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Integrated Weed Management strategies for the control of glyphosate-resistant waterhemp

Principal Investigator

Peter Sikkema

Research Institution

University of Guelph

External Funding Partners

BASF, Bayer Crop Science Inc., Monsanto Inc., Syngenta Inc., Valent Canada Inc.

Project Start

May 2017

Project End

April 2021


  • To develop an Integrated Weed Management (IWM) strategy that will provide near perfect glyphosate-resistant waterhemp control by depleting the seedbank.


  • The study of long-term, sustainable weed management strategies may lead to the development of an Integrated Weed Management strategy to control glyphosate-resistant waterhemp in corn, soybean and wheat rotation.

Scientific Summary

The overreliance on a single weed management strategy or a simplified crop rotation may have short-term advantages such as simplicity and possible short-term profit maximization, but may have long-term detrimental effects due to the evolution of herbicide-resistant biotypes. Herbicides have been a very cost-effective option for weed management in field crops for over 70 years. But the overreliance on herbicides has resulted in the evolution herbicide-resistant biotypes, sometimes multiple-resistant biotypes as is the case with waterhemp (Amaranthus tuberculatus var. rudis) in Ontario. Studies conducted on Ontario farms showed waterhemp pressure can result in up to 48% yield loss in corn and up to 73% yield loss in soybean. Glyphosate-resistant (GR) waterhemp has been confirmed in 40 fields in Essex, Chatham-Kent and Lambton Counties. To make matters worse for Ontario farmers, 61% of seed samples collected had 3-way multiple resistance to Group 2 (Pursuit), Group 5 (Atrazine) and Group 9 (Roundup) herbicides. This dramatically reduces the herbicide options for controlling this competitive weed. This small-seeded, summer annual, broadleaf weed has an extended emergence pattern, has high genetic diversity, is a prolific seed producer, is very competitive and has the potential to spread rapidly throughout Ontario if not properly controlled.

This project aims to study many of the principles of Integrated Weed Management (IWM) to deplete waterhemp seed in the seedbank and to develop a more sustainable approach to weed management using multiple weed management tactics. The diverse crop rotation will include crops with different seeding and harvesting times, crops with different row widths and seeding densities, the inclusion of cover crops, and the use of multiple herbicide modes-of-action. These integrated strategies are expected to limit the selection of herbicide-resistant waterhemp, reduce seed return to the seedbank and reduce its movement from field-to-field in Ontario. Waterhemp seed density in the seedbank will be determined prior to initiating the experiment and after the 3rd year of the study (after one cycle of a 3-year crop rotation). Corn, soybean and wheat will be grown in a 3-year rotation, with a cover crop seeded after winter wheat harvest. The most efficacious herbicides will be used in each crop. As a result of this research, Ontario grain farmers will have data from local studies on the effectiveness of many of the principles of IWM that could lead to the development of long-term, sustainable weed management strategies for control of waterhemp in corn, soybean and wheat rotation.

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