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Grand Challenges in Weed Management

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Bhagirath S Chauhan at University of Queensland
Bhagirath S Chauhan
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SPECIALTY GRAND CHALLENGE
published: 22 January 2020
doi: 10.3389/fagro.2019.00003
Frontiers in Agronomy | www.frontiersin.org 1January 2020 | Volume 1 | Article 3
Edited and reviewed by:
Singarayer Kumardas Florentine,
Federation University, Australia
*Correspondence:
Bhagirath Singh Chauhan
b.chauhan@uq.edu.au
Specialty section:
This article was submitted to
Weed Management,
a section of the journal
Frontiers in Agronomy
Received: 12 November 2019
Accepted: 18 December 2019
Published: 22 January 2020
Citation:
Chauhan BS (2020) Grand Challenges
in Weed Management.
Front. Agron. 1:3.
doi: 10.3389/fagro.2019.00003
Grand Challenges in Weed
Management
Bhagirath Singh Chauhan*
The Centre for Crop Science, Queensland Alliance for Agriculture and Food Innovation, The University of Queensland,
Gatton, QLD, Australia
Keywords: weed management, weed science, crop competition, herbicide, integrated weed management (IWM)
The current global population of 7.7 billion is expected to reach over 9 billion by 2050. To feed this
population, world food production will need to be increased by 70 to 100% (www.fao.org). There
are several biotic and abiotic constraints to crop production, in addition to socioeconomic and
crop management related issues (Ghersa, 2013). Weeds are the most important biotic constraints
to agricultural production in both developing and developed countries. In general, weeds present
the highest potential yield loss to crops along with pathogens (fungi, bacteria, etc.) and animal pests
(insects, rodents, nematodes, mites, birds, etc.) which are of less concern (Oerke, 2006). Weeds
compete with crops for sunlight, water, nutrients, and space. In addition, they harbor insects and
pathogens, which attack crop plants. Furthermore, they destroy native habitats, threatening native
plants and animals.
Yield losses in crops due to weeds depend on several factors such as weed emergence time, weed
density, type of weeds, and crops, etc. Left uncontrolled, weeds can result in 100% yield loss. In
Australia, the overall cost of weeds to Australian grain growers has been estimated at AUD 3.3
billion annually (Llewellyn et al., 2016). In terms of yield losses, weeds amounted to 2.7 million tons
of grain at a national level. In India, these costs were much higher. Weeds cost Indian agricultural
production over USD 11 billion each year (Gharde et al., 2018). In the same study, yield losses due
to weeds were estimated at 36% in peanut (Arachis hypogaea L.), 31% in soybean (Glycine max (L.)
Merr.), 25% in maize (Zea mays L.), and 19% in wheat (Triticum aestivum L.). In the USA, weeds
cost USD 33 billion in lost crop production annually (Pimentel et al., 2005). These studies from
different economies indicate the substantial yield and economic loss caused by weeds.
The current total global grain production is ∼2.1 billion metric tons. Assuming an overall yield
loss of 10% due to weeds (www.fao.org/3/a0884e/a0884e.pdf), the total loss in grain production is
∼200 million metric tons. If this loss can be reduced by half, grain production would increase by
100 million metric tons, which could serve in reducing hunger worldwide.
In developing countries, where farm size is small, weeds are removed manually. This practice
is becoming less common as a result of the urbanization of labor migrating to cities and rising
wage costs in agriculture. Hand weeding is being replaced by herbicide use. In developed countries,
such as Australia and the USA, herbicides are already widely used to control weeds. However,
over-reliance on herbicides with similar modes of action has resulted in the evolution of herbicide-
resistant weeds. At present, more than 500 unique cases of herbicide-resistant weeds have been
reported globally (Heap, 2019). Out of these total cases, more than 160 are from the USA and
over 90 cases are from Australia, making them the two countries with the highest number of cases
of herbicide resistance. These nations are followed by Canada, China, and Brazil. The maximum
number of herbicide-resistant weed species reported in different crops are in the order of: wheat >
maize >rice (Oryza sativa L.) >soybean >spring barley (Hordeum vulgare L.) >canola (Brassica
napus L.) >cotton (Gossypium hirsutum L.) (Heap, 2019). New herbicides with different modes
of action are needed to manage herbicide-resistant weeds; however, no major mode of action
has been introduced in the past three decades (Duke, 2012). These issues suggest the need to
develop different weed management options, and to consider the potential for integrating them
Chauhan Grand Challenges in Weed Management
with herbicide use. These concerns have also encouraged weed
scientists around the world to develop ecologically-based weed
management tools (Chauhan and Gill, 2014).
To develop effective and sustainable weed management
tactics, knowledge of weed biology and ecology is very important
(Chauhan and Johnson, 2010). A recent review highlighted and
prioritized current issues for weed science research (Chauhan
et al., 2017). Therefore, this article discusses only selected issues
and methods.
WEED BIOLOGY AND ECOLOGY
To develop any weed management program, it is essential to
understand the biology and ecology of weeds. The number
of studies into weed biology has increased more recently, as
we still lack a basic information on a number of important
species. A better understanding of the environmental factors
affecting weed seed germination would help to develop
effective management practices through strategies of increasing
germination so that seedlings can be killed or for the purpose of
suppressing germination (Chauhan and Johnson, 2010). Based
on such understanding, strategies to deplete weed seed banks
by influencing weed seed germination could be included in
management programs (Gallandt, 2006). Similarly, information
on weed phenology would allow more specific control methods
to be developed by accurately estimating the timing and effects
of weed competition on crop yield (Ghersa and Holt, 1995).
A recent study on Amaranthus palmeri S. Watson phenology
concluded that while species originating from different regions
of the USA can vary biologically, it was the plant’s environmental
plasticity which contributed to population spread (Spaunhorst
et al., 2018).
Most of the studies on weed biology and ecology have used
a small number of populations; however, populations from
one area may differ from those from other areas because of
differential management practices, rainfall, temperature, soil
type, etc. Therefore, in future studies, there is a need to include
several populations in order to draw conclusions from the
available data.
CROP COMPETITION
Any strategy in which a crop is used to manage weeds is
considered a sustainable weed control practice. Such strategies
need to be integrated with other tools to achieve effective
weed management. In this technique, the effect of weeds on
the crop is reduced through increasing crop competitiveness
or by reducing the competitiveness of weeds (Mortensen
et al., 1998; Gibson et al., 2002). Crop competitiveness can
be increased by narrowing crop row spacing, increasing crop
seeding rate, adjusting crop planting direction, using a weed-
competitive crop cultivar, and increasing precise application
of nutrients so that they are available to crops rather than
weeds. Growing a weed-competitive crop can significantly reduce
weed biomass and weed seed production in-crop. Reduced seed
numbers are always preferred by growers as such strategies
progressively deplete weed seeds in the long run if integrated
with other weed management tools (Mashingaidze et al.,
2009).
Although crop competition is not a new technique, the
potential for more effective use exists, particularly for herbicide-
resistant weeds. A single or double herbicide application would
control weeds at the early stage of the crop before the traits
of a competitive crop would reduce the need for future weed
management 3 to 4 weeks after planting (Chauhan, 2012). The
aim is to close crop canopy as soon as possible. Weeds emerging
after canopy closure are less able to grow and produce biomass
and seeds. Further research is needed where crop competition
components are integrated with herbicide use and other weed
management tools.
THERMAL WEED MANAGEMENT
Plant tissues are susceptible to high temperatures, which
can disrupt physiological functions. Heat can be applied in
different ways to control weeds: direct flaming (Knezevic
et al., 2011), solarization (Horowitz et al., 2017), microwaves
(Brodie et al., 2007), laser radiations (Mathiassen et al., 2006),
steam (Rask and Kristoffersen, 2007), and electrocution
(Parish, 1990). These methods of weed control can be
used in fallows to kill herbicide-resistant weeds. More
research is needed if these techniques are to be used in
field crops.
CLIMATE CHANGE
The main outcomes associated with climate change are an
increase in carbon dioxide (CO2) concentration, temperature,
and the severity and frequency of drought and flooding. Much
of the research on the impact of climate change in weeds has
focused on CO2, mostly conducted in the USA (Chauhan et al.,
2017). There is a need to include other factors (e.g., temperature
and water availability) and global regions in this research
with emphasis on the mechanisms responsible for differential
response to varying climatic conditions. Herbicide efficacy
also stands to be affected by projected climatic conditions.
For example, rising CO2was found to increase glyphosate
tolerance in a C3weedy species, Chenopodium album L. (Ziska
et al., 1999). Such changes in herbicide tolerance suggest that
the efficacy of chemical weed control may be reduced in
the future.
MODELING AND ROBOTICS
The application of modeling and robotics in a highly scientific
and practical manner will help to achieve site-specific and
economical weed management in the future (Bajwa et al., 2015;
Singh et al., 2019). The development of efficient guidance
systems, therefore, is a critical area of research for decision-
support systems and site-specific weed management which may
take some time, particularly in developing countries.
Frontiers in Agronomy | www.frontiersin.org 2January 2020 | Volume 1 | Article 3
Chauhan Grand Challenges in Weed Management
HERBICIDE USE
Herbicides are an integral part of any weed control system.
Current dependence on herbicides requires a more refined
approach, particularly through correct application techniques,
in order to extend the life of many modes of action. Use of
full herbicide rates, herbicide mixtures and herbicide rotations
may reduce the risk of evolution of resistance in weeds. These
strategies need particular attention in developing countries.
Research also needs to be conducted on the development and
application of nanoherbicides in different cropping systems.
INTEGRATED WEED MANAGEMENT (IWM)
IWM is the control of weeds using different, complimentary
methods within a system rather than relying on a single method.
The main aim of IWM is to reduce the selection pressure for the
development of resistance to any single method of weed control
(Chauhan et al., 2017). Unfortunately, weed research in most
countries is oriented toward herbicide research. Effective weed
management and a reduced risk of the evolution of herbicide-
resistant weeds depends on further research into IWM across
global settings.
CONCLUSIONS
Weeds are a major biotic constraint to production in different
cropping systems. A single method of control will not provide
adequate long-term weed management, instead often resulting
in the development of resistance. Weeds are the cause of
significant yield loss, even after the application of a particular
control method. There is a growing necessity to reduce this
yield loss in order to feed an ever-increasing human population.
Therefore, there is a need to develop effective and sustainable
IWM programs.
AUTHOR CONTRIBUTIONS
The author confirms being the sole contributor of this work and
has approved it for publication.
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Conflict of Interest: The author declares that the research was conducted in the
absence of any commercial or financial relationships that could be construed as a
potential conflict of interest.
Copyright © 2020 Chauhan. This is an open-access article distributed under the
terms of the Creative Commons Attribution License (CC BY). The use, distribution
or reproduction in other forums is permitted, provided the original author(s) and
the copyright owner(s) are credited and that the original publication in this journal
is cited, in accordance with accepted academic practice. No use, distribution or
reproduction is permitted which does not comply with these terms.
Frontiers in Agronomy | www.frontiersin.org 4January 2020 | Volume 1 | Article 3
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  • Sep 2017
In modern agriculture, with more emphasis on high input systems, weed problems are likely to increase and become more complex. With heightened awareness of adverse effects of herbicide residues on human health and environment and the evolution of herbicide-resistant weed biotypes, a significant focus within weed science has now shifted to the development of eco-friendly technologies with reduced reliance on herbicides. Further, with the large-scale adoption of herbicide-resistant crops, and uncertain climatic optima under climate change, the problems for weed science have become multi-faceted. To handle these complex weed problems, a holistic line of action with multi-disciplinary approaches is required, including adjustments to technology, management practices, and legislation. Improved knowledge of weed ecology, biology, genetics, and molecular biology is essential for developing sustainable weed control practices. Additionally, judicious use of advanced technologies, such as site-specific weed management systems and decision support modeling, will play a significant role in reducing costs associated with weed control. Further, effective linkages between farmers and weed researchers will be necessary to facilitate the adoption of technological developments. To meet these challenges, priorities in research need to be determined and the education system for weed science needs to be reoriented. In respect of the latter imperative, closer collaboration between weed scientists and other disciplines can help in defining and solving the complex weed management challenges of the 21st century. This consensus will provide more versatile and diverse approaches to innovative teaching and training practices, which will be needed to prepare future weed science graduates who are capable of handling the anticipated challenges of weed science facing in contemporary agriculture. To build this capacity, mobilizing additional funding for both weed research and weed management education is essential.
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Phenology of Five Palmer amaranth (Amaranthus palmeri) Populations Grown in Northern Indiana and Arkansas
Article
  • Mar 2018
  • WEED SCI
Palmer amaranth ( Amaranthus palmeri S. Watson) is a problematic weed encountered in U.S. cotton ( Gossypium hirsutum L.) and soybean [ Glycine max (L.) Merr.] production, with infestations spreading northward. This research investigated the influence of planting date (early, mid-, and late season) and population (AR, IN, MO, MS, NE, and TN) on A. palmeri growth and reproduction at two locations. All populations planted early or midseason at Throckmorton Purdue Agricultural Center (TPAC) and Arkansas Agriculture Research and Extension Center (AAREC) measured 196 and 141 cm or more, respectively . Amaranthus palmeri height did not exceed 168 and 134 cm when planted late season at TPAC and AAREC, respectively. Early season planted A. palmeri from NE grew to 50% of maximum height 8 to 13 d earlier than all other populations under TPAC conditions. In addition, the NE population planted early, mid-, and late season achieved 50% inflorescence emergence 5, 4, and 6 d earlier than all other populations, respectively. All populations established at TPAC produced fewer than 100,000 seeds plant ⁻¹ . No population planted at TPAC and AAREC produced more than 740 and 1,520 g plant ⁻¹ of biomass at 17 and 19 wk after planting, respectively. Planting date influenced the distribution of male and female plants at TPAC, but not at AAREC . Amaranthus palmeri from IN and MS planted late season had male-to-female plant ratios of 1.3:1 and 1.7:1, respectively . Amaranthus palmeri introduced to TPAC from NE can produce up to 7,500 seeds plant ⁻¹ if emergence occurs in mid-July. An NE A. palmeri population exhibited biological characteristics allowing it to be highly competitive if introduced to TPAC due to a similar latitudinal range, but was least competitive when introduced to AAREC. Although A. palmeri originating from different locations can vary biologically, plants exhibited environmental plasticity and could complete their life cycle and contribute to spreading populations.
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Assessment of yield and economic losses in agriculture due to weeds in India
Article
  • Jan 2018
  • CROP PROT
Weeds are notorious yield reducers that are, in many situations, economically more harmful than insects, fungi or other crop pests. Assessment of crop yield and economic losses due to weeds in agriculture is an important aspect of study which helps in devising appropriate management strategies against weeds. A study was conducted to estimate the yield and economic losses due to weeds using the data from 1581 On-Farm Research trials conducted by All India Coordinated Research Project on Weed Management between 2003 and 14 in major field crops in different districts of 18 states of India. The study revealed that potential yield losses were high in case of soybean (50-76%) and groundnut (45-71%). Greater variability in potential yield losses were observed among the different locations (states) in case of direct-seeded rice (15-66%) and maize (18-65%). Three factors viz. location (state), crop, and soil type significantly (p < .0001) explained the variability in actual yield losses due to weeds at farmers' fields. Significant differences were also observed between different locations, crops and soil types. Actual economic losses were high in the case of rice (USD 4420 million) followed by wheat (USD 3376 million) and soybean (USD 1559 million). Thus, total actual economic loss of about USD 11 billion was estimated due to weeds alone in 10 major crops of India viz. groundnut (35.8%), soybean (31.4%), greengram (30.8%), pearlmillet (27.6%), maize (25.3%), sorghum (25.1%), sesame (23.7%), mustard (21.4%), direct-seeded rice (21.4%), wheat (18.6%) and transplanted rice (13.8%).
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Ecologically Based Weed Management Strategies
Article
  • Jul 2014
Weeds are the most important biological constraints in crop production systems. Herbicides are used to manage weeds; however, a heavy reliance on herbicides is not sustainable in the long run. Therefore, there is a need to develop ecologically based weed management strategies in different crop production systems, which aim to reduce the weed seed bank before crop sowing and reduce weed emergence and weed growth in crops. Some of the strategies to reduce the seed bank before crop sowing are the use of the stale seedbed technique and the adoption of practices that enhance seed predation and seed decay. Strategies to reduce weed emergence and weed growth in crops may include the use of appropriate tillage systems; retention of crop residue on the soil surface; the use of crop cultivars with weed competitiveness, allelopathy, and tolerance of germination under anaerobic conditions; the use of a high crop density; and the use of narrow row spacing. This chapter aims to provide a perspective on what can be done to improve ecologically based weed management strategies.
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An assessment of microwave soil pasteurization for killing seeds and weeds
Article
  • Jan 2007
Microwave heating has been applied to various agricultural problems and products since the 1960s. Interest in soil pasteurization as an alternative method of weed control has been proposed for some time. Soil pasteurization requires the projection of microwave energy into the soil using an antenna. The pyramidal horn is probably the easiest microwave antenna to fabricate. This paper explores the use of a pyramidal horn antenna as a microwave applicator for soil pasteurization, with a particular focus on suppression of seed germination and control of already established weed infestations. A laboratory system, energized from the magnetron of a modified microwave oven operating at 2.45 GHz, with a waveguide and pyramidal horn was developed for these experiments. Calibration of the microwave oven revealed that 205 ± 10 Watts of microwave power was delivered from the horn antenna. The H-plane temperature distribution within the soil has the maximum temperature about 3 cm below the surface along the centre line of the horn antenna, which proved to be effective at killing Malva parviflora seedlings and wheat seeds to a depth of about 6 cm.
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Solarization for Weed Control
Article
  • Mar 1983
Solarization is a method of heating moist soil by covering it with plastic sheets to trap solar radiation. In field experiments in Israel during the summer, maximum soil temperature under plastic cover at the 5-cm depth averaged 46 to 49C. No weeds emerged under the plastic cover during solarization and weed emergence was reduced after its removal. The heating effect from solarization decreased with soil depth. Concentration of O 2 in soil under plastic was similar to that in uncovered controls, but the concentration of CO 2 was markedly higher than in control soil, rising up to 2.4%. Higher temperatures and better residual weed control were produced by transparent than by black plastic, with best results from thin (0.03 mm), transparent polyethylene. Under Israeli summer conditions, 2 to 4 weeks of solarization produced effective control of annual weeds that was still appreciable after 1 yr. Narrow sheets of 20 to 50 cm produced effective weed control in bands. on soil irrigated once before placing the plastic sheets, there was no need to irrigate during solarization. The response of weed species to solarization differed. Many annual weeds, both summer species such as pigweed ( Amaranthus spp.) and common purslane ( Portulaca oleracea L.) and winter species as henbit ( Lamium amplexicaule L.) were well controlled by solarization. Broomrape ( Orobanche crenata Forsk.) was controlled in one experiment. on the other hand, horseweed [ Conyza canadensis (L.) Cronq.] and bull mallow ( Malva niceaensis All.) were relatively resistant, and established perennials escaped the treatment.
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Nonconventional Weed Management Strategies for Modern Agriculture
Article
  • Oct 2015
  • WEED SCI
Weeds are a significant problem in crop production and their management in modern agriculture is crucial to avoid yield losses and ensure food security. Intensive agricultural practices, changing climate, and natural disasters affect weed dynamics and that requires a change in weed management protocols. The existing manual control options are no longer viable because of labor shortages; chemical control options are limited by ecodegradation, health hazards, and development of herbicide resistance in weeds. We are therefore reviewing some potential nonconventional weed management strategies for modern agriculture that are viable, feasible, and efficient. Improvement in tillage regimes has long been identified as an impressive weed-control measure. Harvest weed seed control and seed predation have been shown as potential tools for reducing weed emergence and seed bank reserves. Development in the field of allelopathy for weed management has led to new techniques for weed control. The remarkable role of biotechnological advancements in developing herbicide-resistant crops, bioherbicides, and harnessing the allelopathic potential of crops is also worth mentioning in a modern weed management program. Thermal weed management has also been observed as a useful technique, especially under conservation agriculture systems. Last, precision weed management has been elaborated with sufficient details. The role of remote sensing, modeling, and robotics as an integral part of precision weed management has been highlighted in a realistic manner. All these strategies are viable for today's agriculture; however, site-specific selection and the use of right combinations will be the key to success. No single strategy is perfect, and therefore an integrated approach may provide better results. Future research is needed to explore the potential of these strategies and to optimize them on technological and cultural bases. The adoption of such methods may improve the efficiency of cropping systems under sustainable and conservation practices.
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Future atmospheric carbon dioxide may increase tolerance to glyphosate
Article
  • Sep 1999
We tested whether the efficacy of chemical weed control might change as atmospheric CO2 concentration [CO2] increases by determining if tolerance to a widely used, phloem mobile, postemergence herbicide, glyphosate, was altered by a doubling of [CO2]. Tolerance was determined by following the growth of Amaranthus retroflexus L. (redroot pigweed), a C4 species, and Chenopodium album L. (common lambsquarters), a C3 species, grown at near ambient (360 μmol mol-1) and twice ambient (720 μmol mol-1) [CO2] for 14 d following glyphosate application at rates of 0.00 (control), 0.112 kg ai ha-1 (0.1 x the commercial rate), and 1.12 kg ai ha-1 (1.0 x the commercial rate) in four separate trials. Irrespective of [CO2], growth of the C4 species, A. retroflexus, was significantly reduced and was eliminated altogether at glyphosate application rates of 0.112 and 1.12 kg ai ha-1, respectively. However, in contrast to the ambient [CO2] treatment, an application rate of 0.112 kg ai ha-1 had no effect on growth, and a 1.12-kg ai ha-1 rate reduced but did not eliminate growth in elevated [CO2]-grown C. album. Although glyphosate tolerance does increase with plant size at the time of application, differences in glyphosate tolerance between CO2 treatments in C. album cannot be explained by size alone. These data indicate that rising atmospheric [CO2] could increase glyphosate tolerance in a C3 weedy species. Changes in herbicide tolerance at elevated [CO2] could limit chemical weed control efficacy and increase weed-crop competition.
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