Defining and refining the end of the critical period of weed control in soybean for Manitoba

Posted on 23.03.2017 | Last Modified 07.05.2019
Lead Researcher (PI): Robert Gulden
Institution: University of Manitoba
Total WGRF Funding: $57,500
Co-Funders: Manitoba Pulse & Soybean Growers
Start Date: 2017
Project Length: 2 Years

Investigate critical weed free period of soybeans related to variety, row spacing, plant populations and soil nitrogen.

Project Summary:
In these experiments, soybean responded well to cultural weed management practices. Narrowing the row spacing, maintaining adequate soybean stand densities and growing a locally more competitive variety all contributed to reducing the duration of the critical weed free period. This led to significant reduction in the number of in crop herbicide applications required to limit yield losses due to weeds. Individually, using these tools reduced the risk of developing herbicide resistant weeds in soybean and reduced herbicide costs. Their efficacy could be improved even more when used in combination as these tools tend to act synergistically.

The end of the critical period of weed control (CPAC) varied among sites and experiments and ranged from the VE to the R1 developmental stages in soybean.  In addition to the prescribe treatments (row spacing, soybean stand density and soybean variety), precipitation, local weed community and other factors also influenced the end of the CPWC in soybean in Manitoba.

The CPWC in general ended about 1 soybean developmental stage earlier in narrow-row compared with wide-row soybean production and the outcome of weed interference on soybean productivity was more consistent in narrow-row soybean.  Reductions in the CPWC in narrow-row soybean were more prominent at site-years with high mid-season total weed biomass.

Increasing soybean density also reduced to duration of the CPWC by about 1 soybean developmental stage at a number of site-years. The effect of density was more pronounced at site-years with lower overall mid-season weed biomass.

The effects of soybean variety on the end of the CPWC differed among sites, but was relatively consistent among years at each of the sites indicating a regional component to the ability of soybean varieties to interfere with and perform under interference by weeds.

These studies showed that cultural weed management tools are an important component to reducing the duration of the CPWC and thereby reducing the need for additional in-crop applications of herbicides which may lower production costs and lowers the risk for selecting for herbicide-resistant weed biotypes.