For A “Weed-Free” Combine, Feed It Straw – No Kidding

A seemingly thorough cleaning of a combine with compressed air, a leaf blower or pressure washer can still leave millions of weed seeds hiding inside the machine, waiting to be deposited into another field.

But according to research by University of Delaware weed scientists working on a USDA-ARS, areawide project called GROW (Getting Rid of Weeds), the introduction of two to three straw bales into the header after an initial cleaning is a simple, inexpensive method of knocking those weed seeds from their hiding places.

It just goes to show, sometimes a combine – like a human body – needs a little roughage to scrub out the nooks and crannies where bad stuff can hide.

The idea of cleaning combines with straw bales comes from previous work at Argentina’s National Agricultural Technology Institute (known as INTA), according to Claudio Rubione, Extension associate in integrated weed management at the University of Delaware’s Elbert N. & Ann V. Carvel REC.

For a video of the straw bale method click here.

Counting The Differences

Rubione points out that early INTA research did not quantify how much weed seed was removed by the method. “What we did was to just add numbers to what they did in the past,” said Rubione.

The Delaware researchers also found that the combination of wood pellets and straw added to the grain bin auger cleans hard-to-reach places where weed seeds hole up there and in the grain bin.

The researchers ran tests on three random, farmer-owned combines to determine the effectiveness of the straw and wood pellet cleaning method, using a tarp to catch material exiting the combine.

Combine #1

After a thorough cleaning with compressed air, straw bales were introduced into the header and straw bales and wood pellets into the grain bin and auger. An additional 63,500 weed seeds were captured by this method.

This included 3,099 Palmer amaranth seeds, 266 morningglory seeds and 1,009 velvetleaf seeds. The Palmer amaranth seeds were found to be herbicide-resistant.

Like some kind of police lineup, here are the weed seeds collected from Combine #1. From left to right: pigweed, morningglory, velvetleaf, fall panicum, with the last 4 jars holding all the crabgrass seed collected from the combine. Photo: Claudio Rubione

Combine #2

After cleaning with compressed air, the straw bale/wood pellet method removed an additional 3.7 million weed seeds, including 1.7 million Palmer amaranth seeds.

Combine #3

After compressed air and straw were used to clean the combine, no weed seeds were counted. The farmer who owned the combine explained that the machine had most recently harvested around 500 acres that were completely clean with no weeds, according to Rubione.

Drawing Conclusions

Rubione and his team of researchers found these conclusions:

  • Weeds seeds still hide in combines even after being thoroughly cleaned.
  • Straw bales and wood pellets removed weed seeds from what would be considered well-cleaned combines.
  • Farmers should not ignore weed seed volume. Even insignificant volumes can deposit seed in fields the following season.
  • The straw bale method can remove close to 100% of the weed seeds in the combine.

According to Rubione, residue containing weed seeds can accumulate in straw chopper knives, straw walker cranks and straw walkers. Weed seeds also get stuck in the straw chopper and chaff spreader as well as in the unloading auger. The combine cylinder and concave are composed of many parts where weed seeds easily hide.

Here’s more information on the study.

Serious Combine Cleaning In 8 Steps

Here’s how to clean a combine integrating the straw bale/wood pellet method.

Step 1: Open the doors on the sides of the combine.

Step 2: Start cleaning the 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 cleaned 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.

Step 3: 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.

Step 4: With the engine, fans, and all threshing components at normal operating speed and the header turned on, carefully break apart and feed straw bales from the sides of the header to the middle. Depending on how big the combine is, it will take from two to three bales to clean it. Be sure to purchase straw bales that do not contain any seeds, from weeds or otherwise.

Feeding straw. Photo: Claudio Rubione

Step 5: To clean the grain bin and auger, 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 and chaff), moves around the combine more freely to reach those spots containing hidden weed seeds. Wood pellets – the type used for wood stoves – can be purchased locally.

Straw blown out the back of the combine during the cleaning process. Photo: Claudio Rubione

Step 6: After all bales have entered the machine and no material is exiting the combine, 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.

Step 7: Clean out the stone trap door. “It’s not cleaned too often because it’s hard to get to. Usually you can remove 20 pounds of material. Imagine the amount of weed seeds that come with that,” Rubione said.

Step 8: To determine whether to continue cleaning, use a tarp to collect material at the rear of the combine. Inspect the material for weed seeds to determine if cleaning is complete. It typically requires two to three bales.

More On GROW

GROW brings together a team of weed scientists from 14 universities and the USDA’s Agricultural Research Service, whose purpose is more adoption of integrated weed management techniques.

Other research projects include cover crops, crop rotation, use of herbicide effective mode of action and harvest weed seed control. For more information, visit