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Got weeds in your beans? You're not alone

 Seth Naeve, Extension soybean agronomist, Jared Goplen and Dave Nicolai, Extension educators - crops

Patch of volunteer corn in a soybean field. Volunteer
corn, a weed in soybeans, competes for resources,
adds foreign material to the harvested soybeans,
and minimizes crop rotation benefits.
Photo: Jared Goplen
It’s been a memorable year for Minnesota soybean producers, but mostly for the wrong reasons. Dry conditions were problematic most of the year. Soybean emergence was affected by dry soil conditions in some fields. Preemergence herbicides laid dormant until rainfall and hot and dry conditions favored weed development over the crop. Dry soils this spring also delayed weed emergence of some weeds until later in the season. Poor canopy development has allowed waterhemp, volunteer corn, and other weeds to establish and poke out of the canopy in fields statewide.

Do these weed escapes really matter?

While farmers all strive for clean fields, it is likely that the sporadic escaped weeds had little effect on this years’ soybean yields. These weed escapes do have an impact, however. Economic impacts can occur at the local elevator as well as thousands of miles away via export markets. These escapes will also exacerbate weed problems in the future if they are able to produce viable seeds. The good news is that it isn’t too late to keep these weedy plants out of the combine and out of the commodity stream. Watch this video on pre-harvest preparation:  

Escaped waterhemp plant in a Minnesota
soybean field. Controlling weeds before
seed production helps minimize inputs into
the weed seed bank. Photo: Jared Goplen
Herbicide resistance is common in many of Minnesota’s weed species. The best long-term strategy for fighting weeds is to whittle down the weed-seed bank. Even small escaped pigweeds can return thousands of seeds to the soil. 

Ideally, weeds should be pulled prior to harvest and removed from fields if seeds have been produced. However, this may not be a realistic option in many situations. If weeds are present in patches, consider mowing these areas to isolate the weed seed to a limited area where it can be carefully monitored and managed in subsequent crops. Running weed patches through the combine this fall will do an excellent job of dispersing these troublesome weeds throughout the field and into the next one, which is something that no one would want.

Impact on exports

International customers of US soybeans, including China, are rejecting soybean shipments carrying more than one percent foreign material (FM). Since FM includes any material that is not soybean seed, weed seed is an important factor. In fact, the limits on FM in soybean imports are focused on weed seed and other crop seeds, like corn. Weed seed found in shipments will likely lead to rejection of entire vessels. Rejected vessels are extremely costly for the exporter who will certainly turn around and push costs back through the system to the farmer.

Annually, the Naeve Lab at the University of Minnesota analyzes up to 2000 soybean samples from US producers for a wide range of quality traits, including FM and weed seed. Although FM tends to be very low (~0.2% on average), around 25% of the samples contain weed seed, indicating famers are still combining plenty of weeds.

Weeds love combines

Part of the problem is that our common weeds retain seeds until crop harvest, meaning a large percentage of weed seeds pass through the combine. Combines are great at accumulating dust, crop residue, and weed seeds. Research from the University of Wisconsin has documented how important combines are in spreading weed seeds. Of the 31 samples collected from within nine different combines, 97% of them contained viable weed seed. The weed seed most often found included grasses, pigweeds and common lambsquarters. Sanitizing combines between fields can minimize the spread of weeds. Focus sanitation efforts towards the front of the combine if time is limited. The greatest number of weed seeds were found on the header, followed by the feeder house, rock trap, and rotor. Taking even just a small amount of time to clean out combines between fields can be helpful in minimizing the spread of weed problems. Other harvest considerations are highlighted in this short video: 

Keeping weeds out of combines has multiple benefits

Efforts to produce high-quality, weed-free grain serves a dual purpose. Keeping weeds out of soybean fields and out of combines will help maintain important market access while simultaneously improving weed management. Preventing weed seeds from entering the combine is a foundational component for long term weed management, especially when those weeds are herbicide resistant. 

To increase awareness about challenges with FM in soybean while managing herbicide resistant weeds, we have developed a series of six video segments from crop planting through harvest and transportation. View UM Extension's entire Managing foreign material  video series for more information. 

Keep an eye out for the next week’s installment in this series regarding post-harvest weed management and grain handling to ensure continued market access for our soybeans.

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  1. Sixty years ago my neighbor had a combine with a scour clean. He would hang a burlap bag under it and the weed seeds were collected from the screening. The seeds were dumped in a pile where they would rot.

    Why we don't we have a scour clean attachment on our combines now?

    1. Great question! To my knowledge the scour clean attachment hasn't been available for newer combines, though it still could likely help prevent the spread of some weeds via the grain, and can help reduce the amount of FM in the clean grain sample. The issue with the scour clean is that it doesn't address weed seeds going back onto the fields out the back of the combine. There has been increased focus on controlling the weed seeds via harvest weed seed control methods that focuses on weed seeds coming through the entire chaff fraction. Iowa State University Extension had an article posted several years ago that reviews some of these techniques here:


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