Weed Science

There is often strong public dissent to innovations, typically fanned by those who lose out economically, but the reasons they promulgate are not economic and are targeted to public emotions.  Agriculture has some problems that have been intractable to present technologies and we have no choice but to utilize new technologies to overcome them.  These include developing new herbicides that affect multiple targets, new selective synergists and safeners, transgenic herbicide resistant plants that will not have the transgenes expressed in related weeds, using transposons or gene drives to disseminate deleterious genes in weeds, sterile pollen, enhanced-virulence biocontrol agents with sustaining formulations.  These might be workable for multiple resistant Amaranthus and Echinochloa species, parasitic weeds, Phalaris in wheat as well as weedy rice in rice.  Per force, most of the innovations must originate in the public sector, by weed scientists who have a broad training in basic sciences, in collaboration with experts from other fields.

Weed invasion and subsequent infestation represents a major problem in crop production. Chemical weed control is the major management tactic used in conventional agriculture. Complementary strategies to herbicides are increasingly being investigated. The importance of allelopathy has been considered for weed management over the years. However, the relevance of allelopathy has been highly discussed due to the lack of phytotoxic concentrations of allelochemicals under field conditions. Avena fatua, Brassica nigra, Fagopyrum esculentum, Secale cereale, Sorghum bicolor, Triticum aestivum and other cover crops have been used in weed management on a limited basis. Crop residues from existing crop or rotational crops can provide selective weed suppression through their physical presence on the soil surface and/or through the release of allelochemicals. Some of the allelochemicals have been reported to play a role in weed management, including phenolic acids, DIBOA, DIBOA-glycoside, and BOA, dhurrin, fatty acids, hydroxamic acids, isoflavonoids, isothiocyanate, juglone, momilactone, scopoletin, and sorgoleone. The soil system, a living and dynamic, influences the fate and functions of allelochemicals in time and space. The bioavailability of allelochemicals in the soil is dependent on processes such as adsorption, leaching and degradations by abiotic and biotic factors. The clay types, organic matter, and soil pH can affect the bioavailability of allelochemicals in the soil. Thus, the allelopathic potential of many compounds may not be expressed in some soils because of the chemical adsorption to soil colloids. The resulting concentrations (sub-toxic) of any of these allelochemicals in soil matrix may have a variety of functions that influence seed germination, seedling emergence, plant growth suppression, nutrient acquisition or soil microbial activity. Examples of such compounds are benzoic acid, catechin, coumaric acid, dihydroxyphenylalanine, ferulic acid, hydroxybenzoic acid, sorgoleone, vanillic acid, and others.

In my view, future allelopathic research should be focused on mechanisms facilitating persistence of allelochemicals in soil environment and characterization of complementary roles of these compounds in plant growth and development. The bioavailability of allelochemicals under field conditions must be established for its effective role in weed management. Currently, we face challenges and opportunities in using allelopathy as a part of weed management strategies in today’s production agriculture.

Conservation agriculture (CA) was used interchangeably with terms like conservation tillage, no tillage, zero tillage (ZT), direct drilling etc. ZT has been the function of weed management, not just in wheat but in rice too. ZT in India was at the dead end in the early 1990s. Until the evolution of herbicide resistance (HR) in Phalaris minor. By now ZT machines are recognised not only as a commercial venture but also attracts major technological step towards intensification of agriculture. With the availability of the “Happy Seeder”– a ZT machine that can plant rice and wheat in high-residue conditions – has made it possible to retain the residues on the soil surface, thereby providing an alternative to residue burning.  It has been reported that ZT in combination with residue mulch reduced the weed problem over time in ZT wheat than CT wheat. In direct-seeded rice (DSR), no single method can provide effective and sustainable weed management solutions. Therefore, combining cultural methods in tandem with judicious use of modern herbicides is crucial. For successful weed control in DSR,  pre-emergence (pendimethaline or oxadiargyl or pretilachlor with safner) followed by post-emergence (bispyribac or bispyribac based tank mixture including bispyribac + pyrazosulfuron/azimsulfuron/2,4-D/halosulfuron or fenoxaprop with saftner or fenoxaprop based tank mixture including fenoxaprop + ethoxysulfuron) herbicide application has provided effective weed control in DSR.

Herbicide resistant (HR) biotech crops which include both the transgenic and non-transgenic ones are being grown in several countries for over 24 yr. Transgenic biotech crops are derived when an exogenous herbicide-resistant gene/s from non-plant sources is/are inserted into the desired crop plant. When the inserted genes stably integrate and express in the plant genome, the concerned plant behaves like a normal plant but with the acquired character, i.e. herbicide resistance. On the other hand, the non-transgenic biotech crops are generated for some herbicides (ALS-inhibiting and ACCase-inhibiting cyclohexane-diones) by selecting for target mutations in plant populations or by tissue culture or by mutation breeding. HR varieties have been developed for soybean, maize, cotton, canola, wheat, rice, sugar beet, alfalfa, etc. while the herbicides included glufosinate, dicamba, 2,4-D, phenmedipham, paraquat, imidazolinones, mesotrione, sulfonylureas, etc.

About 190 million ha around the world have been under HR transgenic crops in 2017. Around 80% of this area was under HR ones either alone or stacked with insect resistance. Biotech crops have made a positive contribution to global crop production and the economies of farmers, while they certainly raised concerns about biosafety to consumers. Several countries led by USA have widely adopted HR biotech crops, while India has been growing only the insect-resistant (IR) Bt cotton since 2002. With adoption of Bt varieties, the country has achieved a great stride in cotton production, accounting for a quarter of market share in global cotton production in 2017. Although no HR biotech crop is adopted in India, it is grown illegally by farmers in key cotton-growing states.

The concerns and limitations about HR biotech crops are related to agro-ecology, evolution of herbicide-resistant weeds, food safety, soil ecosystem, coexistence of biotech and conventional crops, socio-economic consequences, coexistence of biotech and conventional food products, etc. This paper also discusses management of HR biotech crops in greater detail.

Modern agriculture depends on the four main factors viz: seed, water, fertilizers and pesticides. The total number of pests attacking major crops has increased significantly from 1940s. Therefore, the demand of pesticides especially herbicides in agriculture is increasing. Farmers are facing shortages of labour for hand weeding crop fields as people are moving to urban from rural areas. Herbicides are cheaper and more readily available than labour for hand weeding. This review article focuses on the status of using herbicides vis a vis other pesticides and their uses and potential hazards. All pesticides must be toxic to be effective against the pests they are intended to control. Because of being toxic, pesticides are potentially hazardous to humans, animals, other organisms, and the environment. Therefore, users of the pesticides must understand the relative toxicity and potential health effects of the products they use. Pesticides are classified based on the oral and dermal lethal dose, 50% values (to the rat) of the active principles. Globally, 35% of the 158 insecticides fall under extremely hazardous and highly hazardous categories, compared to only about 4% in case of herbicides. Under the slightly hazardous group, the number of herbicides is two times higher as compared to insecticides. The number of herbicides that are unlikely to present acute hazard is as much as 37.1% of the total as compared to 12.6% insecticides. Thus, herbicides as a pesticide category are safer or less hazardous than other pesticides especially insecticides. But it is not intended to give clear chit to herbicides because the ultimate toxicity depends on the formulation. The formulation of pesticides may be thousand times more toxic than their active principles. Thus, there is need to set maximum residue limits (MRLs) based on formulation rather than on the basis of active principles.

Application of ready-mix herbicides containing two active ingredients in rice to manage broad group of weeds is in practice currently. Among different ready mix formulations, one comprising pyrazosulfuron-ethyl and pretilachlor have been recently registered in India. Though, the persistence of each herbicide in soil under rice as single formulation was reported, the information on persistence of herbicides from ready-mix formulations is not reported under semi-arid tropical conditions of India. Hence, an experiment was conducted to study the dissipation and fate of these ready-mix formulation herbicides in rice soil, water and in rice grain. Pyrazosulfuron-ethyl and pretilachlor residues were determined using high performance liquid chromatograph (HPLC) and gas chromatograph (GC), respectively. The average recoveries of pyrazosulfuron-ethyl and pretilachlor from matrix ranged from 80.3-103.3% with less than 10% standard deviation and sensitivity up to 0.001 µg/g. Both herbicides showed rapid dissipation in rice field water than soil and the degradation followed first order reaction kinetics. While pyrazosulfuron-ethyl dissipated with a half-life of 2.17-5.45 and 0.77-0.79 days respectively in rice field soil and water, pretilachlor dissipated with a half-life of 5.18–6.68 days in soil and 2.59–3.00 days in field water. Pyrazosulfuron-ethyl has shorter half-life than pretilachlor and both the active ingredients dissipated rapidly in rice field soil than water. At harvest, the residues of both the herbicides in rice grain and straw were below the MRLs set by FSSAI

A field experiment was conducted at CCS HAU, Regional Research Station, Karnal during Kharif 2010 to 2014 to evaluate the bio-efficacy of pretilachlor 6.0% + pyrazosulfuron-ethyl 0.15% GR (ready-mix) against complex weed flora in transplanted rice and also to study its residual effects. Results from on-station experiment (2010 and 2011) revealed that the optimum dose of pretilachlor + pyrazosulfuron-ethyl was 615 g/ha which provided effective control (91-96%) of complex weed flora in transplanted rice, with higher grain yield (5.98-6.05 t/ha) and B-C ratio (2.19-2.28). In general, it was comparable to its higher doses, bensulfuron-methyl + pretilachlor 660 g/ha, pretilachlor 1000 g/ha, butachlor 1500 g/ha, bispyribac-sodium 25 g/ha and weed free check in terms of weed control, grain yield of rice and benefit-cost ratio. In comparison to weedy check, there was 54-57% increase in grain yield of transplanted rice under pretilachlor + pyrazosulfuron-ethyl 615 g/ha. On an average of 19 adaptive/farmers-participatory trials, pretilachlor + pyrazosulfuron-ethyl 615 g/ha proved superior to commonly used herbicides butachlor 1500 g/ha in 2013 and pretilachlor 1000 g/ha in 2014 in terms of weed control and grain yield of rice. There was no phyto-toxicity of pretilachlor + pyrazosulfuron-ethyl on transplanted rice up to 1230 g/ha (2x dose) and it was also safe to the succeeding chickpea and wheat (2011-12, 2012-13 and 2012-13) crops in rotation.

The field experiments were conducted during 2015-16 and 2016-17 at Norman E. Borlaug Crop Research Center, Pantnagar G.B Pant University of Agriculture & Technology, Pantnagar, U.S. Nagar (Uttarakhand) India, to study the crop intensification and establishment techniques influence on weed dynamics under irrigated rice-wheat system. In Kharif season, density of total weeds as well as grasses, broad-leaved weeds and sedges was observed lowest in rice – wheat cropping system. Transplanted rice-vegatable pea- groundnut cropping sequence proved superior over those cropping systems where upland direct-seeded rice was included as one of the crop with respect to control of Kharif season weeds. In Rabi season, maize (B) (cob + fodder) + cowpea (B) + Sesbania (F)-2:1:2 - vegetable pea (B) + toria (F)-3:1 – groundnut (B) + mentha (F)-3:1(BBF 105 x 30 cm)] proved to be the most prominent cropping system for controlling broad-leaved weeds and sedges. All the cropping systems proved superior for the control of grassy weeds (Phalaris minor and Avena fatua) in which there was inclusion of either legumes or oilseed crops in place of wheat. During summer season, soybean (B) + rice (DSR) (F)-2:1 – wheat (B) + mentha (F) (3:1) - continue (NBS 60 x 30 cm) cropping system was found better for the control of complex weed flora

A field study was conducted in spring season of 2016 and 2017 on sweet corn to evaluate the effect of different weed management practices upon the weed seed bank dynamics in the soil. In the sample from seed bank studied, the per cent contribution of Cleome viscosa was highest among all the weed species before sowing and harvest stage of the crop   and was followed by Dactyloctenium aegypticum, in both the stages and years. Depth wise weed seed distribution indicated highest weed seed in 10-15 cm depth before sowing and in 0-5 cm depth at harvest stage of the crop. Effect of all the weed control treatments upon previous season’s dormant seeds was non-significant. Twice hand weeding was effective to reduce seed bank in deeper layer. Atrazine 1000 g/ha followed by tembotrione 120 g/ha and tembotrione alone 120 g/ha had caused a significant reduction in weed seed number in 0-5 and 0-10 cm layer but weed seed number at 10-15 cm layer remained  unchanged.

A field experiment was conducted during the Kharif season of 2013 to 2015 at pearl millet Research Station, Jamnagar, Gujarat to study the effect of pre- and post-emergence application of atrazine integrated with manual weeding on weeds, crop productivity, nutrient removal and economics of pearl millet (Pennisetum glaucum L.). The experiment was laid out in a randomized block design comprising of eight weed control treatments with three replications. Lesser weed density (8.22 no./m2 at 30 DAS and 11.89 no./m2 at harvest) and weed-biomass (20.2 g/m2), higher weed control efficiency (84.3%) and lower weed index (7.8%) were observed with post-emergence application of atrazine 0.40 kg/ha followed by (fb) hand weeding (HW) at 35 days after sowing (DAS). The pre-emergence application of atrazine 0.50 kg/ha + HW at 35 DAS was at par with post-emergence application of atrazine 0.40 kg/ha + HW at 35 DAS and recorded maximum net returns (` 40,087/ha) and benefit: cost ratio (2.97).

Results of field experiment carried out during Kharif 2011 and 2012 at Instructional Farm, Rajasthan College of Agriculture, Udaipur, indicated various weed-management treatments significantly enhanced N and P uptake by maize (Zea mays L.) and reduced removal of nutrients by weeds as compared to weedy check. Maximum saving of 52.96% nitrogen and 51.87 % phosphorus was achieved with oxyfluorfen 0.15 kg/ha pre-emergence fb one hoeing 30 DAS. On pooled basis, this treatment gave 163.57% and  95.86%more grain and stover yield, respectively compared to weedy check, which was followed by metribuzin 0.25 kg/ha pre- emergence fb one hoeing 30 DAS. The yield as well as uptake of N and P by the crop was maximum with 150% RDF which were statistically at par with 125% RDF.

A long-term experiment was commenced (2011) and conducted at the Research Farm of Dapoli (Maharashtra) during Kharif and Rabi season in rice–groundnut cropping system, to evaluate the effect of green manuring and different weed control measures on the survival and growth of total bacteria, total fungi, total free living nitrogen fixers and total phosphate solubilizers in rhizospheric soil. Results emerged out from the conduct during 2011 to 2014 indicated that, green manuring (in-situ application of Sesbania rostrata 45 DAS) and without green manuring (control) as a main plot treatment and among the weed control measures and sub plot treatments comprising, comparative effects of hand weeding, fixed herbicide pretilachlor (pre-emergence) for rice crop and pendimethalin (pre-emergence) for groundnut crop, and different rotational herbicides (for rice crop, pyrazosulfuron 0.030 kg/ha at 8-10 DAT (1 year), fenoxaprop -p-ethyl 0.056 kg/ha at 25-30 DAT (2 year), oxadiargyl 0.100 kg/ha at 0-5 DAT (3 year), and for groundnut crop oxadiargyl 0.12 kg/ha at 0-2 DAS-1 year, butachlor 1.0 kg/ha at 0-3 DAS-2 year, alachlor1.5 kg/ha at 0-3 DAS-3 year) application to both the crops were significantly tested along with weedy check. The results concluded that the green manuring significantly increased in microbial populations than without green manuring. There were no adverse effects of herbicidal use on all the estimated microbial population at all the stages of both the crops. In contrast to use of fixed herbicide pretilachlor-S 0.75 kg/ha for rice and pendimethalin 1.00 kg/ha and different rotational herbicides had no long-term adverse effects on rhizosphere micro-flora of rice–groundnut cropping system.

The application of N at recommended or higher dose (33% higher than recommended) effectively enhanced the growth and yield of rice crop by suppressing the weed growth in zero-till transplanted rice. Hand weeding thrice at 25, 55 and 75 days after transplanting (DAT) was the best   in reducing weed growth and ultimately increased the grain yield of zero-till transplanted rice. Among the herbicidal methods, tank mix application of bispyribac-Na + 2,4-D minimized the weed growth at early stage and enhanced rice grain yield.

A field experiment was conducted at Agricultural Research Farm, Institute of Agricultural Sciences, Banaras Hindu University, Varanasi to study effect of nitrogen level and weed management in direct-seeded rice. Treatment consisted of 12 treatment combinations which was laid out in the split-plot design having nitrogen levels (100, 120, 140 and 160 kg N/ha) in main plot and three weed management practices, viz. weedy, two hand weeding and bispyribac-sodium 25g/ha fb Cono-weeding in sub-plots replicated thrice. Two hand weeding recorded significantly lower weed biomass and density and better performance of crop growth and yield attributes and yield followed by bispyribac-Na  25 g/ha fb cono-weeding. In case of nitrogen application of 160 kg N/ha showed better performance of growth, yield attributes and yield as compared to other nitrogen treatments. It was observed that application of nitrogen 160 kg/ha and two hand weeding was found best treatment for higher grain yield and net returns.

A field experiment was conducted during Kharif 2016 and 2017 at Zonal Agriculture Research Station, V.C. Farm, Mandya (Southern dry Zone, Karnataka) to study the efficacy of various weed management practices on weeds in transplanted rice. The experiment consists of ten treatments replicated thrice in a randomized complete block design. Among the various treatments, significantly the lowest weed density (18.3-22.0 no./m2) and weed biomass (5.4-6.3 g/m2) was noticed with hand weeding at 25 and 45 DAS, which was at par with pre-emergence application (PE) of bensulfuron-methyl + pretilachlor fb triafamone + ethoxysulfuron applied at 30 days after transplanting (DAT). Weed management with bensulfuron-methyl + pretilachlor (60 + 600 g/ha) PE fb triafamone + ethoxysulfuron (60 g/ha) applied at 25 DAT recorded significantly higher paddy grain and straw yields and higher economic returns.

A field experiment was conducted at Agricultural Research Station, Utukur, Kadapa, Andhra Pradesh, India during Rabi seasons of 2016 and 2017 to test the efficacy of herbicides in rice-fallow groundnut. It is common that previous rice volunteer plants will come in the succeeding crops after rice. Keeping all this in view, the experiment was conducted with six weed control treatments arranged in a randomized block design (RBD) with four replications. Pre-emergence application of pendimethalin 1.5 kg/ha followed by post-emergence application of imazethapyr 75 g/ha at 18-20 DAS recorded lower density of both broad-leaved and grassy-weeds. Weed free throughout the crop period recorded higher pod yield (4.24 t/ha), which was at par with hand weeding twice at 20 and 40 DAS (4.19t/ha) and pendimethalin 1.5 kg/ha as pre-emergence followed by imazethapyr 75 g/ha as post-emergence at 18-20 DAS (3.91 t/ha). Higher benefit:cost ratio (2.20) was recorded with pre-emergence application of pendimethalin 1.5 kg/ha followed by post-emergence application of imazethapyr 75 g/ha at 18- 20 DAS.

A field experiment was conducted during Rabi seasons of 2015-16 and 2016-17 at Banka (Bihar) to evaluate the weed population, weed biomass and grain yield as influenced by herbicide application in wheat (Triticum aestivum L.). Seven weed species, viz. Phalaris minor, Cynodon dactylon, Chenopodium album, Oxalis purpurea, Anagallis arvensis, Medicago denticulata and Rumex dentatus infested the wheat field. Post-emergence application of sulfosulfuron 75% WG (25 g/ha) + metsulfuron-methyl 20% WP (2 g/ha) at 30 days after sowing (DAS) was very effective against broad-leaf weeds and annual grasses, and recorded significantly lower density and biomass of these weeds at 60 DAS as compared to isoproturon 75% WP 1.0 kg/ha, sulfosulfuron 75% WG 50 g/ha, metsulfuron-methyl 20% WP 4 g/ha and weedy check. Ear length, effective tillers/m2, grains/spike, grain and straw yields were also significantly higher in sulfosulfuron (25 g/ha) + metsulfuron-methyl (2 g/ha) as compared to other herbicide treatments.

The experiment was carried out during Kharif (wet) season of 2014 at Agricultural Farm of Palli Siksha Bhavana, Visva-Bharati, Sriniketan, West Bengal to find the effect of tank mix application of tembotrione and atrazine on weed growth and productivity of Kharif maize. Nine treatments comprising of herbicide tembotrione as early post-emergence at 80, 100, 120 g/ha alone and in combination with atrazine at 500 g/ha, sole application of atrazine at 1000 g/ha and weeding twice at 25 and 40 DAS and unweeded control were assigned in a randomized block design in three replication. Results revealed that the experimental field was infested with all categories of weeds including grassy, broad-leaved and sedges. Among them the most predominant weeds were Ludwigia parviflora, Cynodon doctylon, Cyperus rotundus and Fimbristylis miliacea. Overall weed infestation caused about 48% reduction in yield of maize. Combined application of tembotrione with atrazine was significantly superior to its sole application in all the doses tested. Tembotrione at 100 g/ha + stefes mero surfactant at 733 g/ha + atrazine 500 g/ha considerably reduced the weed infestation- registering lower weed density, dry weight, weed index, higher weed control efficiency and increase in values of growth and yield attributes and yield of maize, which were comparable with tembotrione at 80 g/ha + stefes mero surfactant at 733 g/ha + atrazine 500 g/ha and tembotrione at 120 g/ha + stefes mero surfactant at 733 g/ha + atrazine 500 g/ha. Thus, early post-emergence application of tembotrione at 80-100 g/ha + stefes mero surfactant at 733 g/ha + atrazine 500 g/ha appeared to be the most promising weed management practice for higher weed control efficiency, yield, gross and net return of Kharif (wet) season maize in lateritic soil of West Bengal.