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Integration of cultural weed management methods as a proactive strategy to reduce herbicide resistance risks

Principal Investigator: François Tardif

Research Institution: University of Guelph (U of G)

Timeline: April 2014 – March 2016


  • Determine the impact of three cultural practices (cultivar branching pattern, soybean seeding rate and pre-plant N fertilizer addition) to increase soybean competitiveness.
  • Determine the impact of night tillage on germination and emergence of a weed seedling (weed seedling recruitment).
  • Establish a rotation study of corn/soybean/wheat to measure the effect of crop density and cover crop on light interception and weed seedling recruitment.
  • Evaluate the potential of using seed destruction as a way to reduce weed seedling recruitment.


  • None of the three soybean cultural practices (cultivar branching pattern, increased seeding rate, and pre-plant N) were able to reduce weed biomass by improving crop competitiveness; increased seeding rate improved soybean yield at one site/year out of four; pre-plant N had either neutral, negative or positive effects on yield depending on environment.
  • With the cultivars that were evaluated manipulating soybean cultural has little stable benefits in terms of reducing weed competitiveness.
  • Under Elora conditions, night tillage did not reduce weed recruitment compared to day tillage. It is therefore not a reliable practice to reduce weed impact.
  • Increasing crop density with the addition of cover crop can reduce weed impacts; costs must be taken into consideration as there was not always an increase in yield.
  • Harvest seed destruction using impact technology (Harrington Seed destructor) has efficacy against the most prevalent small seeded Ontario weeds.

Scientific Summary:

Globally, glyphosate resistant (GR) weeds are a real threat to crop production. In Ontario, GR Canada fleabane has reached levels such that the sustainability of soybean production is impacted. Recently we have seen multiple resistant waterhemp spreading rapidly in the province with devastating impact. While there is reliance on new herbicide tolerant crops to address resistance issues these can have limited success if they lead to more resistance. Cultural control methods are often promoted as part of herbicide resistance management systems. It is believed that cultural methods such as competitive cultivars, timely fertilization increased crop seeding rate, especially when combined with each other, can be effective at controlling weeds. Night tillage and harvest seed destruction are also methods that have been promoted to reduce the reliance on herbicides. These were tested in Ontario to determine their fit under our conditions.

Soybean competitiveness could be increased mostly by increasing seeding rate (normal 177,000 seeds/acre to high 233,000 seeds/acre); however, this was not always consistent depending on years and sites. The use of monoammonium phosphate (MAP; 11-52-0) fertilizer at planting was beneficial if there was medium weed pressure but under high weed pressure weed biomass increased and soybean yields decreased by 47%. The likely cause of this is that extra N was captured by the weeds and increased their competitiveness. In contrast, in the presence of medium weed pressure, the addition of N at planting did not have significant effect on yield when averaged over 4 site/years; however, when each site/year was examined individually, the N effect on yield ranged from a 1.5% reduction to a 10% increase. The two cultivars we compared were, according to the seed supplier, differing in branching pattern, but this did not translate in differential competitive ability.

The results of the night-tillage trial showed no positive impact compared to day tillage; this does not appear to be a practice that deserves wider adoption in Ontario.

In the rotation trial, we have had success in establishing the cover crops in corn and we have made observations on light interception and weed populations. It appears that the inclusion of cultural practices (high density, cover crops) in the corn/soybean/wheat rotation could be beneficial as there would be a reduction in weed populations. There are other effects coming from the crop that we have not identified yet, but that are reducing the weed population levels. There was neither reduction nor improvement of crop yields which means the financials impact of these practices must be taken into account.

Harvest Seed Destructor assessment of its efficacy at reducing viability of seeds of common Ontario weed species showed promising results. Average seed viability reductions was 92%, ranging between 84% for green foxtail to 96% for pigweeds.

Funding Partners: Ontario Agri-Food Innovation Alliance (a collaboration between the government of Ontario and the University of Guelph), DuPont Pioneer