There are a limited number of preplant (PP) burndown herbicides, available for soybean and other broadleaf row crops, that are effective on herbicide-resistant broadleaf weeds, provide initial soil residual, and are low cost. Low rates of dicamba fit this description but waiting periods between dicamba application and row-crop planting generally restrict use of the herbicide, due to potential crop injury. Data from this study, which is within the scope of ND Soybean Council research priorities – pest resistance, will be shared with ND farmers and crop advisers to allow them to make decisions with this herbicide use strategy.

Evaluate soybean plant growth and seed yield response based on timing of planting following application of preplant, low-dose soil rate of dicamba with or without water activation.

ND data will provide guidance on soybean tolerance when planted at short intervals following PP-applied dicamba at low rates.

Updated November 24, 2021:
Completed work:
• The initial year of this study was conducted at three NDSU research sites in 2021: Carrington (irrigated), and Minot and Prosper (dryland). Four targeted treatments:
1) untreated and 2) treated (Clarity or generic dicamba preplant applied 4 fl oz product/A mid-May); soybean planted 3) <14 days after dicamba application and before rain or irrigation, and 4) >14 days after dicamba application and rain or irrigation >1”.
Field data included soybean plant development (emergence, flower and maturity) dates, plant density and canopy, injury ratings 1-2, 3-4 and 6-8 weeks after plant emergence, and crop height 3-4 and 6-8 weeks after emergence. At Carrington, soybean seed yield and quality data were generated.

Preliminary results:
• Soybean plant injury (reduction in biomass) with the first planting date (2 days following application of dicamba at Prosper and 7 days at Minot) was 53-73% at Prosper and 85-87% at Minot. Plant injury with the second planting date (16 days and 0.74 inches of rain following application of dicamba at Prosper, and 20 days and 0.96 inches of rain at Minot) was 18-45% at Prosper and 22-39% at Minot. At Carrington, the first evaluation indicated 43% injury with the early planting date (6 days and 0.08 inch of rain after application of dicamba), while minimal injury (<6%) was observed after the second planting date (19 days after application of dicamba and >2 inches of water). Plant density was reduced 17% with the first planting date following application of dicamba at Carrington; 68% at Minot and 55% at Prosper. At Carrington, soybean seed yield from dicamba-treated soil was similar to the untreated checks.

Work to be completed:
• Research reports will be written and available to clientele using Carrington and North Central Research Extension Center websites, ND Soybean Council 2021 annual research report, and 2021 ND Weed Control Research electronic database.
• Data slides developed and presented for crop advisers during professional weed science meetings (NCWSS; Dec 2021) and NDSU Extension’s ‘Wide World of Weed Workshop’ (FargoDome; Jan 2022). The preliminary data will be available for weed scientists and Extension crop specialists to advise farmers and crop advisers on this weed control strategy during the 2022 crop production season.

If soybean will tolerate a prior low-dose PP application of dicamba with limited waiting period for planting or after a significant rainfall, farmers will have another herbicide option to control herbicide-resistant broadleaf weeds including horseweed (marestail), kochia, pigweed species, and wild buckwheat.