Defining and Refining the End of the Critical Period of Weed Control in Soybean for Manitoba

Crop Soybean
Start Date2016
End Date2017
Principal InvestigatorGulden, Rob, University of Manitoba
MPSG Financial Support$57,500
Total Project Funding$115,000
ReportWeed Management for Long-Term Sustainability of Soybean

Research Objectives

  1. Determine how soybean varieties affect the CWFP (critical weed free period) for soybean in Manitoba
  2. Determine what the effect of row spacing is on the CWFP in soybean
  3. Determine how soybean plant populations affect the CWFP
  4. Determine if there is an association between residual soil nitrogen and the CWFP in soybean

Project Description

As soybean production is relatively new in Manitoba, many of the recommendations to produce soybean are being adopted from more southern areas; however, these need to be adapted to our climate and soybean varieties.  The CPWC (critical period of weed control) is generally recognized as lasting from emergence or the 1st trifoliate leaf stage to the 3rd trifoliate leaf stages, but can last well into the reproductive stages. An important reason for the early beginning of the CPWC for soybean is that although soybean is a C3 photosynthesis, cool season plant, its behaviour is more akin to that of a warm season crop with slow biomass accumulation during the early part of the growing season. Most prominent weeds in Manitoba are well adapted and grow quickly at cooler temperatures (e.g., wild oats, wild buckwheat, volunteer canola, and many others). This differential growth in the early part of our growing season allows our prominent weeds to be much more competitive with soybean than in other regions where soybean are better adapted and this is expected to extend the CPWC for soybean in our region. Managing weeds in a crop with a long CPWC with non-residual herbicides can be challenging as the potential need for multiple in-crop applications of the same active ingredient increase the risk for selecting for herbicide-resistant weed biotypes dramatically. In addition, glyphosate is relatively slow acting and correct timing of in-crop applications are critical to minimize yield loss to weeds.

In general, increasing the competitive ability of the crop by integrating weed management techniques is expected to decreases the duration of the critical weed free period. This can decrease the need for multiple in-crop herbicide applications and thereby lower the risk and selection pressure for selecting for herbicide-resistant weed biotypes. Increasing the competitive ability of the crop can be achieved a number of ways including: growing a more competitive variety, increasing the stand density, reducing the row spacing, and manipulating soil fertility. As the CPWC for soybean begins roughly at the time of soybean emergence, defining the end of the CPWC through CWFP experiments is arguably more important and informative than defining the beginning of the CPWC.