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).
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.
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.
Harvest Weed Seed Control
At harvest time, weeds that have escaped season long management often have mature seed still attached to the parent plants. These weed seeds can enter the combine along with the cash crop, exit the back of the combine as chaff (small plant pieces and weed seeds), and be spread across the field, as well as from one field to another. It seems a waste to spend all year spraying weeds with expensive herbicides only to reward the survivors at harvest by spreading their weed seeds out for next year.
An excellent way to stop weeds in their tracks is to collect these weed seeds at harvest and either destroy them or deposit them in a known location where they can be monitored and controlled later. Soybean, wheat, and other crops harvested with a grain header are ideal choices for harvest weed seed control (HWSC). Other crops such as cotton and corn need further equipment development to make HWSC a viable option.
If you are considering adding harvest weed seed control (HWSC) to your weed control program there are excellent resources on the WeedSmart website to help guide you through the initial decisions and the implementation of this important weed control tool.
3 steps to get it working for you
1. Decide which system fits your farm best.
2. Get maximum weed seed into the header.
3. Know how to manage the collected weed seed.
What is Harvest Weed Seed Control?
Choose The Best System For You
Which system is best?
HWSC is being rapidly adopted in Australia and other countries around the world. There are six systems currently being used on Australian farms and they have been initially developed by farmers.
Research has demonstrated that all are very effective weed seedbank management tactics for a range of weed species, achieving over 80 percent control and for some nearly 100 percent.
See HWSC in Action
Over 80% weed control for species that retain seed at harvest
Crop residue management
There are six systems currently used to collect and manage weed seed at harvest. They can be grouped according to the way crop residue is managed: chaff only or chaff + straw.
Chaff carts are a tow-behind unit on the combine that collects the weed seed-laden chaff, which can then be placed into piles that are later either grazed by livestock, burnt, or both and sown through the following season. Chaff carts are often chosen for use on mixed cropping and livestock farms in Australia as the chaff is an excellent livestock feed; however, spreading manure back onto fields can allow for further seed spread.
Chaff lining funnels the chaff and weed seeds into narrow rows behind the combine, where the residue is left to overwinter. The weed seeds are exposed to natural elements that can lead to weed seed decay and predation. Typically a follow-up herbicide application is required. – Chaff lining is usually considered a good entry-level HWSC option.
Chaff decks (chaff tramlining) are similar to chaff lining, but place the chaff in one or both of the combine’s wheel tracks. The added compaction from the wheels can be beneficial in controlled traffic systems.
Impact mills run the chaff through a mill that pulverizes (destroys) the weed seed, which is then spread across the fields. This technology is usually considered the ultimate in HWSC.
CHAFF + STRAW
Bale direct collects all the crop residue directly from the combine and compacts it into large bales suitable for sale.
Narrow windrow burning collects all of the crop chaff and straw residue, and funnels it into narrow rows in the field. These rows are burnt to destroy the weed seed. This method is effective but removes all of the crop residue from the field.
Learn More About Each System in This Research Report
Calculate the cost
While each HWSC tactic is similarly effective in collecting weed seeds , they vary considerably in capital and ownership cost, nutrient removal costs, operational costs, and labor costs. Some HWSC tactics involve the purchase of substantial machinery – such as an impact mill, chaff cart, or chaff deck – but the operational and labor costs might be lower than methods such as narrow windrow burning, which involves low set-up costs but higher nutrient losses and labor costs associated with burning. Chaff lining is often chosen as the best entry-level tactic that requires minimal set-up cost, no additional labor and minimal nutrient loss or redistribution. To calculate the cost of each method for your farm you can use a calculator developed by WeedSmart’s Peter Newman. Download the calculator or learn more @ Calculating the cost of HWSC for your farm.