Weed Control for Conservation Agriculture in Climate Change Scenario

Authored by: Parmeet Singh , Lal Singh

Applied Agricultural Practices for Mitigating Climate Change

Print publication date:  December  2019
Online publication date:  November  2019

Print ISBN: 9780367345297
eBook ISBN: 9780429326400
Adobe ISBN:

10.1201/9780429326400-3

 

Abstract

As the population increases, food demands placed on the agricultural production systems will test the capabilities of current agriculture practices. Moreover, adequate food production in the future can only be achieved through the implementation of sustainable growing practices that minimize environmental degradation and preserve resources while maintaining high-yielding, profitable systems. To this end, conservation agriculture (CA) is a system designed to achieve sustainability by improving the biological functions of the agro-ecosystem with limited mechanical practices and judicious use of chemical inputs. CA is characterized by three linked principles, viz. (i) continuous minimum mechanical soil disturbance, (ii) permanent organic soil cover, and (iii) diversification of crop species grown in sequences and/or associations. While sometimes mistakenly used synonymously, it is the less intensive conservation tillage system that has become more recognized and adopted within the agricultural community. A host of benefits can be achieved through employing components of CA or conservation tillage, including reduced soil erosion and water runoff, increased productivity through improved soil quality, increased water availability, increased biotic diversity, and reduced labor demands. CA systems require a total paradigm shift from conventional agriculture r to management of crops, soil, water, nutrients, weeds, and farm machinery.

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Weed Control for Conservation Agriculture in Climate Change Scenario

3.1  Introduction

As the population increases, food demands placed on the agricultural production systems will test the capabilities of current agriculture practices. Moreover, adequate food production in the future can only be achieved through the implementation of sustainable growing practices that minimize environmental degradation and preserve resources while maintaining high-yielding, profitable systems. To this end, conservation agriculture (CA) is a system designed to achieve sustainability by improving the biological functions of the agro-ecosystem with limited mechanical practices and judicious use of chemical inputs. CA is characterized by three linked principles, viz. (i) continuous minimum mechanical soil disturbance, (ii) permanent organic soil cover, and (iii) diversification of crop species grown in sequences and/or associations. While sometimes mistakenly used synonymously, it is the less intensive conservation tillage system that has become more recognized and adopted within the agricultural community. A host of benefits can be achieved through employing components of CA or conservation tillage, including reduced soil erosion and water runoff, increased productivity through improved soil quality, increased water availability, increased biotic diversity, and reduced labor demands. CA systems require a total paradigm shift from conventional agriculture r to management of crops, soil, water, nutrients, weeds, and farm machinery.

3.2  Prospects of CA

Table 3.1   Global Adoption of Conservation Agriculture Systems

Country

Area (M ha)

% of Global Total

USA

26.5

21.2

Brazil

25.5

20.4

Argentina

25.5

20.4

Australia

17.0

13.6

Canada

13.5

10.8

Russian Federation

4.5

3.6

China

3.1

2.5

Paraguay

2.4

1.9

Kazakhstan

1.6

1.3

Others

5.3

4.2

Total

124.8

100.0

Globally, about 125 million ha area is practiced following the concepts and technologies for CA (Table 3.1). The major countries being USA (26.5 million ha), Brazil (25.5 million ha), Argentina (25.5 ha), Canada (13.5 million ha), and Australia (17.0 million ha), India has also started practicing CA technology, and about 3 million ha area of wheat grown under the rice–wheat system in the Indo-Gangetic plain is believed to be under resourced conservation technologies. The tillage system is gradually undergoing a paradigm shift from frequent tillage operations before sowing crops called as conventional tillage (CT), to no-tillage operation before sowing a crop, called as zero tillage (ZT). The ZT technology in rice–wheat cropping system is now foreshadowing nothing less than the end of an age-old concept, popularly known as more you till, more you eat. The need of the hour now is to infuse new technologies for further enhancing and sustaining the productivity as well as to tap new sources of growth in agricultural productivity. In this context, the role of CA in improving efficiency, equity, and environment is well recognized. The adoption of CA offers avenues for much needed diversification of agriculture, thus expanding the opportunities for cultivation of different crops during different seasons in the year. The prospects for introduction of sugarcane, pulses, vegetables, etc. as intercrop with wheat and winter maize provide good avenues for intensification and diversification of rice–wheat system. Resource conserving technologies help integrate crop, livestock, land, and water management research in agro-ecological intensification of both low- and high-potential environments. Such technologies need to be developed and popularized extensively.

3.3  Weed Management

Weeds are one of the biggest constraints to the adoption of CA. Tillage affects weeds by uprooting, dismembering, and burying them deep enough to prevent emergence, by moving their seeds both vertically and horizontally, and by changing the soil environment and so promoting or inhibiting weed seed germination and emergence. Any reduction in tillage intensity or frequency may, therefore, have an influence on weed management. As the density of certain annual and perennial weeds can increase under CA, effective weed control techniques are required to manage weeds successfully. Crop yield losses in CA due to weeds may vary, depending on weed community and intensity. Weed species shifts and losses in crop yield as a result of increased weed density have been cited as major hurdles to the widespread adoption of CA. Implementation of CA has often caused yield reduction because reduced tillage failed to control weed interference. However, the recent development of post-emergence broad-spectrum herbicides provides an opportunity to control weeds in CA. Crop yields can be similar for conventional and conservation tillage systems if weeds are controlled and crop stands are uniform (Mahajan et al., 2002). The cropping system also plays an important role to influence weed flora in CA. There is also evidence of allelopathic properties of cereal residues in inhibiting weed germination (Jung et al., 2004). Weeds under CA may also be controlled when the cover crop is harvested or killed by herbicides. Farming practices that maintain soil microorganisms and microbial activity can also lead to weed suppression. Various approaches including the use of preventive measures, crop residue as mulches, intercropping, competitive crop cultivars, herbicide-tolerant cultivars, and herbicides are needed to manage weeds effectively in a CA system.

3.3.1  Preventive Measures

Seeds of most crops are contaminated with weeds, especially where weed seeds resemble the shape, size of crop seeds, and have similar life cycles. To obtain weed-free crop seeds, cultural, and mechanical measures need to be adopted. In undisturbed or no-till systems, seeds of weeds and volunteer crops are deposited in the topsoil. There is no weed seed burial by tillage operations under CA. Therefore, an appropriate strategy is needed to avoid high weed infestations and prevent unacceptable competition with the emerging crop.

3.3.2  Cultural Practices

In CA systems, stale seedbed practice is a valuable way of reducing weed pressure. This practice has been found very effective in zero-till wheat in the north-western Indo-Gangetic Plains. The main advantage of the stale seedbed practice is that the crop emerges in weed-free environments, and acquires a competitive advantage over late-emerging weed seedlings. Studies have suggested a small difference in weed populations between conventional and zero-till fields (Derksen et al., 1993), and in some cases, fewer weeds have been observed in zero-till conditions (Hobbs and Gupta, 2001; Singh et al., 2001; Malik et al., 2002). In CA system, time of sowing can be manipulated in such a way that ecological conditions for the germination of weed seeds are not met. In the north-western part of the Indo-Gangetic Plains, farmers advanced wheat seeding by 2 weeks to get a head start over noxious weed, Phalaris minor (Singh et al., 1999). One of the pillars of CA is ground cover with dead or live mulch, which leaves less time for weeds to establish during fallow or a turnaround period. Some other common problems under CA include emergence from recently produced weed seeds that remain near the soil surface, lack of disruption of perennial weed roots, interception of herbicides by thick surface residues, and change in timing of weed emergence. Shrestha et al. (2002) concluded that long-term changes in weed flora are driven by an interaction of several factors, such as tillage, environment, crop rotation, crop type, and the timing and type of weed management practice. Continuous ZT increased the population density of Echinochloa colona and Cyperus iria (C. iria) in rice but reduced the population of Avena ludoviciana and Chenopodium album in subsequent wheat (Mishra and Singh, 2012). Rotational tillage systems significantly reduced the seed density of C. iria, Artemisia ludoviciana, and Montipora hispida compared to continuous ZT or CT. It was concluded that continuous ZT with effective weed management using recommended herbicide + hand weeding was more remunerative and energy efficient in rice-wheat cropping system.

3.3.3  Crop Residues

In CA, crop residues present on the soil surface improve soil and moisture conservation, and soil tilth. The germination response of weeds to residue depends on the quantity, position vertical or flat, and below- or above-weed seeds, allelopathic potential of the residue, and weed biology (Chauhan et al., 2006). Crop residues, when uniformly and densely present under CA, could suppress weed seedling emergence, delay the time of emergence, and allow the crop to gain an initial advantage in terms of early vigor over weeds. Cover crops, such as Sesbania can produce a green biomass of up to 30 t/ha within 60 days, and control most of the weeds, leaving fields almost weed free. In addition to reducing weed emergence, high amount of residue may prolong or delay emergence, which may have implications for weed management in CA. Delayed weed emergence allows the crop to take competitive advantage over weeds, and these weed seedlings are likely to have less impact on crop yield loss and weed seed production. Plants emerging earlier produce a greater number of seeds than the later emerging ones (Chauhan and Johnson, 2010).

3.3.4  Intercropping

Intercrops can be more effective than sole crops in pre-empting resources used by weeds and suppressing weed growth. Intercropping of short-duration, quick-growing, and early-maturing legume crops with long-duration and wide-spaced crops leads to covering ground quickly and suppressing emerging weeds effectively. Maize–legume intercropping results in higher canopy cover and decreased light availability for weeds, leading to reduction in weed density and dry matter compared with sole crops. Brown manuring involving growing of Sesbania along with direct-seeded rice or maize as inter- or mixed-crop for 25–30 days and then killing Sesbania by 2,4-D spray or mechanical means has been found to be a highly beneficial resource-conserving technology for soil and water conservation, weed control, and nutrient supplementation (Sharma et al., 2010).

3.3.5  Crop Diversification

Continuous cultivation of a single crop or crops having similar management practices allow certain weed species to become dominant in the system, and over time, these weed species become hard to control. Therefore, it is very important to rotate crops having a different growing period. Different crops require different management practices, which may help in disturbing the growing cycle of weeds and prevent selection of the weed flora toward increased abundance of problem species. Crop rotation is an effective practice for management of Phalaris minor because selection pressure is diversified by changing patterns of disturbances (Bhan and Kumar, 1997; Chhokar and Malik, 2002). Crop rotation also allows farmers to use new herbicides, and this practice may control problematic weeds.

3.3.6  Chemical Weed Management

Weed management using herbicides has become an integral part of modern agriculture. In CT systems, crop residues generally are not present at the time of pre-emergence herbicide application. However, in CA systems, residues are present at the time of herbicide application, and may decrease the herbicide’s effectiveness as the residues intercept the herbicide, thus reducing the amount of herbicide that can reach the soil surface and kill-germinating seeds. The efficacy of herbicides may also depend on the herbicide formulations under CA systems. For example, pre-emergence herbicides applied as granules may provide better weed control than liquid formations in no-till systems. Depending on the herbicide chemical properties and formulations, some herbicides intercepted by crop residues in CA systems are prone to volatilization, photo degradation, and other losses. As the effect of no-till systems on weed control varies with weed species and herbicides used, choosing an appropriate herbicide and appropriate timing is very critical in CA systems (Chauhan et al., 2006). Nevertheless, injudicious and continuous use of a single herbicide over a long period of time may result in the development of resistant biotypes, shifts in weed flora, and negative effects on the succeeding crop and environment. Therefore, for the sustenance of CA systems, herbicide rotation and/or integration of weed management practices are needed. Any single method of weed control cannot provide season-long and effective weed control under CA systems. Therefore, a combination of different weed management strategies should be evaluated for widening the weed control spectrum and efficacy for sustainable crop production. The use of clean crop seeds and seeders and field sanitation (irrigation canals and bunds free from weeds) should be integrated for effective weed management. Combining good agronomic practices, timeliness of operations, fertilizer, water management, and retaining crop residues on the soil surface improve the weed control efficiency of applied herbicides and competitiveness against weeds. Approaches such as stale seedbed practice, uniform and dense crop establishment, use of cover crops and crop residues as mulch, crop rotations, and practices for enhanced crop competitiveness with a combination of pre- and post-emergence herbicides could be integrated to develop sustainable and effective weed management strategies under CA systems. Based on extensive field experiments on conservation agriculture systems in diversified cropping systems at the IARI, New Delhi during the last decade, the following broad conclusions have been made:

  • It is possible to achieve the same or even higher yield with ZT as with CT.
  • Retention of crop residues on soil surface is essential for success of ZT in the long-run.
  • ZT along with residue has beneficial effects on soil moisture, temperature moderation, and weed control.
  • Zero-till systems cause shift in weed flora and may result in emergence of perennial weeds like Cyperus and Cynodon.
  • Restricting tillage reduces weed control options and increases reliance on herbicides.
  • Altering tillage practices change weed seed depth in the soil, which play a role in weed species shifts and affect the efficacy of control practices.
  • CA is a machine-, herbicide- and management-driven agriculture for its successful adoption.
  • Integrated weed management involving chemical and non-chemical methods (residue, cover crops, varieties, etc.) is essential for success of CA systems in the long run.

3.4  Limitations in Adoption of CA Systems

CA has problems both for scientists and farmers to overcome the past mindset and explore the opportunities. Spread of conservation agriculture is constrained due to non-availability of suitable machinery, competing use of crop residues, weed management problems, particularly of perennial species, localized insect and disease infestation, and more importantly likelihood of lower crop productivity, at least in the short term. Biophysical, economic, social, and cultural constrains limit the adoption of this promising innovation of the 20th century by the resource-poor small land farmers of south and south-east Asia (Lal, 2007). Despite several payoffs, there are also many trade-offs to adoption of CA systems (Table 3.2).

3.5  Climate Change and Weed Management

Global climate change will alter many elements of the future crop production. Atmospheric CO2 concentration, average temperature, and tropospheric ozone (O3) concentration will be higher, droughts will be more frequent and severe, more intense precipitation events will lead to increased flooding, some soils will degrade, and climatic extremes will be more likely to occur (IPCC, 2007). Changes in climate influence not only the performance of individual organism but also impact interactions with other organisms at various stages in their life histories via changes in morphology, physiology, and chemistry. Weeds have better adaptability to the changing environment by virtue of greater genetic diversity. Evidences indicate that few weeds species respond strongly to recent increases in atmospheric CO2. To our belief, weeds may be a better source of genetic materials for genetic engineering of crop plants on account of hardiness to stress factors, relatedness, and their co-existence with crop plants and may offer a better chance of introgression and interaction at macro-molecular level (i.e., up and downstream components which otherwise may be absent in unrelated and non-co-existed species).

Table 3.2   Two Sides of No-Till Conservation Agriculture

Payoffs

Trade-Offs

Reduces soil erosion

Transition from conventional farming to no-till farming is difficult

Conserves water

Necessary equipment is costly

Improves soil health

Heavier reliance on herbicides

Reduces fuel and labor costs

Prevalence of weeds, disease, and other pests may shift in unexpected ways

Reduces sediment and fertilizer pollution of lakes and streams

May initially require more N fertilizers

Sequesters carbon

Can slow germination and reduce yields

Source: Huggins and Reganold (2008).

Numerous studies can be found in literature regarding the crops–weeds interaction in field conditions suggesting that weeds pose a very serious and potential threat to agricultural production resulting approximately one-third yield loss by virtue of their competitiveness with special mention to obnoxious or invasive weeds like Euphorbia geniculata which outplay the crop plants in almost every aspect and led to big loss to crop production in India. Recently, agriculture scientists have started paying more attention toward crop–weed competitiveness in a high CO2 environment (Ziska and George, 2004) and it has been suggested that possibly recent increases in atmospheric CO2 during the 20th century may have been a factor in the selection of weed species and a contributing factor of invasiveness of weed species. Very recently, available literature on crop–weed interaction under climate change pertaining to Indian farming system was reviewed by Mahajan et al. (2012). They opined that productivity of rice and wheat would decline due to the increased climate variability, particularly due to gap in water supply and demand, and weed incidence (2,050 and beyond). Plants with C4 carbon fixation pathway (mostly weeds) have a competitive advantage over crop plants possessing the more common C3 pathway at elevated temperature. For a C3 crop such as rice and wheat, elevated CO2 may have positive effects on crop competitiveness with C4 weeds (Mahajan et al., 2012), however, this may not be so always. All crop–weed competition studies, where the photosynthetic pathway is the same, weed growth is favored as CO2 is increased. Therefore, the problems of Phalaris minor and Avena ludoviciana in wheat would aggravate with increase in CO2 due to climate change. Under water stress conditions, P. minor had advantage over wheat with CO2 enrichment (Naidu and Varshney, 2011). Studies on the effect of CO2 enrichment on weed species at the Directorate of Weed Science, Jabalpur revealed that a few weed species such as Dactyloctenium aegyptium and E. colona responded to elevated CO2, but Cyperus rotundus and Eleusine indica did not respond to CO2 enrichment. In addition, efficacy of several herbicides reduced under high CO2 environment (In house study, DWSR). Many of these weeds reproduce by vegetative means, for example, Cynodon dactylon in rice and Convolvulus arvensis and C. arvense in wheat.

These weeds may show a strong response in growth with increase in atmospheric CO2. India is a country where erratic rainfall is not uncommon. A shift in weed species composition can also be expected under erratic rainfall because of climate change. Owing to a sudden change in climate, environmental stresses on a crop may increase and as a result the crop may become less competitive with weeds. The aberrations in weather conditions not only affect crop–weed competition, but also trigger weed seed germination in several flushes causing serious weed management issues. Three flushes of P. minor are not uncommon in the wheat fields in northwest India, which are not controlled by a single application of herbicide (Mahajan et al., 2012).

3.5.1  Challenges

Many questions are to be answered in context of weeds and weed management under the regime of climate change. Most important include:

  • How an individual factor will affect crop, weeds, and associated microorganisms?
  • How multi-factor climate change (i.e., CO2, O3, UV radiation, other greenhouse gases, temperature, etc.) will affect the relative competitiveness of crop, weeds, and microbes? Who will dominate whom?
  • Weed dynamics under climate conditions.
  • How will a change in precipitation (seems to be almost certain) effect weed growth?
  • What are the physiological, biochemical, and molecular basis and mechanism of dominance?
  • What are ways to sustain/increase the productivity of crops in changing climate?
  • How we can predict the possible losses of crop yields in futuristic climate change conditions?

3.6  Conclusion

From the above mentioned studies, it can be inferred that weeds possess better ability to survive and perform under adverse environmental conditions which make them sturdy and highly competitive with crop plants. Now a big question arises in this context, can we exploit these attributes of weeds for the crop improvement? If yes, then there is no other alternate better than weeds simply because of the co-existence of weed and crop plants. An advantage using weeds as a source of gene(s) may be other co-ordinated regulatory aspects of the transgene(s). As both weeds and crops grow in the same environment, it is expected that internal machinery (at least partly) which is required for the functioning of transgene(s) might be present already in crop plants. Development and availability of the sophisticated molecular tools provide us liberty to play at molecular level and transfer the genetic material into crop plants, thus breaking the reproductive barriers for inter-specific and inter-generic transfer of the genetic material. However, success of such approaches requires integration and collaborative efforts from all the corners of scientists to bring together expertise in weed science, molecular biology, and plant physiology. Following strategies can be beneficial to fight with the problem of climate change which seems to be certain in years to come:

  • A thorough study on weed dynamics under climate conditions.
  • Identification of crop cultivars resilient to climate changes
  • Preventive measures: Early planting of crops can be effective by means of avoiding the high temperature; however, scope of such strategy is limited as it depends on the maturity of the preceding crop also.
  • Return to CT practices: Looks difficult as it required again lots of labor work, fuel, and feed.
  • Engineering of crops for climate resilience: Most viable and dynamic strategy is to engineer.
  • The crop plants which can perform better under futuristic climate change conditions.
  • For this purpose, weeds can be a good source of genetic materials for raising transgenic crops.

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