Sustainable weed management in conservation agriculture

Sustainable crop production is necessary to ensure global food security and environmental safety. Conservation agriculture (CA) is gaining popularity around the globe due to its sustainable approaches such as permanent soil cover, minimal soil disturbance, planned crop rotations and integrated weed management. Weed control is the biggest challenge to CA adoption. Weed ecology and management is different in CA than in conventional agriculture. In CA, weeds expression, seed bank status, distribution, dispersal mechanisms, diversification, growing patterns and competition trends are complex and differ from conventional systems. It is due to reduced tillage of the soil and the flora that thrives in CA. Reduced tillage systems affect the efficacy of herbicides and mechanical weed control measures. So, it is an important task to find out the differences and to fabricate new management options. In this review, changing weed dynamics have been framed. A novel aspect of this review is the comprehensive account of sustainable weed management strategies in relation to CA. Modified tillage operations, improved cultural practices, bioherbicides, chemical herbicides, allelopathy, and crop nutrition have been identified as suitable weed management tools. None of these offers complete control but the integration of these tools in suitable combinations works efficiently. Weeds dominating CA and their responses to CA components are highlighted. For example, small seeded and perennial weeds are more abundant in CA. The role of herbicide resistance in weeds and herbicide tolerant (HT) crops in CA is also highlighted. Allelopathy and crop nutrition are discussed as modern weed management tools for CA. A detailed account of weed responses to fertilizer management options is also given. Integrated weed management compatible to cropping patterns and climatic conditions offers the best results in CA. Future efforts must be directed towards the optimization and integration of these weed management practices.     

我国抗ALS类除草剂油菜种质创制与研究进展

化学除草是控制田间草害的有效手段,但我国油菜化除面积非常有限。创制对特定除草剂具有选择抗性的油菜新种质,选育抗性品种是实现我国油菜化除的有效途径。乙酰乳酸合酶(acetolactate synthase,ALS) 类除草剂具有高效低量、选择性强、杀草谱广等优点,在生产上得到广泛应用。本文简述了ALS 基因及其突变产生抗性的机制,着重介绍我国基于ALS靶酶突变的抗除草剂油菜种质创制、抗性生理生化和分子机理等方面的研究进展,并探讨了抗除草剂新种质在油菜抗性新品种选育、杂交油菜制种及抗性基因在转基因工程中的应用。     

Altered target sites as a mechanism of herbicide resistance

Over 200 distinct herbicide resistant weed biotypes have evolved worldwide. In most of these, resistance is conferred by an altered target site, i.e. a modified target protein with reduced affinity for the herbicide(s) in question. This has been documented for herbicides that target most major known sites of action, including those that inhibit photosynthetic electron transfer at photosystem II, acetyl-CoA carboxylase, acetolactate synthase, and tubulin polymerization. Patterns of cross-resistance to structurally similar herbicides and those from other chemical classes that target the same site vary, depending on the mutation and its effect on protein steric and electronic properties. Mechanisms of target site-based herbicide resistance are reviewed, with emphasis on the biochemical and molecular basis for resistance.     

Bioherbicides: Dead in the water? A review of the existing products for integrated weed management

The intensive use of synthetic herbicides is questioned for many reasons. Bioherbicides, as integrated weed management tools, however, have the potential to offer a number of benefits such as increased target specificity and rapid degradation. Despite the efforts to identify effective bioherbicide agents in laboratory and field, only thirteen bioherbicides are currently available on the market. Since 1980, the number of biopesticides has increased around the world, while the market share of bioherbicides represents less than 10% of all biopesticides. Nevertheless, weed management implemented at the cropping systems scale needs bioherbicides because of legislation to drive weed management away from heavy reliance on chemicals, the global increase in organic agriculture, the need of both organic and conventional agriculture to increase weed control efficiency, concerns about herbicide resistance, and concern from the public about environmental safety of herbicides. Consequently, we review here the existing products on the market and describe their history, mode of action, efficacy and target weeds. This review is unique because we also discuss the role of bioherbicides in integrated weed management: to manage soil weed seedbanks with seed-targeted agents in addition to primary tillage, to increase the efficacy of mechanical weeding because bioherbicides are more effective on seedlings, to increase the suppression effect of crop cultivars by first slowing weed growth, to terminate cover crops particularly in conservation agriculture, and finally to manage herbicide resistant populations.     

Minimizing environmental impacts of grassland weed management: can Cirsium arvense be controlled without herbicides?

Invasion by undesirable plants, such as Cirsium arvense, can constrain attempts to conserve and restore biodiversity in extensively managed temperate grasslands, but control with herbicides can cause environmental harm. We contrasted herbicides with more environmentally sustainable weed control strategies. Six‐year, large‐scale randomized block experiments were established to determine optimum combinations of grazing management and mechanical or herbicide treatments to control thistles within lowland and upland grazing systems. Factorial combinations of tight vs. lenient grazing in spring and autumn with additional treatments of winter grazing were compared. Thistle control methods were applied in sub‐treatments for the first 2 years: cutting twice yearly, herbicide wiping, and cutting followed by herbicide application. Thistle abundance decreased under lenient grazing in spring, autumn and winter at the lowland site, under lenient spring and winter grazing in the uplands and under cattle compared with sheep grazing. Herbicide wiping was the most effective control measure and cutting the least, but effects of all weed control sub‐treatments were lost rapidly, so lenient grazing was sufficient to give long‐term thistle control. Lenient grazing and herbicide wiping also caused small declines in non‐target forb diversity. Control of creeping thistle can therefore be achieved without herbicides because lenient grazing in spring and autumn can decrease thistle populations to sufficiently low levels. Severe infestations can be more rapidly controlled using herbicides, but are better avoided at botanically diverse sites. Ecologically‐based weed control strategies have great potential, but require well designed field experiments, which run for sufficiently long periods to allow community‐level impacts to develop.     

Weed management challenges in rice (Oryza sativa L.) for food security in Bhutan: A review

Rice is the most important and staple cereal in Bhutan, currently grown over an area of 19,549 hectares with a total production of 77,038 tonnes. However, its yield is limited in certain regions due to weed competition with losses of up to 50% reported through infestation from both annual and perennial weeds. In this article, we describe the current weed management practices in rice in Bhutan and identify challenges and prospects to guide future weed control research given that weed science in Bhutan is in its infancy. Generally, the most common form of weed control is hand weeding. However, herbicide use is also increasing as farmers face labour shortages on farms and realize the benefits of herbicides, especially in reducing the time taken to weed crops. But the continued use of the single chemical butachlor is a concern given the possible development of herbicide resistance. Research priorities for weed management in rice in Bhutan include a systematic assessment of crop yield losses; prioritization of weed importance and species shifts; mechanical and cultural alternatives to hand weeding; herbicide resistance testing and management; collective community approaches to area-wide management; and weed management options to suit alternative rice production systems. The ecology and management of the perennial semi-aquatic Potamogeton distinctus needs serious attention owing to its prevalence in major rice areas and potential alone to cause significant yield losses of up to 37%. The successful experiences and challenges in neighbouring Asian countries should also help to guide future weed control research in Bhutan.     

Distribution and frequency of resistance to four herbicide modes of action in Lolium rigidum Gaud. accessions randomly collected in winter cereal fields in Spain

Lolium rigidum is the most prevalent and damaging grass weed of winter cereals in Spain. L. rigidum infestations are frequently treated with herbicides and, consequently, populations have evolved resistance. The objective of this study was to determine the extent and frequency of herbicide resistance in L. rigidum populations in Spain to the selective herbicides chlortoluron, diclofop-methyl and chlorsulfuron, commonly used for its control in-crop, and to glyphosate. The response to these herbicides was evaluated on 123 accessions surveyed randomly across cereal cropping areas of the regions of Castile and León, Catalonia and Andalusia. The fresh weight and the frequency of undamaged plants were calculated for each accession and herbicide. At the regional level, higher frequencies of accessions displaying resistance occurred in Catalonia, an intensively cropped region with a greater herbicide selection pressure. Of concern is that in this region the 60% of the accessions displayed some level of resistance to the ALS-inhibiting herbicide chlorsulfuron. The 6.9% of the accessions found in Castile and León with some resistance to glyphosate could also indicate an incipient problem of resistance to this herbicide. For the other herbicides and regions the majority of the accessions remained susceptible. The possible mechanisms of herbicide resistance development in L. rigidum accessions (target-site versus non-target-site resistance) and their variation among regions was discussed. This study can be used to generate herbicide resistance-management schemes for farmers, based upon the herbicide the site and the potential for resistance development.     

Herbicide savings and economic benefits of several strategies to control Sorghum halepense in maize crops

This study was conducted to assess the herbicide savings and the cost efficiency of site-specific herbicide application strategies in comparison with other strategies based on uniform application of herbicides throughout the whole field. The specific situation considered was Sorghum halepense infested maize fields in Spain. The results from a theoretical economic model were contrasted with the information derived from a S. halepense survey conducted in 37 commercial maize fields distributed over three Spanish maize production areas. Seven application strategies were simulated: 1) no herbicide; 2) overall full-rate; 3) overall half-rate; 4) and 5) site-specific spraying infested cells with full- and half-rate, respectively; 6) and 7) site-specific spraying infested cells plus adjacent buffer areas with full- and half-rate, respectively. The simulation results showed that site-specific weed management was the most profitable strategy when S. halepense infested area ranged between 6.5 and 18.7%. This scenario was present in 22% of the surveyed fields. In fields with less than 6.5% infestation (a situation present in 51% of the surveyed fields), yield losses were slight and the most profitable strategy was using no herbicide. When the infested area ranged between 18.7 and 40.8% (19% of the surveyed fields), no significant differences were observed between the net benefits of the various strategies. Full rate herbicide applications throughout the entire field resulted in the highest net returns in fields with more than 40.8% weed infestation (8% of the surveyed fields). In these cases, the added costs of weed detection, mapping and site-specific herbicide application were not justified by the herbicide savings obtained. Likewise, herbicide savings obtained with the various strategies depended on the proportion of the field infested. Considering the high difference between site-specific treatments and uniform application of herbicides when the infested area ranged between 18.7 and 40.8%, and that no significant differences in net returns were observed in this range, site-specific treatments should be preferred. Adding a buffer area to the site-specific treatments does not seem a suitable strategy due to the considerable increases in costs and reductions in herbicide savings.     

Economic implications of herbicide resistance and high labour costs for management of annual barnyardgrass (Echinochloa crus-galli) in Philippine rice farming systems

Implications of increasing labour costs and the development of herbicide resistance for profitable weed management in Philippine rice farming systems are investigated. The study employs RIMPhil (Resistance and Integrated Management in the Philippines), a bioeconomic simulation model developed to provide a comprehensive assessment of integrated weed management programmes for the control of annual barnyardgrass (Echinochloa crus-galli) in rice crops. Results indicate that herbicide application will become increasingly economically attractive, relative to manual weeding, as labour cost increases. This is important since urban migration in the Philippines continues to increase the scarcity of rural labour. Results also show that the onset of herbicide resistance results in substantial losses in farm profit. It is worthwhile for farmers to take management actions to prevent or delay the onset of herbicide resistance, provided that these changes are effective and not too costly. The study highlights the complexity of decision making about integrated weed management on rice farms in the Philippines.     

东北部分春玉米地乙莠合剂防效下降的原因和对策

在我国的东北春玉米区,长时间使用乙草胺和莠去津,导致部分地区防效下降。在吉林省农安县通过定点试验、大面积试验和大范围调查发现,乙莠合剂防效下降的主要原因是因为杂草群落结构的变化引起的,同时降水的减少和药剂使用方法不当也起了一定的作用,未发现产生明显抗性的迹象。在未来,乙莠合剂仍是该地区玉米地除草剂的主要药剂,对于杂草群落尚未发生变化的地块仍然有很好的防效,而在杂草群落发生变化的地区,应适当更换除草剂或适当增加轮作;针对当地特点,适当推迟乙莠合剂的喷施时间,采用机械喷雾、加大用水量等措施来提高乙莠合剂的防效。     

A review of the biology and control of selected weed species in the genus Oxalis: O. stricta L., O. latifolia H.B.K. and O. pes-caprae L.

This review considers the distribution, weed problems, biology and control of Oxalis stricta, O. latifolia and O. pes-caprae. The rhizomatous O. stricta is a persistent seed-producing weed which invades horticultural crops in North America. Chemical control is generally most effective in situations where soil-applied residual herbicides can be used; in turfgrass however, foliage-applied herbicides provide suppression only. O. latifolia is widely distributed in a range of annual and perennial crops growing in temperate and tropical locations. This pernicious perennial weed reproduces exclusively by asexual structures, underground bulbs, stolons and tubers. The complexities of the perennating system have rendered ineffective a wide range of herbicide families as a means of restricting the spread of O. latifolia. Similarly, cultural methods of control have not been successful. O. pes-caprae also grows and perennates from bulbs. This weed is essentially a problem in cereal crops in Southern Australia and Mediterranean fruit orchards. Cultural and chemical control difficulties and inadequacies are similar to those in O. latifolia. Future research strategies involving weed physiology, herbicide action and cultural control including biological control are discussed in relation to obtaining improved control methods for the selected Oxalis species.     

Integration of soil-applied herbicides at the reduced rates with physical control for weed management in fennel (Foeniculum vulgare Mill.)

Fennel has been widely used in traditional medicine for their antimicrobial effects. Since fennel is long duration crop and have slow initial growth, it protection from weed is essential. Experiments were conducted for two consecutive seasons to evaluate the efficacy of soil-applied herbicides at the reduced rates in combination with physical control for weed management and optimizing the yield of fennel. Treatments were type of herbicide (trifluralin and pendimethalin), application dose (recommend dose (R), 75% R, 50% R, and 0% R) and physical weed control (none, one hand-weeding at 50 day after planting (DAP), wheat straw mulch). Weed-free control treatment was also included in each year. The results showed that the use of soil-applied herbicides resulted in reduced weed biomass but did not provide season long weed control without an additional physical control. In both seasons, pendimethalin provided better weeds control than trifluralin. Reduced herbicide rates were found to be more effective when herbicides application followed by hand-weeding than when were used alone or combined with mulch. Experimental results also showed that one time increasing in herbicide rates increased seed yield by 17.5 and 7.5% in 2012 and 16.5 and 6.3% in 2013, when one hand-weeding and mulching were used as supplemental control, respectively. Overall, the 75% of the labeled recommended rate of herbicides followed by one hand-weeding at 50 DAP produced consistently high yields and less weed biomass, reflecting both superior weed control and crop safety.     

Can wild species become problem weeds because of herbicide resistance? Brachypodium distachyon: a case study

A purported drawback to the use of transgenic herbicide-resistant crops is the fear that the crop or interbreeding wild relatives will become weedy. It has been posited that a change even in a single trait can confer weediness. This hypothesis was tested with Brachypodium distachyon. This innocuous species came into contact with herbicides through the use of crushed rock from its habitat for road foundations. It evolved s-triazine resistance and developed as a monoculture. When true weeds later evolved simazine resistance, B. distachyon was partially competed from the ecosystem and then disappeared upon the use of non-triazine herbicides. Thus, this wild species remained a weed only until true weeds evolved resistance or until other herbicides were used. One gene mutation did not convert it into a weed, which implies that this will be equally improbable in other cases, when the gene codes for an otherwise neutral trait such as herbicide resistance.     

Diclofop resistance in sterile wild oat (Avena sterilis L.) in wheat fields in Greece and its management by other post-emergence herbicides

In 2009, a survey was conducted of cereal fields in five prefectures of Greece to establish the frequency and distribution of herbicide-resistant sterile wild oat (Avena sterilis L.). In total, 104 sterile wild oat accessions were collected and screened in a field experiment (conducted in 2009 and repeated in 2010) with several herbicides commonly used to control this weed. Most of the sterile wild oat accessions (89%) were classed as resistant (or developing resistant) to the ACCase-inhibiting herbicide diclofop, while resistance to other ACCase-inhibiting herbicides was markedly lower. The results of the pot experiments showed that some of the sterile wild oat accessions were found to have a very high level of diclofop resistance (resistance index up to 28.6), while cross-resistance with other herbicides was common. The levels of resistance and cross-resistance patterns varied among biotypes with different amount and time of selection pressure, indicating either more than one mechanism of resistance or different resistance mutations in these sterile wild oat biotypes. LA14, which had the highest diclofop resistance level (28.6 resistance index), showed resistance to all APP herbicides applied and non-ACCase inhibitors. Alternative ACCase-inhibiting herbicides, namely tralkoxydim and pinoxaden remain effective on 86 and 92% of the tested sterile wild oat populations, respectively. For the ALS-inhibiting herbicide mesosulfuron + iodosulfuron, nearly all the sterile wild oat accessions were susceptible (97%), with only 3 of them classed as developing resistance. Therefore, there is an opportunity to effectively control sterile wild oat by selecting from a wide range of herbicides and other cultural practices. Early post-emergence herbicide application is strongly suggested, since it could decrease the number of resistant seeds in the field and slow down the dispersal of this major problem.     

Pre and postemergence herbicides for weed control in castor crop

Weed management is among the main factors limiting cultivation of castor (Ricinus communis) in extensive fields, particularly when labor is scarce or expensive. This experiment evaluated the efficiency of weed management programs using preemergence (clomazone, pendimethalin, and trifluralin) and a postemergence herbicide (chlorimuron-ethyl) applied at 20 days after emergence in castor plants cv. BRS Energia under rainfed conditions in Apodi, Brazil. No phytotoxicity was observed on the castor plants, and the postemergence herbicide significantly increased castor seed yield to 1466 kg ha⁻¹ complementing the weed control of preemergence herbicides treatments in which seed yield was 1207 kg ha⁻¹. Seed yield on weedy and weed-free treatments was 760 and 1971 kg ha⁻¹, respectively. Weeds were kept under a satisfactory control up to 40 days after emergence. This program resulted in reasonable weed control because the preemergence herbicides controled monocotyledon weeds, while the postemergence herbicide controlled broad leafed species being selective to castor plants.     

Clearfield® rice: Its development,success, and key challenges on a global perspective

Weedy rice (Oryza sativa) is a close relative of domesticated rice and a noxious weed prevalent in rice fields in world regions where rice is grown. Weedy rice management has remained challenging to farmers, mainly due to the weed's physiological and morphological resemblance to rice cultivars. The introduction of Clearfield^® rice provides an alternative solution and an additional tool for integrated weed management. Clearfield^® rice-based programs result in the cleanest rice fields in the southern U.S. However, persistent application of the imidazolinone herbicides (imazethapyr, imazamox, and imazapic) in Clearfield^® rice raises concerns about the possible evolution of resistance to ALS-inhibitor herbicides in weedy rice and the transfer of resistance trait. The risk of resistant weedy rice evolution is much higher in Asia, Latin America, and other tropical regions where there is no winterkill and rice is planted at least twice each year. Herbicide carryover to rotational crops is also a concern. We summarized the progress of commercialization of Clearfield^® rice in 15 countries across the continents of America, Asia and Europe. In some countries, imidazolinone-resistant weedy rice outcrosses have been found abundant, thereby negating the utility of Clearfield^® technology. The persistence of imidazolinone herbicide residues in the soil is a concern in regions where multiple crops are planted in a year, or the following year. These challenges should be anticipated by countries that are considering adopting Clearfield^® rice technology. Issues associated with gene escape, resistance evolution in weedy rice, and herbicide carryover to rotational crops remain to be resolved. Research to achieve sustainable solutions for weedy rice management, must be continued and intense educational programs for growers must be sustained.     

Cyperus difformis evolves resistance to propanil

Cyperus difformis L. is one of the worst weeds of rice world-wide and has evolved resistance to acetolactate synthase (ALS)-inhibiting herbicides in rice fields of California. Propanil use was intensified to control the widespread resistant biotypes. Rice growers have recently experienced poor control, suggesting resistance to this photosystem II-inhibiting herbicide may have evolved in C. difformis populations. The objectives of this study were to detect the presence of propanil resistance, to establish resistance levels, and to investigate involvement of enhanced herbicide detoxification as mechanism of resistance through the use of metabolic inhibitors. Four C. difformis populations collected in rice fields from the Sacramento Valley of California were confirmed resistant to propanil. This is the first case of such resistance outside the Poaceae and the first time C. difformis exhibits resistance to an herbicide mechanism of action other than ALS inhibition. Carbaryl and malathion applied individually in mixture with propanil had minor effects on herbicide toxicity suggesting metabolic detoxification was not a resistance mechanism. A resistant biotype produced more than 80% biomass after a propanil (6.7 kg a.i. ha⁻¹) and carbaryl (1.9 kg a.i. ha⁻¹) or propanil and malathion (1.0 kg a.i. ha⁻¹) treatment compared to <20% by a susceptible biotype, suggesting substantial resistance still persisted in spite of insecticide addition. Propanil-resistant plants were cross-resistant to bensulfuron-methyl, imazosulfuron, halosulfuron-methyl and penoxsulam, but susceptible to carfentrazone. The loss of propanil to control this important weed of rice underscores the fragility of herbicide-based weed control in monoculture rice. Integrated weed management approaches to decrease herbicide selection pressure are needed to mitigate the evolution of multiple-herbicide resistance in C. difformis of California rice.     

Interactions of agrochemicals applied to peanut; part 1: Effects on herbicides

Numerous agrochemicals are applied in peanut production systems. Field and laboratory experiments were conducted in North Carolina to characterize biological and physicochemical interactions when the herbicides clethodim, imazapic, imazethapyr, lactofen, sethoxydim, and 2,4-DB were applied in combination with adjuvants, fungicides, insecticides, and micronutrients. A wide range of interactions was noted when comparing across herbicides, weed species, and agrochemical combinations. There was little consistency across weed species for a herbicide or across herbicides for a weed species when comparing significant main effects and interactions. In most instances, when compared with the standard herbicide treatment and adjuvant applied alone, herbicide efficacy was not affected in the presence of other agrochemicals. Changes in solution pH and formation of precipitates varied according to the herbicide combinations used. Boron, manganese, and 2,4-DB often caused dramatic changes in solution pH.     

Weed control in conservation agriculture systems of Zimbabwe: Identifying economical best strategies

Weed management under conservation agriculture (CA), especially when manually controlled is one of the major setbacks for the widespread adoption of CA in southern Africa. This study was conducted at three on-station and three on-farm sites: CIMMYT-Harare, Domboshawa Training Centre and Henderson Research Station (on-station sites), Hereford farm, Madziva communal area and Shamva communal area (on-farm sites). The evaluation focused on the effect of initial herbicide application and succeeding manual weeding whenever weeds were 10 cm tall or 10 cm in length for grasses with stoloniferous–rhizomatous growth habit. Weeds counts, weeding time and grain yields were collected at all on-station sites. At the on-farm sites, weed counts were done before weeding and a number of farmers were timed during weeding. The results showed that herbicides use reduced the weed density and time taken on weeding at all sites. Combining herbicides e.g. atrazine, glyphosate and metalachlor had the lowest weed density and weeding time at all sites. However, the treatments had no effect on maize grain yields suggesting that appropriate and timely manual weeding reduced crop/weed competition. Herbicides treatments had higher input costs than manual weeding due to the additional cost of herbicide but the treatment with manual weeding only had more overall labour days compared to the mixture of three herbicides. In order to achieve economic benefits, smallholder farmers may use the time for value addition e.g. expand cropped land area, use time for value addition, or sell new products on the market. Herbicides use reduces the manual labour needed to control weeds and minimise total crop failure due to untimely weeding hence, herbicides are an important but not the only weed control option under CA systems in Zimbabwe.     

Weed management in no-till winter wheat (Triticum aestivum L.)

In general, effective weed control in no-tillage systems is based on the use of herbicides. However, the development of appropriate weed management strategies can help to reduce the amount of herbicides applied.