Preventing the distribution of weed seeds from combines – AgFax

Weeds spread by combines often result in linear weed distributions in the field the following season. Photo: University of Nebraska-Lincoln

Combines are very good at what they do, which is to separate seeds from plants during the harvesting process. Unfortunately, this can include separating weed seeds from weeds in the field. As a result, combines can spread weed seeds directly to the rear of the combine or carry weed seeds to new areas, resulting in weed escapes.

Weed Seed Introductions

Weeds can enter and establish themselves in fields in several ways. Some weeds can be introduced by wildlife, wind or water and can be carried tens of feet or even kilometers.

Another method of seed transport is human activity. Weeds can attach themselves to vehicles, which then spread the seeds along roads or in fields. Weed seeds are also commonly carried on agricultural equipment, such as tractors or tillage tools, through soil attached to machinery, or by transporting plant material.

As the prevalence of resistant and difficult to manage weeds such as Palmer pigweed and common hemp increases, the need for weed management tools in addition to herbicide is increasingly important. These weeds are known to produce up to 500,000 seeds per plant, so preventing their spread in a field or across a farm will help manage these difficult weed species.

How Combines Spread Weeds

Combines widespread weeds mainly in two ways. One way is to spread weed seeds in individual fields. This has the potential to turn an isolated weed problem, such as an individual plant, into a field-wide problem in just a few seasons. Another way combines can spread weed seeds is by transporting them between fields, introducing weeds to new areas of the farm.

Regardless of how weeds initially establish themselves in a field, combines are very effective spreaders because many of our common agronomic weed species do not break and remain on the plant until the harvest or other physical separation. In addition to harvesting crops, they can also harvest weeds if present in the field, thresh their seeds, and distribute the seeds. Seed will mostly be pushed out the back of the combine along with straw and residue, creating linear patches of weeds the following season.

However, a study conducted at Iowa State University showed that significant amounts of foreign material – which could include weed seeds – accumulate in different parts of the machine. These data show the relative percentage of material in different areas of the combine after harvesting corn and soybeans (Figures 1-2).

The results of this study show that a significant amount of material can accumulate in certain areas of the combine, such as the rock trap, rotor/cylinder, grain tank and residue elevator. However, weed seeds in these areas may not be the biggest concern, as they are largely held in these areas and not distributed.

The biggest concern may be foreign material and therefore weed seeds that may accumulate on the grain platform or on the corn head and conveyor. Weed seeds in these locations can spread to fields but also to new fields as combines move out to harvest grain across the farm. Fortunately, these areas are relatively easy to access and can be cleaned quickly.

Figure 2. Percentage distribution of foreign matter, and possibly weed seeds, in various areas of the combine after corn harvest. Adapted from: Hanna, HM et al. (2009). Click on the image to enlarge

Combine distribution of foreign matter for soybeans

Figure 2. Percentage distribution of foreign matter, and possibly weed seeds, in various areas of the combine after soybean harvest. Adapted from: Hanna, HM et al. (2009). Click on the image to enlarge

Where the weeds hide in a combine harvester

In response to the Iowa study on the distribution of foreign materiala Wisconsin 2020 study by Dr. Rodrigo Werle and Nick Arneson went further to quantify weed seeds on a combine and found that the majority of weed seeds in a combine are on the head/platform, rock trap and conveyor . The rotor contained a small percentage of weed seeds. These results can provide additional guidance on where to focus when cleaning a combine for weed seeds and how much time to spend cleaning the combine for weed seeds.

Cleaning a combine harvester to prevent the distribution of weed seeds

Combine cleaning is a relatively simple management practice that can save a lot of time, money and headaches in the future. In as little as 20-30 minutes, major areas of the combine that often contain weed seeds can be cleared.

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It is recommended that these areas be inspected and cleaned to a minimum before using a used combine for the first time on your farm, before harvesting, after combining fields or very weedy areas of a field , and at the end of the harvest. A thorough cleaning of the combine to identify persevered grain or an end-of-season cleaning for maintenance is a much more complex process and will take longer.

When cleaning a combine, make sure the combine is in a safe area where weed seeds can fall to the ground and not be a problem, such as the field where the machine harvested last or back to the farm store. Also, be sure to wear appropriate personal protective equipment (PPE) and read owner’s manuals, especially the sections on cleaning and equipment safety.

To perform a quick cleanup of the combine, follow these procedures and refer to Figure 3.

  1. Run the unloading auger to make sure it is empty.
  2. Working from the front of the combine, blow the corn head or soybean mat. One of the best tools for cleaning the combine is a leaf blower or air compressor. When blowing the machine, look for areas where residue naturally accumulates, such as under the corn snouts.
  3. Clean the conveyor, again looking for areas where residue may have accumulated. At the front of the machine, open the rock trap and drop any collected material.
  4. Moving to the side of the machine, open the residue return, clean out the grain elevator and unloading auger sump and let any trapped residue escape.
  5. Once the previous steps are complete, go full circle around the exterior using a leaf blower or air compressor to clean any surfaces where residue may have accumulated, such as the axles and management system residues.
  6. Finally, close any access panels that were previously opened.
Clean up a combo chart

Figure 5. Areas of the combine to be cleaned for a quick 20-30 minute cleaning process. Click on the image to enlarge


While it can be daunting to add another task while harvesting, in as little as 20-30 minutes and with the proper equipment and safety precautions, a combine can be cleaned to reduce the risk of carrying weed seeds to the next field or farm. This will reduce the number of headaches for future seasons by reducing weed introductions and distribution, thereby reducing herbicide costs and improving crop quality.

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