The Straw Bale Methodology for Cleaning Weed Seeds Out of a Combine

In this video we describe how to deep-clean your combine from weed seeds with the use of straw bales.

We often see weeds visible above a cash crop canopy, particularly in soybean fields. Sometimes these weeds are solitary specimens, while other times they are distributed in patches or even rows. Farmers have told us that these weed species are new to their fields and may have been carried into the fields by a combine. It is a plausible scenario: combines have been documented to spread weed seeds from infested fields to new fields. Weed seeds can survive in debris on a combine across seasons.

Most people think that weed seeds are hidden somewhere in the combine and that is almost impossible to remove, even during a thorough cleaning-up. This is not true. It is both crucial and possible to deep-clean the combine before moving to another field, as well as at the end of the season.

A soybean combine works by cutting off both soybeans and weeds near ground level and bringing the material into the header. All the material then feeds into the threshing cylinder. While grain is collected in a bin, straw and chaff are dropped behind the combine as it moves along the field.

Moving from the rear of the combine to the header, straw chopper knives, walker cranks, and straw walkers can allow residue to accumulate. Weed seeds get stuck in the straw chopper and chaff spreader as well as in the unloading auger. To reach many of those parts, the different access gates on the sides of the combine should be opened. Cylinder and concave are composed of many parts where weed seeds easily hide.  Since they are difficult to access they should be cleaned with compressed air whenever possible.

Sieves, tailing return chains, drives, tank auger, and transmission are also important parts that must be considered in the cleaning process.

Plant residues containing weed seeds can be removed by opening the stone trapdoor.

Start cleaning a combine from the top and from the header to the rear, following the normal circulation of the material. Certain parts are better cleaned with an air compressor, while others could be done with a leaf blower. Cleaning the grain bin and augers as well as the moisture sensor is essential to prevent wagon and truck contamination with weed seeds.

After the combine is cleaned with a blower or compressed air, let the fans work until no more residue is coming out the back. A tarp could be helpful to see when that happens.

The last step is to introduce hay through the header and a combination of hay and wood pellets to the grain bin. Be sure to take the bale pieces apart and feed them into the machine from either the feeding house opening or the header. Depending on how big the combine is, it will take from two to three bales to clean it. With the engine, fans, and all threshing components at normal operating speed and the header turned on, carefully feed bales from the sides of the header to the middle. To clean the grain bin, be sure the auger is not running, mix 25 pounds of wood pellets with half a bale, and introduce the mixture into the grain tank auger. Then start operating the auger to clean it. Straw fed into a combine after harvest, as opposed to straw as part of the harvest (grain + straw + chaff), moves around the combine more freely to reach those spots containing hidden weed seeds.

Once the bales have entered the machine, watch the rear of the combine to see when no more material comes out. When that happens, a final, cosmetic cleaning step with the help of a blower may be necessary to remove a few large pieces of straw which might be stuck in different parts of the combine.

Results From Three Combines (pdf)

Prior to storing combines at the end of the season, three previously cleaned combines were tested for weed seed retention. Initially combines were run until no residue fell on the tarp used to collect the weed seeds, then straw bales and wood chips were fed through the combine in an attempt to catch and remove any remaining weed seed.

Research has shown the straw bale cleaning method to be effective.

After running this test from a first combine, weed seeds were identified and counted. Over 3,000 Palmer amaranth seeds along with other weed seeds were collected. A second combine cleaning resulted in more than 1,700,000 Palmer amaranth seeds collected in total over 3,5 million weed seeds were removed from this cleaning. The third combine received a thorough cleaning and resulted in no weed seed retention.

Combines have been designed for harvesting crops, separating grains from stems and pods, and cleaning the grain of unwanted material. Unfortunately, they are not designed for weed seed self-cleaning. Consequently, the machine must be carefully cleaned, keeping in mind where weed seeds can hide. The time it takes to deep-clean a combine to remove weed seeds could range from minutes to hours, depending on how often the combine is cleaned and how weedy the fields are. Remember that prevention is the key to managing weed seed dispersal, and combine cleaning is a major part of prevention.

Final considerations

  • Prevention is the key to avoid weed seed dispersal and contamination.
  • Plan ahead and be proactive.
  • Implement a plan for keeping weeds under control, managing fields strategically as well as cleaning the machinery involved.
  • Follow a “Weed Seed Cleaning Protocol” with all machinery which could be contaminated with weed seeds.


Claudio Rubione (University of Delaware)

Mark VanGessel (University of Delaware)

Edits: Victoria Ackroyd, Mark VanGessel, Barb Scott