Sow and Grow: Controlling weeds and soybean pests

Sara Bauder, South Dakota Extension forage field specialist
Posted 9/13/23

Fall is a great time to control many noxious and winter annual weeds.

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Sow and Grow: Controlling weeds and soybean pests


Fall is a great time to control many noxious and winter annual weeds.

If weeds were sprayed or clipped (set back in some way) previously this year and have significant regrowth on them at this time, fall is an excellent time for spraying to achieve a good kill.

If controlled earlier in the season, but plant regrowth is minimal or the plant is drying up, spraying may or may not be effective. If you spray these areas, using a chemical with residual activity can provide control when plants begin actively growing again.

Depending on weed species and your 2024 intended crop, residual control options may not be viable to you and a herbicide with minimal residual activity could be the best option.

Be sure to check with an agronomist and/or read through pesticide labels/check the South Dakota Pest Management guides ( to ensure you are not putting your 2024 plans at risk due to carry- over.

If weeds were not controlled earlier and are tall and quite mature by fall, herbicide control is probably not worth the investment.

Since hard frost is likely to occur soon for much of the state, remember that perennial plants will begin to send nutrients to the roots to prepare for winter. This means spraying perennials (Leafy Spurge, Canada Thistle, Sow Thistle, etc.) soon (early October) is imperative. Keep in mind that if a light frost occurs, leaves appear healthy, and temps are predicted to remain above freezing, a herbicide application may still be effective.

While we’re talking about adding to late season fieldwork … fall is also the ideal time of year to test your fields for soybean cyst nematode. Testing is a simple process that may save you money in the long run.

SCN sampling can actually take place anytime in any crop, but there is likely to be a higher and more detectable occurrence of SCN in the fall following a soybean crop.

Soybean Cyst Nematode is an all-too-common yield robber; in fact, it’s considered the most damaging soybean pathogen in North America with research showing that it can cause over 40% yield loss in some cases. This pest can quietly sneak up on growers as it causes yield loss with no obvious above ground symptoms.

SCN poses a threat to soybean growers throughout South Dakota as it has been positively confirmed in almost all counties where soybeans are currently grown.

This nematode attacks the root of the soybean plant causing physiological damage to the root cells. Damage from the nematode on the root can pose a greater risk for other fungal pathogens to easily infest the plant through open wounds.

SCN tends to spread from field to field by equipment, erosion, wildlife, and other environmental factors. Once a field is infested with SCN, it is highly unlikely to eradicate it; however, reducing the population is possible. If SCN is detected in a field, there are several integrated pest management practices that should be considered.

Lengthening crop rotation (three crops or more) removes the host crop (soybean) for a longer period of time, which can help to decrease SCN populations. Other options include purchasing SCN resistant soybean cultivars for any level of infestation and/or the use of nematicide seed treatments in heavy infestations (>10,000 eggs/100 cc of soil).

Soybean Cyst Nematode lives in the top 8 inches of the soil, so when testing, use a soil probe or spade to take at least 20 soil samples at an 8 inch depth in areas no larger than 15 to 20 acres. Field entrances, low lying areas, and fence lines are good areas to target. When sampling, angling the probe or spade into the soybean row is an ideal practice. Soil should be mixed well and placed in a soil sample bag.

South Dakota growers can obtain free SCN soil sample bags and receive free SCN testing through the SDSU Plant Diagnostic Clinic, courtesy of the South Dakota Soybean Research and Promotion Council.

To obtain SCN soil sample bags and submission form/instructions call 605-688-5545 or 605-688-4521 or stop by a SDSU Extension Regional Office. The sample submission form can also be found at SDSU Plant Diagnostic Clinic website (go to and search “SDSU Plant Diagnostic Clinic”). Sampling fields every 2-3 years can help monitor populations (or lack thereof) and help explain yield losses in question.

For additional information on SCN and to better understand SCN test results, visit and search "SCN."