Making its first appearance in the top 20 most abundant weeds list, according to Manitoba’s 2016 survey, biennial wormwood is up from 49th place in 2002. In 2016, it was ranked as the 9th most abundant weed in soybeans. The threshold for biennial wormwood is 10 plants/m2, which can cause 44% yield loss in soybeans (NDSU). Despite its name, wormwood actually behaves as an annual plant in Manitoba—it emerges very late in June and often escapes in-crop herbicides. A combination of pre-emergent residual and in-crop herbicides (e.g., Valtera followed by bentazon) can offer effective control. Wormwood is often confused with common ragweed; however, wormwood leaves are more finely divided (Figure A).
Although ranked lower in abundance than other weeds, kochia (Figure B) is poised to be problematic. It thrives in conditions considered poor for most crops, such as drought and salinity. Kochia emerges very early, often prior to crop seeding, meaning seedlings are often too large to effectively control with in-crop herbicides. Therefore, pre-emergent herbicides are often required to control kochia in pulses and soybeans. In addition, glyphosate resistance has been confirmed in twelve rural municipalities including Emerson-Franklin, Montcalm, Rhineland, Stanley, Pembina, Louise, MacDonald, Grey, Dufferin, North Norfolk, Two Borders and Brenda-Waskada. If you suspect glyphosate-resistant (GR) kochia, contact an MPSG Production Specialist and take samples to the Pest Surveillance Initiative lab to confirm resistance via qPCR tissue testing.
Ranked as the number one weed in soybeans, volunteer canola (Figure C) is also a concern in most crops. It was ranked as the 5th most abundant weed in Manitoba overall. Research by Dr. Rob Gulden’s lab at the U of M has found that 2.5 and 3.2 canola plants/m2 will reduce soybean yield by 5% in wide (30”) and narrow (7.5”) rows, respectively. Increasing seeding rate, managing residual N and using inter-row tillage on wide rows will also increase the competitive ability of soybeans against volunteer canola. Tank-mix options are available to effectively control volunteer canola in soybeans and pulses and independent evaluation of tank mix efficacy can be found in the latest issue of the Pulse Beat Science Edition. If growing Xtend soybeans, be aware that glyphosate-resistant (GR) volunteer canola is not on the dicamba label. Enlist soybeans are tolerant to 2,4-D, which is registered for control of GR volunteer canola.
Another weed that recently increased in relative abundance is yellow foxtail, currently ranked 6th, up from 30th place in 2002. Yellow foxtail is morphologically similar to green foxtail. However, yellow foxtail can be distinguished by examining the ligule, which has long conspicuous hairs (Figure D). High populations of yellow foxtail (48 plants/ft2) typically cause only minimal yield loss (5%) in soybeans, unless soil moisture is adequate until July then limiting from late July to maturity, which can result in 15% soybean yield loss. The 2016 herbicide resistance weed survey found 42% of yellow foxtail populations to be resistant to group 1 and/or 2 herbicides.