Removing biomaterial to avoid moving weed seeds between fields can be a daunting task. Typically, combines hold 125-150 lbs of grain and biomaterial after the unloading auger has operated “empty” for one minute.
Trail of weeds from seed shot out back of combine
A true clean-out
A true clean-out requires about six hours to remove as much biomaterial as possible. A clean-out process may be necessary to prevent the spread of weeds to new fields. A first line-of-defense is to avoid harvesting fields with an offending weed species, such as burcucumber or Palmer amaranth, until the end of the season to avoid introducing seeds of new weeds to other fields.
Steps for limited cleaning
While a thorough, top-to-bottom cleaning of the combine is best to avoid the spread of weed seeds between fields, spending 15 to 30 minutes cleaning the combine before moving it out of the field will still remove some biomaterial. First, allow the combine to do some “self-cleaning” by opening doors at the bottom of the clean grain elevator and unloading auger sump on the clean grain tank. Remove the gathering head if time allows. Then, start up the combine; operating the thresher and separator at full speed. Have the cleaning shoe sieves fully open and the fan adjusted to maximum speed. Make sure no one is within 100 ft of the area to avoid being hit by flying debris. Use the rest of the time available to clean the outside surfaces of the combine, the gathering head, and inside the rock trap. Head removal allows cleaning of the feederhouse and easier access to the rock trap. Remember to close elevator and sump doors when finished.
Removing larger weed seeds (e.g. burr cucumber) will probably be more effective than smaller weed seeds (e.g. Palmer amaranth) with this type of limited cleaning. However, any effort is likely more effective than none.
(Source – http://www.farms.com/news/clean-combines-to-avoid-weed-seed-contamination-across-fields-113477.aspx)