Weedy rice (Oryza sativa f. spontanea) is the complex of morphotypes of Oryza species, evolved largely by natural hybridization between wild and cultivated rice. With diverse biotypes, weedy rice has already infested large rice growing areas across the globe. It has also become a threat in major rice tracts of eastern and southern India. The weed has distributed in the commercial rice fields especially in areas where farmers have switched to direct-seeding due to labour shortage and high cost. Weedy rice has competitive advantage over cultivated rice as it grows taller and faster, tillers profusely and competes with cultivated rice for nutrients, light and space. It flowers much earlier than cultivated rice and produces grain that shatter easily thus enhancing the weed seed bank. Survey conducted has revealed the presence of weedy rice variants with respect to morphological characters like number of tillers per plant, height of plant, length of ligule, panicle characters, colour of grains, and length and colour of awns. Management of weedy rice infestation is complex mainly because of its morphological similarities to cultivated rice and lack of herbicides for selective control of weedy rice in cropped fields. Management options found effective for the control of weedy rice in direct seeded puddled rice include pre sowing surface application of oxyfluorfen  0.3 kg/ha, three DBS in thin film of water and selective drying of weedy rice panicles by direct contact application of glufosinate-ammonium or glyphosate or paraquat 15- 20% concentration at 60-65 DAS using specially designed wick applicator. Stale seedbed technique with dry and wet ploughing followed by the application of a broad spectrum herbicide and flooding proved to be effective in exhausting soil seed bank. Integrated management strategies are to be adopted for effective control of weedy rice.

Lowland rice ecosystem in India is infested with complex weed flora including semi-aquatic and aquatic weeds. Recently weeds having mimics with rice, perennial and other weeds which propagate by vegetative means are emerging as major threat. Weeds cause yield losses from 15 to 76% in rice crop. Besides, weeds remove about 21-42 kg N, 10-13.5 kg P and 17-27 kg K/ha in transplanted rice. Research on weed management in lowland rice in India has been reviewed in this paper with respect to weed flora and their impact, biology and ecology of weeds, weed management methods and future thrust areas of research and management. The weed flora in lowland rice is very much diverse and dynamic over times and places. Very limited information is available on biology and ecology of major weeds. Studies have been carried out on cultural, manual, mechanical, chemical means of weed management. Shortage of labours, lack of suitable weed control implements and problem of specific weeds have compelled the farmers to think for alternative strategies and herbicides have been the obvious choice to the farmers. Many herbicides either alone or in combinations as ready or tank mixed have been recommended in India. Studies on integrated weed management have also been undertaken. But majority of researches focused on herbicide based IWM. Future research and weed management in lowland rice should be focused on ecophysiology and biology of major weeds, HR genetically modified rice, integrated weed management, exploring biocontrol agents and screening and use of allelopathic and weed competitive rice cultivars.  

In India, wheat covers an area of 30 Mha with 3.1 t/ha productivity. Rice–wheat system has started showing the signs of fatigue. Certain reports say that the wheat yield reduces by 8% when sown after puddled transplanted rice compared to wheat sown after direct-seeded rice. The conventional method of wheat sowing by repeated tillage delays the sowing by 10 to 15 days, which adversely affect yield. To curtail problems faced by intensive tillage in rice and delayed sowing of wheat, adoption of no or reduced tillage is a viable option. The high input requirement and less competitive nature of high yielding dwarf wheat varieties have provided conducive environment for weed infestation. The average yield losses caused by weeds in different wheat growing zones ranged from 20 to 32%. Uncontrolled weeds in wheat caused 60.5% reduction in wheat grain yield under CT and 70% in ZT conditions. Potential solutions include a shift from intensive tillage to no or reduced tillage and/or from transplanting to direct-seeding. Zero tillage ameliorates the problem of delayed sowing as well as reduces weeds like Phalaris minor in wheat. A shift from an intensive tillage to reduced/no tillage system cause major changes in weed dynamics, herbicide efficacy and weed seed recruitment. Therefore, an attempt has been made in this article to review works done on several aspects of weed management in zero-till wheat.

Rainy season maize contributes 85% of the total maize area in India. Among major Rabi maize growing States, earlier Andhra Pradesh contributed maximum with 45.5% share. The present review reveals that Echinochloa colona L. is the most dominant weed species with importance value index (IVI) of 37.64 followed by Panicum repens L. (32.29), Trianthema portulacastrum L. (16.33) and Digera arvensis L Forsk (13.37). Wider spacing and initial slow growth of maize during the first 3-4 weeks provides enough opportunity for weeds to invade and offer severe competition, resulting in 30-93% yield losses. Among weed management practices, hand weeding twice at 15-21 DAS and 30-42 DAS, and integrated weed management like pre-emergence application of  atrazine 1.5 kg/ha, pendimethalin 1.50 kg/ha, atrazine + alachlor 0.75 + 1.25 kg/ha, or alachlor 1.5 kg/ha followed by hand weeding at 30 DAS was found effective. Among sequential applications, atrazine as pre-emergence 1.25 kg/ha, or pendimethalin as pre-emergence 1.5 kg/ha followed by paraquat 0.6 kg/ha at 3 weeks after sowing or atrazine 1.0 kg/ha as pre-emergence followed by topramazone 0.030 kg/ha at 30 DAS were found economical with higher gross returns, net returns and B:C.

Millets are important staple foods in semi-arid tropics of Asia and Africa. Low productivity and susceptibility to biotic and abiotic factors are the major reasons for declining area and productivity of millets in India. As the millets are grown predominantly in the hot and humid rainy season, weeds deprive these crops of vital nutrients and moisture and reduce the yield considerably. Because of wider row spacing and slow initial growth in millets, weeds are more problematic during initial crop growth period, and hence, early control is needed to optimize the yield. The objective of this paper is to review the research that has been conducted pertaining to various aspects of weed management in different millets while also identifying key knowledge gaps that should be addressed in future research. Literature suggests that satisfactory weed control can be achieved with integration of pre-emergence herbicides with one manual/mechanical weeding. Additionally, future research is needed to evaluate the post-emergence herbicides that are the best suited for different millets and millet-based intercropping systems to improve weed control and reduce environmental impacts, including herbicide residues.

Maize (Zea mays L), being a C4 plant, is one of the most vibrant food grain crops under diverse edaphological conditions. In India, maize-wheat is by and large a predominant cropping system that is followed on a large scale, particularly in central and northern part of the country. The low productivity of maize in India, as compared to major maize growing countries of the world, can be attributed to several limiting factors, of which poor weed management poses a major threat to crop productivity. The most important weeds that can be associated with maize/maize-based cropping systems in the country are Echinochloa colonum, Brachiaria ramosa, Digitaria sanguinalis, Dactyloctenium aegyptium, Eleusine indica, Setaria glauca, Sorghum halepense, Panicum spp. Cynodon dactylon, Digitaria setigera, Digitaria ciliaris, and Leptochloa chinensis among grasses; Ageratum conyzoides, Galinsoga parvif]ora, CommeIina benghalensis, Undernia cilata, Polygonum hydropiper, Euphorbia geniculata, Oxalis latifolia, Celosia argentea, Cleome viscose, Sida acuta, Aschynomene indica, Acanthospermum hispidum, Portulaca oleracea, Phyllanthus niruri, Amaranthus viridis, Acalypha indica, Tridax procumbens, Ipomoea pestigridis, Parthenium hysterophorus and Euphorbia hirta among non-grassy weeds and Cyperus rotundus and Cyperus iria among sedges. In the rainy season, it was reported thet the emergence of maize and weeds was simultaneous and the first 20-60 days was the most critical period of competition for the crop. However, in winter maize the period beyond 30 days and up to 45 days after sowing was detrimental to maize growth. In India, presence of weeds reduce the maize yields by 27-60%, depending upon the growth and persistence of weed population. The agronomic manipulations, viz. tillage and inter-cultivation, intercropping, mulching, cover crops, crop rotation, higher seed rate or plant populations, planting at closer spacing, nutrient management, planting methods, and other agro-techniques are used for weed management in maize/maize-based cropping systems. However, herbicides play a key role providing an option of economical weed management.

Pigeonpea (Cajanus cajan (L.) Millsp., syn. Cajanus indicus Spreng), also known as arhar, tur, redgram, congopea, no eye pea, is one of the most important pulse crop of India in terms of acreage and production. Worldwide, it is grown on an area of 4.75 million hectares with 3.68 million tonnes of production (FAO 2012). Its grains are highly nutritious and rich in protein (21.7%), carbohydrates, fibre and minerals that constitute the main source of dietary protein to all vegetarian people, especially in developing countries. Weed infestation in pigeonpea is severe at the initial period during first 6-8 weeks, when the crop requires to be kept free from weeds. Chemical weed control is most promising, although there are cultural options like intercropping, crop rotation, closer spacing, tillage, etc. which could reduce the weed infestation in pigeonpea and pigeonpea-based cropping systems. Intercropping of pigeonpea with soybean (2:4) had smothering effect on weeds and resulted in 32% more grain yield than in sole crop. In pigeonpea, pre-emergence applications of pendimethalin 1.25 kg/ha was found most effective with 21.4% higher grain yield. Integration of the components of production technologies enhanced the productivity of pigeonpea by 29.8% with 27.2% higher net returns. Therefore, an attempt has been made in this article to review works done on several aspects of weed management in pigeonpea-based systems.

Vegetable and floricultural crops are major components of the horticultural industry in India. Weed management is an important aspect in the successful production of these crops. Weeds reduce crop yields, lower their quality and increase costs of production. They host pests and diseases thereby raising the need to control them as well. Weed management may involve non-chemical and or chemical methods. The decision of method to be used depends on the environmental conditions, available labour, weed population, the crop, desired management practices and the cost of controlling weeds. The major aim is to manage the weed population to a level below that will cause a reduction in economic return for the farmer. An integration of different control methods, therefore, needs to be addressed in future research. Furthermore, specific researches on weed management in horticultural crops in India need to be addressed. This article attempts to highlight important weed flora of vegetables and flower crops in India and some of the management strategies that could be used to manage these weeds.

Ever increasing global population, rapid industrialization, increased fossil fuel consumption, deforestation etc. lead to the increased concentration of greenhouse gases in the atmosphere. IPCC reports provide strong evidence that rising CO2 and other trace gases could lead to a 3±12°C increase in global surface temperatures with subsequent effects on climate. Relationship between climate change and agriculture is of particular importance as the world population and world food production showing imbalance under pressure. As mean temperature increases, weeds expand their range into new areas. Climate change is likely to trigger differential growth in crops and weeds and may have more implications on weed management in crops and cropping systems. Growth at elevated CO2 and elevated temperatures would result in anatomical, morphological and physiological changes that could influence herbicidal uptake rates, besides translocation and overall effectiveness. Climate change has an indirect influence on the occurrence of weeds via crop management and land use. There is a possibility that agricultural weed populations will evolve new traits in response to emerging climate and non-climate selection pressures. Reducing the impacts of weeds and preventing new weeds are essential to increasing the resilience of ecosystems and giving native species the best chance to deal with the adverse impacts of climate change.

The North East Himalayan region recognized as one of the seven biodiversity hotspots of the world where Indo-Malayan, Indo-Chinese, and Indian bio-geographical realms have converged. Based on the elevation, climate and soil conditions, water availability and socio-economic aspects, different agro-ecosystems are prevalent in the north eastern India. However, the  agro-ecosystems of the region can be broadly classified as   i)  Jhum agro-ecosystem,  ii) Terrace land agro-ecosystem, and iii) Valley land agro-ecosystem. The major production ecosystems of the region are: rice, jute, sugarcane, tea, horticultural crops and forest and wetland. Shifting cultivation is a primitive agricultural system still practiced in some of the hill areas which is characterized by uniqueness of weed problem and its weed flora is primarily governed by altitude, slope of the land, jhum-cycle and fallowing, burning, rainfall, run-off, crops and cropping geometry and many other relevant issues like biotic interference coupled with climatic factors. The nature and severity of weed menace in different crop ecosystems vary according to the agro-ecosystems in which the crop is grown. Weeds are one of the main production constraints in all crop ecosystems. The common agronomic factors contributing to weed problems in different crops are inadequate land preparation, seed contamination with weed seeds, use of poor quality seeds, broadcast seeding in lowlands, use of overage rice seedlings for transplanting, inadequate water management, inadequate fertilizer management, mono-cropping, labour shortages for weeding operations, delayed herbicide applications and other interventions. In this article, the distribution of weeds and their management practices are reviewed exhaustively.

First successful classical biological control of a weed (prickly pear) was achieved unintentionally in India when cochineal insect, Dactylopius ceylonicus was mistakenly introduced from Brazil in place of D. cacti to produce dye from Opuntia vulgaris. This incident led to biological control of weeds. From 1863 to 1868, it was introduced to southern India, which was first successful intentional use of an insect to control a weed. In 1926, D. opuntiae, a North American species, was imported from Sri Lanka and its colonization resulted in spectacular suppression of Opuntia stricta and related O. elatior. So far in India, about 30 exotic biological control agents have been introduced against weeds, of which six could not be released in the field, 3 could not be recovered after release while 21 were recovered and established. From these established bioagents, 7 are providing excellent control, 4 substantial control and 9 partial control. Biological agents, mainly insects provided excellent biological control of prickly pear, Opuntia elatior and O. vulgaris by D. ceylonicus and  D. opuntiae; Salvinia molesta by weevil, Cyrtobagous salviniae; water hyacinth by weevils Neochetina bruchi and N. eichhorniae and galumnid mite Orthogalumna terebrantis; and Parthenium hysterophorus by chrysomelid beetle Zygogramma bicolorata. Some introduced bioagents did not prove success but providing partial control like of Lantana by agromyzid seedfly, Ophiomyia Lantanae, tingid lace bug, Teleonemia scrupulosa, Diastema tigris, Uroplata girardi, Octotoma scabripennis and Epinotia lantanae; Chromolaena odorata by Pareuchaetes pseudoinsulata; Ageratina adenophora by gallfly, Procecidochares utilis; submerged aquatic weeds such as Vallisneria spp. and Hydrilla verticillata in fish ponds by grass carp. There are many bioagents which have been introduced in other countries and have shown varying degree of success through combined effect. In Australia, 9 bioagents have been introduced against Parthenium alone. Such successful bioagents need to be introduced in India against some of the problematic weeds like Parthenium, water hyacinth, Pistia, alligator weed etc.

Water hyacinth is an aquatic plant coined to multifarious activities including its role as an obnoxious weed with tremendous economic and aesthetic implications. Parthenium (Parthenium hysterophorus L.) is a terrestrial weed often put in the category of world’s worst weeds now assuming status of India’s national weed affecting human health, agriculture, environment and natural biodiversity with tremendous economic implications. The weed has toxic and phytotoxic constituents comprising of phenolics and terpenoids- two major chemical classes implicated in toxic and allelopathic interactions of the species. The species has wide range of constituents in its plant parts, but a few allelochemicals have been investigated for water hyacinth control. Many phenolic acids have been investigated for inhibitory activity on water hyacinth, of which p-hydroxybenzoic acid appeared to be of potential herbicidal activity at 100 ppm, a lethal level for the aquatic weed. Major sesquiterpene lactone parthenin is another allelochemical which has been shown to be a potential herbicidal for water hyacinth at 100 ppm, which killed the weed irrecoverably.  Investigations undertaken on the aspect of control of water hyacinth by Parthenium allelochemicals showed that allelochemical crude and constituent allelochemicals and other secondary metabolites from Parthenium leaf herbicidal for the water hyacinth. The allelochemicals could be used under certain situations for managing a weed (water hyacinth) by another weed (Parthenium) fostering a concept of weed against weed. The little work undertaken so far on screening of Parthenium constituents including allelochemicals for herbicidal activity on water hyacinth and other aquatic weeds pointer to necessity of taking up the investigations on these lines intensively, which might facilitate the development of natural herbicides and their formulations and provide lead for the development of newer synthetic herbicides with novel chemistry for more effective and environment friendly management of water hyacinth and other aquatic weeds.

Herbicides have become obligatory for increasing the agricultural production and to maintain the non-cropped area free from weeds and pests. In general, herbicides are formulated in such a way that they degrade from the environment after completion of their intended work, but a few of them persist in the environment and pose a serious hazard to the succeeding crop and also to the surroundings. Mostly the triaizines, isoxazolidinones, imidazolinones and a few of sulfonylureas are persistent herbicides.  Hence, it is essential to compile the available literature on the management of herbicide residues in the soil environment. In this review, the management aspects were covered under five broad categories, viz. cultural and mechanical, enhanced degradation, deactivation, reducing the availability in soil, and removing from the site of contamination. From the review, it was found that the integration of mechanical and cultural management practices with herbicides for managing weeds is a viable protecting option since the safeners exhibit varying behaviour in soil on influencing the herbicide persistence. Further, the combination of bioaugmentation and biostimulation along with the organic matter addition might be a promising technology to accelerate the biodegradation. Although it requires extensive field evaluation studies, biostimulation in conjunction with other tools like crop rotation and increasing the organic matter content is definitely a promising technique for managing the herbicide persistence minimizing its residue in the soil.