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Ontario Grain Farmer Magazine

Integrated weed management strategies for the control of glyphosate-resistant waterhemp

Principal Investigator: Peter Sikkema

Research Institution: University of Guelph

Timeline: April 2021 – March 2026

Objectives:

  • To develop IWM strategies, based on a more holistic approach, that includes crop rotation, cover crops, reduced soybean row width, and efficacious herbicides from eight herbicide groups for control of MHR waterhemp.
  • To document the economic benefit to Ontario grain farmers of using IWM strategies to manage MHR waterhemp.
  • To disseminate information on an IWM approach to manage MHR waterhemp to growers and agri-business personnel in Ontario.

Impacts:

  • An IWM strategy will be developed for the control of MHR waterhemp which will result in near-perfect control and depletion of waterhemp seed in the seedbank. Consequently, the spread of MHR across Ontario will be slowed due to improved control and reduced weed seed return to the soil. The number of Ontario counties with MHR waterhemp from 2014-2021 was 1, 3, 3, 4, 7, 13, 14, and 15, respectively.
  • The monetary benefit of this research to Ontario corn farmers is tremendous. In studies conducted on commercial farms in Ontario, waterhemp interference in corn caused an average yield loss of 19% which increased to 99% in fields with heavy waterhemp pressure. Based on the most recent OMAFRA data (2016-2020), the five-year average corn yield was 163 bu/ac and the average selling price was $5.21/bu for a value of $848/ac. For Ontario corn farmers, the monetary loss with average and heavy waterhemp pressure would be $161 and $840/acre. Over the same five-year period corn was grown on 2.14 million acres. Assuming, an average yield loss of 19% due to waterhemp interference, with waterhemp on 1 and 5% of the Ontario corn acres there would be an annual farm-gate loss of $3.5 and 17.3 million, respectively.
  • The monetary benefit of this research to Ontario soybean farmers is tremendous. In studies conducted on commercial farms in Ontario, waterhemp interference in soybean caused an average yield loss of 43% which increased to 99% in fields with heavy waterhemp pressure. Based on the most recent OMAFRA data (2016-2020), the five-year average soybean yield was 48 bu/ac and the average selling price was $12.56/bu for a value of $597/ac. For Ontario soybean farmers, the monetary loss with average and heavy waterhemp pressure would be $257 and $591/acre. Over the same five-year period soybean was grown on 2.9 million acres. Assuming, an average yield loss of 43% due to waterhemp interference, with waterhemp on 1 and 5% of the Ontario soybean acres there would be an annual farm-gate loss of $7.6 and 37.9 million, respectively.
  • A more sustainable MHR waterhemp strategy will be developed that includes three crops, cover crops, reduced soybean row width, and eight herbicide modes-of-action. IWM strategies developed from this study will a) enhance the control of MHR waterhemp, b) increase corn and soybean yields, c) increase net returns to Ontario grain farmers, and d) reduce the selection intensity for additional HR weeds in Ontario. Additionally, this research will determine the effectiveness of IWM strategies for the depletion of MHR waterhemp in the seedbank.
  • Another benefit of this study for Ontario corn, soybean and wheat producers is that they will be able to develop an effective IWM program for their farm based on sound science rather than anecdotal stories. Growers will knowledgeably develop IWM systems that result in excellent weed control while maintaining overall farm profitability.

Scientific Summary:

Glyphosate-resistant (GR) waterhemp was first confirmed in Ontario from seed collected from one field on Walpole Island in 2014. GR waterhemp has now been confirmed in 15 Ontario counties; it was found east of Toronto for the first time in 2019 in Northumberland County and in 2021 it was found in Glengarry County adjacent to the Quebec border. Subsequently, the first 4-way herbicide-resistant (HR) weed biotype in Ontario – Group 2, 5, 9, and 14-resistant waterhemp was confirmed in Middlesex county. Four-way multiple-herbicide-resistant (MHR) waterhemp has now been confirmed in eight Ontario counties. Studies conducted on Ontario farms found that waterhemp interference causes an average yield loss of 43 and 19% in soybean and corn, respectively. 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.

This 9-year project has been established on two commercial farms in Ontario (the first 5 years have been completed) and will study the impact of many IWM practices on the depletion of waterhemp seed in the seedbank. Corn, soybean, and wheat is being grown in a 3-year rotation, with a cover crop seeded after winter wheat harvest. The most efficacious herbicides from eight herbicide groups will be used in each crop. The objective of this IWM project will be to obtain near-perfect waterhemp control in each crop to minimize weed seed return to the soil. Waterhemp seed density in the seedbank was determined prior to initiating the experiment and will be determined after the 3rd, 6th, and 9th year of the study (after one, two and three cycles of a 3-year crop rotation). Based on this study, Ontario Grain Farmers will have local data on the effectiveness of IWM strategies to control MHR waterhemp and deplete MHR waterhemp seed in the soil seedbank.

After the first cycle of a three-crop rotation the number of waterhemp seeds in the seed bank decreased from 165 to 30 million seeds/acre, or a decrease of 82% at the trial established on a commercial farm near Cottam, ON. 

External Funding Partners:

BASF

Bayer

Syngenta

Valent