Weed control ahead of the combine

Dave Nicolai, Extension educator - crops, Tom Peters, Extension sugarbeet agronomist, Liz Stahl, Extension educator - crops, Debalin Sarangi, Extension weed scientist, and Ryan Miller, Extension educator - crops

Waterhemp weed escapes in corn.
Photo: Ryan Miller

While it is too late to control weeds in row crops this year, there is still time to limit weed seed production and prevent additions to the weed seedbank. Combine harvesters are notorious for spreading weed seeds. Hand-pulling or mowing weed patches are some of the last lines of defense before harvest.

Surveys of Private Pesticide Applicators (Figure 1) across Minnesota show that although a smaller percentage of respondents report they harvest weed areas separately (5 to 10%), a growing percentage of respondents report hand-pulling weeds as part of their weed management program (39% in 2024, the highest percentage reported since 2017). Taking some extra time now through these tactics can pay dividends for weed management in the upcoming years.

Figure 1. Survey results from Private Pesticide Applicators across MN when asked about their use of non-chemical weed control strategies such as hand-pulling weeds or harvesting weedy areas separately (n= 925 in 2024, 691 in 2023, 823 in 2022, 896 in 2021, 883 in 2020, 684 in 2019, 907 in 2018, 698 in 2017). (Stahl, Hanson, Miller, Nicolai, and Peltier, 2024).

Weed biology is important

Know the biology of the weeds you are trying to control. Although waterhemp can produce 250,000 to one million seeds per plant, over 95% of the seed bank can be degraded in 4 years if waterhemp is prevented from producing seed and replenishing the seedbank during this time. The giant ragweed seed bank is degraded even faster, with over 95% degraded in just 2 years. However, a few weed escapes can produce enough seed to replenish the seedbank and negate past weed control success. 

Image 1. This late-germinating waterhemp plant produced 88
seeds in late September, 2020. Image credit: Joe Ikley, NDSU

Waterhemp creates a unique challenge in that it germinates and emerges in response to changes in soil temperature following rain events, emerging from May 1 to August 15. Even late-emerging pigweed can produce viable seeds (Image 1). Further, pigweed species,  including waterhemp, Powell amaranth, and redroot pigweed, are able to produce viable seeds within 14 days after pollination. Although seed production began in late July, there is still benefit in managing pigweed escapes in fields. Weeds need to be pulled or mowed, or small grain stubble needs to be sprayed immediately to prevent viable seed production.

Manage fencerows and field edges now

Field edges are often where weed infestations start (image 2). Now is the time to mow fence lines or field edges to prevent or minimize seed production. Left untouched, these weeds can be gathered by the combine, resulting in the spreading of weed seeds well into the field. Fall and spring tillage can then spread weed seeds even further across the field. Giant ragweed and waterhemp in particular can invade corn and soybean fields from fencerows and field edges. Also, discuss ideas for controlling weeds along field edges that you share with your neighbors (image 3). As we say, someone needs to own seed production along the fence line.

Image 2. Giant ragweed along the field edge in McLeod County.

Image 3. Weeds along edges of neighboring fields, Norman County.

2,4-D applied after brown-silk corn

Applying 2,4-D to field corn once silks have turned brown is an option for late-season weed suppression to reduce weed seed production. This application might be done with a high- clearance sprayer using drop nozzles or an airplane. Apply a single 2,4-D application at up to 3 pints per acre depending on the product type and formulation. Be aware of surrounding crops and do not use a treated crop as fodder for 7 days following application. The published literature is ‘mixed’ on whether a late-stage 2,4-D application reduces seed production or viability of waterhemp seed. This application will not improve the corn yield or control taller weeds at this point.

Scout for escaped weeds and maintain records

Image 4. A patch of waterhemp is easily spread by a combine
 at harvest. Use your field mapping software to mark areas of
the field with waterhemp patches so special considerations can
be made next season. Cass County, ND.

Make maps of weeds along roadsides, in shelterbelts and waterways, and in areas of fields that are subject to flooding or where wildlife may congregate or travel (Image 4). Maps of troublesome spots can be helpful when planning weed management strategies for next year. 

Since combines are an effective dispersal mechanism, check field entrances for new weeds and be on the lookout for noxious weeds like Palmer amaranth. Now is a good time to detect new weed infestations before seeds are moved further into fields with equipment. 

Having this record not only allows you to catch resistant populations sooner but these records can also help save money on herbicide application costs if weed problems are confined to a specific area in the field. If heavy weed infestations are present only in some areas, such as stressed areas of the field or headlands, perhaps a spot application on these areas should be considered for next year. 

 Are your weeds herbicide-resistant? Populations of waterhemp, kochia, and common and giant ragweed have developed resistance to multiple herbicide sites of action in Minnesota. Mowing or pulling weed escapes can be a great way to get a better handle on herbicide-resistant weed populations by preventing seed production and depleting the weed seed bank.

Hand-pulling

As our survey results show, many farmers have been “hand-pulling” problem weeds in their fields in addition to other weed control methods. Some farmers have wondered at what point mature weeds need to be carried out of the field to remove viable seeds from the field. Many weed seeds need to be “ripe” or physiologically mature before they are of concern for next year’s crop. If not yet mature, weeds don’t need to be bagged and/or carried out of the field. If weed seeds have a darkened appearance, they are most likely viable. Many weeds in Minnesota have produced viable seeds by early September, or will very soon, so when pulling weeds, the best option now is to bag and carry weeds out of the field.

Don’t combine through weed patches

Combines are one of the best ways to spread weed seed if escaped weeds are not controlled prior to seed production. Consider combining around the weed patch if there is a confined patch of weeds, especially if there is little crop to harvest in these spots, to prevent spreading weed seeds throughout the field. Consider mowing down weed patches or weedy fence lines ahead of harvest to prevent seeds from entering the combine. It is VERY tempting to just combine straight through weed patches once you’re in the combine seat, but resisting the temptation can help confine troublesome spots to smaller areas of the field.

Harvest weedy fields last

To keep clean fields clean, combine weedy fields last to prevent spreading seeds from field to field. Keep in mind that an uncleaned combine can harbor over 150 pounds of dust, debris, and weed seeds. Simple cleanout techniques (i.e., running the combine empty for several minutes with clean grain, tailings elevator, rock trap, and unloading auger sump open) between fields can go a long way in preventing weed seed movement. This video, “How to Clean a Combine to Limit the Spread of Weed Seeds”, by University of WI Extension details combine cleanout procedures.