Site-specific physical weed control

Take home messages

• Developments in sensing technology will soon allow the direct targeting of weeds within cropping systems
• Site-specific weed control creates the opportunity to use alternate physical weed control technologies
• Energy required to effectively control weeds is an effective approach to identifying suitable physical weed control techniques

Background

The reliance on herbicidal weed control has resulted in the widespread evolution of herbicide-resistant weed populations (Boutsalis et al., 2012; Broster et al., 2013; Owen et al., 2014). Changing regulations and expensive herbicidal development costs combined with the perennial threat of herbicide resistance, ensures future effective weed control is reliant on the inclusion of alternate weed control technologies in weed management programs.

Physical weed control techniques were in use well before herbicides were introduced and the development of new options has continued throughout the era of herbicides. However, most of these new technologies have not been adopted, primarily due to cost, speed of operation and fit with new farming systems. The introduction of weed detection and actuation technologies creates the opportunity to target individual weeds i.e. site-specific weed management. This greatly increases the potential cost-effectiveness of many directional physical weed control techniques in conservation cropping systems.

Comparison of physical weed control technologies

There is a diverse array of effective physical weed control options with a proven ability to control weeds. The majority of these have not been commercialized and evidence of their efficacy relates to research findings, making cost-effectiveness comparisons difficult. While inputs and control methods differ significantly between physical control options, all systems share an energy requirement value for activation and use. Therefore, the energy required for effective weed control can be a reasonably accurate approach to comparing the efficiency and efficacy of physical control systems on an energy consumed per weed or hectare basis.

The direct energy requirements for the control of two-leaf weed seedlings were estimated from published reports on the weed control efficacy of a comprehensive range of physical weed control techniques (Table 1). To determine the energy requirement per unit area, a weed density of 5.0 plants m^-2 was chosen to represent a typical weed density in Australian grain fields, based on results from a recent survey of Australian grain growers (Llewellyn et al., 2016).

Broadcast weed control

Broadcast weed control is defined as the indiscriminate use of a control method on a whole paddock basis when controlling weeds within crops or in fallow situations.

Chemical weed control

Herbicides are the most commonly used form of weed control in global cropping systems primarily due to their high efficacy and reliability. Herbicides are highly cost effective and have a relatively low energy cost of approximately 220 MJ ha^-1, covering manufacture and application. Importantly, herbicides remain the only broadcast weed control option that provides highly selective in-crop weed control and, therefore have been critical to the adoption of highly productive conservation cropping systems. No other currently available form of weed control offers similar weed control efficacy with equivalent crop safety.

Physical weed control

Historically, tillage was relied on for weed control as well as seedbed preparation and continues to be used extensively in global cropping systems despite the extensive reliance on herbicides. As a group, soil disturbance-based options are the most energy efficient form of physical weed control (Table 1) with no additional energy inputs beside the draft force requirements. Tillage acts to control weeds by uprooting plants, severing roots and shoots and/or burial of plants. Consequently, the efficacy and impact of this approach is reliant on rainfall and soil moisture. Effective control can only be achieved when disturbed weeds are exposed to a drying environment after the tillage operation. Although tillage can be a highly effective weed control option the soil disturbance involved is not compatible with conservation cropping systems and, therefore this approach needs to be used sparingly.

There are a group of thermal weed control technologies (flaming, hot water foaming and steaming etc.) using chemical or electrical energy that may be used for broadcast weed control (Table 1). In comparison to tillage and herbicide-based options these approaches are considerably more energy expensive. With 100 to 1000-fold higher energy requirements it is not surprising that these technologies have not been widely adopted for use in large-scale cropping systems, although in more intensive operations flaming is used to some extent.

Table 1. Total energy requirement estimates for physical weed control options currently available for broadcast application. Estimates are based on the control of two-leaf weeds present
at 5 plants m^-2.

Site-specific weed control

The opportunity for substantial cost savings and the introduction of novel tactics are driving the future of weed control towards site-specific weed management. This approach is made possible by the accurate identification of weeds in cropping systems using machine vision typically incorporating artificial intelligence. Once identified, these weeds can be controlled through the strategic application of weed control treatments. This precision approach to weed control creates the potential for substantial cost savings (up to 90%) and the reduction in environmental and off-target impacts (Keller et al., 2014). More importantly for weed control sustainability, site-specific weed management creates the opportunity to use alternate physical weed control options that currently are not suited for whole paddock use.

Accurate weed detection allows physical weed control treatments to be applied specifically to the targeted weed. As weed identification processes develop to include weed species, size and growth stage, there exists the potential for some approaches (such as electrical weeding, microwaving and lasers) to be applied at a prescribed lethal dose. This dramatically reduces the amount of energy required for effective weed control (Table 2). For example, microwaving, as the most energy expensive weed control treatment as a broadcast treatment (42,001 MJ ha^-1), requires substantially less energy when applied directly to the weed targets (3.4MJ ha^-1). Thus, even though the same number of weeds are being controlled (5 plants m^-2) the specific targeting of these weeds results in a 99% reduction in energy requirements.

The accurate identification of weeds allows the use of alternate weed control technologies that are not practically suited for use as whole paddock treatments. For example, lasers are typically a narrow beam of light that is focussed on a point target. In a site-specific weed management approach with highly accurate weed identification and actuation, lasers can be focussed precisely on the growing points of targeted weeds, concentrating thermal damage. By reducing the treated area of the weed, off-target losses are further reduced allowing additional energy savings.

Table 2. Total energy requirement estimates for physical weed control options when used for site-specific weed control treatment. Estimates are based on the control of two-leaf weeds present at 5 plants m^-2.

* Different laser weeding systems

Conclusions

By using energy requirements as a level playing field for comparison, the various efficiencies of each control method became more apparent. Furthermore, this approach enabled a better understanding of site-specific opportunities for physical weed control. Targeting treatments on individual plants results in significant energy savings and makes previously impractical options on a broadcast basis, available for use on a site-specific basis. The opportunities here are immense for the future management of problem weeds.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

References

Boutsalis P, Gill GS, Preston C (2012) Incidence of herbicide resistance in rigid ryegrass (lolium rigidum) across South Eastern Australia. Weed Technol. 26:391-398

Broster JC, Koetz EA, Wu H (2013) Herbicide resistance levels in annual ryegrass (Lolium rigidum Gaud.) and wild oat (Avena spp.) in southwestern New South Wales. Plant Prot. Quart. 28:126-132

Keller M, Gutjahr C, Möhring J, Weis M, Sökefeld M, Gerhards R (2014) Estimating economic thresholds for site-specific weed control using manual weed counts and sensor technology: An example based on three winter wheat trials. Pest Management Science 70:200-211

Llewellyn R, Ronning D, Clarke M, Mayfield A, Walker S, Ouzman J (2016) Impact of weeds in Australian grain production: the cost of weeds to Australian grain growers and the adoption of weed management and tillage practices. CSIRO, Australia.

Owen MJ, Martinez NJ, Powles SB (2014) Multiple herbicide-resistant Lolium rigidum (annual ryegrass) now dominates across the Western Australian grain belt. Weed Res. 54:314-324

Contact details

Michael Walsh
University of Sydney
12656 Newell Highway
Narrabri 2390
Ph: 02 6799 2201
Mb: 0448 847 272
Email: m.j.walsh@sydney.edu.au