Tools for detection and mitigation of Palmer amaranth and waterhemp
In Canada, 14 species of Palmer amaranth have evolved herbicide resistance against eight modes of action, while waterhemp has evolved resistance against seven. The review showed that detection and control of any populations should be executed rapidly to prevent establishment and spread of infestations into new areas and reduce selection pressure on herbicides.
Herbicide-resistant weeds continue to evolve and spread across Canadian agro-ecosystems, which can increase management inputs and costs, reduce yields and crop quality and impede harvest efficiency. Two Amaranthus species including Palmer amaranth (Amaranthus palmeri) and the closely related species waterhemp (Amaranthus tuberculatus) are particularly concerning.
Although Palmer amaranth has not yet established a stable foothold in Canada, waterhemp is established in the provinces of Ontario and Quebec, and is spreading in Manitoba. These species have a history of invasion and spread into agro-ecosystems including much of the USA, and a propensity towards herbicide resistance evolution. Common pathways for Palmer amaranth introductions in the northern USA included animal feed systems with grain screenings, crop production systems including seed and equipment contamination, and spread by waterfowl and other animals.
For this project, a biovigilance framework approach was taken to build awareness of these two pigweeds, their initial invasion and spread in the USA, and to develop an understanding of how these weeds could be a threat to Canadian agriculture. A review of their key identification characteristics, potential hybridization, known herbicide resistance evolution, as well as the identification of potential tools for detection, assessment and mitigation options were also addressed.
From the review, 14 species of Amaranthus were found in Canada, nine having herbicide-resistant biotypes, including waterhemp. In addition, a total of 45 hybrids between various Canadian Amaranthus species with each other or Palmer amaranth have been noted. Both Palmer Amaranth and waterhemp show herbicide resistance to multiple modes of actions through a variety of mechanisms including metabolism, target-site mutations, altered absorption and translocation, and gene amplification. Palmer amaranth has evolved resistance against nine modes of action: Group 2, 3, 4, 5, 9, 10, 14, 15, and 27, while waterhemp has evolved resistance towards seven of the same modes of action, with the exception of Groups 3 and 10.
Integrated control required
Both Palmer amaranth and waterhemp tend to germinate and emerge later in the growing season and exhibit season-long emergence patterns compared with most summer-annual weeds in western Canada. Integrated management strategies for Palmer amaranth will include a chemical component. Proper dose and spray coverage with contact herbicides such as Groups 10, 14 and 22 will be important since this weed has the propensity to develop resistance to phloem-mobile, translocating herbicides such as Groups 2, 4, and 9. Due to the evolution of multiple herbicide-resistant biotypes, there are some parallel management considerations for Palmer amaranth and current kochia management on the Canadian Prairies.
Other management strategies could include herbicide-resistant crops, however there is limited availability. For those crops that do have herbicide-resistant crop options, such as in eastern Canadian row cropping systems, herbicide control of multiple-resistant Amaranthus species remains challenging. In some other crops there are limited options overall for registered herbicides. There may be opportunities to include other integrated weed management strategies such as cover crops, alternative crops, and cropping systems that may help limit the potential for Palmer amaranth to get established in Canadian cropping systems. However, strategies such as increased seeding rates and row spacing were not successful at reducing Palmer amaranth densities in the USA. Crops such as soybean in eastern Canada and pulses in western Canada may also be more vulnerable to competition with Palmer Amaranth.
Overall, the review showed that with the evolution of multiple herbicide-resistant biotypes for both Palmer amaranth and waterhemp, detection and control of any populations should be executed rapidly. Strategies such as regional awareness campaigns will be critical to support Canadian farmers in identifying and quickly mitigating invasions of Palmer amaranth and waterhemp to prevent establishment and spread of infestations into new areas.
Other monitoring strategies maybe be developed through further research, including expanded genetic testing capabilities to support the seed industry to ensure rapid detection of contaminants of these invasive amaranths. Monitoring and management control strategies will be key to preventing seedbank establishment and reducing selection pressure on herbicides in Canadian agro-ecosystems.
Shaun M. Sharpe, Sara L. Martin, Eric R. Page, and Charles M. Geddes. 2024. Palmer amaranth (Amaranthus palmeri S. Wats.) and waterhemp (Amaranthus tuberculatus (Moq.) J.D. Sauer) biovigilance in Canadian Agro-ecosystems. Canadian Journal of Plant Science. 104(4): 277-297. https://doi.org/10.1139/cjps-2023-0193
Waterhemp photo by Ingrid Kristjanson.
