Anna M. Szmigielski

Phytotoxicity and persistence of flucarbazone-sodium in soil

Abstract Flucarbazone-sodium, a new herbicide, exhibits high bioactivity at low concentrations. To elucidate potential carryover and crop injury, the behavior of flucarbazone in six Western Canadian soils was studied in the laboratory. A sensitive bioassay was developed for the detection of flucarbazone. Of five crops tested, oriental mustard showed the highest degree of root and shoot inhibition from the presence of flucarbazone in soil. Flucarbazone concentrations as low as 1 μg kg−1 were detected by the mustard root inhibition method. This bioassay was used to examine phytotoxicity and persistence of flucarbazone. Phytotoxicity was related to soil organic carbon content. Concentrations corresponding to 50% inhibition (I50 values) were estimated after fitting the data to a log-logistic model. I50 estimates ranged from 6.0 to 27.5 μg kg−1 for soils containing 1.1 to 4.3% organic carbon, respectively, and were correlated (R = 0.979) with percent organic carbon in the investigated soils. Persistence of flucarbazone was examined in soils incubated at 25 C and moisture content of 85% field capacity (FC). Flucarbazone dissipation followed first-order kinetics in one soil, but a two-compartment model provided the best fit for dissipation in the other soils. Half-lives (t0.5), calculated from dissipation curves in each soil, ranged from 6 to 110 d. Half-lives were correlated (R = 0.776) with soil organic carbon. Flucarbazone dissipation was more rapid in soils containing less organic carbon. Flucarbazone was more persistent in drier soil; t0.5 was 11 d in soil at 85% FC and was 25 d in soil at 50% FC. Soil characteristics and environmental conditions will affect the degree of plant injury to sensitive crops the year after flucarbazone application. Nomenclature: Flucarbazone-sodium; oriental mustard, Brassica juncea L. ‘AC Vulcan’.

Development of a Laboratory Bioassay and Effect of Soil Properties on Sulfentrazone Phytotoxicity in Soil

Abstract Sulfentrazone is a phenyl triazolinone herbicide used for control of certain broadleaf and grass weed species. Sulfentrazone persists in soil and has residual activity beyond the season of application. A laboratory bioassay was developed for the detection of sulfentrazone in soil using root and shoot response of several crops. Shoot length inhibition of sugar beet was found to be the most sensitive and reproducible parameter for measurement of soil-incorporated sulfentrazone. The sugar beet bioassay was then used to examine the effect of soil properties on sulfentrazone phytotoxicity using 10 different Canadian prairie soils. Concentrations corresponding to 50% inhibition (I50 values) were obtained from the dose–response curves constructed for the soils. Sulfentrazone phytotoxicity was strongly correlated to the percentage organic carbon (P  =  0.01) and also to percentage clay content (P  =  0.05), whereas correlation with soil pH was nonsignificant (P  =  0.21). Because sulfentrazone phytotoxicity was found to be soil dependent, the efficacy of sulfentrazone for weed control and sulfentrazone potential carryover injury will vary with soil type in the Canadian prairies. Nomenclature: Sulfentrazone, N-[2,4-dichloro-5-[4-(difluoromethyl)-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-1-yl]phenyl]methanesulfonamide; sugar beet, Beta vulgaris L. ‘Beta 1385’.

Evaluating a Mustard Root-Length Bioassay for Predicting Crop Injury from Soil Residual Flucarbazone

Abstract A simple mustard root‐length bioassay was developed for detection of flucarbazone residues in soil. This bioassay is completed in 3 days using 200 g of soil for four replicate measurements. The results of the bioassay and chemical analysis were compared with the field data (yield and crop injury symptoms) obtained for subsequent crops grown 1 year after flucarbazone application in experimental plots and farm fields in western Canada. For crops grown in replicated experimental field trials, the bioassay was a better predictive tool for yield reduction than chemical analysis (88.5% and 26.9% agreement, respectively), whereas in farm fields where history of other herbicide applications was not identified, bioassay and chemical analysis were similar in their agreement with visual injury symptoms (70.2% and 80.8% agreement, respectively). The mustard root‐length bioassay has potential as a simple tool in research and routine testing for identifying plant injury due to residual herbicide.

Effects of Soil Factors on Phytotoxicity and Dissipation of Sulfentrazone in Canadian Prairie Soils

Studies were conducted to examine the effects of soil properties on sulfentrazone phytotoxicity and dissipation under laboratory conditions. The pH values of five soils from Saskatchewan were altered through acidification with hydrochloric acid (HCl) and alkalization with calcium carbonate (CaCO3). The phytotoxicity of sulfentrazone to sugar beet (Beta vulgaris L. Beta 1385), determined using a shoot length bioassay, was reduced when soil pH was lowered and was greater when soil pH increased. Concentrations corresponding to 50% inhibition (I50 values) obtained from the dose–response curves were correlated with soil pH, demonstrating the relationship between soil pH and sulfentrazone phytotoxicity. Dissipation of sulfentrazone was examined in soils incubated at 25 °C and moisture content of 85% field capacity. Sulfentrazone dissipation followed a two-compartment model, and sulfentrazone half-lives estimated from the dissipation curves ranged from 21 to 111 days. Half-lives were correlated with soil pH (R = –0.857, p = 0.014) and soil organic carbon content (R = 0.790, p = 0.034) but not with clay content (R = 0.287, p = 0.533). Soil characteristics, particularly soil pH and organic carbon content, affect the bioactivity of sulfentrazone and influence both sulfentrazone efficacy in weed control and its potential for carry-over injury to subsequent crops.

Determination of Thiencarbazone in Soil by Oriental Mustard Root Length Bioassay

Abstract Using an oriental mustard root length bioassay, thiencarbazone bioavailability and dissipation in five Saskatchewan soils was investigated under laboratory conditions. Thiencarbazone bioavailability was assessed at 0 to 3.9 µg ai kg−1. Thiencarbazone concentrations corresponding to 50% inhibition (I50 values) obtained from dose-response curves varied from 0.56 to 1.71 µg kg−1. Multiple regression analysis indicated that organic carbon content (P  =  0.018) and soil pH (P  =  0.017) predicted thiencarbazone bioavailability. Thiencarbazone dissipation was examined in soils incubated at 23 C and moisture content of 85% field capacity. Thiencarbazone half-lives estimated from dissipation curves were 9 to 50 d, and organic carbon content (P  =  0.002) and soil pH (P  =  0.008) were significant factors affecting thiencarbazone dissipation. Thiencarbazone bioavailability decreases and dissipation rate is slower in Canadian prairie soils of high organic matter content and low soil pH. Because root length of oriental mustard plants also was reduced by ammonium, therefore ammonium-containing or -producing fertilizers can cause false positive results for thiencarbazone soil residues. Canaryseed roots had the same sensitivity to ammonium as oriental mustard roots but were not inhibited by thiencarbazone. Therefore canaryseed root length bioassay was effective in identifying inhibition caused by ammonium toxicity. Use of oriental mustard root and canaryseed root bioassays together can aid in interpreting bioassay results for detection of thiencarbazone residues. Nomenclature: Thiencarbazone, oriental mustard, Brassica juncea (L.) Czern. ‘Cutlass’; canaryseed, Phalaris canariensis L. ‘CDC Togo’.

Application of a Laboratory Bioassay for Assessment of Bioactivity of ALS-Inhibiting Herbicides in Soil

Acetolactate synthase (ALS) herbicides inhibit the biosynthesis of branched chain amino acids (valine, leucine and isoleucine) in sensitive plants. The ALS-inhibitor group of herbicides includes sulfonylureas, imidazolinones, triazolopyrimidines, pyrimidinyl oxybenzoates , and sulfonylamino carbonyl trizolinones. They control a wide spectrum of broadleaf weeds and grasses and are commonly used in cereal and pulse crops, soybean and rice. Tolerant plants rapidly metabolize ALS-herbicides to an inactive product while sensitive plants show little or no metabolism of ALS-herbicides (Sweetser et al., 1981). ALS inhibition is a biological pathway that exists only in plants and not in animals, and therefore the ALS-inhibiting herbicides are considered to be safe (Colborn & Short, 1999). Because of the very high plant toxicity of ALS-inhibiting herbicides to susceptible plants, the application rates of these herbicides are remarkably low, typically between 3 to 150 g ai/ha (Senseman, 2007) making these herbicides environmentally attractive. The bioavailability of ALS-herbicides to plants is soil dependent, and the efficacy in weed control may decrease in soils of high organic matter and clay content and low pH. Dissipation of ALS-herbicides varies greatly with environmental conditions, soil characteristics and type of herbicide. Although the half-lives are relatively short, the small residual quantities remaining in soil may be of agronomic concern due to the high potency of these herbicides at low concentrations. The expected levels of soil residual ALS-inhibiting herbicides one year after application are at or below one part per billion concentrations. Addressing concerns regarding possible damage to successive crops requires the ability to detect extremely low concentrations of these herbicides in soil.

Relationship of soil properties to pyroxasulfone bioactivity in a range of prairie soils

ABSTRACT The relationship between pyroxasulfone bioactivity and soil properties has not been investigated in a wide range of soils typical of western Canada. In this study, 47 soils from Saskatchewan, Manitoba and Alberta, with varying organic matter content (1.5%–22.1%), pH (5.0–7.9), and clay content (6.8%–59.4%) were used to evaluate the effect of soil properties on pyroxasulfone bioactivity and its relevance to field application rates. Bioactivity was assessed by measuring the reduction of sugar beet shoot length after 7 days in response to 0, 92, 184, and 368 µg ai kg−1 pyroxasulfone concentration in soil. Multiple regression analysis showed that pyroxasulfone bioactivity was related to soil organic matter content, pH and clay content. Grouping the soils according to these properties allowed for a summarization of pyroxasulfone field application rates required to achieve bioactivity based on the magnitude of sugar beet shoot length inhibition (%). The estimated field application rates ranged from less than 120–480 g ai ha−1.

Assessment of Wild Mustard (Sinapis arvensis L.) Resistance to ALS-inhibiting Herbicides

There is an urgent need for rapid, accurate, and economical screening tests that can deter‐ mine if weeds surviving a herbicide application are resistant. This chapter describes de‐ velopment and application of a simple root length bioassay technique for detection of wild mustard (Sinapis arvensis L.) resistance to ALS-inhibiting herbicides. This bioassay was performed in 2-oz WhirlPak® bags filled with 50 g of soil wetted to 100% moisture content at field capacity. Wild mustard seeds were pre-germinated in darkness in Petri dishes lined with moist filter paper for 2 days. Six seeds with well-developed radicles were planted in the non-treated soil and in soil with added herbicide, and plants were grown in a laboratory under fluorescent lights. After 4 days of growth, WhirlPak® bags were cut open, soil was washed away, intact plants were removed, and root length was measured with a ruler. The concentration of each herbicide in soil at which a significant root inhibition of susceptible biotype, but no root inhibition of a resistant biotype occur‐ red was selected. Susceptibility/resistance of wild mustard populations was estimated by calculating the percentage of uninhibited roots of plants grown in the herbicide-treated soil as compared to the plants grown in the non-treated soil.

Use of Sugar Beet as a Bioindicator Plant for Detection of Flucarbazone and Sulfentrazone Herbicides in Soil

Determination of herbicide residues can be challenging due to the very low herbicide concentrations that can persist and remain bioactive in soil. Detection of residual herbicides is of great importance since these miniscule herbicide amounts may cause injury to sensitive rotational crops. Plant bioassays are a valuable alternative to instrumental procedures for determination of herbicides in soil. Instrumental methods such as gas chromatography or high performance liquid chromatography require solvent or solid phase extractions before sample analysis, and these highly efficient extractions enable the determination of total amount of herbicide in soil. In contrast, bioavailable herbicide is determined by bioassay procedures because plant response varies with soil type and generally decreases in soils of high organic matter and clay contents and low soil pH (Thirunarayanan et al. 1985; Renner et al. 1988; Che et al. 1992; Wang & Liu 1999; Wehtje et al. 1987; Grey et al. 1997; Szmigielski et al. 2009). Typically bioassay detection of herbicides that belong to different groups with different modes of action requires use of different plant species and/or measuring different plant parameters. Use of herbicides with different modes of action applied either in rotation or as pre-mixed combinations has become a common practice in farming to combat weed resistance problems. Thus performing more than one bioassay may be necessary for assessment of herbicide residues in soil after field applications of herbicides with different modes of action.