The following is from Jeff Burbrink, Elkhart County Extension educator.
If you were to design a machine to spread weed seeds across a field, a combine would be close the ultimate design, although I am sure you could come up with a cheaper alternative.
When a combine leaves an average corn or bean field, it carries with it approximately 150 to 200 pounds of plant material. Much of that is crop residue, of course, but there may be 5-6 pounds of weed seed hitching a ride in the various nooks and crannies.
Given the situation with a number of weeds now resistant to some of our more common herbicides, it makes economic sense to give the combine a thorough cleaning between farms and before the winter. You can view a short, well-done video by the University of Wisconsin on how to do a top to bottom, front to rear cleaning of a modern combine at https://goo.gl/L4Y7Gj. The steps below are similar across most models of combines, and are very similar to protocols used when "identity protected" seed are harvested.
Safety is the first concern. Much of the cleaning procedure involves blowing the dust from crop residue with compressed air or a leaf blower. Eye and lung protection are important to keep you healthy. Do not forget to protect your hearing. Leaf blowers are among the loudest of equipment and even short-term exposure can do damage to your hearing.
The first step to clean a combine begins by running the machine for approximately a minute to clean out matter along the main grain path. Next, engage the safety stop and open the rock trap near the header. After that, open the unloading auger sump. The tailings elevator door and the clean grain elevator are opened. Once these steps are completed, the combine is started, the separator is engaged, and the fan speed is set to high to blow out reside in these locations.
The next phase is to move to the top of the combine. Run the combine over some bumpy terrain for a few minutes to help dislodge any residue first. Shut the machine down, then use a leaf blower to blow debris from the grain tank. You may have to use a knife or screwdriver to dislodge residue that the blower cannot move.
The next step is to clean out the unloading auger. This will require about 1.5 bushels of wood chips or sawdust to be put into the sump. Run the unloading auger and collect the sawdust to keep the weed seeds for spreading.
Moving to the front of the machine, lift the snouts to expose the plant debris and use a leaf blower to dislodge the residue. This is a dusty operation, so be sure to wear eye and lung protection. Separate the head from the combine and clean the interior of the head. The exterior of the feeder housing can be cleaned with compressed air.
The access doors to the rotor and cylinder area are next. Once removed, you can clean the threshing area with compressed air. There is usually quite a bit of residue in this area.
The clean grain augers are the next phase. Using a combination of compressed air and vacuums, this area does not have a lot of residue, but often has a few fines, which may harbor weed seeds. Following the grain path, the chopper and sieves area, removing the cover from the bottom of the chamber. The elevators, which transfer the grain up to the holding tank, and the moisture sensors are the final areas to be cleaned, followed by a once-over of the exterior of the machine.