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Pest Patrol: Weed control in identity-preserved, non-GMO soybeans

#PestPatrol with Mike Cowbrough, OMAFRA

Figure 1: This unsprayed control treatment consists mainly of common ragweed but with green foxtail, common lamb’s-quarters, witchgrass, crabgrass and red-root and green pigweed.

A decade of public research trials evaluating weed control in non-GMO soybeans at the University of Guelph has demonstrated that the key to success is to start with a soil-applied herbicide program that has three or more different modes of action.

In Ontario, there are a number of common weeds (e.g. common ragweed, lamb’s-quarters, foxtail and pigweed species) that are resistant to multiple herbicide groups and they simply won’t be controlled to a satisfactory level with herbicide programs having single or double modes of action.

During the 2018 season, this was demonstrated at the Wellington County research site. Several common weeds were present, with common ragweed being the most dominant species (Figure 1 at top). Since this particular population of common ragweed was resistant to two different modes of action (WSSA Groups 2 and 5), a dual mode of action soil-applied herbicide like Conquest LQ (WSSA Groups 2 and 5) provided unsatisfactory control (Figure 2 below), although it effectively controlled other species. When a third mode of action was added (WSSA Group 14), control of common ragweed was excellent (Figure 3 at bottom), and if there was the odd plant found, it had emerged well after the residual activity of the herbicide program had dissipated.

Figure 2: A pre-emergence application of Conquest LQ where the main escape is common ragweed that is unfortunately resistant to both modes of action in the herbicide. photo: Mike Cowbrough, OMAFRA

The following guidelines to achieve effective weed control in non-GMO soybeans have been consistent throughout the past 10 seasons:

  • Budget around $70-$80 per acre for herbicide costs.
  • Plan on applying a soil-applied herbicide program with a minimum of three modes of action to eliminate an early-season weed competition and to protect yield potential. Will these programs sometimes deliver less-than-exceptional control due to lack of rainfall? Yes, but they will still provide some level of control and the seedling “escapes” will be more successfully controlled with a post-emergence herbicide program than if that soil-applied herbicide had not been applied.
  • Relying exclusively on post-emergence herbicides to control weeds in non-GMO soybeans has provided the poorest results in the University of Guelph trials.
  • More often than not, a post-emergence herbicide will be needed to control late-emerging weeds and/or volunteer corn.
  • If you have an appreciable level of perennial weeds (e.g. perennial sow-thistle) or weeds that stain (e.g. eastern black nightshade) and for which there is zero tolerance, then don’t grow non-GMO, identity preserved soybeans.
Figure 3: The addition of a third mode of action, Valtera, to Conquest LQ has controlled the majority of the common ragweed (Note: In each experimental unit, the treated area consists of 6" x 15" rows of soybean and to a length of 30'.) photo: Mike Cowbrough, OMAFRA

Have a question you want answered? Hashtag #PestPatrol on Twitter to @cowbrough or email Mike at [email protected].

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