PROJECT RESULTS

Residual Activity of Herbicides in Soil

Applicant: 

Dr. Annemieke Farenhorst

Department of Soil Science

University of Manitoba

Winnipeg, Manitoba  R3T 2N2  Canada

Table of Contents:

• Background and Objectives
• Procedure and Project Activities
• Results and Discussion
• Conclusion

ARDI Project:

Total Approved:

Date Approved:

Project Status:

#98-188

$47,124

January 15, 1999

Completed December, 2003

Background and Objectives:

This study was conducted from 1999 to 2001 and included two growing seasons.  The research project focused on herbicide residual effects of group 2 herbicide products in soil.  Herbicide residual effect is here defined as the portion of herbicide residues in the soil that is available to plants by root uptake and will cause adverse effects on sensitive plants grown in that soil.

The specific objective of the research project was to compare herbicide residual effects in conventional-till versus reduced-till fields to assess the effect of the tillage system on the effectiveness of control of multiple flushes of weed emergence and growth, and on herbicide carry-over from one year to the next.  Since field trials were conducted using two different herbicide products and four different soil types, secondary objectives of this research project were:

1. To compare herbicide residual effects for Odyssey versus Pursuit in soil.

2. To compare herbicide residual effects in soils with different clay, soil pH and soil organic matter contents.

3. To compare the effect of slope position on herbicide residual effects in fine-textured soils.

The results reported here are specific to Odyssey and Pursuit herbicide products.

Procedure and Project Activities:

Both Odyssey and Pursuit were applied at recommended field rates onto four different soil types, each under two different tillage systems.  The four soil types were a well-drained, sandy-loam Hallboro soil; a well-drained, clay-loam Newdale soil; an imperfectly-drained, clay/clay-loam Fairland soil; and an imperfectly drained, clay-loam Varcoe soil.  The two tillage systems were fields under conventional-till (heavy cultivator) and fields under reduced-till (direct seeding).  The reduced-till fields were chosen because they had been under reduced till management for at least 6 years.  The conventional-till fields were selected if they were adjacent to a reduced-till field.  During both growing seasons, soil moisture contents in fields were very high due to frequent rainfall events.  It is important to realize that herbicide persistence and residual effects are generally less in moist soils than when soils are very dry.  However, soils that are waterlogged for a long-time may show greater persistence of herbicides because the lack of oxygen in these soils will reduce soil microbial communities that are otherwise capable of degrading the herbicide.

Results and Discussion:

Soil was sampled throughout the growing season and also in the year following herbicide application.  The herbicide residual effects of Odyssey and Pursuit in soils were assessed by comparing the growth of oat (Avena sativa L.) seedlings in treated-soil with the growth of oat seedlings in untreated soil.  Based on these measurements, the following information is important to Manitoba farmers:

Weed control efficacy due to herbicide residues in soil during the growing season:

• For either Odyssey or Pursuit, the type of tillage system had no clear influence on the level of weed control efficacy during the growing season.

• Both Pursuit and Odyssey products provided better long-term weed control in clay-rich soils with large amounts of soil organic matter than in coarse-textured soils with lesser clay and soil organic matter contents.

• Pursuit provided a better long-term weed control than Odyssey in both conventional-till and reduced-till fields.  The benefit of increased weed control when using Pursuit was more pronounced in soils that have lesser clay and organic matter contents.

• In order to achieve better long-term weed control in coarse-textured soils, it is advised to apply Pursuit rather than Odyssey.  This is true for both conventional-till and reduced-till fields.

• For fine-textured soils, Pursuit and Odyssey provided better long-term weed control in slope positions with large amounts of soil organic matter than in slope positions with less soil organic matter.

Herbicide carry-over risks due to herbicide residues in soil in the year following herbicide applications:

• Herbicide residues present in soil at the end of the growing season remain in soil over the winter and were still present in soil in the spring.  The potential for herbicide carry-over and crop injury risks can therefore be assessed at the end of the growing season.
• Herbicide carry-over and crop injury risks were greatest for Pursuit applications in fields that have clay-rich soils with large amounts of soil organic matter.  Herbicide carry-over and crop injury risks were relatively small for Odyssey applications onto coarse-textured soils with less clay and soil organic matter contents.
• For fine-textured soils, the risk of herbicide carry-over and crop injury was not influenced by slope position or by the soil organic matter content in these soils.
• The risk of herbicide carry-over and crop injury was greatly reduced when soils were tilled in the spring prior to seeding.

Conclusion:

In summary, in order to improve long-term weed control during the growing season, it is better to apply Pursuit rather than Odyssey, particularly for coarse-textured soils that have lower clay and organic matter contents.  Tillage system has little impact on weed control efficacy during the growing season.  In order to reduce herbicide carry-over from one year to the next, it is recommended that farmers use spring tillage prior to seeding, particularly for soils that have higher clay and organic matter contents. Using Odyssey rather than Pursuit could additionally reduce the risk of herbicide carry-over and crop injury.

Acknowledgements:

The Department of Soil Science, University of Manitoba gratefully acknowledges the funding that was provided for this research by the Agri-Food Research and Development Initiative, the Alberta Canola Producers Commission, the Manitoba Canola Growers Association, BASF, and the University of Manitoba.

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