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. California Small Grain Production
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
Concerns about a growing resistance to herbicides
In Mediterranean or arid climates, particularly in areas with marginal soils, crop rotations are often limited to a narrow range of hay, pasture, a handful of winter legumes, or rainy-season grasses. Arid conditions and weathered soils drove Australia’s rainfed grain growers to adopt no-till strategies earlier than their counterparts in California. While beneficial from a water use perspective, successful no-till systems depend on herbicides to control weeds that were traditionally kept in check with tillage.
Dependence on herbicides alone in these systems has resulted in weeds with resistance to multiple modes of action. In Australia, there is one documented population of rigid ryegrass (Lolium rigidum) that is resistant to 15 different herbicides, covering seven different modes of action. Italian ryegrass (Lolium multiflorum), an annual winter species commonly found in small grain production systems in California is also notorious for its ability to develop resistance to entire groups of herbicides. One population collected in California orchards has resistance to four modes of action, but this population is not yet widespread.
One advantage is that California is still a long way from Australia in terms of herbicide resistance in our small grain cropping systems. We can look to Australia for 20 years of methods and data that have arisen in an effort to combat herbicide-resistant weeds. And, if we can find a way to slow the spread of herbicide resistance, we may be able to continue relying on some of the herbicides that are available in our area.
What is Harvest Weed Seed Control?
When growers need to manage herbicide-resistant populations of weeds without tillage, one strategy is to reduce the amount of seed returned to the seedbank by destroying, or removing weed seed caught during harvest operations. Collectively these strategies are referred to as Harvest Weed Seed Control (HWSC). These methods have long been studied in Australia. Researchers at Washington State University, Virginia, Texas, and other parts of the US have been looking into these methods for several years. With increasing herbicide resistance developing in Italian ryegrass populations in California, similar approaches may be worth exploring in the Sacramento Valley.
HWSC methods include several different strategies. Each have their own constraints and challenges, and all are designed to physically destroy seeds during or after small grain harvest. This prevents new seeds from entering the seedbank, including those that are from plants partially or fully resistant to herbicides used in the field. This reduces the rate of resistance development and the spread of resistant genes.
Narrow-windrowing – Sorting straw and chaff into narrow windrows, in an effort to contain weed seed in a small area. Windrows can be burned, grazed, or if narrow enough, allowed to compost/ rot in place. If the windrows are left alone, the high rates of carbon will drive soil microbes to scavenge for nitrogen, which subsequently immobilizes plant available nitrogen. Low available nitrogen adds to a crowded, competitive, and less-than-ideal environment for a large number weed individuals that otherwise thrive in high-nitrogen environments. Although this reduces the overall area for the crop, the tradeoff is reduced weed pressure in the rest of the field. This method is improved by the use of controlled-traffic farming, which ensures that the chaff lines end up in the same area after every harvest (maintaining the competitive environment and containing the weed seed). The material in the windrows can also be destroyed with burning or by grazing. Burning in windrows has been shown to greatly reduce the survival rate of Italian ryegrass seeds, while additional studies have shown that other species of ryegrass have a roughly 10% survival rate in ruminant stomachs. However, grazing and burning, particularly in the context of California’s wildfire risks, have their own understandable challenges associated with them.
Chaff lining/ Chaff tramlining – This technique is similar to narrow windrowing, but in this case only the chaff is funneled into a narrow area and straw is spread throughout the field as normal. Chaff can also be moved to the sides of the combine (using a lateral conveyor belt), being deposited under the path of the wheels (this process is referred to as “tramlining”). Controlled traffic farming ensures that the seed is concentrated in the same place every year. In the case of tramlining, tractor wheels run over the weedy areas with every field operation, ensuring that any weed seeds that germinate end up growing in compacted, competitive, and highly-trafficked soil.
Direct Baling – Collecting straw and chaff immediately into bales, usually using a tow-along baler behind the combine. After harvest, bales can be moved off the field, thereby removing a large proportion of weed seed before it can disperse into the soil. However, removal of this much biomass can be problematic for growers with low organic matter soils that otherwise benefit from maintaining residues on the ground.
One version of an impact mill attachment added to the back of a combine behind the cleaning sieves (photo: Redekop Mfg)
Integrated impact mills – In these systems, chaff is still ejected back into the field, but only after initially going through a hammer mill that shatters and destroys the seed. High initial equipment investments can cool grower enthusiasm. However, one 2017 report from Australia indicated that 29% of growers considered impact mills to be part of their future operations, indicating that at least some growers consider the benefits to outweigh the costs. Combine compatibility can also be a problem as most of the mills are designed to accommodate the larger combines of the midwest, whereas most California combines are typically optimized for relatively smaller acreage.
Limitations and project work
The caveat to all of this is that in order for HWSC to be effective, fields need to be harvested before weed seeds shatter (falling from the plant to the ground). As of yet the shatter patterns of Italian ryegrass in California are not well known. Additionally, previous research has indicated that Italian ryegrass seed retention at small grains harvest can be highly variable across different locations. For example, researchers from the inland Pacific Northwest have reported Italian ryegrass seed retention rates at harvest of 27-50% whereas a 58% of seed retention has been found in Australia. Furthermore, grains in California are planted at different times of the year and our harvest season occurs at different times of the year relative to Australia (not to mention the Pacific Northwest).
Shatter patterns might well be the same as those in other areas of the world but calling for a full scale HWSC revolution isn’t very useful unless we know that a good amount of the weed seed has not yet shattered in the field when small grain crops are harvested.
In an effort to address this, UC researchers will begin collecting data on the shatter status of Italian ryegrass populations in several areas of the Sacramento Valley leading up to harvest. If we consistently see that a significant portion of the Italian ryegrass seed remains attached to the plant at harvest, then HWSC strategies may offer viable control options for California. Conversely, if ryegrass seed has largely shattered by the time our grain crops are harvested, then California growers may need to consider other options for control.
Of course, the best control strategies for herbicide-resistant weeds still include a mixture of different tools such as: the use of herbicides with different modes of action (including pre-emergent and post-emergent types where possible), the use of diversified crop rotations, and well-timed mechanical control. However, a better understanding of the potential for HWSC in California may give growers an edge in reducing the spread of herbicide resistance.
If growers in Sacramento, Solano, or Yolo County are interested in collaborating with UC Cooperative Extension Agronomists in trials focusing on control of Italian ryegrass in small grain crops or have related questions, please reach out to Konrad Mathesius ([email protected]).
 : ACCase inhibitors, ALS inhibitors (imazamox, mesosulfuron), PS1 inhibitors (paraquat), and EPSP synthase inhibitors (glyphosate)
 My thanks to Dr. Ian Burke and Prof. Drew Lyon of WSU for sharing their research and insight regarding HWSC of Italian Ryegrass and other troublesome weed species.