Dynamics of canopy-dwelling arthropods under different weed management options,including glyphosate,in conventional and genetically modified insect-resistant maize

The use of genetically modified varieties tolerant to herbicides (HT varieties) and resistant to insects (Bt varieties) in combination with application of a broad-spectrum herbicide such as glyphosate could be an effective option for the simultaneous control of weeds and pests in maize. Nevertheless, the possible impact of these tools on nontarget arthropods still needs to be evaluated. In a field study in central Spain, potential changes in populations of canopy-dwelling arthropods in Bt maize under different weed management options, including glyphosate application, were investigated. Canopy-dwelling arthropods were sampled by visual inspection and yellow sticky traps. The Bt variety had no effect on any group of studied arthropods, except for the expected case of corn borers—the target pests of Bt maize. Regarding the effects of herbicide regimes, the only observed difference was a lower abundance of Cicadellidae and Mymaridae on yellow sticky traps in plots not treated with pre-emergence herbicides. This effect was especially pronounced in a treatment involving two glyphosate applications. The decrease in Cicadellidae and Mymaridae populations was associated with a higher density of weeds in plots, which may have hindered colonization of the crop by leafhoppers. These differences, however, were only significant in the last year of the study. The low likelihood of the use of glyphosate- and herbicide-tolerant varieties for weed control triggering important effects on the nontarget arthropod fauna of the maize canopy is discussed.     

Integrating perennial biomass crops into crop rotations: How to remove miscanthus and switchgrass without glyphosate

Perennial energy grasses have gained attention in recent years as a promising resource for the bioeconomy because of their benign environmental profile, high stress tolerance, high biomass yields and low input requirements. Currently, strong breeding efforts are being made to extend the range of commercially available miscanthus and switchgrass genotypes. In order to foster farmers' acceptance of these crops, and especially of novel hybrids, more information is required about how they can be efficiently integrated into cropping rotations, how they can be removed at the end of their productive lifespan, and what effect they have on subsequently grown crops. Farmers in Europe are meanwhile increasingly constrained in the methods available to them to remove these crops, and there is a risk that the herbicide glyphosate, which has been used in many studies to remove them, will be banned in coming years. This study looks at the removal of seven-year-old stands of miscanthus and switchgrass over 1 year at an experimental site in Southern-Germany. Three novel miscanthus genotypes were studied, alongside one variety of switchgrass, and the impact of each crop's removal on the yield of maize grown as a follow-on crop was examined. A combination of soil tillage and grass herbicides for maize cultivation was successful in controlling miscanthus regrowth, such that yields of maize grown after miscanthus did not differ significantly from yields of maize grown in monoculture rotation (18.1 t dry biomass ha⁻¹). Yields of maize grown after switchgrass (14.4 t dry biomass ha⁻¹) were significantly lower than maize in monoculture rotation caused by insufficient control of switchgrass regrowth by the applied maize herbicide. Although some regrowth of miscanthus and switchgrass was observed in the follow-on crop maize, complete eradication of both crops was achieved by subsequent winter wheat cultivation.     

Crop management and agronomic context of the Farm Scale Evaluations of genetically modified herbicide-tolerant crops

The Farm Scale Evaluations of genetically modified herbicide-tolerant crops (GMHT) were conducted in the UK from 2000 to 2002 on beet (sugar and fodder), spring oilseed rape and forage maize. The management of the crops studied is described and compared with current conventional commercial practice. The distribution of field sites adequately represented the areas currently growing these crops, and the sample contained sites operated at a range of management intensities, including low intensity. Herbicide inputs were audited, and the active ingredients used and the rates and the timings of applications compared well with current practice for both GMHT and conventional crops. Inputs on sugar beet were lower than, and inputs on spring oilseed rape and forage maize were consistent with, national averages. Regression analysis of herbicide-application strategies and weed emergence showed that inputs applied by farmers increased with weed densities in beet and forage maize. GMHT crops generally received only one herbicide active ingredient per crop, later and fewer herbicide sprays and less active ingredient (for beet and maize) than the conventional treatments. The audit of inputs found no evidence of bias.     

Weed Management in Rice: Wheat Rotation by Allelopathy

Rice is major crop in India and its cultivation in northwest India started 25 to 30 years ago in assured irrigation areas during the summer rainy season. In this region, rice-wheat rotation became most popular owing to its high yields; however, these crops are highly infested by the weeds, thus farmers use herbicides for their control. Hence, this rotation consumes a maximum quantity of herbicides in this region, which has resulted in several problems (environmental pollution, human health hazards, development of herbicide resistance in weeds). Thus, serious ecological questions about the reliance on herbicides for weed control in this rotation have been raised. One of the alternatives to overcome these problems is with the use of allelopathic strategies, including the use of weed-smothering crops for weed management and for the sustainability of agriculture. The field, pot culture, and laboratory studies have shown that inclusion of weed-smothering crops in rotation considerably reduced the weed population in the current and succeeding crops. Early summer (April-June) fodder crops of sorghum, pearlmillet and maize drastically smothered the weed population and biomass. The residual suppression effect of peralmillet also persisted in the next crop up to 45 days. Thus, it is conceptualized that the inclusion of such summer fodder crops before the rice crop in the rice-wheat rotation may provide satisfactory weed control in the succeeding rice crop and may minimize the use of herbicides. Likewise, the replacement of wheat by winter fodder crops of oat and berseem (Trifolium alexandrinum) may also help in the control of winter weeds. Hence, further studies in this direction may provide satisfactory weed management in rice-wheat rotation and may minimize the use of herbicides and thereby help indeveloping sustainable agricultural practices.     

Terbuthylazine interferes with iron nutrition in maize (<Emphasis Type="Italic">Zea mays</Emphasis>) plants

Many chemicals, including herbicides, are routinely applied to crops for weed management and food production improvement. However, the intensive use of herbicides could lead eventually to a great environmental threat due to their persistence and accumulation in the ecosystems and contamination of soils. Furthermore, the possible effect of these chemicals on nutrient uptake and assimilation in crops has only recently been discussed. The present study aimed at understanding the effect of the herbicide terbuthylazine (TBA), a herbicide commonly applied to control weeds in leguminous species and triazine tolerant crops, on the capability of maize plants to cope with iron (Fe) shortage. The application of 2 and 5 mg L^?1 TBA caused a significant reduction of root Fe concentration. This reduction might be attributed to a decreased release of phytosiderophores, which in turn could be ascribed to a reduced sulfur assimilation. Results provide evidence that TBA impairs Fe uptake and accumulation in non-target plants most likely interacting with sulfur-assimilating enzymes [ATP sulfurylase and O-acetylserine(thiol)lyase].     

A new weed control strategy in onion culture

A new strategy combining modem hoeing technique and spray application has been developed in order to reduce the amount of herbicides down to 20% compared to common practice. The effects on weed control have been investigated as well as the impact on qualitative and quantitative harvest. In two large scale field trials and two years of testing the authors evaluated different hoeing techniques combined with band spray application and standard spray application, the minimal lethal herbicide dose method (MLHD). All varieties have been calculated for environmental impact as well as practical and economical means. These studies reveal crop losses due to improper weed control as well as losses due to herbicide stress. Detailed information on concentration depending impact of several herbicides have been correlated to their control of different weeds and the achieved yield. Two contrary effects influencing the total yield have been identified. The novel strategy is based on the knowledge of these complex effects which finally led to a well practicable and highly economic strategy that enables onion farmers to control weeds while reducing the amounts of herbicides down to approximately 20%.     

A novel approach to the use of genetically modified herbicide tolerant crops for environmental benefit

The proposed introduction of genetically modified herbicide tolerant (GMHT) crops, with claims of improved weed control, has prompted fears about possible environmental impacts of their widespread adoption, particularly on arable weeds, insects and associated farmland birds. In response to this, we have developed a novel weed-management system for GMHT sugar beet, based on band spraying, which exploits the flexibility offered by the broad-spectrum partner herbicides. Here, we show the results from two series of field experiments which, taken together, demonstrate that, by using this system, crops can be managed for enhanced weed and insect biomass without compromising yield, thus potentially offering food and shelter to farmland birds and other wildlife. These results could be applicable widely to other row crops, and indicate that creative use of GMHT technology could be a powerful tool for developing more sustainable farming systems in the future.     

Transfer of auxinic herbicide resistance from Brassica kaber to Brassica juncea and Brassica rapa through embryo rescue

Auxinic herbicides are widely used in agriculture to selectively control broadleaf weeds. Prolonged use of auxinic herbicides has resulted in the evolution of resistance to these herbicides in some biotypes of Brassica kaber (wild mustard), a common weed in agricultural crops. In this study, auxinic herbicide resistance from B. kaber was transferred to Brassica juncea and Brassica rapa, two commercially important Brassica crops, by traditional breeding coupled with in vitro embryo rescue. A high frequency of embryo regeneration and hybrid plant establishment was achieved. Transfer of auxinic herbicide resistance from B. kaber to the hybrids was assessed by whole-plant screening of hybrids with dicamba, a widely used auxinic herbicide. Furthermore, the hybrids were tested for fertility (both pollen and pistil) and their ability to produce backcross progeny. The auxinic herbicide-resistant trait was introgressed into B. juncea by backcross breeding. DNA ploidy of the hybrids as well as of the backcross progeny was estimated by flow cytometry. Creation of auxinic herbicide-resistant Brassica crops by non-transgenic approaches should facilitate effective weed control, encourage less tillage, provide herbicide rotation options, minimize occurrence of herbicide resistance, and increase acceptance of these crops.     

Quantitative Evaluation of the Environmental Impact Quotient (EIQ) for Comparing Herbicides

Various indicators of pesticide environmental risk have been proposed, and one of the most widely known and used is the environmental impact quotient (EIQ). The EIQ has been criticized by others in the past, but it continues to be used regularly in the weed science literature. The EIQ is typically considered an improvement over simply comparing the amount of herbicides applied by weight. Herbicides are treated differently compared to other pesticide groups when calculating the EIQ, and therefore, it is important to understand how different risk factors affect the EIQ for herbicides. The purpose of this work was to evaluate the suitability of the EIQ as an environmental indicator for herbicides. Simulation analysis was conducted to quantify relative sensitivity of the EIQ to changes in risk factors, and actual herbicide EIQ values were used to quantify the impact of herbicide application rate on the EIQ Field Use Rating. Herbicide use rate was highly correlated with the EIQ Field Use Rating (Spearman’s rho >0.96, P-value <0.001) for two herbicide datasets. Two important risk factors for herbicides, leaching and surface runoff potential, are included in the EIQ calculation but explain less than 1% of total variation in the EIQ. Plant surface half-life was the risk factor with the greatest relative influence on herbicide EIQ, explaining 26 to 28% of the total variation in EIQ for actual and simulated EIQ values, respectively. For herbicides, the plant surface half-life risk factor is assigned values without any supporting quantitative data, and can result in EIQ estimates that are contrary to quantitative risk estimates for some herbicides. In its current form, the EIQ is a poor measure of herbicide environmental impact.     

Weed Dynamics during Transition to Conservation Agriculture in Western Kenya Maize Production

Weed competition is a significant problem in maize (Zea mays, L.) production in Sub-Saharan Africa. Better understanding of weed management and costs in maize intercropped with beans (Phaseolus vulgaris, L.) during transition to conservation agricultural systems is needed. Changes in weed population and maize growth were assessed for a period of three years at Bungoma where crops are grown twice per year and at Trans-Nzoia where crops are grown once per year. Treatments included three tillage practices: minimum (MT), no-till (NT) and conventional (CT) applied to three cropping systems: continuous maize/bean intercropping (TYPICAL), maize/bean intercropping with relayed mucuna after bean harvest (RELAY) and maize, bean and mucuna planted in a strip intercropping arrangement (STRIP). Herbicides were used in NT, shallow hand hoeing and herbicides were used in MT and deep hoeing with no herbicides were used in CT. Weed and maize performance in the maize phase of each cropping system were assessed at both locations and costs of weed control were estimated at Manor House only. Weed density of grass and forb species declined significantly under MT and NT at Manor House and of grass species only at Mabanga. The greatest declines of more than 50% were observed as early as within one year of the transition to MT and NT in STRIP and TYPICAL cropping systems at Manor House. Transitioning to conservation based systems resulted in a decline of four out of five most dominant weed species. At the same time, no negative impact of MT or NT on maize growth was observed. Corresponding costs of weed management were reduced by $148.40 ha^-1 in MT and $149.60 ha^-1 in NT compared with CT. In conclusion, farmers can benefit from effective and less expensive weed management alternatives early in the process of transitioning to reduced tillage operations.     

农作物抗除草剂遗传工程研究进展

控制杂草提高农作物产量是农业生产中共同面临的问题,发展抗除草剂农作物将是最经济最方便控制杂草的技术。由于对除草剂的作用模式和除草剂代谢途径的了解,弄清了除草剂的关键靶酶及其基因,因此分离除草剂靶酶基因,克隆能解毒除草剂的酶基因,通过转化技术可获得抗除草剂农作物,大量的抗除草剂转基因农作物大田试验表明,将最有希望在２０００年进入市场。     

Effects of tillage intensity on nitrogen dynamics and productivity in legume-based grain systems

In 1988 an experiment was established at the Rodale Institute Experimental Farm to study weed control and nitrogen (N) management in rotations with grain crops and N-fixing green manures under reduced tillage without the use of herbicides. Tillage intensities ranging from moldboard plow (MP) to continuous no-till (NT) were compared. We present results for maize production in 1994, the seventh year of the experiment. Our goal was to further investigate reduced tillage regimes that alternated no-till with different forms of primary tillage in legume-based systems. In the chisel-disc (CD) and MP treatments comparable yields were achieved under so-called organic (weeds controlled with cultivation and green manure N source) and conventional management (weeds controlled with herbicides and mineral N fertilizer applied). Weed competition in these treatments was minimal and the N status of maize plants was essentially the same regardless of the N source (fertilizer or green manure). Of the four organic no-till maize treatments, only the mixed-tillage system with cultivation for weed control (CD-NTc) produced yields comparable to conventional NT maize. The fate of vetch N as well as temporal N dynamics were largely determined by tillage intensity and the handling of the vetch residues at maize planting. Treatments with primary tillage (CD and MP) had extremely high levels of mineral N early in the season and had greater average net N-mineralization, even though N content of hairy vetch in these treatments was equal to or lower than that in treatments with mow-killed vetch. In terms of soil mineral N concentrations, the CD-NTc treatment was similar to the other mow-killed vetch/no-till maize treatments. However, N availability in this treatment was greater, probably due to more complete decomposition of green manure residues. Cultivation for weeds not only helped control weeds but also increased mineralization of the vetch residues, which in turn increased the N supply during the period of maximum N demand by the maize. Carefully designed rotations combining tillage reductions with the use of leguminous N sources can have multiple benefits, including improved timing of N availability, reduced herbicide applications, and improved soil quality in the long term. This revised version was published online in June 2006 with corrections to the Cover Date.     

Herbicide resistant rice (Oryza sativa L.): Global implications for weedy rice and weed management*

Rice cultivars resistant to broad‐spectrum herbicides have been developed and their commercial release is imminent, especially for imidazolinone and glufosinate resistant varieties in the USA and Latin America. Glyphosate‐resistant rice should follow within a few years. Rice growers throughout the world could benefit from the introduction of herbicide‐resistant rice cultivars that would allow in‐crop, selective control of weedy Oryza species. Other perceived benefits are the possibility to control ‘hard‐to‐kill’ weed species and weed populations that have already evolved resistance to herbicides currently used in rice production, especially those of the Echinochloa species complex. Weed management could also be improved by more efficient post‐emergence control. Introduction of herbicide resistant rice could also bring areas heavily infested with weedy rice that have been abandoned back to rice production, allow longer term crop rotations, reduce consumption of fossil fuels, promote the replacement of traditional chemicals by more environmentally benign products, and provide more rice grain without adding new land to production. There are also concerns, however, about the impact of releasing herbicide‐resistant rice on weed problems. Of most concern is the possibility of rapid transfer of the resistance trait to compatible weedy Oryza species. Development of such herbicide resistant weedy rice populations would substantially limit the chemical weed management options for farmers. Herbicide‐resistant rice volunteers also could become problematic, and added selection pressure to weed populations could aggravate already serious weed resistance problems. Because of the risk of weedy Oryza species becoming resistant to broad‐spectrum herbicides, mitigating measures to prevent gene flow, eventually attainable by both conventional breeding and molecular genetics, have been proposed. With commercialisation of the first herbicide resistant varieties planned for 2001, these mitigating measures will not be available for use with this first generation of herbicide resistant rice products. Release of herbicide resistant rice should depend on a thorough risk assessment especially in areas infested with con‐specific weedy rice or intercrossing weedy Oryza species. Regulators will have to balance risks and benefits based on local needs and conditions before allowing commercialisation of herbicide‐resistant rice varieties. If accepted, these varieties should be considered as components of integrated weed management, and a rational herbicide use and weedy rice control should be promoted to prevent losing this novel tool.     

“使它隆”对玉米根部不同微生环境细菌群落多样性的影响

【目的】研究除草剂"使它隆"施用后对玉米根部不同微生环境细菌群落结构和多样性的影响。【方法】利用Illumina Miseq高通量测序技术,对玉米根系内生菌、根际和非根际土壤细菌16S rRNA的V4–V5可变区序列进行测定,分析不同生长期喷施除草剂使它隆对玉米土壤细菌及根系内生菌群落结构和多样性的影响。【结果】本研究15个样品共得到544393条有效序列,333565条优质序列。多样本共有OTU分析表明,非根际和根际土壤的群落结构更为相似,在一定程度上说明玉米根部相关细菌的定殖具有选择性并且是从根际到根系逐步专一化。丰度等级曲线和Alpha多样性结果显示非根际和根际土壤群落的丰富度和均匀度较高,而玉米根系内生菌群落的丰富度和均匀度都比较低,且成熟期玉米根系内生菌群落的丰富度在施用除草剂使它隆后下降比较剧烈。群落组成分析发现,使它隆除草后,玉米根部相关细菌各时期在门及属水平上的分布都发生了较大变化。菌群代谢功能预测结果表明玉米生长从苗期到成熟期,微生物的生长压力逐渐加大,需要消耗更多的能量用于新陈代谢和环境适应。【结论】施用除草剂使它隆后会降低玉米根部土壤细菌群落的多样性,使它隆对成熟期玉米根系内生菌群落影响最为显著。     

Circadian response of annual weeds in a natural setting to high and low application rates of four herbicides with different modes of actions

Four herbicides [glyphosate (GLYT), an amino acid synthesis inhibitor; glufosinate (GLUF), a glutamine synthetase inhibitor; fomesafen (FOME), a protoporphyrinogen oxidase inhibitor; and chlorimuron ethyl (CLIM), an acetolactate synthase inhibitor] were used to examine the influence of time of day of application on the control of a variety of annual broadleaf weeds in field studies conducted in Minnesota (five studies on GLYT and GLUF, three studies on FOME and CLIM). All herbicides were applied with an adjuvant at recommended high and low (half or quarter strength) rates every 3h between 06:00 and 24:00h local time. Visual ratings of percent weed control evaluated at 14d were analyzed by herbicide and application rate for each study and across studies for time-of-day effect by analysis of variance (ANOVA) and single cosinor. A circadian response to each herbicide was found, with greatest weed control observed between 09:00 and 18:00h. Increasing the herbicide application rate did not overcome the time-of-day effect (ANOVA: p≤0.008 for time-of-day effect for each herbicide and application rate). The least-squares fit of a 24h cosine was significant (p≤0.001) for each herbicide and application rate, with double amplitudes of 18-82% (units=% visual control) and estimated peaks (acrophases) near midday between 12:40 and 13:35h. Analysis of residuals obtained from multiple regression that included weed height, herbicide rate, temperature, and relative humidity as independent factors also found a significant time-effect by both ANOVA and cosinor for each herbicide and rate, with acrophases advancing significantly by 3 to 7h for GLYT and GLUF, but not for FOME or CLIM. These results suggest that the four herbicides, while belonging to different families with different modes of action, may reveal different peak times of efficacy when adjusting for environmental factors. Nonetheless, each displays similar circadian patterns when influenced by these factors under natural seasonal field conditions. The within-day rhythmic differences found in weed control are large enough to warrant consideration of the practical financial and environmental importance of the time-of-day that these and other herbicides are applied.     

The chemical weed control of maize culture in the Danube meadow

In the world, maize covers about 150 million hectares, following close to rice and wheat. (Balteanu 2000) In Romania, maize culture covers an area of more than 3,500,000 hectares from the overall arable soil, being cultivated in all Romanian districts. Chemical weed control took the greatest extension in the world, at the same time with the synthesis of Atrazine, in 1956, in the laboratories of J.R. Geigy Company, as a super-selective herbicide for the maize culture. This is why many researchers from all continents studied weed control of maize culture, using Atrazine together with other 50 herbicides synthesized until 2004. In the embanked meadow of the Danube, from the 500,000 hectares of arable soil, the maize covers the greatest area. For this reasons, the chemical weed control was granted a great attention by using different herbicides based on Atrazine, Alachlor, Acetochlor, Butilat, Pendimethalin, Dicamba, 2,4-D. At the same time, in the experiments from the Danube Meadow, the authors also studied the efficiency of some combined herbicides: Butizin, Magnific, Guardian Extra is Tazastomp.     

Assessing broomrape risk due to weeds in cropping systems with an indicator linked to a simulation model

Integrated crop protection tolerates residual weed floras if they are not harmful for crop production. These weeds can host harmful crop pests, among which parasitic plants such as branched broomrape (Phelipanche ramosa). This holoparasite is responsible for large yield losses in French crops such as oilseed rape. To date, there are no herbicides available to control it. To evaluate ex ante the impact of crop management practices on weed-mediated parasite infection of crops, we developed an indicator calculated from outputs of the weed dynamics model FlorSys. It consists of three components assessing weed impact on (1) stimulation of parasite germination during the whole cropping season, i.e. the potential risk reduction for future crops via a reduction of the parasite seed bank, (2) the stimulation of parasite germination in host crops, i.e. the potential risk increase for the current crop, (3) parasite reproduction on weed plants, i.e. the potential risk increase for future crops. This indicator was then used to predict weed-mediated broomrape risk in cropping systems from six regions from France and one from Spain. Antagonisms and synergies with other indicators of weed-harmfulness for crop production and weed contribution to plant and functional biodiversity were investigated with Pearson correlation analyses. For instance, cropping systems with a high parasite risk also had a high functional biodiversity (e.g. weed-based food offer for bees). Effects of crop management practices on the weed-mediated parasite risk indicator were identified with linear models; regression trees were used to identify the combinations of management practices that maximised or minimised weed-mediated broomrape risk. Parasite risk depended on crop rotation, sowing and harvest dates, tillage, herbicides and mechanical weeding. The lowest risk was observed in fields that were last tilled less than 21 days before sowing, with more than 0.6 herbicides per year (i.e. 3 applications in 5 years) with multiple entry modes into the weeds (e.g. leaves and roots) and the last herbicide sprayed no later than 127 days before harvest. RLQ analyses were used to identify correlations between weed species traits (Q matrix) and simulated parasite risk (R matrix), via simulated weed densities (L matrix). Early summer-emerging weed species increased parasite risk. No other notable correlations were found, indicating that parasite risk results from a weed community of interacting species, and not simply from individual weed species. An advice table was built to summarize and explain the effects of crop management practices on weed-mediated parasite risk.     

Engineering dicamba selectivity in crops: a search for appropriate degradative enzyme(s)

The biotechnologial approaches to conferring crop selectivity to herbicides have been demonstrated for a number of compounds such as glyphosate, glufosinate, imidazolinones and cyclohexanediones. Imidazolinone-resistant and cyclohexanedione-resistant maize lines are already in the market. There are several other effective and environmentally benign herbicides such as dicamba, for which engineering crop selectivity is desirable, to broaden the product utility in different crops and provide new solutions for weed control. One of the most effective approaches to conferring dicamba selectivity in crops is to incorporate a gene for its rapid metabolism. It is advantageous to have different dicamba-metabolizing enzymes in order to maximize the chances of at least one functioning optimally in a plant environment. Three different metabolizing enzymes are currently available to engineer crop selectivity. The first one is the folate-dependent O-demethylase from Clostridium thermoaceticum, that converts dicamba to herbicidally inactive 3,6-dichlorosalicylate. The second enzyme is the NADH-dependent, multi-component monooxygenase from Pseudomonas maltophilia DI-6 that also converts dicamba to 3,6-dichlorosalicylate. The third enzyme is from corn endosperm cultures that catalyzes the 5-hydroxylation of dicamba. The merits of these three enzymes are discussed with respect to conferring crop selectivity to dicamba. In addition, a rapid microbial screen was conceived for discovery of new dicamba-degrading bacteria, which resulted in identification of Pseudomonas orvilla. This bacteria degraded dicamba by the same pathway, perhaps using a similar enzyme system as Pseudomonas maltophilia DI-6. However, the microbial screen has the potential to identify novel bacteria that degrade dicamba by a different pathway, providing more options for metabolizing enzymes to confer herbicide selectivity in crops. Received 13 February 1997/ Accepted in revised form 26 June 1997     

Circadian Response of Annual Weeds in a Natural Setting to High and Low Application Rates of Four Herbicides with Different Modes of Action

Four herbicides [glyphosate (GLYT), an amino acid synthesis inhibitor; glufosinate (GLUF), a glutamine synthetase inhibitor; fomesafen (FOME), a protoporphyrinogen oxidase inhibitor; and chlorimuron ethyl (CLIM), an acetolactate synthase inhibitor] were used to examine the influence of time of day of application on the control of a variety of annual broadleaf weeds in field studies conducted in Minnesota (five studies on GLYT and GLUF, three studies on FOME and CLIM). All herbicides were applied with an adjuvant at recommended high and low (half or quarter strength) rates every 3h between 06:00 and 24:00h local time. Visual ratings of percent weed control evaluated at 14d were analyzed by herbicide and application rate for each study and across studies for time-of-day effect by analysis of variance (ANOVA) and single cosinor. A circadian response to each herbicide was found, with greatest weed control observed between 09:00 and 18:00h. Increasing the herbicide application rate did not overcome the time-of-day effect (ANOVA: p≤0.008 for time-of-day effect for each herbicide and application rate). The least-squares fit of a 24h cosine was significant (p≤0.001) for each herbicide and application rate, with double amplitudes of 18–82% (units=% visual control) and estimated peaks (acrophases) near midday between 12:40 and 13:35h. Analysis of residuals obtained from multiple regression that included weed height, herbicide rate, temperature, and relative humidity as independent factors also found a significant time-effect by both ANOVA and cosinor for each herbicide and rate, with acrophases advancing significantly by 3 to 7h for GLYT and GLUF, but not for FOME or CLIM. These results suggest that the four herbicides, while belonging to different families with different modes of action, may reveal different peak times of efficacy when adjusting for environmental factors. Nonetheless, each displays similar circadian patterns when influenced by these factors under natural seasonal field conditions. The within-day rhythmic differences found in weed control are large enough to warrant consideration of the practical financial and environmental importance of the time-of-day that these and other herbicides are applied.     

An individual‐based model of seed‐ and rhizome‐propagated perennial plant species and sustainable management of Sorghum halepense in soybean production systems in Argentina

Perennial plants which propagate through both seeds and rhizomes are common in agricultural and nonagricultural systems. Due to their multifaceted life cycle, few population models are available for studying such species. We constructed a novel individual‐based model to examine the effects of ecological, evolutionary, and anthropogenic factors on the population dynamics of perennial species. To exemplify the application of the model, we presented a case study of an important weed, Sorghum halepense (L.) Pers. (Johnsongrass), in soybean productions in Argentina. The model encompasses a full perennial weed life cycle with both sexual (seeds) and asexual (rhizomes) propagations. The evolution of herbicide resistance was modeled based on either single genes or quantitative effects. Field experiments were conducted in the species' native environment in Argentina to parameterize the model. Simulation results showed that resistance conferred by single‐gene mutations was predominantly affected by the initial frequency of resistance alleles and the associated fitness cost. Population dynamics were influenced by evolved resistance, soil tillage, and rhizome fecundity. Despite the pivotal role of rhizomes in driving the population dynamics of Johnsongrass, most herbicides target the aboveground biomass, and chemical solutions to control rhizomes are still very limited. To maintain effective (short‐term) and sustainable (long‐term) weed management, it is recommended to combine soil tillage with herbicide applications for suppressing the rhizomes and delaying the evolution of resistance. This novel model of seed‐ and rhizome‐propagated plants will also be a useful tool for studying the evolutionary processes of other perennial weeds, cash crops, and invasive species.