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

Objectives:

• To complete the final five years of a GFO-funded project that was established in 2017 to determine the depletion of multi-herbicide resistant (MHR) waterhemp seed in the seedbank using integrated weed management (IWM) practices. The decrease of waterhemp seed in the soil seedbank after each cycle of a three-crop rotation of corn, soybean and wheat will be ascertained. The overall objective is to reduce the number of waterhemp seeds in the seedbank by greater than 90%.

• To develop IWM strategies, based on a more holistic approach, which 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 was developed for the control of MHR waterhemp which resulted 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 Ontario Ministry of Agriculture, Food, and Rural Affairs (OMAFRA) data (2019-2023), the five-year average corn yield was 168.1 bu/ac and the average selling price was $6.56/bu for a value of $1103/ac. For Ontario corn farmers, the monetary loss with average and heavy waterhemp pressure would be $210 and $1092/acre, respectively. Over the same five-year period corn was grown on 2.23 million acres. Assuming, an average yield loss of 19% due to waterhemp interference, with waterhemp on 4% of the Ontario corn acres (survey of Ontario CCAs) there would be an annual farm-gate loss of $18 million.
• 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 93% in fields with heavy waterhemp pressure. Based on the most recent OMAFRA data (2019-2023), the five-year average soybean yield was 49.2 bu/ac and the average selling price was $15.60/bu for a value of $768/ac. For Ontario soybean farmers, the monetary loss with average and heavy waterhemp pressure would be $330 and $714/acre. Over the same five-year period soybean was grown on 2.95 million acres. Assuming, an average yield loss of 43% due to waterhemp interference, with waterhemp on 4% of the Ontario soybean acres (survey of Ontario CCAs) there would be an annual farm-gate loss of $39 million.
• A more sustainable MHR waterhemp strategy was developed that includes three crops, cover crops, reduced soybean row width, and eight herbicide modes-of-action. The 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 herbicide resistant (HR) weeds in Ontario. Additionally, this research demonstrated 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, 5-way herbicide-resistant (HR) waterhemp (Group 2, 5, 9, 14, and 27-resistant waterhemp) has been confirmed in seven Ontario counties (Essex, Chatham-Kent, Lambton, Elgin, Middlesex, Northumberland and Stormont/Dunda/Glengarry). 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 spread rapidly across Ontario.

The primary goal of this study was to determine if the number of MHR waterhemp seeds in the soil seedbank could be reduced using a diversified, integrated weed management (IWM) program. The study included continuous soybean, two-crop rotations (corn/soybean and soybean/wheat), and three-crop rotations (corn/soybean/wheat and corn/soybean/wheat+cover crop), narrow row widths, and the use of efficacious herbicides. In the most diverse crop rotations, ten different herbicide active ingredients were used (dicamba, atrazine, bromoxynil, glyphosate, flumioxazin, pyroxasulfone, S-metolachlor, mesotrione, bicyclopyrone, and pyrasulfotole) from seven herbicide groups (4, 5, 6, 9, 14, 15 and 27).

Results:

The depletion of waterhemp seed in the seedbank was determined after years 3 and 6 of this study. To document the decrease in the seedbank, the number of waterhemp seeds in the soil when the study was initiated had to be determined. When the study was initiated, the average number of waterhemp seeds in the seedbank at the Cottam and Walpole sites were 164 million seeds/acre and 12 million seeds/acre, respectively.

On September 1, at the Cottam site, the waterhemp ground cover in the continuous soybean with only glyphosate applied was 83% in 2017, 78% in 2018, 73% in 2019, 79% in 2020, 85% in 2021 and 68% in 2022. In 2022, the waterhemp ground cover was 0% with the remaining 11 treatments evaluated.

On September 1, at the Walpole Island site, the waterhemp ground cover in the continuous soybean with only glyphosate applied was 69% in 2017, 21% in 2018, 24% in 2019, 8% in 2020, 5% in 2021 and 6% in 2022. In 2022, the waterhemp ground cover was 0% with the remaining 11 treatments evaluated.

At the Cottam site, after three years in the continuous soybean with Roundup there was a decrease in the number of waterhemp seeds in the seed bank of 7%. In contrast, in the continuous soybean with Fierce followed by Roundup Xtend, the two-crop rotation of corn/soybean, the two-crop rotation of soybean/wheat, the three-crop rotation of corn/soybean/wheat and the three-crop rotation of corn/soybean/wheat followed by an oat/tillage radish cover crop caused a decrease in the number of waterhemp seeds in the seedbank of 76, 80, 68, 77 and 82% respectively. At the Cottam site, after six years in the continuous soybean with Roundup there was a decrease in the number of waterhemp seeds in the seed bank of 95%. In the continuous soybean with Fierce followed by Roundup Xtend, the two-crop rotation of corn/soybean, the two-crop rotation of soybean/wheat, the three-crop rotation of corn/soybean/wheat and the three-crop rotation of corn/soybean/wheat followed by an oat/tillage radish cover crop caused a decrease in the number of waterhemp seeds in the seedbank of 99, 98, 96, 97 and 97% respectively.  

At the Walpole Island site, after three years, the continuous soybean with Roundup there was a decrease in the number of waterhemp seeds in the seed bank of 22%. In the continuous soybean with Fierce followed by Roundup Xtend, the two-crop rotation of corn/soybean, the two-crop rotation of soybean/wheat, the three-crop rotation of corn/soybean/wheat and the three-crop rotation of corn/soybean/wheat followed by an oat/tillage radish cover crop caused a decrease in the number of waterhemp seeds in the seedbank of 36, 46, 47, 26 and 8% respectively. At the Walpole Island site, after six years, in the continuous soybean with Roundup there was a decrease in the number of waterhemp seeds in the seedbank of 89%. In the continuous soybean with Fierce followed by Roundup Xtend, the two-crop rotation of corn/soybean, the two-crop rotation of soybean/wheat, the three-crop rotation of corn/soybean/wheat and the three-crop rotation of corn/soybean/wheat followed by an oat/tillage radish cover crop caused a decrease in the number of waterhemp seeds in the seed bank of 77, 83, 82, 77 and 81% respectively.

This research conclusively shows that waterhemp ground cover can be reduced dramatically by using an integrated weed management program. The depletion in waterhemp seed in the soil seedbank at the Cottam site was very encouraging after year three of this study; however, the results after year six were extremely disappointing. The researcher does not know why. Possibly it was due to waterhemp seed decay in the seedbank due to natural causes, but the researcher thinks that is unlikely. Possibly the innate dormancy of the seed collected was extremely high and seed dormancy was not broken with the methodology used.

Recommendations:

• Farmers should proactively introduce more diversity in their crop/weed management programs.
• Implement a diverse crop rotation with a combination of summer annual crops (corn, soybean) and winter annual crops (winter wheat). Include other crops as they fit your farming operation.
• Utilize multiple herbicide modes of action over time.
• Plan your crop/weed management program 5 years into the future. Minimize the repeated use of herbicides with the same mode of action.
• Use tillage at strategic points in the rotation.
• Plant cover crops after winter wheat combining.
• Plant in narrow rows where possible (soybean).
• Purchase a combine with harvest weed seed control.
• Implementation of IWM strategies can provide near-perfect MHR waterhemp control in corn, soybean, and wheat in Ontario.

External Funding Partners:

BASF

Bayer

Syngenta

Valent

Project Related Publications:

Soltani, N., Shropshire, C. and Sikkema, P. 2023. Integrated weed management strategies for the depletion of multiple-herbicide-resistant waterhemp seed in the soil seed bank. Weed Technology. 37:108-112.