Brookings Record | Planting and Grow: Control of weeds and soybean pests

Soybean cyst nematodes can rob producers of some of their produce. (courtesy photo)

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

If the weeds were sprayed or mowed (receded in some way) earlier this year and you see significant growth on them at this time, fall is a good time to spray for a good kill.

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

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

Be sure to consult an agronomist and/or read pesticide labels/consult the South Dakota Pest Management Guides ( to ensure you are not putting your 2024 plans at risk. Because of deportation

If weeds are not controlled early and are tall and fully mature by fall, herbicide control may not be worth the investment.

Since a hard freeze is likely soon in much of the state, remember that perennials will begin sending nutrients to the roots to prepare for winter. This means that spraying perennials (Leafy Spurge, Canada Thistle, Sow Thistle, etc.) soon (early October) is essential. Keep in mind that if a light frost has occurred, the leaves appear healthy, and temperatures are expected to remain above freezing, herbicide application may still be effective.

While we’re talking about adding to late-season fieldwork…fall is also the perfect 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 can be sampled virtually at any time in any crop, but there is likely to be a higher and more detectable incidence of SCN in the fall after the soybean crop.

Soybean cyst nematode is a very common yield thief; In fact, it is considered one of the most harmful soybean pathogens in North America with research showing that it can cause more than 40% crop loss in some cases. This pest can quietly creep into farmers because it causes crop loss without visible symptoms above ground.

SCN poses a threat to soybean farmers throughout South Dakota as it has been confirmed positive in nearly all counties where soybeans are currently grown.

These nematodes attack the root of soybean plants, causing physiological damage to the root cells. Damage caused by nematodes to the root can pose a greater risk to other fungal pathogens that easily infect 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 infected with an SCN, it is unlikely to be eradicated; However, population reduction is possible. If SCN is discovered in a field, there are several IPM practices that should be considered.

Lengthening the crop rotation (three or more crops) removes the host crop (soybeans) for a longer period of time, which can help reduce SCN numbers. Other options include purchasing SCN-resistant soybean varieties for any level of infestation and/or using nematicidal seed treatments for severe infestations (>10,000 eggs/100 cc of soil).

Soybean cyst nematode lives in the top 8 inches of soil, so when testing, use a soil probe or spade to take at least 20 soil samples 8 inches deep in areas no larger than 15 to 20 acres. Field entrances, low areas and fence lines are good areas to target. When sampling, placing a probe or spade in the soybean row is ideal practice. The soil should be mixed well and placed in a soil sample bag.

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

To obtain SCN soil sample bags and an application form/instructions, call 605-688-5545 or 605-688-4521 or visit the SDSU Extension Regional Office. The sample submission form can also be found on the 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 the yield losses involved.

For additional information about the SCN and to better understand your SCN test results, visit and search for “SCN.”

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