Herbicide resistance basics

Glyphosate- and atrazine-resistant.

Palmer amaranth is a C4, summer annual pigweed species that under ideal conditions can grow 3 inches a day and reach 10 feet tall. Being dioecious, it produces male and female flowers on separate plants, and is a prolific seed producer. Female plants can produce over 500,000 seeds when growing alone and even under competition, can produce 100,000 

Due to its ability to evolve resistance to herbicides, rapid growth, and abundant seed production, Palmer amaranth presents a serious threat to annual field and specialty crop production. If poorly managed, Palmer amaranth can reduce soybean yields by up to 79% and corn yields by 91%.

Glyphosate resistance has been confirmed in three Palmer amaranth populations collected from soybean fields in New York's Genesee, Orange, and Seneca Counties  Two of these populations are also atrazine resistant (PSII – Group 5). Ongoing herbicide screening research suggests that these populations may be resistant to other herbicides including chlorimuron/cloransulam/halosulfuron/rimsulfuron (ALS – Group 2) and mesotrione (HPPD – Group 27). Full dose-response assays are ongoing to confirm and describe the level of resistance to these herbicides.

--Aleah Butler-Jones, Sosnoskie Lab

Glyphosate-resistant.

Waterhemp is a summer annual pigweed species. It is dioecious, producing male and female flowers on separate plants. As a C4 plant, it grows rapidly, about an inch a day. A single female plant can produce more than one million seeds and reach heights up to 12 feet. Due to its fast growth and ability to evolve resistance to herbicides, waterhemp presents a serious risk to soybean and corn fields. If not managed properly, waterhemp can lower crop yields by up to 43% in soybean and 74% in corn.

Glyphosate resistance has recently been confirmed among two waterhemp populations from soybean fields in Seneca County, New York. Ongoing herbicide screening work further suggests that majority of the waterhemp populations in New York are glyphosate-resistant as well as potentially resistant to other herbicides, including chlorimuron/thifensulfuron (ALS – Group 2) and mesotrione (HPPD – Group 27), and atrazine (PSII – Group 5). Full dose-response assays are underway to confirm and characterize the level of resistance to these herbicides.

--Henrique Scatena, Kumar Lab

Glyphosate-, paraquat-, and diquat-resistant.

Horseweed is a winter or summer annual with dense leafy foliage and mostly unbranched stems. Seeds germinate and emerge often in fall or spring. Small overwintering rosettes are easily managed with spring cultivation. But horseweed is a frequent problem in perennial crops such as grapes and strawberries or no-till grain crops. Horseweed is sensitive to competition. So management may include rotations of competitive crops with dense canopies to suppress horseweed growth. 

Horseweed herbicide resistance has been reported in 27 states (as of April 2025), with widespread resistance to glyphosate. Cornell’s Specialty Crops Weed Science Lab recently confirmed multiple populations of paraquat- and diquat-resistant horseweed in New York grapes and apples (Schulteis et al. 2025).  The lab also recently confirmed several horseweed populations in a New York soybean field with multiple resistance to glyphosate and a premix of chlorimuron and thifensulfuron herbicides.  

--Jane Petzholdt, Sosnoskie Lab