Harvest Weed Seed Control

There are four other methods of harvest weed seed control in use today, though less common than seed impact mills and chaff lining.

Four of the six HWSC systems—chaff lining, seed impact mills, chaff tramlining, and chaff carts—target only the chaff, whereas bale direct and narrow windrow burning target both the chaff and the straw residues leaving the combine. Many combines require the installation of a baffle to keep the chaff and straw separate. For a quick overview of the pros and cons of each method, see Figure 1.

Chaff Tramlining: Here, chaff is funneled into narrow rows that align with the two wheel tracks of a farmer’s equipment. In a “controlled traffic farming system,” a grower uses only these dedicated wheel tracks—or tramlines—as they move equipment through the field. This reduces overall soil compaction and produces an unfavorable environment for weed seeds within the tramlines, due to factors such as soil compaction and plant injury from equipment.

Chaff Carts: Imported from Canada, chaff carts (sometimes called “chaff wagons”) follow behind a combine and collect chaff harvest materials, where most weed seeds end up. Chaff can be dumped in the field for subsequent management or removed from the field. Chaff can be a valuable feedstuff for livestock.

Bale Direct: A baler is attached directly to the combine, which bales chaff and straw residues as well as weed seeds in the harvest residues. If a market exists for them, the bales can be sold for bedding or livestock feed.

Narrow Windrow Burning: In this method, all harvest residues—including chaff, straw, and weed seeds contained within them—are funneled into a narrow row, where weed seeds can be destroyed by burning. This method has proven effective at destroying weed seeds, but comes with significant fire risks and other trade-offs to soil health. Local burning regulations may also exist.

Watch this video clip of narrow windrow burning:

For harvest weed seed control to be effective, seeds must be retained on the weed plant at harvest. Seeds on the ground or lower than the cutter bar cannot be controlled with these methods. The timing of weeds’ seed drop can vary by species, region, and even season to season. But in general, upright annual broadleaf weeds are the best targets, followed by annual grasses. While harvest weed seed control methods do not control perennial structures such as tubers and rhizomes, they can help reduce the spread of perennial weeds by their seed. Lastly, we don’t know how well wind-dispersed seeds, such as horseweed, are captured by the combine. See the following table for a breakdown of soybean and wheat weeds by their seed retention at harvest, how long their seeds remain viable in the seedbank, and their overall harvest weed seed control potential (Figure 2).

In general, harvest weed seed control technology relies on the likelihood of weed seeds making it into and through a combine during harvest. As a result, crops where HWSC methods work best tend to be grain crops harvested by combine headers that maximize the capture of weed seeds (see Figure 3).

Small grains such as wheat, soybeans, canola, rice, sorghum, and dry beans—typically harvested with platform headers (also called draper or grain headers)—have proven to be good candidates for harvest weed seed control. Likewise, stripper headers have proven adept at capturing weed seeds. Although less research exists on corn harvest weed seed capture, recent findings led by the University of Missouri and Virginia Tech suggest that weed seed capture with corn headers is variable and significantly lower than with platform or stripper headers, though it can be improved with certain header modifications. Preliminary findings show that 20% to 80% of weed seeds escape at the corn header and thus are not subjected to HWSC.

Finally, crops harvested with entirely unique harvest equipment—such as cotton and peanuts—are not good candidates for harvest weed seed control methods, due to a lack of compatibility with commercially available HWSC equipment options today. However, researchers from Texas A&M University are analyzing the ability of cotton pickers and strippers to capture weed seeds to aid future exploration of the use of seed impact mills in cotton. Canadian researchers are also evaluating HWSC in potato production.

Check out these resources to set up your combine. Adjusting your combine for maximum weed seed capture can actually reduce fuel use and reduce grain losses, too. 

• Farmer Forum Recap: How to Set Up Combine Headers for Weed Seed Capture
• Farmer Forum Recap: How to Optimize Threshing and Separating for Harvest Weed Seed Control

This question is currently the focus of ongoing research in the U.S. So far, researchers have found that waterhemp in the seed bank has been reduced by 96% to 98% in just two years with seed impact mill use in Missouri. However, while rigid ryegrass density declined every year in Australia, it took five to eight years before fully realizing the potential of HWSC. Other research has seen about a 30% reduction in weed density after one harvest with simulated seed impact mills.

The cost of harvest weed seed control varies widely by method used.
The chart below (Figure 4) provides a general comparison of the cost per acre between the different methods in use today, in U.S. dollars.
Up-front costs can vary substantially—from less than $1,000 for do-it-yourself chaff lining to around $70,000 for a seed impact mill.

You can use this digital calculator, adapted from WeedSmart, to customize the cost of HWSC for your farm.