. The increasing spread of waterhemp in New York (NY) cropping systems has become a concern as it is a highly prolific weed that has evolved resistance to many herbicides (seven groups: 2, 4, 5, 9, 14, 15 and 27). A recent survey by the Weed Science Society of America has ranked waterhemp as the most troublesome and hard-to-control weed in agronomic crops.

Identifying features

Waterhemp is often confused with other pigweeds. Unlike other pigweeds, waterhemp leaves and stems are smooth, green, red, green-red striped and hairless, whereas redroot and smooth pigweed have hair on stem. The leaf stalk (petiole) of waterhemp is usually shorter (half as long as the blade than the leaf blade) and is one of the key characteristics to distinguish it from Palmer amaranth. The inflorescences on waterhemp have slender branches that are typically less than 6 inches. For more details on waterhemp identification, read Cornell University pigweed identification fact sheet.
 

waterhemp plant and infestation in soybean field
waterhemp infestation in young corn

Figure 1. Above left to right: Waterhemp plant, and infestations of waterhemp in soybean field and in corn field in New York.

Waterhemp biology 

Waterhemp is an erect plant that grows up to 8 feet tall, with glossy alternate-ovate shaped leaves (Figure 1). Seedlings are light green to red-pink with prominent veins, and a shallow notch at the leaf tip growing 1.24 inches per day. Mature plants can reach up to 2.5 to 8 feet tall with male and female green flowers produced by separate plants. Female waterhemp plant under favorable conditions can produce as many as 1 million seeds, but 35,000-200,000 seeds are usually reported when they grow in corn-soybean fields. Waterhemp seeds remain viable in the soil for 5 to 6 years. If left uncontrolled, waterhemp can significantly reduce soybean yields by as much as 63% and corn up to 74%.

Herbicide resistance status

Herbicide-resistant waterhemp has become a significant problem in the United States. Waterhemp populations resistant to ALS inhibitors (Group 2: Harmony, Classic), synthetic auxins (Group 4: Clarity, 2,4-D), photosystem ll inhibitors (Group 5: Atrazine), 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) inhibitors (Group 9: Roundup), PPO inhibitors (Group 14: Cobra, Flexstar), long-chain fatty acid inhibitors (Group 15: Dual II Magnum), and HPPD inhibitors (Group 27: Callisto) have been reported in the United States. More recently, glyphosate-resistant waterhemp has been reported in several western and central NY counties. For instance, two waterhemp populations from Seneca County, NY have been found 5.6- to 8.3-fold resistance to glyphosate1. Furthermore, glyphosate-resistant populations are also potentially resistant to other herbicides, including atrazine (Group 5), chlorimuron/thifensulfuron (Group 2) and mesotrione (Group 27).  

Management options

Sanitation and cleaning. Farm equipment and machinery (such as combines) are known to transport weed seeds from contaminated fields to cleaned fields. Proper cleaning and sanitation of farm equipment after use in waterhemp infested fields can help to reduce its further spread.

Cultural practices. Cultural practices such as rotation of annual grain crops with perennial forage crops, fall-seeded cover crops and termination after corn-soybean planting2, and deep tillage as needed can help reduce waterhemp densities. The use of best agronomic practices, including proper selection of competitive crop variety/hybrid, optimum seeding rates, reduced row spacing, planting times, and proper fertility and irrigation management can increase corn and soybean competitiveness with waterhemp.

Mechanical practices. Tillage greatly reduces waterhemp emergence, and moldboard plowing is more effective than chisel plowing for reducing emergence. Inversion tillage, ridge-tillage, and interrow mowing in row crops can be a component of waterhemp management strategies. Recent work has shown that Weed Zapper (electric weeder) and harvest weed seed control (both seed impact mill and chaff lining) can help managing waterhemp in soybean by reducing seedbank in the soil 3,4,5. 

Chemical options. Herbicide programs that utilize more than two effective sites of action, and a management plan that is diverse and includes multiple control tactics, are generally most effective for glyphosate-resistant waterhemp management.

  • Soybean: Preemergence herbicides which are premix of 2 to 3 active ingredients such as acetochlor, chlorimuron, cloransulam-methyl, S-metolachlor, metribuzin, flumioxazin and postemergence herbicides such as glufosinate (Liberty Link soybean)6, 2,4-D choline (Enlist E3 soybean), lactofen, and fomesafen can effectively control glyphosate-resistant waterhemp.
  • Corn: Preemergence herbicides which are premix of 2 to 3 active ingredients such as acetochlor, atrazine, bicyclopyrone, clopyralid, S-metolachlor, mesotrione, and postemergence herbicides such as glufosinate (Liberty Link corn), 2,4-D (Enlist corn), dicamba, tembotrione, tolpyralate, etc. can provide effective control of glyphosate-resistant waterhemp.

Soil residual herbicides (acetochlor, dimethenamid-P) applied postemergence in corn and soybean can provide extended overlapping residual activity for season-long control of waterhemp.

Citations:

  • 1Kumar V, Aulakh J, Stanyard M, et al. (2025) Confirmation of glyphosate-resistant waterhemp (Amaranthus tuberculatus) in New York. Weed Technology, 39:e28
  • 2Stephens T, Blanco-Canqui H, Knezevic SZ, Rees J, Kohler-Cole K, Jhala AJ (2024) Integrating fall-planted cereal rye cover crop with herbicides for reducing Palmer amaranth seed production in soybean under planting green conditions. Agrosystems, Geosciences & Environment, 7, e20507
  • 3Bennett AJ, Yadav R, Jha P (2023) Using soybean chaff lining to manage waterhemp (Amaranthus tuberculatus) in a soybean–corn rotation. Weed Science, 71(4):395-402
  • 4Schreier H, Bish M, Bradley KW (2022) The impact of electrocution treatments on weed control and weed seed viability in soybean. Weed Technology, 36(4):481-489
  • 5Winans T, Massey R, Schreier H, Bish M, Bradley KW (2023) Harvest weed seed control in soybean with an impact mill. Weed Technology, 37(2):113-122
  • 6Aulakh JS, Jhala AJ (2015) Comparison of glufosinate-based herbicide programs for broad-spectrum weed control in glufosinate-resistant soybean. Weed Technology, 29:419–430

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