Photo: Kondinin Group Research Report

Photo: Kondinin Group Research Report

Updated: September 26, 2022
By Mark Townsend , Ben Beale , and Kurt Vollmer

Minimizing Combine Weed-Seed Movement

Harvest is fast approaching for grain producers; some of which have already begun shelling corn in recent days. Though this time of year often marks the end of the season, some decisions we make at harvest have lasting effects on subsequent growing seasons; one of which is weed control. If weeds are allowed to mature and set-seed, harvest equipment can be highly effective at retaining, and transporting weed seeds from field to field, as well as dispersing weed seeds across the field, further increasing the weed seed bank.

In a previous Agronomy News article, we offered some cultural Palmer amaranth control options at harvest. This article will expand on these strategies and provide additional context on the impacts of weed seed movement. Before the combine ever reaches the field, determining the order in which fields will be harvested based on weed pressure can reduce weed seed movement.

Determining the relative weed pressure and species composition of fields offer some guidance to a harvest order. Infested areas should not be harvested before clean areas, as weed seeds can easily be transported to the clean field. If possible, save the worst for last: highest weed pressure fields should be combined last to minimize spread to the other less pressured fields. Finally, combine operator judgment should not be understated: it may be prudent not to harvest some portions of the field heavily infested with difficult weeds and a poor crop stand. What you do not harvest in crop, may be repaid in weed seeds unscattered. This is especially true in operations where only a small percent of the fields have palmer amaranth present. Our experience suggests the most common and costly mistakes when dealing with early infestations of Palmer amaranth is harvesting an infested field and then driving directly to clean fields.

Once out of the field, cleaning the combine before moving to another field can significantly reduce the potential for weed seed transport. Following harvest of a field, combines can retain as much as 150 lbs of biomaterial on and in combine components. This additional material protects and holds weed seeds that can be later dislodged at other locations. An efficient, yet thorough, cleanout takes only 20-30 minutes with a leaf blower and air compressor:

  • With the header removed, and after running the unloading auger for at least a minute, open the clean grain door, tailings door, rock trap, and unloading auger sump.
  • Run the separator, adjust the shoe-fan and rotor to full speed while electronically opening and closing the sieves and concaves.
  • Run in this configuration for at least two minutes. Operators may also choose to drive in this state to dislodge more material during this operation.
  • Wearing personal protective equipment (dust mask, eye protection, ear protection, and gloves), blow off material from the external components of the combine–remember to work from top to bottom, and to focus on high-chaff regions like the feeder-house, and straw-chopper. Video of the process may be found at:

At the end of the harvest season or between highly infested and lower pressure fields, a more thorough combine clean out should occur following the above steps with the addition of the straw bale method before the blower clean-out procedure.

  • With the combine components at normal operating speeds, begin feeding flakes from small straw bales into the raised header (between 2-3 bales required depending on combine size).
  • Allow straw to work through the combine and flow out of the machine onto an area appropriate to receive the straw and additional harvest chaff (tarps, concrete pad, etc).
  • To clean the grain tank, mix 25 lbs of wood pellets with half a small straw or hay bale and place the mixture at the grain tank auger. Run the auger to remove the mixture along with any additional harvest residue from the grain tank. Video of the process may be found at:

Additionally, given that some weeds are especially difficult to control in certain crops (ex. Palmer Amaranth in a soybean crop), observe the planned crop rotation when determining a harvest order. Ensure that fields containing potentially difficult to manage weed species will be rotated to a crop in which the weed species may be more manageable.

New technology is being developed that can further reduce seed viability during harvest. While in the field, combine add-ons have shown efficacy in reducing weed seed spread. Seed impact mills and chaff lining devices are two common options of combine modifications. Seed impact mills were devised and first implemented by Australian growers, and have received significant attention world-wide in recent years. Seed impact mills pulverize weed seeds that enter the combine with an aggressive chopping and hammer-mill-like system powered by a drive belt and fits behind the combine’s straw-chopper. There is a growing number of brands; however, the documented research focuses on the Harrington Seed Destructor (now iHID) and the Reddekopp Seed Control Unit. Recent research from Virginia Tech with trailing the Reddekopp system indicates a 99% kill rate for Palmer amaranth and common ragweed seeds, with a subsequent 42% reduction in common ragweed density the following year. Similar research from the University of Missouri documents a 98% reduction in viable waterhemp weed seeds. These systems, though effective, come at a significant capital expense. Additionally, these systems place an additional load on the combine engine increasing fuel consumption, but does not significantly impact combine performance, as measured by harvested acres per hour.

Locally-designed chaff line chute working in Delaware. Photo: Claudio Rubione, GROW.
Locally-designed chaff line chute working in Delaware. Photo: Claudio Rubione, GROW.

Chaff Lining devices funnel the chaff exiting the straw-chopper into wind-rows directly behind the combine, or in the compacted wheel tracks while the straw is distributed evenly behind the combine. Chaff lining does not directly destroy weed seeds, instead the device concentrates the weed seed containing chaff to less than 10% of the field. The layer of debris slowly decomposes and exposes the weed seeds to decay and a less suitable environment for germination. Ongoing research from Iowa State finds that 95% of weed seeds can be concentrated behind wheel tram-lines, reducing the distribution of waterhemp and aboveground biomass of germinated seeds. Currently, these systems are the most inexpensive harvest weed seed management device available, with DIY plans available online and other relatively inexpensive commercial options for sale as well. However, these systems are more well-suited for species that retain a majority of their seed at harvest, such as Palmer amaranth and waterhemp harvest of small-grain and other relatively low-biomass producing crops.

These harvest weed seed management methods will require multiple seasons of good stewardship to effectively reduce weed populations. They will not substitute for preventative or in-season control treatments, but are a core component of effective integrated weed management programs.

This article appears in September 2022, Volume 13, Issue 6 of the Agronomy news.

Agronomy News, September 2022, Vol. 13, Issue 6

Agronomy News is a statewide newsletter for farmers, consultants, researchers, and educators interested in grain and row crop forage production systems. This newsletter is published once a month during the growing season and will include topics pertinent to agronomic crop production. Subscribers will receive an email with the latest edition.