By Megan Pearce, Summer Student
Soybean production is becoming more common in both Saskatchewan and Alberta, so producers need to be prepared for one of the biggest threats to their soybean yields: weeds. Soybeans are among the most vulnerable crops for high yield loss percentage due to uncontrolled weed populations. Since soybeans compete poorly with weeds, implementing proper weed management early in the crop development process is crucial to minimizing yield losses.
The critical period of and for weed control is the interval of the crop growth cycle, when weeds must be controlled in order to prevent significant yield loss. An Ontario study on the critical period for weed control in soybeans determined that if weeds can be controlled up to the fourth node stage, yield loss would be kept at 2.5 per cent or less. The critical weed-free period (CWFP) is the length of time the crop must be kept weed-free, after which controlling the weeds will not have an effect on crop yields. In soybeans, the CWFP is variable but usually lies between the first unrolled trifoliate leaf (V1) and the third unrolled trifoliate leaf (V3).
In years of low moisture, yield loss can be further increased by dry soils combined with poorly controlled weeds. The critical period for weed control is affected by weed species and density, and should be based on the stage of crop growth. The type of weed and population per metre-squared has a significant effect on the percentage of yield loss associated with weed competition in soybean fields. For example, in soybeans, one lamb’s-quarter plant/m² can result in approximately 13 per cent yield loss, where one green foxtail plant/m² is likely to result in two per cent yield loss.
Herbicide application is currently the most effective method to manage weeds for soybeans. However, soybean’s herbicide sensitivity has led to a lack of herbicide variety and the consequent overuse of Group 2 herbicides and glyphosate. Repeated use of mainly glyphosate with Round-Up Ready soybeans has led to the development of glyphosate-resistant weeds. Kochia is a glyphosate-resistant weed that is a problem in Saskatchewan (Photo 1). Kochia is also resistant to Group 2 herbicides, making it very difficult to control. Another challenge when growing glyphosate-resistant soybeans is the presence of glyphosate-resistant volunteer canola. There are limited herbicides available to control volunteer glyphosate-resistant (Round-Up Ready) canola in soybean crops. However, in a collaborative study conducted by the University of Saskatchewan, University of Manitoba, and Agriculture and Agri‐Food Canada, pre-emergence application of cloransulam-methyl and florasulam increased soybean yields and controlled volunteer Round-Up Ready canola consistently. Integrated weed management strategies such as crop rotation, herbicide rotation and cultivation are practices that can help manage herbicide-resistant weeds. Residual and post emergence herbicides, mechanical weed control, and post-harvest herbicide application are recommended as part of an integrated weed management program.
A study conducted at the University of Manitoba suggested that narrow row spacing has an effect on shortening the critical weed-free period. Even with little weed control requirements, soybeans planted in narrow rows result in higher average yields per acre, as the plants hold their pods higher, leading to decreased harvest losses. The study also concluded that increasing soybean target densities reduced the critical weed-free period and can even decrease the number of in-crop herbicide applications. Low soybean densities (below 180,000 plants/ac) were shown to have less yield protection compared to those of higher soybean density (270,000 plants/ac). Higher seeding densities have also decreased biomass of volunteer glyphosate-resistant canola. A seeding rate of 200,000 to 240,000 plants/ac will decrease volunteer canola competition, increase soybean yields significantly and decrease volunteer canola numbers in subsequent years.
Lastly, selecting the appropriate soybean variety can decrease the number of herbicide applications needed to limit yield losses and can reduce the critical weed-free period. Cohesive use of these weed management strategies to keep weed populations low has the potential to maximize soybean yields.
Information about herbicides used to treat weeds for soybeans can be found in the 2019 Guide to Crop Protection.