Harvesting Weed Seeds

Keeping your corn and soybean fields free of unwanted weed seed is an important task, as seeds in your harvest can lead to dockage at elevators. Harvest weed seed control has been a popular topic at grower’s meetings this winter and a few early adopters in the PNW are experimenting with this new (to us) technology. While chaff lining, tram lining… While herbicide resistance is not a new issue, the rapid increase in multiple resistant biotypes in waterhemp, giant ragweed, and horseweed (marestail) is limiting herbicidal options. A waterhemp biotype resistant to six herbicide groups (2, 4, 5, 9, 14, and 27) was recently identified in Missouri. The majority of Iowa fields have waterhemp resistant to three herbicide groups, and five-way resistant populations have been found.

Keep Weed Seed Out of Your Harvest

Keeping your corn and soybean fields free of unwanted weed seed is an important task, as seeds in your harvest can lead to dockage at elevators. You’ve worked hard this season to keep your fields free of unwanted weeds, especially during a year where weed escapes might be more prevalent because of weather challenges. To get every possible bushel out of your acres, you’ll need to be extra vigilant about what goes into the combine this season.

If you do see weeds in your fields at harvest, consider some of these harvesting tips outlined by the United States Department of Agriculture:

  • Avoid harvesting crops that are located in areas that are densely populated with weeds – the risk of spreading weed seed is likely not worth the bushels you might get from that area of the field.
  • Adjust your combine’s cutting height settings to minimize the amount of weed seed that is harvested. Consulting your combine provider will help determine the right height for your field conditions.
  • Regularly clean your farm equipment between harvesting different fields to avoid contamination from weed seed.
  • Destroy all weed seed that is left in the field after harvest to prevent it from entering the soil seed bank. Burning weed seed is a common way to get rid of it.

Before taking your crops to the elevator, consider these additional precautions:

  • Examine corn and soybeans for weed seed upon arrival.
  • Separate weed seed from corn and soybeans through mechanical cleaning or other means.

Weed scientists are recommending a zero tolerance for weed escapes. Ridding fields of weed presence not only benefits your yield potential, but also lowers the chance of future seeds getting added to the seed bank. Incorporating a burndown harvest aid like Gramoxone ® SL 2.0 herbicide helps target difficult-to-control weeds and keeps the combine running smoothly.

Taking these preventive actions to minimize contamination from weed seed will help ensure the hard work you’ve put into this season pays off, and will help prevent weeds from growing on your farm in the future.

To learn more about managing weeds while preventing resistance development, visit ResistanceFighter.com.

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Looking Critically at Harvest Weed Seed Control: What Benefits Can be Realized in PNW Dryland Cropping Systems?

Harvest weed seed control has been a popular topic at grower’s meetings this winter and a few early adopters in the PNW are experimenting with this new (to us) technology. While chaff lining, tram lining, narrow-windrow burning, and integrated impact mills are the most referred to under the term “post-harvest weed seed control”, impact mill units have attracted the most buzz at the events I have attended. Regional weed scientists published an excellent Extension Bulletin titled, Harvest Weed Seed Control: Application for PNW (Pacific Northwest) Wheat Production Systems on the practices and given a concise overview in the blog post by Judit Barroso, OSU, titled, “A Potential Tool to Control Herbicide Resistant Weeds During Harvest?” as well. Since there seems to be an abundance of interest, I want to discuss some of the issues surrounding what this technology can and can’t be expected to do for us.

First of all, and key to understanding the potential effectiveness of these technologies, weed seeds need to get into the combine in order to be chaff lined, tram lined, or impact milled. Producers in the southern US using impact mills have had great success controlling herbicide-resistant Palmer Amaranth, but it retains 98% of its seed at the time of harvest. In contrast, our problematic grassy weeds, Italian rye, downy brome, and wild oat tend to lose some significant percentage of seeds to shatter before harvest is begun. The extent of seed drop depends on a number of factors such as weed maturity at time of harvest and localized weather events (think hot, dry August winds or late summer thunderstorms). This has led to skepticism of the effectiveness of harvest weed seed control at making substantial gains in managing problematic grassy weeds considering how a few weed escapes can proliferate, especially if the escaped weeds are herbicide resistant. I agree we should not expect harvest weed seed control alone to solve our grassy weed problems, but there are potential benefits that should be considered before leaving this tool in the box.

See also  Haze Weed Seeds

Consideration #1: Volunteer Crop Reduction?

These systems should greatly reduce unintentional re-seeding of fields with the fine fraction of seed harvested, reducing the total area of nutrient robbing and green-bridge disease hosting volunteers that need to be managed.

Consideration #2: Lower Quality Weed Seeds in the Bank?

We don’t know yet what impact, over time, these practices will have on the soil seed bank. It is possible that the seeds retained on our grassy weeds at time of harvest are more robust and that eliminating them will provide advantages over time by leaving less thrifty weed seeds in the seed bank. This is an area where more research is very much needed on a species-by-species basis.

Consideration #3: Cumulative Benefits?

When used in combination with crop rotation, an herbicide program utilizing multiple-sites of action and other technologies like smart spraying, can we realize benefits over time that allow us to reduce input costs or boost yields? We know there are increased capital and operation costs with adding equipment and practices. We don’t know what the agronomic and economic benefits are over time. Again, this is an area where multi-disciplinary research is needed.

Will harvest weed seed control solve all our weed problems? No, but that doesn’t mean it should be ignored out of hand as a potentially useful tool. As with any weed control activity, be it cultural, mechanical or chemical, the practice is only as good as the extent to which it is integrated into a well thought out integrated weed management program supported by targeted, relevant, and timely research.

Harvest Weed Seed Control

While herbicide resistance is not a new issue, the rapid increase in multiple resistant biotypes in waterhemp, giant ragweed, and horseweed (marestail) is limiting herbicidal options. A waterhemp biotype resistant to six herbicide groups (2, 4, 5, 9, 14, and 27) was recently identified in Missouri. The majority of Iowa fields have waterhemp resistant to three herbicide groups, and five-way resistant populations have been found. While herbicides will remain the backbone of weed management systems for the foreseeable future, it should be clear that additional tactics are needed. This article focuses on alternative strategies that fit our current cropping system, with an emphasis on harvest weed seed control (HWSC). While none are as easily adopted as changing herbicide programs, they will be essential for preserving herbicides as effective management tools.

Identifying weak links in a weed’s life cycle

Summer annuals are the dominant weed problem in the corn-soybean rotation since these pests establish and mature at the same time as the crops. The annual life cycle has several distinct stages, the success at transitioning between these stages determines whether a weed increases or decreases within a field (Figure 1). Population dynamic models allow researchers to investigate how control tactics targeting different ‘choke points’ influence the long-term success or failure of a weed.

Current weed management programs primarily target weeds as they emerge from the seed bank (preemergence herbicides) or as they develop from seedlings (postemergence herbicides). While these tools are highly effective, the increase in herbicide resistance raises concern regarding their sustainability. Population dynamic models reveal that weeds are very sensitive to changes in the number of seed that enter the seed bank, thus tactics that reduce seed production or their entry into the seedbank have a large impact on the long-term success of the weed. A population model evaluating the benefit of HWSC in delaying the onset of resistance showed that destroying approximately 50% of weed seed at harvest delayed resistance evolution by nearly 10 years (Somerville et al. 2018).

See also  Weed Seed Packet

Harvest weed seed control (HWSC)
Preventing seed produced by weed escapes from entering the seed bank is an effective approach to weed management. The first mechanized combines often were equipped with weed seed collectors to prevent weed seed from being returned to fields. Walking beans was a rite of passage for generations of Iowa farm kids. However, with the introduction of modern herbicides and increases in farm size, these tactics have fallen out of favor. HWSC is one of the few alternatives to herbicides that can be incorporated within Iowa’s production system without significantly increasing labor requirements.

The interest in HWSC has been driven by Australia’s struggle with herbicide resistant weeds. Western Australia is recognized as the herbicide resistant weed capital of the world due to the widespread occurrence of multiple resistant weeds. The loss of effective herbicides for several important weed species, especially annual ryegrass (Lolium rigidum) in wheat production, has forced the development of innovative approaches to managing weed seed before they enter the seedbank. A survey of Australian farmers in 2014 found that 82% of respondents expected to use some form of HWSC within 5 years (Walsh et al. 2017).

Australians have developed several HWSC techniques, including chaff carts, baling of crop residues, chaff tramlining, narrow-windrow burning, and weed seed destruction (Walsh et al. 2017). Narrow-windrow burning is the most widely used HWSC practice in Australia (Table 1). This strategy involves altering how the combine manages crop residue during harvest (Walsh and Newman 2007). Relatively simple modifications are made to concentrate the chaff in a narrow windrow that is later burned. Research has shown that 70 to 80 percent of weed seed is collected by the combine and concentrated in the chaff, and nearly all of these seeds are killed by fire. The use of this practice in Western Australia has increased from 15% of farmers in 2004 to more than 50% of farmers in 2014. Due to differences in crops and climate, this tactic may not be effective in Iowa, but its widespread adoption demonstrates the value of targeting weed seed at harvest.

Probably most applicable for Midwest cropping systems are chaff mills, devices that destroy weed seed during harvest (Schwartz-Lazaro et al. 2017). Chaff, which contains the majority of weed seed, is separated from other plant material as it moves through the combine. The chaff is run through a rolling cage mill that damages seed sufficiently to render them non-viable. The original design, the Harrington Seed Destructor, was a separate unit pulled behind the combine. More recently, chaff mills are integrated into the back of the combine.

Factors that influence the effectiveness of chaff mills are how long seed are retained on weeds and the ability of the mill to destroy seed. Seed retention varies widely among species. Chaff mills or other forms of HWSC would have little value for managing giant foxtail since most seeds fall from plants prior to harvest. Weed scientists at the University of Illinois found that 72, 92, and 95% of waterhemp seed remained on the plant at the time of soybean harvest in three years of research (A.S. Davis, unpublished data). The value of HWSC for waterhemp would be diminished in years with late harvest, such as in north central Iowa in 2018. Tests with a wide range of weed seeds have shown that greater than 95% of seed entering the mill is rendered non-viable, thus retention on the plants is the limiting factor in the effectiveness of this tactic.

Destroying weed seeds with combine modifications, therefore reducing the return of seeds to the soil weed seedbank, is an effective way of reducing herbicide selection pressure and the resultant evolution of herbicide resistant weeds. But as they say, there is no free lunch. Early versions of chaff mills cost approximately $120,000, but people estimate that with mass production the cost could be reduced by at least 50%. In addition to the cost of the equipment, there are several factors that pose challenges to this technology. Current versions require approximately 80 to 100 horsepower, this power drain can result in a 12-20% reduction in combine capacity (Anonymous 2018). The increased power requirement associated with weed seed destruction results in an average increase in fuel consumption of 0.4 gallon per ton of grain.

See also  Cotton Candy Weed Seeds

Currently there is limited experience using the chaff mills in corn or soybean, thus it is unclear how well they will perform in our system. Green stems of crops and weeds negatively impact mill performance, frequently blocking flow of the chaff through the mill. The University of Arkansas has the only integrated chaff mill in the United States, and they report that it has worked better in corn than in soybean. The problems in soybean likely are associated with the green material frequently present during soybean harvest.

Another issue with the chaff mills has been rapid wear of the mill components. Manufacturers estimate that rotors should have a useable life of 800 hours, but most Australian owners of the equipment report lives of less than half that. Replacement cost for rotors is more than $2000. The low cutting height of soybean probably would reduce the lifespan of rotors more than when harvesting corn due to greater amount of soil on the soybean residue.

At this time, most Iowa farmers would not consider weed management a big enough problem to warrant the expense and inconvenience of adopting HWSC. Australia’s farmers went this route only after weeds developed resistance to nearly all herbicides available for their cropping system. The increasing pace of herbicide resistance evolution and the slow introduction of new herbicide sites of action suggest we will face the same dilemma in the not-too-distant future. To prevent further loss of effective herbicides, attitudes towards weed management need to change. Reducing the size of the weed seedbank must be viewed as just as important as protecting crop yields.

There are several issues with HWSC that limit its utility in our system at this time. However, it is important to recognize that this is new technology for which the bugs are being worked out. Weed problems are severe enough in Australia that many farmers are willing to tolerate the problems associated with the equipment. Currently there are only two versions of the chaff mills on the market. At least three other companies are involved in designing new equipment; these companies have greater resources available for supporting development than the initial companies. In addition to reducing the cost of the equipment, it is likely that some of the current limitations to the internal chaff mills will be resolved. Until HWSC is more widely available and convenient to implement, farmers must take steps to optimize both the effectiveness of herbicide programs and the suppressive ability of the crop.


Anonymous. 2018. Residue management at harvest. Weed seed options. Kondinin Group. Online https://weedsmart.org.au/wp-content/uploads/2018/06/RR_1802_weedsmart.pdf

Schwartz-Lazaro, L.M., J.K. Norsworthy, Walsh, M.J., and Bagavathinnan, M.V. 2017. Efficacy of the Integrated Harrington Seed Destructor on Weeds of Soybean and Rice Production Systems in the Southern United States. Crop Sci. 57:2812-2818.

Somerville, G.J., S.B. Powles, M.J. Walsh and M. Renton. 2018. Modeling the impact of harvest weed seed control on herbicide-resistance evolution. Weed Sci. 66:395-403.

Walsh, M. and Newman, P. 2007. Burning narrow windrows for weed seed destruction. Field Crops Res. 104:24-30.

Walsh, M., Ouzman, J., Newman, P., Powles, S., and Llewellyn, R. 2017. High levels of adoption indicated that harvest weed seed control is now an established weed control practice in Australian Cropping. Weed Technol. 31:341-347.

Prepared for the 2018 Integrated Crop Management Conference. Ames, IA.