ND crop acreage that includes cover crops is rapidly expanding due to benefits including reduction in soil erosion, soil moisture management, weed suppression, and long-term improvement in soil productivity. Research-based information to assist with successful establishment of cover crops is needed by farmers. Management and input costs are involved with incorporating cover crops into a cropping system that normally includes soybean. A risk often overlooked is herbicide residual that can greatly reduce cover crop stands. This study will provide information that will help soybean farmers successfully establish cover crops during late summer following soybean production. The information on tolerance of selected cool-season cover crops to commonly used soybean herbicides with soil residual will be widely available and useful for ND farmers and crop advisers.

Document the tolerance of late-summer planted, cool-season cover crops following the normally timed application of soybean PRE- and POST-applied herbicides that have soil residual.

Field data of cover crop tolerance to soybean herbicides will be compiled across several site-years (Fargo, 2016 and 2018-20; and Carrington REC, 2018-20) to generate a reference table published in the 2021 ND Weed Control Guide (NDSU Extension circular W253).

Updated January 8, 2021:
FY 2021 Mid-Year Report ND Soybean Council
Late season planted cover crop tolerance to soybean herbicides
Objectives of the research:

The goal of this study was to build a North Dakota State University database on late-season planted cover crop tolerance to previously applied soybean herbicides that have soil residues potentially harmful to cover crops. This database will aid farmers and crop advisers as plans are made for adding cover crops into cropping systems that include soybean.

Completed work:
Field trials were conducted at the Carrington Research Extension Center and Fargo during 2018-2020. Soybean were planted late May followed by application of soil-applied herbicides (metribuzin [Sencor], sulfentrazone [Spartan], flumioxazin [Valor], pyroxasulfone [Zidua], and imazethapyr [Pursuit]) and post-emergence herbicides (dicamba [Engenia], fomesafen [Flexstar], imazamox [Raptor], and glufosinate [Liberty 280]) at labeled rates for soybean. Soybean (seedfill growth stages) were terminated by mowing during August. Cover crops (barley, winter rye, field pea, flax, radish, rapeseed/canola and turnip) were direct-planted into soybean stubble perpendicular to herbicide strips generally late August to early September. Visual evaluation of injury (biomass and/or stand reduction) began generally in late September (3-4 months after application of herbicides).

Preliminary results:
All herbicides, except Liberty 280, injured cover crops. Barley, winter rye and field pea had the greatest tolerance to herbicides. Radish, turnip and rapeseed/canola generally had the least tolerance to herbicides.
These herbicides potentially have high risk of injury for cover crops:
Spartan=radish
Valor=radish, turnip and rapeseed/canola
Pursuit=flax, radish and turnip
Flexstar=radish

Work to be completed:
• Table summarizing study results will be published in the NDSU 2021 ND Weed Control Guide.
• Research reports written and published in Carrington Research Extension Center 2020 Annual Report, 2020 ND Weed Control Research and ND Soybean Council 2020 annual research report.
• Data to be presented during 2021 NDSU (virtual) Weed Workshop (January 19) and other NDSU Extension soybean production and cover crop educational events. Research paper to be tentatively presented in March, 2021 at (virtual) Western Society of Weed Science annual meeting.

Updated June 30, 2021:

View uploaded report

Late-season planted cover crop tolerance to soybean herbicides

Why the research is important to ND soybean farmers:
The goal of this project is to build a NDSU database on late-season planted cover crop tolerance to early season applied soybean herbicides that have soil residues. This database will aid farmers and crop advisers as plans are made for adding cover crops into their cropping system.

Research conducted:
• Study was conducted at Carrington and Fargo during 2018-20 and included nine soybean herbicides and eight cover crops. Soil and post-emergence herbicides were applied at labeled rates and timings to soybean. The crop was mowed during August (seed-fill stages) and cover crops direct planted into the soybean stubble generally late August to early September. Visual evaluation of injury (biomass and/or stand reduction) began generally in late September (3-4 months after application of herbicides).

Research findings:
• All herbicides, except Liberty 280, injured cover crops. Barley, winter rye and field pea had the greatest tolerance to herbicides. Radish, turnip and rapeseed/canola generally had the least tolerance to herbicides. These herbicides potentially have high risk of injury for cover crops: Spartan=radish; Valor=radish, turnip and rapeseed/canola; Pursuit=flax, radish and turnip; and Flexstar=radish.
Benefits/Recommendations to ND soybean farmers and industry:
Research reports for individual trials were written and published in ‘ND Weed Control Research’ (www.ag.ndsu.edu/weeds/nd-weed-control-research). After completion of the field study, a table was published in ‘2021 ND Weed Control Guide’ as a reference for farmers and crops advisers when selecting cover crops for fall establishment following soybean. This database will aid in successful establishment of fall cover crops, which will reduce soil erosion, help manage soil moisture, increase long-term productivity of soil, plus other benefits.

If farmers are including cover crops in their farm management plans, the data generated from this study will aid in increasing their success with establishing cover crops following soybean production. By increasing the potential to successfully establish cover crops, this provides the benefits of cover crops. The information gained from the data will aid farmers in correctly selecting cover crop species after soybean weed management with herbicides. Extra costs of replacement seed and replanting will be avoided by proper cover crop selection. Effective establishment and use of cover crops will aid in a more sustainable environment for soybean and other crop production.