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Review

Integrated Weed Seed Impact Mills for Southeast Asian Rice Systems: Could They Aid Sustainable Weed Management?

by
Leigh Vial
1,*,
Jhoana Opeña
2 and
Jaquie Mitchell
2
1
Research Institute for Environment and Livelihoods, Charles Darwin University, Ellengowan Dr, Casuarina, NT 0810, Australia
2
School of Agriculture and Food Sustainability, University of Queensland, St Lucia, QLD 4072, Australia
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(6), 1333; https://doi.org/10.3390/agronomy15061333
Submission received: 12 April 2025 / Revised: 25 May 2025 / Accepted: 28 May 2025 / Published: 29 May 2025
(This article belongs to the Special Issue Agricultural Innovation and Sustainable Development in Tropical Cropping Systems)
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Abstract

Weed management is a persistent challenge in Southeast Asian rice production, particularly in direct-seeded rice (DSR), due to the diversity of weed species and variable field and environmental conditions that can compromise weed control, necessitating innovative solutions. An integrated weed seed impact mill (iWSIM) reduces weed seed banks by destroying weed seeds during the harvest process. This mixed study is the first to fully explore the applicability of iWSIM technology in Southeast Asian rice systems, focusing on both combine harvester and iWSIM specifications and operation, determinants of efficacy, and field and harvest conditions. Weed seed bank reduction with an iWSIM depends on several factors, including weed seed retention and subsequent capture by the combine at harvest, weed seed separation into the chaff fraction, and the iWSIM’s efficacy against weed seeds captured in the chaff fraction. Observations from Southeast Asia indicate variable seed retention among key weed species, presenting challenges for harvesting strategies and iWSIM effectiveness. To optimize the iWSIM efficacy, recommendations include larger fields to reduce the weed seed produced on bunds, achieving complete early-season weed control, lowering the harvest header height to about 15 cm to capture more weed seeds, cleaning mechanism adjustments to ensure weed seeds are retained in the chaff fraction, and greater combine harvester engine power to allow a lower header height and power the iWSIM. The estimated weed control benefits of the iWSIM should also be weighed against additional equipment operating costs. iWSIM technology holds promise as part of a sustainable solution for weed control in Southeast Asian rice, contingent upon further region-specific research and adaptation.

1. Introduction

Effective weed management remains a critical challenge in Southeast Asian rice farming systems, where direct-seeded rice has been adopted [1]. Manual weeding is no longer viable, given much less on-farm labour availability than in the past [2] and consequent higher labour costs [3]. Herbicides are now commonly used, but non-uniform spray coverage due to manual techniques, along with a lack of systematic herbicide rotation and unpredictable weather before and after application, often compromises efficacy [4] and has the potential to hasten herbicide resistance development [5], which threatens the sustainability of direct-seeded rice systems. The widespread and sometimes indiscriminate use of herbicides also poses significant environmental sustainability risks, including soil and water contamination, effects on non-target species, and the disruption of local biodiversity [6,7].
In Southeast Asian rice systems, climatic conditions and diverse weed ecologies present additional weed control challenges. A wet season characterized by large, unpredictable rainfall events, interspersed with periods of drought, can easily compromise herbicide efficacy, and leave few other wet season crop rotation options other than paddy rice [2]. Predominant weed species include barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.], slender fimbristylis (Fimbristylis miliacea L.), and rice flatsedge (Cyperus iria L.), which exhibit varying germination timing, seed retention rates, and seed-shedding behaviours [8]. Finally, Southeast Asian smallholder rice fields are typically small, with numerous bunds that harbour weeds, and field-to-field crop timing is often asynchronous over small distances [7]. Hence, innovative and sustainable approaches to integrated weed management (IWM) remain necessary.
One such innovation is the integrated weed seed impact mill (iWSIM), a device designed to render unviable in-crop weed seeds captured during combine harvesting [9]. By physically disrupting the seed viability of weeds that have survived in-crop weed control, the iWSIM can reduce weed populations by reducing the weed seed bank after harvest that can then germinate in the following season, complementing existing IWM strategies. This technology has shown promise, particularly in temperate winter cropping systems, demonstrating high efficacy in reducing seed viability and consequent weed seed bank reduction for major weed species such as annual ryegrass (Lolium rigidum Gaudin) and wild radish (Raphanus raphanistrum L.) [10,11], but, to our knowledge, an iWSIM has not yet been studied in Southeast Asian rice systems.
The efficacy of the iWSIM depends on three factors: weed seed retention at harvest and consequent capture by the combine, seed separation to the chaff fraction in the combine, and iWSIM seed destruction efficacy against weed seeds in the chaff fraction [12,13]. Southeast Asian rice systems present challenges for all three factors.
The application of harvest weed seed control technologies like the iWSIM in Southeast Asian rice systems has not yet been thoroughly considered, despite the aforementioned weed management challenges in direct-seeded rice. This mixed study evaluates the iWSIM’s potential to manage weeds in Southeast Asian rice systems and identifies practical barriers and opportunities for its integration into direct-seeded rice systems. It aims to combine the current literature and field observations in Lao PDR to evaluate the potential of the iWSIM in addressing the weed management challenges faced by Southeast Asian rice farmers. Much of the analysis and conclusions will also be applicable to other rice systems, but this study’s focus remains on Southeast Asia, as that is where the field and combine harvester observations were made. Specifically, it achieves the following:
  • Examines the mechanics and the efficacy of the iWSIM’s proven in various cropping systems to date.
  • Assesses its integration into Southeast Asian rice farming, focusing on smallholder fields, smaller combine harvesters, Southeast Asian weed species, and higher moisture straw, chaff, and weed seeds.
  • Identifies research gaps and proposes strategies for iWSIM efficacy in Southeast Asian rice systems.

2. Materials and Methods

This mixed study synthesizes findings from global and regional studies on iWSIM technology, focusing on its mechanics, efficacy, and integration into Southeast Asian rice systems. Data were collected from peer-reviewed journals, industry reports, iWSIM manufacturers, and some strategic observations of field conditions and a case study on a common combine harvester (Kubota DC Series) in Lao PDR. Specific attention was given to weed species prevalent in Southeast Asia, harvest weed seed control strategies, and operational challenges unique to rice farming.
Empirical observations were collected from visual field observations conducted in Lao PDR, when rice had reached physiological maturity, during 2024. In the dry season of 2023/24, 3 randomly selected fields were observed in Wattana Village, Champhone District, Savannakhet Province, and 2 fields in Napok Village, Vientiane Capital. In the wet season of 2024, observations were taken in 5 randomly selected transplanted fields and 5 randomly selected direct-seeded fields in each of Nonvilay Village, Outumphone; Kanthajan Village, Xaiphouthong; and Gangsoung Village, Khanthabouly. Three observations were made in a transect of each field. These visual observations estimated weed seed retention rates and heights for various weed species in both direct-seeded and transplanted rice systems. Weed species were identified using standard taxonomic keys, and seed retention was estimated through visual assessment and seed counts at harvest time.
Data on iWSIM efficacy, weed seed retention, and operational considerations were compiled and analyzed to evaluate the potential application of this technology in Southeast Asian rice systems. In the 2024 wet season, the data were analyzed with one-tailed, heteroscedastic Student’s t-Tests comparing each establishment method for each species, for all locations. Statistical analysis was conducted with Statistix 2.0 software package (Tallahassee, FL, USA).

3. Overview of the Integrated Weed Seed Impact Mill (iWSIM) Technology

3.1. Mechanics and Operation of iWSIM

An iWSIM is a powered device, mounted on the rear of a combine harvester, that processes the chaff fraction of harvested material [14]. It kills weed seeds in the chaff fraction through crushing, shearing, grinding, and hitting the weed seed in the chaff [10,15,16]. The iWSIM technology has seen increasing application in a variety of global cropping systems, particularly temperate winter crops, but has not yet been widely applied in rice systems. An iWSIM consists of a high-speed flail spinning in one direction, and a two-part concentric rotor spinning in the other direction, between static stators separated by a small clearance (Figure 1). First, the flail impacts the chaff and expels it through apertures in the inside stator to reach the rotor. Then, the rotor impacts the chaff again, before expelling it through the intermediate and then the outside stator. Modern weed seed impact mills are integrated into combine harvesters (iWSIM) and powered by them, in contrast to early versions that were self-powered and trailed behind the combine harvester [16]. The iWSIM processes only the chaff fraction; weed seeds are separated into the chaff fraction from the straw and grain fraction during the combine harvester’s threshing and separation process and are confined to the chaff fraction. Hence, weed seeds of similar dimensions and density to rice (such as weedy rice) are unlikely to be separated into the chaff fraction and hence are unlikely to be destroyed by an iWSIM. An iWSIM is one form of harvest weed seed control (HWSC) that treats the chaff fraction from combine harvesting, which also includes narrow windrow burning and baling the windrow; these may also have merit in complementing other weed management in some Southeast Asian rice systems but are not considered in this review.

3.2. Commercially Available iWSIM Models

Three primary iWSIM models are currently available in Australia, the USA, and Europe: the Seed Terminator (Figure 2a), the Harrington Seed Destructor (Figure 2b), and the Redekop Seed Control Unit (Figure 2c). Each model is designed for seamless integration with all major modern combine harvesters, providing farmers with various options to implement this technology based on their specific machinery and operational needs. All of these iWSIM models require a significant power input of about 55–60 kW [18]. Hence, their use is currently confined to the larger Class 8 and 9 combines that have an excess of 400 kW engine power, which are capable of providing the required power to the iWSIM with only limited reduction in harvest capacity [18].

4. Factors Affecting iWSIM Effectiveness in Destroying Fresh Weed Seeds

The effectiveness of the integrated Weed Seed Impact Mill (iWSIM) in reducing fresh weed seed input to the weed seed bank depends on three sequential factors: weed seed capture, weed seed separation into the chaff fraction, and weed seed destruction in the iWSIM (Figure 3). Successful capture is determined by seed retention at harvest, the cut height of the header relative to the weed seed height, and header-induced shattering. Only the weed seeds separated into the chaff fraction during harvest are processed by the iWSIM, as seeds in the straw bypass the mill. Finally, the rate of seed destruction (expressed as a proportion of the processed seed) is influenced by the size, shape, moisture content, and the energy applied during milling.

4.1. Weed Seed Retention and Capture at Harvest

Weed seed retention rates at harvest vary widely across species, regions, and cropping systems, underscoring the need for localized data to optimize harvest timing and strategies.
Studies in Italian rice systems showed high retention rates of 95% for species like southern cutgrass [Leersia oryzoides (L.) Sw.], E. crus-galli, and bulrush [Scirpus spp.] at harvest [19]. In Australian rice systems, an average seed retention rate of 50% (ranging from 10% to 90%) was reported for water plantain [Alisma plantago-aquatica L.] in the Riverina region [13]. Moreover, most summer annual grasses, including Echinochloa spp., exhibit insufficient retention for effective iWSIM implementation [15].
Retention rates of 96% were reported for L. rigidum in winter cereals [20], demonstrating successful iWSIM efficacy in reducing weed populations over time. In Australia, retention rates for several key species in temperate winter crops were reported as follows: 86% for L. rigidum, 100% for R. raphanistrum L., 80% for brome grass [Bromus spp.], and 85% for wild oats [Avena fatua L.]. Notably, these rates declined by approximately 1% per day following crop maturity [12].
In contrast, the retention rates in U.S. wheat systems are notably variable, as reported by [21], who found wide annual fluctuations in seed retention at typical harvest times. For example, downy brome [Bromus tectorum L.] exhibited retention rates ranging from 3.7% to 87%, feral rye [Secale cereale L.] from 34% to 70%, Italian ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot] from 29% to 48%, and rattail fescue [Vulpia myuros (L.) C.C. Gmel.] from 13% to 87%. European winter crop weeds generally had retention rates generally below 70%, although these rates also varied significantly by species and year [22].
Research in Arkansas soybean [Glycine max (L.) Merr.] systems by [11] showed that Palmer amaranth [Amaranthus palmeri S. Watson] retained 98% of its seeds at 146–148 days after sowing (DAS), while E. crus-galli, a prevalent rice weed, retained only 43%. A month later, A. palmeri maintained a high seed retention rate of 95%, whereas E. crus-galli retention dropped further to 32%. A. palmeri was rated as highly suitable for harvest weed seed control (HWSC) methods due to its high retention, whereas E. crus-galli was deemed less suitable [14].
These variations underscore the importance of region-specific studies, particularly in Southeast Asian rice systems. Weed seed retention data remain limited but weed seed contamination in combine harvested rice [9] suggests significant quantities of weed seeds are retained at the time of harvest. Understanding retention rates for major weed species in these systems is essential for determining the potential effectiveness of iWSIMs. For instance, the observed lower retention rates in many summer annual grasses, such as Echinochloa spp., suggest that iWSIMs may be less effective in environments where these species dominate. Comprehensive, localized research would help identify whether adaptations, such as modified harvest timing or techniques, could improve seed capture rates and enhance the iWSIM performance in SE Asian rice systems.
The header cutter bar height must be low enough to capture the weed seeds with cut crop biomass. This height may be a similar height to that suitable for just harvesting the crop, or it may need to be lowered; this requires further investigation.

4.2. Separation of Weed Seeds into the Chaff Fraction

The successful separation of weed seeds into the chaff fraction enables iWSIM effectiveness. In well-maintained combines, almost all weed seeds are concentrated in the chaff, which the iWSIM processes. However, poor combine adjustments, worn components, heavy combine loading, or design limitations may lead to seed dispersal into straw or grain fractions, hence bypassing the iWSIM [14]. Southeast Asian combines, like the Kubota DC series, may require modifications or different operating settings to minimize weed seeds in grain and straw fractions.

4.3. Weed Seed Destruction in the iWSIM

A Harrington Seed Destructor achieved a 98–100% seed destruction efficiency for both large and small seeds of various rice weed species in the USA, including E. crus-galli, weedy rice [Oryza sativa L.], hemp sesbania [Sesbania herbacea (Mill.) McVaugh], C. iria, Nealley’s sprangletop [Leptochloa nealleyi Vasey], tall waterhemp [Amaranthus tuberculatus (Moq.) J.D. Sauer], and Johnsongrass [Sorghum halepense (L.) Pers. [11].
The Seed Terminator achieved a similar result with waterhemp (Amaranthus tuberculatus) in soybean; destroying 77–99%, with an average of 94% [18].
A Redekop Seed Control Unit destroyed 99.8% of redroot pigweed (Amaranthus retroflexus) seed, 99.8% of common ragweed (Ambrosia artemisiifolia) seed, and 99.4% of barnyardgrass (Echinochloa crus-galli) seed in soybean residue. The chaff moisture content had little effect on the weed seed destruction rate, but the authors did not mention the seed moisture content [23].
A Harrington Seed Destructor likewise destroyed 99% of Echinochloa colona seeds, but these were in wheat straw and the weed seeds were also sourced separately and likely under low moisture to conserve seed viability in storage (the seed moisture content was not specified) [24]. This compares with the destruction of weed seeds in Australian winter cereals: annual ryegrass 95%, wild radish 93%, wild oats 99%, and brome grass 99% [25]. A 2024 test of the two major Australian iWSIMs found an average weed seed destruction efficiency of 82% for the Harrington Seed Destructor and 92% for the Seed Terminator [26].
The energy applied to each seed by the iWSIM rotor, seed characteristics, and moisture content determine the effectiveness of the iWSIM [26]. There is a clear relationship between the energy applied per unit of weed seed, and the weed seed kill rate. Reduced energy input into the iWSIM, a worn rotor or stator, and/or higher chaff flow rates through the iWSIM can reduce iWSIM efficacy below acceptable thresholds [26]. Larger and spherical seeds are generally easier to destroy than smaller or non-spherical seeds, as it is easier to apply the required energy to render them non-viable [27]. The moisture content of seeds and chaff also influences the iWSIM efficacy, with a higher moisture content potentially reducing kill rates [16,19]. Studies have also shown varying effects of chaff moisture on kill rates for different species. For instance, an increased wheat chaff moisture content reduced kill rates for rigid ryegrass and Italian ryegrass, but did not affect the kill rates for canola and hairy vetch [14]. In soybean systems, some studies found that the chaff moisture content did not influence the efficacy of killing seeds of A. palmeri, morning glory [Ipomoea spp.], E. crus-galli, and common ragweed [Ambrosia artemisiifolia L.] [11,15]. However, rice weeds and chaff generally have a higher moisture content at harvest compared to winter crop weeds and chaff, which may limit the iWSIM’s efficacy in some conditions [14]. Due to its high moisture content, rice chaff does not break up into small particles like winter cereals, which easily pass through the iWSIM’s stator. This may reduce the efficacious chaff flow rate or require a stator with larger gaps (as is used often for canola chaff) that applies less impact energy and is hence less efficacious [27].

5. Weed Seed Retention and Capture in Southeast Asian Rice Systems

5.1. Current Data on Seed Retention

Research on weed seed retention in Southeast Asian rice systems remains limited. Observational studies in Lao PDR, summarized in Table 1, were only performed in five fields in the dry season, and only ten fields in each of three locations in the following wet season, but they did suggest significant variability in seed retention among major weed species, with many species having seed retention heights below the standard 50–60 cm harvest cut height. Notwithstanding the limited scope of the observations, there were two findings from them (Table 1):
  • Early-season weed control, as was achieved in the observed transplanted rice fields, generally led to consistently higher weed seed retention at harvest.
  • Minimal retained weed seeds were observed below a height of 15 cm, indicating that a lowered harvest cut from the standard 50–60 cm could potentially capture more weed seeds.
The data collected in Lao PDR highlight that weed seed retention rates vary not only by species but also between direct-seeded and transplanted rice systems (Table 1). For instance, F. milliacea, smallflower umbrella sedge [Cyperus difformis L.] and C. iria showed 100% seed retention in transplanted systems. In contrast, these species in direct-seeded rice displayed inconsistent retention, with rates ranging from 21 to 51%. These transplanted fields had excellent early-season weed control, whereas the direct-seeded fields did not. This complete early-season weed control observed in the transplanted rice should improve the effectiveness of the iWSIM by delaying weed establishment and, hence, increasing the proportion of weed seeds retained on plants at harvest.
The variability in weed seed retention rates and seed heights observed in Lao PDR suggests that standard harvest practices may not be sufficient to capture all weed seeds in Southeast Asian rice systems. Lowering the harvest cut height to approximately 15 cm could capture a greater proportion of weed seeds, especially for shorter species (Table 1). Implementing this approach would require a reduced harvest rate or additional engine power to manage the additional biomass load and the consequent extra harvest cost. Alternatively, a stripper header could be used, which can harvest both the grain and weed seed down to a low height but not cut and intake the biomass at that low height. A stripper front was found to harvest the same proportion of ryegrass seed in wheat as a conventional (draper) header with the correct header height and sufficient crop biomass [28]. Lowering the harvest cut height for better weed capture aligns with contemporary practices in Australian winter cropping systems, where harvest cuts lower than necessary for grain harvest are often used to capture weed seeds if using an iWSIM, but also to process straw residues as part of preparation for the subsequent crop. The harvest process achieves more than just harvesting the grain. More powerful combines and advances in header design like double-cut knives, draper belt feed, and additional feeding augers have made it possible to adopt lower cut heights without compromising field productivity [29]. Similar modifications could enable a lower harvest cut height in Southeast Asia for weed management, although the subsequent management of the extra processed rice residue in the rice-based system will need to be considered.
There is a need to further evaluate the synergy between early-season control and iWSIMs to develop region-specific integrated weed management (IWM) strategies. Additionally, conducting longitudinal studies to measure the cumulative impact of combining early-season control and preventing seed set on bunds with iWSIMs on weed populations over multiple growing seasons will be crucial. These research efforts will help optimize the integration of iWSIM technology into Southeast Asian rice production systems, potentially leading to more sustainable and effective weed management practices.

5.2. Weed Seed Shattering During Harvest

The harvest process can cause weed seed shatters as they are contacted by the header, with studies indicating that some retained weed seeds shatter during the harvest process prior to capture. For instance, the header dislodged 22–40% of the retained A. tuberculatus seeds during harvest [18]. HWSC methods were noted as being less effective in crops like cotton [15], where the picker removes bolls without capturing weed seeds. Corn (Zea mays L.) headers that only removed the ears are likewise less effective than cereal headers at collecting weed seeds [30]. While iWSIMs may effectively reduce weed seed banks in some contexts, alterations to the headers or harvesting technique may be necessary for crops or weeds prone to significant seed shattering at harvest.

6. Efficacy of iWSIMs in Reducing Weed Seed Banks

6.1. Seed Life and Environment

Even with a high iWSIM efficacy at harvest reducing the fresh weed seed input to the weed seed bank, the rate of reducing weed seed banks is influenced by the longevity of weed seeds in the soil; conditions that reduce seed longevity hasten the effect of destroying weed seeds at harvest on the weed seed bank. Tropical weed seeds generally have a shorter viable seed life compared to those in temperate regions due to several factors:
  • Higher seedling recruitment from favourable germination conditions.
  • Increased predation by diverse biota.
  • Accelerated seed decay from higher temperatures and relative humidity.
  • Seedling mortality during dry periods following wet season germination.
  • Reduced seed dormancy [31].
These characteristics suggest that tropical rice weed seed banks may be more rapidly depleted with effective iWSIM use as part of IWM, compared to temperate weed seed banks. Additionally, no-till rice systems should hasten weed seed bank depletion by keeping weed seeds at or near the soil surface, exposing them to predation, light, and greater soil moisture and temperature variations [32,33].

6.2. Integrated Weed Management

The integration of iWSIMs with other weed management practices is essential for optimal results. Good weed control prior to harvest ensures only small numbers of weed seedsets to be managed at harvest, amplifying the effect of the iWSIM [14]. Complete early-season weed control can improve seed retention at harvest by reducing the age of weeds, enhancing iWSIM efficacy. The application of, and rotation between, other forms of weed control will reduce the number of weed seeds at harvest and counter any selection for traits that reduce the iWSIM efficacy. Additionally, increasing the field size, which has been shown to improve combine field productivity [34], will also reduce the area of bunds that can produce weed seeds that are not captured by the combine.

6.3. Selection Pressure and Weed Adaptation

The iWSIM technology has demonstrated promising results in reducing weed seed banks in winter cropping systems. However, the long-term use of iWSIMs may exert selection pressure on weed populations, potentially favouring traits that allow seeds to escape processing. The most recalcitrant 1–2% of seeds often require significantly more energy to destroy, which may lead to selection pressure for a weed seed morphology that requires more energy to kill, potentially reducing the iWSIM’s effectiveness over time [14]. There are several possible adaptations to avoid seed capture, including reduced plant height and more seed shattering from earlier seed maturity or other means [10].
While iWSIM technology shows potential for reducing weed seed banks in rice systems, further research is needed to evaluate this in Southeast Asian conditions, in combination with other weed management techniques. Factors such as the weed species composition, seed retention rates, and harvester modifications for tropical rice production require additional investigation to maximize the effectiveness of iWSIMs at reducing weed seed banks in these agroecosystems.

7. Optional Considerations for iWSIMs in Rice Systems

7.1. Case Study: An iWSIM on a Kubota DC-Series Combine

Dismantling and inspecting Kubota DC70, DC105, and DC108 combines at a combine sales dealership in Lao PDR showed that a smaller (for example, 6–10 kW, representing approximately 10% of the engine power, like current commercial iWSIMs on current commercial combines) iWSIM can be realistically operated on these very common Southeast Asian combines. No installation nor operation of an iWSIM was performed. An iWSIM could be reliably mounted on the rear of the machine in an appropriate place to receive the chaff fraction (Figure 4a). The chaff fraction can be reliably kept separate from the straw fraction and conveyed to the iWSIM in a shroud, with the straw fraction directed past the iWSIM. The iWSIM could be powered by a pair of C-section V-belts, driven by an additional pair of pulleys on the jackshaft that also drives the cleaning fan (Figure 4b). More engine power is likely required to power the iWSIM whilst adequately powering the combine, especially if the crop cut height is lowered [29] and/or a stripper header is used [28], so a heavier-duty belt may be required from the engine to the jackshaft, and a larger jackshaft and heavier-duty bearings may be required to tolerate the extra power. These adaptations require no further alteration to the standard combine, as they have no effect on the specification or operation of the remainder of the combine, propulsion, harvest, separation and cleaning. The alterations could be offered as a factory specification, or as an after-market product like the combine alterations for current iWSIMs [27].

7.2. Weed Seed Separation into the Chaff Fraction

It appears to be accepted that in Australian winter crop conditions, negligible weed seeds are present in the straw fraction of modern combine harvesters [13,24]. That assumption appears empirically correct, given the overall effectiveness of iWSIMs in reducing most weed seed banks in Australian winter crop systems [14]. Only 0–1.15% of weed seeds escaped in wheat straw in Virginia, USA, and 0.57–4.28% of weed seeds escaped in soybean straw with a John Deere S680 rotary combine [23]. A poorly adjusted, worn, or overloaded combine may not succeed in separating almost all weed seeds into the chaff fraction.
There are no specific considerations for weed seed separation into the chaff fraction in rice. The weed seed and chaff usually has higher moisture than in winter cereal systems, particularly in tropical systems, which may make separation more difficult, particularly with heavy loads of chaff and weed seed. However, the chaff makes up a lesser proportion of the harvested biomass [35], which eases the separation task.

8. Recommendations and Future Research

8.1. Enhancing iWSIM Efficiency in Rice Systems

iWSIM chaff processing for rice may be challenging, given the unique characteristics of rice chaff: It is both silicaceous and has a high moisture content. This likely requires further development of iWSIM components, specifically high-capacity or adjustable stators tailored to rice systems. Such adaptations may enhance the chaff flow while maintaining an acceptable operational efficiency. Also, research into modifications or optimizations of existing combine models commonly used in Southeast Asia, like the Kubota DC series and Claas Crop Tiger, to reliably separate weed seed into the chaff fraction, will aid the iWSIM efficacy in Southeast Asian rice systems.

8.2. Weed Seed Retention and Capture

Data on seed retention at harvest for prevalent Southeast Asian rice weed species, such as Echinochloa spp., F. milliacea, and C. difformis, remain limited. Detailed studies on retention rates, plant heights, and shedding behaviours across different rice systems (direct-seeded and transplanted) are essential for fine-tuning iWSIM applications. Additionally, investigating the operational feasibility and effectiveness of reducing the harvest cut heights will enable greater weed seed capture.
Combining an iWSIM with early-season weed control measures should increase the weed seed retention at harvest time by delaying weed germination. Conducting field trials to evaluate the combined efficacy of iWSIMs, early-season weed control, and weed control on bunds would provide a strong foundation for integrated weed management (IWM) strategies specific to rice. Understanding the iWSIM’s impact in rotations with other crops, such as corn or soybeans, could broaden its applicability and clarify its role within diverse IWM practices across different cropping systems.

8.3. Potential Weed Adaptations to iWSIMs

Over time, the use of iWSIMs may apply selection pressure on rice weed populations, potentially favouring traits like lower seed height, earlier maturation, and increased shattering before harvest. Monitoring these evolutionary adaptations in due course would be essential to guide adaptive management strategies, mitigating the risk of resistance. Testing alternative harvesting and collection methods, such as modified combine header designs or stripper fronts, could also reduce the adaptation risks and improve weed seed capture, making iWSIMs more effective in the long term.

8.4. Technical and Economic Viability

Smaller, less power-demanding iWSIM models compatible with the engine power of Southeast Asian combine harvesters, such as the Kubota DC series, are necessary for adoption. Even 6–10 kW represents about 9–12% of engine power of the DC108; a significant extra load on top of normal power requirements. A smaller, single-rotor iWSIM should be feasible, given they do exist for other applications [29], but further engineering research is needed to explore adaptations that reduce power requirements while maintaining efficacy.
Conducting a cost–benefit analysis of iWSIMs in rice systems can evaluate the technology’s feasibility, considering additional fuel use, capital and maintenance costs from engaging an iWSIM, and perhaps cutting the crop lower. Understanding the cost–benefit ratios for both smallholder and large-scale farms would support a more targeted approach to adoption.

9. Conclusions

An iWSIM has potential for managing weed seed banks in Southeast Asian rice systems. Specifically, the following points are raised:
  • A smaller iWSIM, of similar-but-smaller specifications to current models but only demanding 6–10 kW, can be designed to mount and operate on Southeast Asian combine harvesters, such as the Kubota DC series combines.
  • While iWSIM technology has shown success in temperate cropping systems, its application in tropical rice production requires a consideration of combine design and required engine power, given the iWSIM’s power requirements, and more biomass intake, given the likely lower harvest cut height and the need to separate high-moisture chaff and weed seeds.
  • Research is required to understand the proportion of weed seeds directed into the chaff fraction under different harvest conditions in Southeast Asian rice systems.
  • Research is required to examine the iWSIM’s effectiveness on high-moisture seeds and green chaff encountered in tropical environments. Higher moisture levels in rice chaff and seeds could reduce the impact mill’s efficacy and material flow rate. Previous studies have generally used low-moisture seeds and dry chaff.
  • Future studies are needed to investigate the weed seed retention of major rice weeds during both wet and dry seasons across Southeast Asia, with and without robust early-season weed control. Detailed data on seed retention rates, shattering during harvest, the effectiveness of a low cut height to capture weed seeds, and the effect of weed seed production on bunds will further clarify the advantages and potential constraints of implementing iWSIMs in regional rice systems, and assist with a consequent cost–benefit analysis.
A well-adapted iWSIM could provide Southeast Asian rice farmers with an additional tool to manage weeds and enhance crop productivity. This will aid the sustainability of Southeast Asian direct-seeded rice systems, particularly as herbicide resistance increases.

Author Contributions

Conceptualization, J.M. and L.V.; methodology, L.V.; investigation, L.V.; data curation, L.V.; writing—original draft preparation, L.V.; writing—review and editing, J.O. and J.M.; project administration, J.M.; funding acquisition, J.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Australian Centre for International Agricultural Research (ACIAR) through project CROP/2019/145. The authors disclose that all funding received to support the research described in this manuscript has been acknowledged herein.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

The authors would like to thank Phatsalakone Manivong for observing the weed seed retention and Phetmanyseng Xangsayasane for help in identifying field sites, both from the National Agriculture and Forestry Research Institute in Lao PDR. They also thank Ned Jeffrey, Commercial Manager for Victoria, Seed Terminator, for his technical and commercial insights into the iWSIM.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
DSRDirect-seeded rice
iWSIMIntegrated weed seed impact mill
IWMIntegrated weed management

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Agronomy 15 01333 g001
Figure 1. The internal mechanism of the Seed Terminator (Seed Terminator P/L, Lonsdale, South Australia), with anti-clockwise-rotating internal flail (A) and two-stage clockwise-rotating concentric rotor (B), separated by three stator screens (C). Arrows indicate directions of rotation [17].
Figure 1. The internal mechanism of the Seed Terminator (Seed Terminator P/L, Lonsdale, South Australia), with anti-clockwise-rotating internal flail (A) and two-stage clockwise-rotating concentric rotor (B), separated by three stator screens (C). Arrows indicate directions of rotation [17].
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Figure 2. Commercially available iWSIMs: (a) Seed Terminator (Source: Seed Terminator P/L, Lonsdale, South Australia), (b) Harrington Seed Destructor (Source: De Bruin Engineering, Mount Gambier, South Australia), and (c) Redekop Seed Control Unit (Source: Redekop Manufacturing, Martensville, SK, Canada).
Figure 2. Commercially available iWSIMs: (a) Seed Terminator (Source: Seed Terminator P/L, Lonsdale, South Australia), (b) Harrington Seed Destructor (Source: De Bruin Engineering, Mount Gambier, South Australia), and (c) Redekop Seed Control Unit (Source: Redekop Manufacturing, Martensville, SK, Canada).
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Figure 3. The fate of viable weed seedset in the field, both before and during harvest with a combine fitted with an iWSIM, and the three means of weed seeds surviving the process.
Figure 3. The fate of viable weed seedset in the field, both before and during harvest with a combine fitted with an iWSIM, and the three means of weed seeds surviving the process.
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Figure 4. (a). The approximate arrangement of the pair of C-section V-belts (in blue), driven by C-section pulleys (in green) to power the iWSIM on a Kubota DC70 combine. (b) The approximate mounting position on an iWSIM on a Kubota DC-70 combine for receiving the chaff fraction and allowing straw past the unit. The chaff shroud is in yellow, the iWSIM is in white, and the power supply via a pulley and shaft is in green.
Figure 4. (a). The approximate arrangement of the pair of C-section V-belts (in blue), driven by C-section pulleys (in green) to power the iWSIM on a Kubota DC70 combine. (b) The approximate mounting position on an iWSIM on a Kubota DC-70 combine for receiving the chaff fraction and allowing straw past the unit. The chaff shroud is in yellow, the iWSIM is in white, and the power supply via a pulley and shaft is in green.
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Table 1. Mean estimated weed seed retention and weed seed height relative to a 50 cm cutter bar height for 5 dry season fields in 2023/24 and mean estimated weed seed retention and minimum seed height in 30 wet season fields in 2024, Lao PDR.
Table 1. Mean estimated weed seed retention and weed seed height relative to a 50 cm cutter bar height for 5 dry season fields in 2023/24 and mean estimated weed seed retention and minimum seed height in 30 wet season fields in 2024, Lao PDR.
Dry Season 2023–2024Wet Season 2024
Weed SpeciesWeed Seed Retention %Seed Height Weed Seed Retention %
Direct-Seeded (n = 15)Transplanted (n = 15)
Echinochloa crus-galli>20AboveNot observed Not observed
Fimbristylis milliacea20Above and below51b100a
Cyperus difformis5Below25b100a
Cyperus iria0Below21b100a
Scripus juncoides30Below100a100a
Jussiaea linifolia0BelowNot flowered Not observed
Cynodon dactylon0Below0a3a
Ischaemum rogosumNot observed 100a100a
Fuirena ciliarisNot observed 100a100a
Cyperus pulcherimusNot observed 11b100a
Leptochloa chinensisNot observed 0 Not observed
Mean minimum seed height (cm) 18.0 17.3
Range of minimum seed heights (cm) 11.7–25.0 11.7–21.7
Means in each row for wet season 2024 followed by the same letter are not different based on one-tailed heteroscedastic Student’s t-Test (p = 0.001).
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MDPI and ACS Style

Vial, L.; Opeña, J.; Mitchell, J. Integrated Weed Seed Impact Mills for Southeast Asian Rice Systems: Could They Aid Sustainable Weed Management? Agronomy 2025, 15, 1333. https://doi.org/10.3390/agronomy15061333

AMA Style

Vial L, Opeña J, Mitchell J. Integrated Weed Seed Impact Mills for Southeast Asian Rice Systems: Could They Aid Sustainable Weed Management? Agronomy. 2025; 15(6):1333. https://doi.org/10.3390/agronomy15061333

Chicago/Turabian Style

Vial, Leigh, Jhoana Opeña, and Jaquie Mitchell. 2025. "Integrated Weed Seed Impact Mills for Southeast Asian Rice Systems: Could They Aid Sustainable Weed Management?" Agronomy 15, no. 6: 1333. https://doi.org/10.3390/agronomy15061333

APA Style

Vial, L., Opeña, J., & Mitchell, J. (2025). Integrated Weed Seed Impact Mills for Southeast Asian Rice Systems: Could They Aid Sustainable Weed Management? Agronomy, 15(6), 1333. https://doi.org/10.3390/agronomy15061333

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