Weed Science

Weeds are the most important biological constraints to increasing rice productivity in Asia. They are managed by using herbicides; however, reliance on herbicides alone is not sustainable in the long run. There is thus, a need to develop sustainable weed management strategies in different rice-based cropping systems. The development and adoption of improved weed management strategies must form an integral part of sustainable rice production. Improved weed management techniques in rice should focus on shifting the crop-weed balance in favour of rice by integrating possible cultural, physical, and biological weed management tools with judicious use of herbicides. Together, these approaches may be used as components of an integrated package in the future to slow down the evolution of new weed problems in rice production. The improved weed management approaches should aim to reduce the weed seed bank before crop sowing and reduce weed emergence and weed growth in rice

Weedy rice belongs to the same genus and species as cultivated rice but with different forms. It appears as hybrid swarms due to introgression of genes between wild and cultivated species in nature. In Asian rice, it is known as Oryza sativa  var. spontanea whereas in African context it is said as O. sativa var. stapfii. It grows faster; produces more tillers, panicles and biomass; makes better use of available N; shatters earlier; has better resistance to adverse conditions; and possesses longer dormancy in soil. Because of its high competitive ability, it becomes a serious threat to rice growers worldwide. Great morphological variability, similar growth behavior and high biological affinity with cultivated varieties make its control difficult. No single management technique can effectively control weedy rice. An appropriate combination of preventive, cultural, mechanical and chemical control measures is essential.

Conservation agriculture (CA) technologies involve minimum soil disturbance, soil cover through crop residues or other cover crops, and crop rotations. Weeds are a major constraint in adoption of CA-based technologies. Conservation tillage influences weed infestation, and thus interactions between tillage and weed control practices are commonly observed in crop production. There are reports available that zero tillage increases as well as reduces infestation of certain weed species in different crops. In rainy season when the weed problem is generally more, growing crops with zero tillage requires additional measures for effective weed control, including use of non-selective herbicides like paraquat and glyphosate. Zero-till sowing in standing crop residues along with application of herbicides in proper combination, sequence or in rotation leads to lower weed population and higher yield than conventional planting. However, changing from tillage-based farming to no-till farming is not easy. No-till incurs a greater risk of crop failure or lower net returns than conventional agriculture, and this perception has seriously hindered its adoption in countries outside north and south America. Yields of no-till crops may be lower by 5-10% in the initial years, especially on fine-textured and poorly-drained soils. No-till farming demands use of extra N fertilizer and heavy reliance on herbicides. The continued practice of no-till is, therefore, highly dependent on development of new herbicide formulations and integrated weed management options.

It was 20 years ago which marked the beginning of conservation agriculture (CA) with introduction of zero-tillage (ZT) in wheat to (1) reduce cultivation cost so that farmers can afford to purchase new but expensive alternate herbicides for the control of herbicide-resistant population of Phalaris minor Retz., the most troublesome weed of wheat, and (2) reduce land preparation period for timely wheat planting. Worldwide, CA has spread mostly in the rain-fed agriculture but India witnessed its success more in irrigated rice-wheat cropping systems (RWCS) of the Indo-Gangetic Plains (IGP). High input based crop culture in the North West IGP has enabled weeds such as P. minor in wheat and Echinochloa crusgalli (L.) Beauv. in rice to dominate the weed flora. In wheat, zero tillage (ZT) is widely adopted by farmers in North West India and recently it is widely accepted by farmers in the eastern IGP also. In North West India, under ZT wheat, emergence and biomass of P. minor was reduced, but weed flora shifted toward more broad-leaf weeds such as Rumex dentatus (L.). In the Eastern IGP, perennial weeds such as Cynodon dactylon L. Pers. and Cyperus rotundus L. are also problematic weeds in some cases under ZT. In rice, the focus now is on dry direct-seeded rice (DSR) and machine transplanting of non-puddled rice (MTNPR) as an alternate option to puddled transplanted rice (PTR). Shifting from PTR to DSR results in changes in tillage, crop establishment method, water and weed management which often results in changes in weed composition and diversity. Weedy rice has emerged as a major threat for DSR in countries where DSR is widely adopted. In the eastern IGP, Physallis minima and Cyperus rotundus are also becoming major problematic weeds in DSR. Increased net profit for farmers by using this new technology was the main reason for rapid adoption of ZT. Since 2009, the Cereal Systems Initiatives for South Asia (CSISA), project funded by Gates Foundation and USAID and implemented by four consultative group on International Agricultural Research (CGIAR) (CG) Centers (CIMMYT, IRRI, IFPRI and ILRI) in collaboration with national partners, has explored options for sustainable intensification across the IGP, including CA-based crop management. This paper highlights the weed management scenario in conservation agriculture in India.

Orobanche or broomrape obligate, troublesome root parasite which completely depends on the host plant to complete its life cycle. The host plants of Orobanche includes crucifers such as oilseed rape (Brassica spp.), broad bean (Vicia faba) and other crops belonging to Apiaceae, Asteraceae, and Solanaceae families. In India, Orobanche has emerged as a major threat to rapeseed mustard production. Many farmers have abandoned the cultivation of mustard under the threat of this parasitic weed. Orobanche  infestation is mostly confined to major mustard growing states of northern Rajasthan, Haryana, Punjab, Western UP, and North East Madhya Pradesh. In Andhra Pardesh, 50% area under tabacco (40,000 ha) is infested with Orobanche and causing 50% crop losses. In Karnataka state, 90% area under tobacco is infested with this weed with 50-60% yield losses. Tomato crop is also infested with Orobanche spp. in Mewat and Bhiwani districts of Haryana. Depending upon the extent of infestation, environmental factors, soil fertility, and the crops’ response damage from Orobanche can range from zero to complete crop failure. Orobanche  aegyptiaca is the most dominating species in India; however, localized infestation of two other species namely O. cernua and O. ramosa has also been observed to some extent. In spite of continuous and extensive research by the scientists, no single method for effective and economical management of Orobanche is  available. Integration of cultural, preventive and chemical methods is required in spite of its  costly inputs. Following methods may be adopted in integration fashion: crop rotation with non-host crops like wheat, barley and chickpea depending on the irrigation facilities;  delayed sowing (25 October - 10 November) of mustard supplemented with higher seed rate;  use of organic manures in combination with increased fertilizer N dose for enhancing crop vigour; two sprays of glyphosate at 25 g/ha at 30 DAS and 50 g/ha at 55 days after sowing provided the crop does not experience any moisture stress at the time of spray; and hand removal/pulling of left-over emerging shoots before flowering to reduce weed seed bank in the soil

Lantana camara L.var. aculeata, Parthenium hysterophorus (L.), Chromolaena adenophorum Spreng., Imperata cylindrica (L.) Beauv,Urtica dioca (L.) and Ageratum houstonianum (Mill.) are the major obnoxious perennial weeds of non-cropland hill ecosystems. These weeds are difficult to control and have spread like a wild fire in almost all the state because of the favourable climatic conditions, ability to propagate by seeds, stems and roots, faster dissemination by wind, water, birds, animals, machinery etc. and ability to adapt adverse conditions of hills. These weeds have become more problematic in hilly regions due to availability of more uncultivated land. These weeds are responsible to suppress useful vegetation in pasture and grasslands, orchards, forests, tea gardens, field bunds and other cropped and non-cropped lands by their competitive and allelopathic effects. These are responsible to threat plant biodiversity, shrinkage of grazing land, economic losses to the forest wealth, reduction in productivity of grasslands up to 90%. The toxins present in these weeds are proving hazardous to the health of animals and human beings. Preventive, mechanical, chemical, biological, utilization and integrated methods to manage these obnoxious perennial weeds have been discussed in this paper. These weeds should be cut at frequent intervals before flowering to exhaust food reserves in their vegetative propagules, check production of seeds and their dissemination. The cut biomass should be utilized to prepare compost, as mulch, biogas production, making furniture, as fuel wood and other industrial uses as per the property of weed species. A three phased integrated technology to manage Lantana camara under different hill ecosystems has been developed and demonstrated in large areas. In waste lands and forestland ecosystems, biological agents like Zygogramma bicolorata, Cassia tora or Cassia sericea are effective to manage Parthenium, hence should be introduced to check the rampant growth of this weed. In pasture and grasslands, herbicides should only be used in integration with plantation of fast growing forage species, recommended fertilizer, and harvesting or grazing schedules. These integrated technologies to manage Parthenium, Lantana and Ageratum have been demonstrated on large scale in hilly regions. However, for effective results, these technologies need to be adopted on campaign basis with the active participation of public, Government, scientists and policy makers.

Weeds are major threat to agriculture and biodiversity as they out-compete crops and native species and contribute to land degradation. Changes in geographic distributions, abundances and life-cycles of weeds are the likely outcome of the effect of climate change. Natural evolution and certain specific characteristics such as short life cycles, dispersal mechanisms, may give the weeds a competitive advantage over less aggressive species under changing climate. Climate change may favour certain native plants to such an extent that they then become weeds. The dynamics of competition between weed and crop plants are affected by environmental conditions, and have been shown to change with atmospheric CO2 concentration, temperature, precipitation and adaphic factors. Invasive weeds like Lantana and Parthenium may become more aggressive under climate change especially due to increases in atmospheric CO2. Growth at elevated CO2 would result in anatomical, morphological and physiological changes that could influence herbicidal uptake rates, besides translocation and overall effectiveness. The physiological plasticity of weeds and their greater intraspecific genetic variation compared with most crops could provide weeds with a competitive advantage in a changing environment. There is a possibility that agricultural weed populations will evolve new traits in response to emerging climate and non-climate selection pressures.

Herbicides use is increasing throughout the globe due to increasing labour cost, choice of application of herbicides, quick weed control in crop and non-crop areas. In India, herbicide use has increased up to 30% during the last 10 years in the country. Herbicides are chemical in nature, therefore, excessive and repeated use may pose residue problems, phytotoxicity to crop plants, residual effects on susceptible intercrops or succeeding crops, adverse effects on non-target organisms and ultimately health hazards to human and animals. Many herbicides are found as bound residues which make them not only unavailable to the targets but also polluting the soil ecosystem in a number of ways. Thus monitoring of these residues in soil, water, plants, fishes and other matrixes is very much important.  The fate of herbicide in soil depends on adsorption, absorption, volatilization, leaching, runoff, photodecomposition, degradation by microbial and chemical processes etc. In Indian tropical conditions, the half-life of imadazoline, phynylureas, sulfonylureas, triazines, chloroacetinalides, dinitroanilines, diethyl ethers, thiocarbamates, and fop group of herbicides in soil are found to varied 57-71, 13-60, 13-147, 12-58, 5-60, 12-77, 19-29, 19-24, and 8-24 days. At harvest, herbicides in various commodities were found either below the maximum residue limit or below detectable limits. Indirect effects of  herbicides are not common in India. However increasing incidences of intentionally acute pisioning by some of the herbicide such as butachlor, fluchloralin, paraquat, 2,4-D, pendimethalin, glyphosate etc. are emerging problem in India. Paraquat poisoning is an uncommon entity in India, and is associated with a high mortality rate. It can be concluded that in India herbicide contamination of soil, plants and natural waters occurs infrequently and at low levels.

Crops made resistant to herbicides by biotechnology are being widely adopted in various parts of the world. Those containing transgenes that impart resistance to post-emergence, non-selective herbicides such as glyphosate and glufosinate will have the major impact. These products allow the farmer to more effectively use reduced or no-tillage cultural practices, eliminate use of some of the more environmentally suspect herbicides and use fewer herbicides to manage nearly the entire spectrum of weed species. In some cases, non-selective herbicides used with herbicide resistant crops reduce plant pathogen problems because of the chemicals’ toxicity to certain microbes Herbicide tolerant crops can be produced by either insertion of a “foreign” gene (transgene) from another organism into a crop, or by regenerating herbicide tolerant mutants from existing crop germplasm. Biotech crops reached 160 million hectares, up 12 million hectares on 8% growth, from 2010 and 94 fold increase in hectarage from 1.7 million hectares in 1996 to 160 million hectares in 2011, makes biotech crops the fastest adopted crop technology in the history of modern agriculture. From the genesis of commercialization in 1996 to 2011, herbicide tolerance has consistently been the dominant trait. In 2011, herbicide tolerance deployed in soybean, maize, canola, cotton, sugar beet and alfalfa, occupied 59% or 93.9 million hectares of the global biotech area of 160 million hectares. Over the past few years, several herbicide resistant crops (HRCs), both transgenic and non-transgenic, have become available in many countries for commercial cultivation. But in India, the technology of herbicide tolerant crops is in initial stage of field evaluation.

Seeds and planting materials of different plant species are being imported into India. Many of these plants have the potential to become agricultural or environmental weeds and this risk needs to be assessed before allowing their entry. Weed risk assessment is a question based scoring system, containing 49 questions about the species. The questions include details of the plant’s climatic preferences, biological attributes, dispersal methods and reproduction. A minimum number of questions must be answered before an assessment is made. The weed risk assessment uses responses to the questions to generate a numerical score that is positively correlated with weediness. The assessment method was tested against 170 plants representing both weeds and useful plants from agriculture and environment. The method was judged on its ability to correctly reject weeds and accept non weeds. A total of 40% plants were classified as serious weeds, 30% as common weeds and remaining 30% were non weeds. The system is designed to be operated by plant quarantine officers. The weed risk assessment system with explicit scoring of biological, ecological and geographical attributes is a useful tool for detecting potentially invasive weeds in other areas of the world.

Some people, particularly in developed countries, have strong negative attitudes towards weeds, and a tendency to label potentially useful plant resources as invasive ‘aliens’, which are to be controlled at any cost. This undesirable attitude ignores the considerable evidence of beneficial uses of weed species to many societies, over a long period of human history. The recent application of ‘species-focused’ weed risk assessments have contributed to the maligning of many plant taxa as ‘invaders’ in the public’s mind, undermining their worth as biological resources. Some of the methods used in the blitz against weeds, including the excessive use of herbicides, have resulted in undesirable consequences, such as herbicide resistance, and negative impacts on biodiversity in farming landscapes. Weeds maintain the biological diversity of farming landscapes, providing food and shelter for a variety of animals. Insects, which pollinate crops, extensively use weeds as a source of nectar, when crops are not in flower. Weeds also attract crop pests; and there is evidence that pest populations in some crops are much lower in ‘weedy fields’ than in ‘weed-free’ crops. As many of our primary crops have ‘weedy-relatives’, the genes present in weeds appear crucial for future evolution of crops, particularly to confer ‘hardiness’ (ability to tolerate variable environmental conditions). Some weed species contribute to aesthetic pleasure, as part of ‘wild nature’, while others provide culinary delights for humans, and are important as food sources for both vertebrate and invertebrate animals. Many weeds with medicinal values continue to be used either as traditional ‘herbal’ remedies, or extracted for secondary metabolites. The colonising strengths of several species are being used in the remediation of water and terrestrial environments to scavenge soil pollutants. Globally, there is considerable interest in using the large biomass produced by these species as raw materials for countless household products, including bricks, paper and furniture; and as future bio-fuels.Therefore, within the field of weed science, a fresh look at weeds is essential. Perhaps, a new and bold paradigm should be ‘co-existing’ or ‘living with weeds’, recognising their intrinsic worth as part of biodiversity, and the many possible uses as bio-resources.