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Extension > Local Extension Offices > Meeker > County Agriculture Educator > Articles > Late Season Soybean Management

Late Season Soybean Management

Soybean aphid watch 2017
Bruce Potter, University of Minnesota Extension Integrated Pest Management Specialist, reports that over the past week pockets of soybean aphids reaching economic threshold began to appear in southwest and southern Minnesota. As always, not having to treat a field is preferred but not always economically viable. Because soybean development in many fields is further behind than most years, the longer you can wait to treat, the better. Watch for re-infestation of insecticide treated fields or late developing economic populations. Fields that are late planted or otherwise later maturing deserve extra attention.

While the recent cool weather will slow soybean development, it might have a positive effect on aphid management.  Most aphid fungal pathogens are most effective in cool, wet weather. Look for gray to brown dead aphids. Fungal diseases can cause the rapid collapse of aphid populations. Soybean aphids develop slower but will produce more young in cool weather.

Through R5 (soybean seeds developing, but pod cavity not filled), use the economic threshold of an average of 250 aphids/plant AND more than 80% of plants having aphids AND aphid populations increasing. Count both adults and nymphs, including any white dwarves.

Late season weed escapes in soybeans
Is there a herbicide available for rescue treatment in soybeans? The short answer is no. Although it would be great to have a herbicide rescue treatment to control escaped waterhemp and other weed species that are now 3+ feet in height in soybeans, there are no control options available. The crop stage at this point is generally advanced far enough that products, such as Roundup, Liberty, Flexstar, dicamba, and others are no longer labeled for use in soybean. Although some may be inclined to spray Cobra or other lactofen-containing products in an attempt to control weed escapes, they will not provide effective control of large weeds and have significant crop-injury potential (up to a 14% yield reduction).

Will soybean yield be impacted by weed escapes?
Probably not. If weeds were controlled earlier in the season and there are few sporadic escapes, there will likely not be an effect on soybean yield. However, one weed can make weed control more difficult in the future if seed production occurs. Although seed production of weeds will vary widely, waterhemp can produce over 200,000 seeds per plant in competition with soybean, lambsquarters can produce over 70,000 seeds per plant, and giant ragweed can produce over 10,000 seeds per plant.

What can a soybean grower do?
Herbicides are not an option, but seed production can have lasting impacts on future weed control. Unfortunately, the best option to control escaped weeds involves physical labor and pulling the weeds by hand. If soybean canopies are lagging behind and have not closed in, cultivation can still be used as an option, although cultivation will probably still not be effective on large weeds or useful in large soybeans. Most weeds in Minnesota will begin producing viable seeds in the next 1-2 weeks, so the time to pull or control weeds is now. If escaped weeds are not controlled prior to seed production, keep in mind that combines are one of the best ways to spread weed seed. If there is a confined patch of weeds, consider going around the weed patch to prevent spreading weed seeds throughout the field. Consider harvesting order too. To keep clean fields clean, combine weedy fields last to prevent spreading seeds from field to field.


David Nicolai
Extension Educator, Crops & Institute for Ag Prof Coord
(651) 480-7706
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