Glyphosate-resistant and glyphosate-susceptible Palmer amaranth (Amaranthus palmeri S. Wats.): hyperspectral reflectance properties of plants and potential for classification

BACKGROUND

Palmer amaranth (Amaranthus palmeri S. Wats.) is a troublesome agronomic weed in the southern United States, and several populations have evolved resistance to glyphosate. This paper reports on spectral signatures of glyphosate-resistant (GR) and glyphosate-sensitive (GS) plants, and explores the potential of using hyperspectral sensors to distinguish GR from GS plants.

RESULTS

GS plants have higher light reflectance in the visible region and lower light reflectance in the infrared region of the spectrum compared with GR plants. The normalized reflectance spectrum of the GR and GS plants had best separability in the 400-500 nm, 650-690 nm, 730-740 nm and 800-900 nm spectral regions. Fourteen wavebands from within or near these four spectral regions provided a classification of unknown set of GR and GS plants, with a validation accuracy of 94% for greenhouse-grown plants and 96% for field-grown plants.

CONCLUSIONS

GR and GS Palmer amaranth plants have unique hyperspectral reflectance properties, and there are four distinct regions of the spectrum that can separate the GR from GS plants. These results demonstrate that hyperspectral imaging has potential application to distinguish GR from GS Palmer amaranth plants (without a glyphosate treatment), with future implications for glyphosate resistance management. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.

Tillage, Fungicide, and Cultivar Effects on Frogeye Leaf Spot Severity and Yield in Soybean

Frogeye leaf spot (FLS) of soybean, caused by Cercospora sojina, has been a problem in the southern United States for many years but has become an increasing problem in the northern United States more recently, causing significant yield losses. This increase in disease severity in the northern United States has been attributed to increased utilization of no-till planting and changes in climate. A field study was conducted at the University of Tennessee, Research and Education Center in Milan, TN from 2007 to 2010 to determine severity in tilled and no-till plots treated with or without fungicide at R3 and R5 growth stages. Three FLS-susceptible cultivars, one each in Maturity Groups III, IV, and V, were treated with pyraclostrobin (Headline) fungicide. Analysis of variance using the area under the disease progress curve (AUDPC) indicated no significant difference (P ≤ 0.05) in disease severity between tilled and no-till plots without fungicide. Fungicide did not significantly reduce disease under no-till, but did under tilled plots. This is the first study showing that no-till plots did not reduce or enhance the severity of FLS when no fungicide was applied. Fungicide application significantly reduced (P ≤ 0.05) disease severity and AUDPC and increased yield in tilled plots. The yield gains in tilled, fungicide-treated plots ranged from 1 to 17%. When fungicide was applied, disease severity was not reduced as significantly in no-till as in treated tilled plots, suggesting that fungicide programs under a no-till system may require further study to minimize the risk of FLS severity.

Responses of nitrogen metabolism and seed nutrition to drought stress in soybean genotypes differing in slow-wilting phenotype

Recent advances in soybean breeding have resulted in genotypes that express the slow-wilting phenotype (trait) under drought stress conditions. The physiological mechanisms of this trait remain unknown due to the complexity of trait × environment interactions. The objective of this research was to investigate nitrogen metabolism and leaf and seed nutrients composition of the slow-wilting soybean genotypes under drought stress conditions. A repeated greenhouse experiment was conducted using check genotypes: NC-Roy (fast wilting), Boggs (intermediate in wilting); and NTCPR94-5157 and N04-9646 (slow-wilting, SLW) genotypes. Plants were either well-watered or drought stressed. Results showed that under well-watered conditions, nitrogen fixation (NF), nitrogen assimilation (NA), and leaf and seed composition differed between genotypes. Under drought stress, NF and NA were higher in NTCPR94-5157 and N04-9646 than in NC-Roy and Boggs. Under severe water stress, however, NA was low in all genotypes. Leaf water potential was significantly lower in checks (-2.00 MPa) than in the SLW genotypes (-1.68 MPa). Leaf and seed concentrations of K, P, Ca, Cu, Na, B were higher in SLW genotypes than in the checks under drought stress conditions. Seed protein, oleic acid, and sugars were higher in SLW genotypes, and oil, linoleic and linolenic acids were lower in SLW genotypes. This research demonstrated that K, P, Ca, Cu, Na, and B may be involved in SLW trait by maintaining homeostasis and osmotic regulation. Maintaining higher leaf water potential in NTCPR94-5157 and N04-9646 under drought stress could be a possible water conservation mechanism to maintain leaf turgor pressure. The increase in osmoregulators such as minerals, raffinose, and stachyose, and oleic acid could be beneficial for soybean breeders in selecting for drought stress tolerance.

Analytical study of Kuberaksha/Kantaki Karanja Patra Churna [Caesalpinia Bonduc (L.) Roxb. leaf powder]

Caesalpinia bonduc (L.) Roxb. (Kuberaksha) is an Ayurvedic herb used in the management of malaria, liver disorders, worms, edematous conditions, etc. Based on classical Ayurvedic textual indications and recent pharmacological studies, its leaf powder was selected for studying its effect clinically on filaria. Before conducting the clinical trails, this leaf powder was subjected to certain chemical studies to find the pH, ash value, extractive values, High Performance Thin Layer Chromatography (HPTLC), etc. for standardization of the drug.

Effects of glyphosate on the mineral content of glyphosate-resistant soybeans (Glycine max)

There are conflicting claims as to whether treatment with glyphosate adversely affects mineral nutrition of glyphosate-resistant (GR) crops. Those who have made claims of adverse effects have argued links between reduced Mn and diseases in these crops. This article describes experiments designed to determine the effects of a recommended rate (0.86 kg ha(-1)) of glyphosate applied once or twice on the mineral content of young and mature leaves, as well as in seeds produced by GR soybeans (Glycine max) in both the greenhouse and field using inductively coupled plasma mass spectrometry (ICP-MS). In the greenhouse, there were no effects of either one application (at 3 weeks after planting, WAP) or two applications (at 3 and 6 WAP) of glyphosate on Ca, Mg, Mn, Zn, Fe, Cu, Sr, Ba, Al, Cd, Cr, Co, or Ni content of young or old leaves sampled at 6, 9, and 12 WAP and in harvested seed. Se concentrations were too low for accurate detection in leaves, but there was also no effect of glyphosate applications on Se in the seeds. In the field study, there were no effects of two applications (at 3 and 6 WAP) of glyphosate on Ca, Mg, Mn, Zn, Fe, Cu, Sr, Ba, Al, Cd, Cr, Co, or Ni content of young or old leaves at either 9 or 12 WAP. There was also no effect on Se in the seeds. There was no difference in yield between control and glyphosate-treated GR soybeans in the field. The results indicate that glyphosate does not influence mineral nutrition of GR soybean at recommended rates for weed management in the field. Furthermore, the field studies confirm the results of greenhouse studies.

Polysaccharide-free nucleic acids and proteins of Abelmoschus esculentus for versatile molecular studies

Abelmoschus esculentus (okra) is one of the polysaccharide rich crop plants. The polysaccharides interfere with nucleic acids and protein isolation thereby affecting the downstream molecular analysis. So, to understand the molecular systematics of okra, high quality DNA, RNA and proteins are essential. In this study we present a method for extracting genomic DNA, RNA and proteins from polysaccharide rich okra tissues. The conventional extraction procedures were integrated with purification treatments with pectinase, RNase and proteinase K, which improved the quality and quantity of DNA as well. Using SDS, additional washes with CIA and NaCl precipitation improved the RNA isolation both quantitatively and qualitatively. Finally, ammonium acetate mediated protein precipitation and re-solubilization increased the quality of total protein extracts from the okra leaves. All of the methods above not only eliminated the impurities but also improved the quality and quantity of nucleic acids and proteins. Further, we subjected these samples to versatile downstream molecular analyses such as restriction endonuclease digestion, RAPD, Southern, reverse transcription-PCR and Western analysis and were proved to be successful.

Physiological responses of glyphosate-resistant and glyphosate-sensitive soybean to aminomethylphosphonic acid, a metabolite of glyphosate

Aminomethylphosphonic acid (AMPA) is formed in glyphosate-treated glyphosate-resistant (GR) and glyphosate-sensitive (GS) soybean [Glycine max (L.) Merr.] plants and is known to cause yellowing in soybean. Although, AMPA is less phytotoxic than glyphosate, its mode of action is different from that of glyphosate and is still unknown. Greenhouse studies were conducted at Stoneville, MS to determine the effects of AMPA on plant growth, chlorophyll content, photosynthesis, nodulation, nitrogenase activity, nitrate reductase activity, and shoot nitrogen content in GR and GS soybeans. AMPA was applied to one- to two-trifoliolate leaf stage soybeans at 0.1 and 1.0 kg ha(-1), representing a scenario of 10% and 100% degradation of glyphosate (1.0 kg ae ha(-1) use rate) to AMPA, respectively. Overall, AMPA effects were more pronounced at 1.0 kg ha(-1) than at 0.1 kg ha(-1) rate. Visual plant injury (18-27%) was observed on young leaves within 3d after treatment (DAT) with AMPA at the higher rate regardless of soybean type. AMPA injury peaked to 46-49% at 14 DAT and decreased to 17-18% by 28 DAT, in both soybean types. AMPA reduced the chlorophyll content by 37%, 48%, 66%, and 23% in GR soybean, and 17%, 48%, 57%, and 22% in GS soybean at 3, 7, 14, and 28 DAT, respectively. AMPA reduced the photosynthesis rate by 65%, 85%, and 77% in GR soybean and 59%, 88%, and 69% in GS soybean at 3, 7, and 14 DAT, respectively, compared to non-treated plants. Similarly, AMPA reduced stomatal conductance to water vapor and transpiration rates at 3, 7, and 14 DAT compared to non-treated plants in both soybean types. Photosynthesis rate, stomatal conductance, and transpiration rate recovered to the levels of non-treated plants by 28 DAT. Plant height and shoot dry weight at 28 DAT; nodulation, nitrogenase activity at 10 DAT, and nitrate reductase activity at 3 and 14 DAT were unaffected by AMPA. AMPA reduced root respiration and shoot nitrogen content at 10 DAT. These results suggest that a foliar application of AMPA could indirectly reduce photosynthesis through decreased chlorophyll content in GR and GS soybean up to 14 DAT, but affected plants can recover to normal growth by 28 DAT.

Biological responses to glyphosate drift from aerial application in non-glyphosate-resistant corn

BACKGROUND

Glyphosate drift from aerial application onto susceptible crops is inevitable, yet the biological responses to glyphosate drift in crops are not well characterized. The objectives of this research were to determine the effects of glyphosate drift from a single aerial application (18.3 m swath, 866 g AE ha(-1)) on corn injury, chlorophyll content, shikimate level, plant height and shoot dry weight in non-glyphosate-resistant (non-GR) corn.

RESULTS

One week after application (WAA), corn was killed at 3 m from the edge of the spray swath, with injury decreasing to 18% at 35.4 m downwind. Chlorophyll content decreased from 78% at 6 m to 22% at 15.8 m, and it was unaffected beyond 25.6 m at 1 WAA. Shikimate accumulation in corn decreased from 349% at 0 m to 93% at 15.8 m, and shikimate levels were unaffected beyond 25.6 m downwind. Plant height and shoot dry weight decreased gradually with increasing distance. At a distance of 35.4 m, corn height was reduced by 14% and shoot dry weight by 10% at 3 WAA.

CONCLUSIONS

Corn injury and other biological responses point to the same conclusion, that is, injury from glyphosate aerial drift is highest at the edge of the spray swath and decreases gradually with distance. The LD(50) (the lethal distance that drift must travel to cause a 50% reduction in biological response) ranged from 12 to 26 m among the biological parameters when wind speed was 11.2 km h(-1) and using a complement of CP-09 spray nozzles on spray aircraft.

Agronomic and environmental implications of enhanced s-triazine degradation

Novel catabolic pathways enabling rapid detoxification of s-triazine herbicides have been elucidated and detected at a growing number of locations. The genes responsible for s-triazine mineralization, i.e. atzABCDEF and trzNDF, occur in at least four bacterial phyla and are implicated in the development of enhanced degradation in agricultural soils from all continents except Antarctica. Enhanced degradation occurs in at least nine crops and six crop rotation systems that rely on s-triazine herbicides for weed control, and, with the exception of acidic soil conditions and s-triazine application frequency, adaptation of the microbial population is independent of soil physiochemical properties and cultural management practices. From an agronomic perspective, residual weed control could be reduced tenfold in s-triazine-adapted relative to non-adapted soils. From an environmental standpoint, the off-site loss of total s-triazine residues could be overestimated 13-fold in adapted soils if altered persistence estimates and metabolic pathways are not reflected in fate and transport models. Empirical models requiring soil pH and s-triazine use history as input parameters predict atrazine persistence more accurately than historical estimates, thereby allowing practitioners to adjust weed control strategies and model input values when warranted.

Glyphosate effect on shikimate, nitrate reductase activity, yield, and seed composition in corn

When glyphosate is applied to glyphosate-resistant (GR) crops, drift to nonglyphosate-resistant (non-GR) crops may cause significant injury and reduce yields. Tools are needed to quantify injury and predict crop losses. In this study, glyphosate drift was simulated by direct application at 12.5% of the recommended label rate to non-GR corn (Zea mays L.) at 3 or 6 weeks after planting (WAP) during two field seasons in the Mississippi delta region of the southeastern USA. Visual plant injury, shikimate accumulation, nitrate reductase activity, leaf nitrogen, yield, and seed composition were evaluated. Effects were also evaluated in GR corn and GR corn with stacked glufosinate-resistant gene at the recommended label rate at 3 and 6 WAP. Glyphosate at 105 g ae/ha was applied once at 3 or 6 weeks after planting to non-GR corn. Glyphosate at 840 (lower label limit) or 1260 (upper label limit) g ae/ha was applied twice at 3 and 6 WAP to transgenic corn. Glyphosate caused injury (45-55%) and increased shikimate levels (24-86%) in non-GR compared to nontreated corn. In non-GR corn, glyphosate drift did not affect starch content but increased seed protein 8-21% while reducing leaf nitrogen reductase activity 46-64%, leaf nitrogen 7-16%, grain yield 49-54%, and seed oil 18-23%. In GR and GR stacked with glufosinate-resistant corn, glyphosate applied at label rates did not affect corn yield, leaf and seed nitrogen, or seed composition (protein, oil, and starch content). Yet, nitrate reductase activity was reduced 5-19% with glyphosate at 840 + 840 g/ha rate and 8-42% with glyphosate at 1260 + 1260 g/ha rate in both GR and GR stacked corn. These results demonstrate the potential for severe yield loss in non-GR corn exposed to glyphosate drift.

Effects of glyphosate application on seed iron and root ferric (III) reductase in soybean cultivars

Previous research demonstrated that plant nutrient assimilation was reduced by glyphosate (Gly). A 2 year field experiment investigated the effects of Gly at drift rate (12.5% of commercial use rate) on Fe concentrations in leaves and seeds of Gly-sensitive (GS) soybean, and a greenhouse experiment evaluated Gly effects on Fe assimilation using root in vivo ferric reductase activity (FRA) in two GS and one Gly-resistant (GR) soybean cultivars. Field studies showed that Gly drift rates resulted in a significant decrease in the Fe concentration in seeds and leaves compared to the nontreated plants. In greenhouse studies, leaf Fe and FRA were inhibited in GS cultivars Hutcheson and DP 5110 and the GR cultivar AG 4604RR and leaf Fe was positively correlated with root FRA (p < 0.0001). These results indicate that Gly can interfere with Fe assimilation in both GS and GR soybean. Understanding the implication of Gly on Fe nutrition in soybean seed would help soybean agronomists and breeders seeking to improve seed mineral nutrition qualities.

Soil depth and tillage effects on glyphosate degradation

The use of glyphosate-resistant crops facilitated the widespread adoption of no-tillage (NT) cropping systems. The experimental objectives were to determine glyphosate sorption, mineralization, and persistence at two depths [0-2 cm (A) and 2-10 cm (B)] in a silt loam managed under long-term conventional tillage (CT) or NT soybean. Relative to the other soils, organic carbon (OC) and fluorescein diacetate (FDA) hydrolytic activity were at least 1.4-fold higher in NT-A. Glyphosate K(d) values ranged from 78.2 to 48.1 and were not correlated with OC. Cumulative glyphosate mineralized after 35 days was highest in NT-A soil (70%), intermediate in CT-A and CT-B (63%), and least in NT-B (51%). Mineralization was positively correlated with OC and FDA activity, but negatively correlated with K(d), indicating that sorption decreased bioavailability. Independent of tillage and depth, the half-lives for 0.01 N CaCl(2) and 0.1 N NaOH extractable residues (bioavailable residues and residues bound to iron and aluminum oxides, respectively) were Collapse

Key Words Collapse

MESH Headings

• Adsorption
• Agriculture/methods
• Biodegradation, Environmental
• Glycine/analogs & derivatives
• Glycine/chemistry
• Herbicides/chemistry
• Soil/analysis
• Glyphosate

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Bromoxynil degradation in a Mississippi silt loam soil

BACKGROUND

The objectives of these laboratory experiments were: (1) to assess bromoxynil sorption, mineralization, bound residue formation and extractable residue persistence in a Dundee silt loam collected from 0-2 cm and 2-10 cm depths under continuous conventional tillage and no-tillage; (2) to assess the effects of autoclaving on bromoxynil mineralization and bound residue formation; (3) to determine the partitioning of non-extractable residues; and (4) to ascertain the effects of bromoxynil concentration on extractable and bound residues and metabolite formation.

RESULTS

Bromoxynil K(d) values ranged from 0.7 to 1.4 L kg(-1) and were positively correlated with soil organic carbon. Cumulative mineralization (38.5% +/- 1.5), bound residue formation (46.5% +/- 0.5) and persistence of extractable residues (T(1/2) < 1 day) in non-autoclaved soils were independent of tillage and depth. Autoclaving decreased mineralization and bound residue formation 257-fold and 6.0-fold respectively. Bromoxynil persistence in soil was rate independent (T(1/2) < 1 day), and the majority of non-extractable residues (87%) were associated with the humic acid fraction of soil organic matter.

CONCLUSIONS

Irrespective of tillage or depth, bromoxynil half-life in native soil is less than 1 day owing to rapid incorporation of the herbicide into non-extractable residues. Bound residue formation is governed principally by biochemical metabolite formation and primarily associated with soil humic acids that are moderately bioavailable for mineralization. These data indicate that the risk of off-site transport of bromoxynil residues is low owing to rapid incorporation into non-extractable residues.

Evidence for cross-adaptation between s-triazine herbicides resulting in reduced efficacy under field conditions

BACKGROUND

Enhanced atrazine degradation has been observed in agricultural soils from around the globe. Soils exhibiting enhanced atrazine degradation may be cross-adapted with other s-triazine herbicides, thereby reducing their control of sensitive weed species. The aims of this study were (1) to determine the field persistence of simazine in atrazine-adapted and non-adapted soils, (2) to compare mineralization of ring-labeled (14)C-simazine and (14)C-atrazine between atrazine-adapted and non-adapted soils and (3) to evaluate prickly sida control with simazine in atrazine-adapted and non-adapted soils.

RESULTS

Pooled over two pre-emergent (PRE) application dates, simazine field persistence was 1.4-fold lower in atrazine-adapted than in non-adapted soils. For both simazine and atrazine, the mineralization lag phase was 4.3-fold shorter and the mineralization rate constant was 3.5-fold higher in atrazine-adapted than in non-adapted soils. Collectively, the persistence and mineralization data confirm cross-adaptation between these s-triazine herbicides. In non-adapted soils, simazine PRE at the 15 March and 17 April planting dates reduced prickly sida density at least 5.4-fold compared with the no simazine PRE treatment. Conversely, in atrazine-adapted soils, prickly sida densities were not statistically different between simazine PRE and no simazine PRE at either planting date, thereby indicating reduced simazine efficacy in atrazine-adapted soils.

CONCLUSIONS

Results demonstrate the potential for cross-adaptation among s-triazine herbicides and the subsequent reduction in the control of otherwise sensitive weed species.

Nitrogen metabolism and seed composition as influenced by glyphosate application in glyphosate-resistant soybean

Previous research has demonstrated that glyphosate can affect nitrogen fixation or nitrogen assimilation in soybean. This 2-year field study investigated the effects of glyphosate application of 1.12 and 3.36 kg of ae ha(-1) on nitrogen metabolism and seed composition in glyphosate-resistant (GR) soybean. There was no effect of glyphosate application on nitrogen fixation as measured by acetylene reduction assay, soybean yield, or seed nitrogen content. However, there were significant effects of glyphosate application on nitrogen assimilation, as measured by in vivo nitrate reductase activity (NRA) in leaves, roots, and nodules, especially at high rate. Transiently lower leaf nitrogen or (15)N natural abundance in high glyphosate application soybean supports the inhibition of NRA. With the higher glyphosate application level protein was significantly higher (10.3%) in treated soybean compared to untreated soybean. Inversely, total oil and linolenic acid were lowest at the high glyphosate application rate, but oleic acid was greatest (22%) in treated soybean. These results suggest that nitrate assimilation in GR soybean was more affected than nitrogen fixation by glyphosate application and that glyphosate application may alter nitrogen and carbon metabolism.

Nitrogen metabolism and seed composition as influenced by glyphosate application in glyphosate-resistant soybean

Previous research has demonstrated that glyphosate can affect nitrogen fixation or nitrogen assimilation in soybean. This 2-year field study investigated the effects of glyphosate application of 1.12 and 3.36 kg of ae ha(-1) on nitrogen metabolism and seed composition in glyphosate-resistant (GR) soybean. There was no effect of glyphosate application on nitrogen fixation as measured by acetylene reduction assay, soybean yield, or seed nitrogen content. However, there were significant effects of glyphosate application on nitrogen assimilation, as measured by in vivo nitrate reductase activity (NRA) in leaves, roots, and nodules, especially at high rate. Transiently lower leaf nitrogen or (15)N natural abundance in high glyphosate application soybean supports the inhibition of NRA. With the higher glyphosate application level protein was significantly higher (10.3%) in treated soybean compared to untreated soybean. Inversely, total oil and linolenic acid were lowest at the high glyphosate application rate, but oleic acid was greatest (22%) in treated soybean. These results suggest that nitrate assimilation in GR soybean was more affected than nitrogen fixation by glyphosate application and that glyphosate application may alter nitrogen and carbon metabolism.

Integrating soil conservation practices and glyphosate-resistant crops: impacts on soil

BACKGROUND

Conservation practices often associated with glyphosate-resistant crops, e.g. limited tillage and crop cover, improve soil conditions, but only limited research has evaluated their effects on soil in combination with glyphosate-resistant crops. It is assumed that conservation practices have similar benefits to soil whether or not glyphosate-resistant crops are used. This paper reviews the impact on soil of conservation practices and glyphosate-resistant crops, and presents data from a Mississippi field trial comparing glyphosate-resistant and non-glyphosate-resistant maize (Zea mays L.) and cotton (Gossypium hirsutum L.) under limited tillage management.

RESULTS

Results from the reduced-tillage study indicate differences in soil biological and chemical properties owing to glyphosate-resistant crops. Under continuous glyphosate-resistant maize, soils maintained greater soil organic carbon and nitrogen as compared with continuous non-glyphosate-resistant maize, but no differences were measured in continuous cotton or in cotton rotated with maize. Soil microbial community structure based on total fatty acid methyl ester analysis indicated a significant effect of glyphosate-resistant crop following 5 years of continuous glyphosate-resistant crop as compared with the non-glyphosate-resistant crop system. Results from this study, as well as the literature review, indicate differences attributable to the interaction of conservation practices and glyphosate-resistant crop, but many are transient and benign for the soil ecosystem.

CONCLUSIONS

Glyphosate use may result in minor effects on soil biological/chemical properties. However, enhanced organic carbon and plant residues in surface soils under conservation practices may buffer potential effects of glyphosate. Long-term field research established under various cropping systems and ecological regions is needed for critical assessment of glyphosate-resistant crop and conservation practice interactions.

Aminomethylphosphonic acid accumulation in plant species treated with glyphosate

Aminomethylphosphonic acid (AMPA) is the most frequently detected metabolite of glyphosate in plants. The objective of this study was to determine if there is any correlation of metabolism of glyphosate to AMPA in different plant species and their natural level of resistance to glyphosate. Greenhouse studies were conducted to determine the glyphosate I 50 values (rate required to cause a 50% reduction in plant growth) and to quantify AMPA and shikimate concentrations in selected leguminous and nonleguminous species treated with glyphosate at respective I 50 rates. Coffee senna [ Cassia occidentalis (L.) Link] was the most sensitive ( I 50 = 75 g/ha) and hemp sesbania [ Sesbania herbacea (P.Mill.) McVaugh] was the most resistant ( I 50 = 456 g/ha) to glyphosate. Hemp sesbania was 6-fold and Illinois bundleflower [ Desmanthus illinoensis (Michx.) MacM. ex B.L.Robins. & Fern.] was 4-fold more resistant to glyphosate than coffee senna. Glyphosate was present in all plant species, and its concentration ranged from 0.308 to 38.7 microg/g of tissue. AMPA was present in all leguminous species studied except hemp sesbania. AMPA concentration ranged from 0.119 to 4.77 microg/g of tissue. Shikimate was present in all plant species treated with glyphosate, and levels ranged from 0.053 to 16.5 mg/g of tissue. Non-glyphosate-resistant (non-GR) soybean accumulated much higher shikimate than glyphosate-resistant (GR) soybean. Although some leguminous species were found to be more resistant to glyphosate than others, and there was considerable variation between species in the glyphosate to AMPA levels found, metabolism of glyphosate to AMPA did not appear to be a common factor in explaining natural resistance levels.

Tillage management to mitigate herbicide loss in runoff under simulated rainfall conditions

Conservation tillage mitigates soil loss in cropland because plant residues help protect the soil, but effects on pesticide movement in surface runoff are not as straightforward. Effects of soil disturbance on surface runoff loss of chlorimuron and alachlor were evaluated utilizing runoff trays. Soil in the trays was either disturbed (tilled) and kept bare or was not tilled, and existing decomposed plant residue was left on the surface. Rainfall (25mm, 20min) was simulated 1d after alachlor (2.8kg ha(-1)) or chlorimuron (54g ha(-1)) application, and runoff was collected. Runoff fractions were analyzed for herbicide and sediment. Total alachlor loss from bare plots was greater than that in no-tillage plots (4.5% vs. 2.3%, respectively). More than one-third of total alachlor lost from bare plots occurred in the first l of runoff, while no-tillage plots had less runoff volume with a more even distribution of alachlor concentration in the runoff during the rainfall simulation and subsequent runoff period. In contrast, more chlorimuron was lost from no-tillage plots than bare plots (12% vs. 1.5%) even though total runoff volume was lower in the no-tillage plots (10.6mm vs. 13.6mm). This was attributed to dense coverage with partially decomposed plant residue in no-tillage plots (1652kg ha(-1)) that intercepted chlorimuron. It was likely that chlorimuron, a polar compound, was more easily washed off surface plant residues and transported in runoff.

Mycotoxin occurrence and Aspergillus flavus soil propagules in a corn and cotton glyphosate-resistant cropping systems

The effects of cotton-corn rotation and glyphosate use on levels of soil-borne Aspergillus flavus, aflatoxin and fumonisin contamination in corn and cotton seed were determined during 2002-2005 in Stoneville, Mississippi (USA). There were four rotation systems (continuous cotton, continuous corn, cotton-corn and corn-cotton) for both glyphosate-resistant (GR) and non-GR cultivars-herbicide system arranged in a randomized complete block design with four replications. Aspergillus flavus populations in surface (5-cm depth) soil, sampled before planting (March/April), mid-season June) and after harvest (September), ranged from 1.47 to 2.99 log (10) cfu g(-1) soil in the four rotation systems. Propagules of A. flavus were higher in the continuous corn system compared to the continuous cotton system on three sample dates, and cotton rotated with corn decreased A. flavus propagules in three of nine sample dates. Propagules of A. flavus were significantly greater in plots with GR cultivars compared to non-GR cultivars in three samples. In cotton seed, aflatoxin and fumonisin levels were similar (< or = 4 microg kg(-1) and non-detectable, respectively) regardless of rotation and glyphosate. In corn grain, aflatoxin was above the regulatory level (> or = 20 microg kg(-1)) only in GR cultivar in 2004 and 2005. Fumonisin was higher in non-GR cultivar (4 mg kg(-1)) regardless of rotation in 2004; however, in 2002, 2003 and 2005, aflatoxin and fumonisin levels were similar regardless of rotation and glyphosate. These results indicate the potential for increased aflatoxin and fumonisin levels (1 of 4 years) in corn; however, climatic conditions encountered during this study did not allow for mycotoxin production. In laboratory incubation studies, fairly high concentrations of glyphosate were required to inhibit A. flavus growth; however no short-term effect of soil treatment with glyphosate on A. flavus populations were observed. These data suggest that altered populations of A. flavus or higher aflatoxin concentrations in corn grain were due to indirect effects of the GR cropping system.