Waterhemp weed seed removal

Can Weed Seed Production be Reduced with Late-season Herbicide Applications?

Problems caused by unfavorable conditions this season have resulted in greater than normal weed escapes. These weeds may reduce crop yields and definitely will contribute to future weed problems via new seed. While it is too late to protect crop yields, a common question is whether herbicides can be used to reduce the quantity of viable weed seed produced by weeds. While there is no simple answer due to the many different scenarios across the state, in most situations late-season applications are not warranted.

The potential to limit seed production is affected by two main factors: 1) susceptibility of the weed to the herbicide, and 2) stage of seed development at the time of application. It is important to recognize that herbicides that are effective early in the season will be much less, if at all, effective on the mature weeds in fields now. Waterhemp is unlikely to be killed by any labeled herbicides at this time of the year. While other species may be killed with herbicides, the impact on seed production will be highly variable.

Research in the early 1980’s looked at the effect of late-season 2,4-D applications in corn on seed production by velvetleaf and cocklebur (the 1980’s equivalent of today’s waterhemp). While brown-silk 2,4-D applications were able to kill both species, the treatments were much more effective at preventing seed production by cocklebur than velvetleaf. Much of the velvetleaf seed had filled by the time of the 2,4-D application, whereas cocklebur was still in early flowering stage. Seed that had filled prior to 2,4-D retained their viability even if the parent plant was killed prematurely. Before committing to any late-season treatments, examine weeds to determine the stage of seed development. Both waterhemp and giant ragweed present in fields in central Iowa had fully developed seed during the week of August 26 (Fig 1 and 2). The 2,4-D label for late-season applications was changed in the 1980’s from after brown-silk to after the dent stage, this change will reduce the effectiveness of the treatments since it provides more time for weed seeds to mature.

Figure 1. Waterhemp seed, Aug 29.

Figure 2. Giant ragweed seed, Aug 29.

Reducing weed seed production is essential in order to minimize the seed bank. By reducing the size of the seed bank, future weed management will be simplified and the risk of new herbicide resistant biotypes will be reduced. However, late-season herbicide applications are unlikely to provide significant benefits for most fields. Hand pulling weeds is an alternative, but at this time of year the plants would need to be removed from the field. In situations where a patch of a weed is present that is suspected to possess a new resistant trait for that field, removal of the weeds probably would be worth the effort. Rather than spending money on a questionable treatment, spend time determining why this year’s program failed and develop an effective weed management plan for 2020.

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on August 30, 2019. The information contained within may not be the most current and accurate depending on when it is accessed.

Minnesota Crop News

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.