We determined feeding preferences of invertebrate seed predators and the effect of seed predation on weed emergence. Feeding choice studies were completed with three species of common ground beetles: (Amara aenea DeGeer, Anisodactylus sanctaecrucis F., and Harpalus pensylvanicus DeGeer) (Coleoptera: Carabidae) and the northern field cricket (Gryllus pennsylvanicus DeGeer) (Orthoptera: Gryllidae). Anisodactylus sanctaecrucis, H. pensylvanicus, and the female and male G. pennsylvanicus consumed more redroot pigweed seeds compared with giant foxtail seeds; A. aenea seed consumption did not differ between these two weed species. All invertebrates consumed fewer velvetleaf seeds compared with redroot pigweed and giant foxtail seeds; however, when seed biomass was compared, A. aenea consumed similar biomass of velvetleaf, giant foxtail, and redroot pigweed, whereas A. sanctaecrucis and H. pensylvanicus consumed greater biomass of velvetleaf compared with giant foxtail seed. Seed burial depths of 0.5 or 1.0 cm reduced redroot pigweed and giant foxtail seed consumption by A. aenea and A. sanctaecrucis but not by the larger carabid beetle, H. pensylvanicus. In a greenhouse study, A. sanctaecrucis decreased total weed emergence by 15%, and G. pennsylvanicus females and males decreased weed emergence by 16 and 5%, respectively. Emergence of redroot pigweed, but not velvetleaf or giant foxtail, decreased when A. sanctaecrucis and the male G. pennsylvanicus were present, whereas the emergence of all three weed species decreased in the presence of the female G. pennsylvanicus. In field experiments, vertebrate access to velvetleaf seeds reduced emergence from 4 to 9% across field sites; invertebrate access reduced emergence 4 to 6%. Vertebrate access to giant foxtail seeds reduced emergence 3 to 7%, and invertebrate access reduced emergence 4 to 13%. These results suggest that predation of weed seeds by both vertebrates and invertebrates may reduce weed emergence and influence the weed community.

Competition studies were conducted in Brazil during 1972 and 1973 between high populations (160 plants/0.1 m2 at 5 to 7 weeks after planting) of purple nutsedge (Cyperus rotundus L.) and the following vegetable crops: garlic (Allium sativum L. ‘Mineiro’), okra (Hibiscus esculentus L. ‘UFV 1152’), carrot (Daucus carota L. ‘Nantes’ and ‘Kuroda’), bush-type green bean (Phaseolus vulgaris L. ‘Topcrop’), cucumber (Cucumis sativus L. ‘Aldai’), transplanted cabbage (Brassica oleracea L. var. capitata ‘Louco’) and transplanted tomato (Lycopersicon esculentum Mill. ‘Santa Rita’). Purple nutsedge grew all year with irrigation, although growth was greater during the warm, wet season (October to March). Crop losses due to purple nutsedge competition during the entire cropping season were as follows: garlic 89%; okra 62%; two carrot cultivars, ‘Kuroda’ and ‘Nantes’ 39% and 50%, respectively; green bean 41%; cucumber 43%; cabbage 35%; and tomato 53%. Critical periods of purple nutsedge competition occurred between 3 and 13 weeks for garlic; 3 and 7 weeks for okra, cucumber and the carrot cultivar ‘Nantes’; 3 and 5 weeks for tomato and the carrot cultivar ‘Kuroda’; and at approximately 4 weeks for cabbage and green bean. Purple nutsedge competed for light in the slow-growing, non-competitive crops and for nutrients in all crops. Competition for water was reduced because the vegetables were irrigated regularly. The rate of leaf area development for a competitive crop, green bean, was similar to the rate for purple nutsedge, whereas the rate was much lower for the non-competitive okra.

A number of redroot pigweed and Powell amaranth populations from various locations in Ontario, Canada, have distinct patterns of resistance to the acetolactate synthase–inhibiting herbicides imazethapyr and thifensulfuron. This suggested the presence of diverse ALS gene mutations among these populations. Seven polymerase chain reaction primer pairs were used to amplify the gene to obtain full sequence information and to determine the identity of resistance-conferring mutations. There was a high degree of similarity in the ALS gene of the two species with only five nucleotides and one amino acid differing. A total of four herbicide resistance-conferring mutations were identified in the two species. The Ala122Thr, Ala205Val, and Trp574Leu amino acid substitutions were found in redroot pigweed whereas Ala122Thr, Trp574Leu, and Ser653Thr were detected in Powell amaranth. The pattern of resistance known to be conferred by the mutations concurred with the resistance level observed at the whole plant level. Distinct mutations being found in geographically separated populations suggest that selection for resistance occurred simultaneously in different locations. It reinforces the fact that resistance to ALS inhibitors is easily selected and that growers need to take this into account when formulating weed management strategies.

Several inbred lines of acetolactate synthase (ALS)-inhibiting herbicide-resistant (ALS-R) Palmer amaranth and ALS-susceptible (ALS-S) common waterhemp were developed in the greenhouse. Interspecific hybrids were obtained by allowing several ALS-S common waterhemp females to be pollinated only by ALS-R Palmer amaranth in a growth chamber. Putative hybrid progeny were treated with an ALS-inhibiting herbicide, and the hybrid nature verified using a polymorphism found in the parental ALS gene. Polymerase chain reaction (PCR) was used to amplify a region of the ALS gene in both parental plants and putative hybrids. Restriction enzyme digestion of the ALS-R Palmer amaranth PCR fragment resulted in two smaller fragments, whereas the PCR fragment in the ALS-S common waterhemp was not cut. Restriction digestion of the putative hybrid PCR fragment showed a combination of ALS-R Palmer amaranth double fragments and an ALS-S common waterhemp single fragment. Approximately 4 million flowers were present on 11 common waterhemp females and produced about 44,000 seeds that appeared viable. From the approximately 3,500 putative hybrid seedlings that were screened, 35 were confirmed as hybrids using herbicide resistance as a phenotypic and molecular marker. The data collected here verify that interspecific hybridization does occur between these two species, and even at a low rate, it could contribute to the rapid spread of ALS resistance in these species.

The weedy Setaria species (giant, green, yellow, knotroot, and bristly foxtail) compose one of the worst weed groups interfering with world agriculture and in other disturbed and managed habitats. These species, together with their crop counterparts (foxtail millet, korali), form the foxtail species-group (spp.-gp). Five successive waves of Setaria spp. invasion from preagricultural times to the present have resulted in widespread infestation of the disturbed, arable, temperate regions of the earth. These invasions have resulted in considerable economic and environmental costs. The success of the Setaria spp.-gp is because of their intimate evolutionary relationship with humans, disturbance, agriculture, and land management. The ability to adapt rapidly to local conditions is the hallmark of this weedy group. Genotypic and phenotypic biodiversity provides this spp.-gp with traits that allow it to invade, colonize, adapt to, and endure in a wide range of habitats around the world. The phenotypic life-history traits important to the success of weedy Setaria spp. begin with the induction of dormancy in seed during embryogenesis. The formation of long-lived, heterogeneous seed pools in the soil is the inevitable consequence of the dormant seed rain. In soil seed pools, after-ripening, the occurrence and timing of seedling emergence, and the induction of secondary (summer) dormancy are regulated by seasonally and diurnally varying soil oxygen, water, and temperature signals. Precise and variable timing of seedling emergence ensures Setaria a dominant place in disturbed and managed communities during the growth and reproductive phases that follow. Once established in a community, phenotypic plasticity inherent in an individual weedy Setaria sp. allows it to maximize its growth, form, and reproduction to the specific local conditions it encounters, including competitive interactions with neighbors. Traits controlling the plastic development of plant architecture include the ability to form one or more tillering shoots whose stature and number are precisely sized to local conditions. A complex pattern of branching, from plant to spikelet, provides diverse microenvironments within which different levels of dormancy are induced in individual seeds on a panicle and among panicles on a common plant. Traits for adaptation to stress in weedy Setaria spp. include tolerance to many inhibitory chemicals (e.g., herbicides, salt), mechanical damage, and drought. Genetic traits such as self-pollination and small genome size contribute to a highly diverse collection of locally adapted genotypes and phenotypes ready to exploit any opportunities provided by a cropping system. Self-pollinating Setaria spp. exist in wild, weed, and crop variants, an ideal genetic condition ensuring both long-term stability and novelty. Weedy Setaria spp. populations have low to exceedingly low amounts of total genetic variation, unusually low intrapopulation genetic diversity, and unusually high genetic diversity between populations compared with an average plant species. These traits result spatially in local populations that are unusually homogeneous, typically consisting of a single multilocus genotype. Either a generally or a specifically adapted genotype of an individual species might predominate in that local population. Across the landscape, different single-genotype populations dominate particular local sites, providing novel genetics to the region by dispersal and gene flow when conditions change. Across North America, populations of green foxtail and knotroot foxtail are genetically differentiated along a north–south gradient. The history of invasion and colonization, the successful life histories of locally adapted weedy Setaria spp., and the evolutionary potential of this weed group emphasize the need for accurate prediction of its behavior. Weedy Setaria spp. management is the management of local selection pressure and consequent adaptation. Farmers, land managers, policy makers and regulators, homeowners, and consumers need accurate information about weedy Setaria spp. to predict and guide management decisions based on economics, risk, and environmental sustainability.

A field study was conducted during 2000 and 2001 at Stoneville, MS, to determine the effects of isopropylamine, trimethylsulfonium (Tms), diammonium, and aminomethanamide dihydrogen tetraoxosulfate (Adt) salt formulations of glyphosate on weed control, growth, chlorophyll content, nodulation, nitrogen content, and grain yield in glyphosate-resistant soybean and to assess potential glyphosate accumulation in soybean nodules. Glyphosate-Tms and glyphosate-Adt injured soybean, and visible injury ranged from 29 to 38% 2 d after late postemergence (LPOST) application; however, soybean recovered by 14 d. Glyphosate formulations had no effect on chlorophyll content, root and shoot dry weight, or nodule number but reduced nodule biomass by 21 to 28% 14 d LPOST. Glyphosate levels in nodules from treated plants ranged from 39 to 147 ng g−1 (dry weight), and leghemoglobin content was reduced by as much as 10%. Control of five predominant weed species 14 d after LPOST was > 83% with one application and > 96% with two applications regardless of the glyphosate salts used. Soybean yields were generally higher with two applications than with one application regardless of glyphosate formulation. These results indicate that soybean injury and inhibition of nodule development with certain glyphosate formulations can occur, but soybean has the potential to recover from glyphosate stress.

Glyphosate is often applied with diammonium sulfate to increase weed control. However, many other salts in the spray carrier have antagonized glyphosate phytotoxicity. Research was conducted with wheat as a bioassay species to further determine the influence of various salts on glyphosate phytotoxicity. Cation antagonism of glyphosate occurred with iron > zinc > calcium ≥ magnesium > sodium > potassium. Ammonium cation with hydroxide or most other anions was not antagonistic. Anions of ammonium compounds were of primary importance in overcoming glyphosate antagonistic salts, while the ammonium cation was neutral or slightly stimulatory with certain anions. Sulfate, phosphate, citrate, and acetate anions were not antagonistic, but nitrate and chloride anions were slightly antagonistic when applied as ammonium salts or acids. Antagonism of glyphosate action by sodium bicarbonate and calcium chloride was overcome by phosphoric, sulfuric, and citric acid and phosphate, sulfate, and citrate ammonium salts. Acid and ammonium salts of nitrate and chloride were more effective in overcoming sodium bicarbonate than calcium chloride antagonists of glyphosate. Ferric sulfate antagonism was overcome only by citric, partly by phosphoric and sulfuric but not by nitric and hydrochloric acids or their ammonium salts. Acetic acid, ammonium acetate, and ammonium hydroxide did not overcome any salt antagonism of glyphosate. Glyphosate response to salts was independent of spray carrier pH.

Three approaches to data analysis were compared to describe competitive interactions between wheat and Italian ryegrass. Replacement series were performed using the two species at total densities of 100, 200, and 400 plants/ m2, and separate monoculture experiments for each species at densities from 33 to 800 plants/m2. Approaches to data analysis included: 1) conventional analysis of replacement series experiments, 2) development of synthetic no-interaction responses from monoculture experiments for comparison with results from mixtures, and 3) responses of the reciprocal yield of individual plants to variation in densities of the two species. Wheat was the superior competitor to ryegrass; however, the three approaches varied in ability to quantify this competitive relationship. The conventional replacement series analysis was least sensitive in describing the influences of either density or proportion on the plant association. The synthetic no-interaction approach provided the most detailed analysis of the influence of proportion on the species interaction. The reciprocal yield approach provided the simplest and most sensitive analysis of the joint influences of density and proportion. The latter approach also provided the most quantitative analysis of the influence of density on the species interaction. Plant density and species proportion are important variables for interpreting the process of plant competition.

Three empirical crop yield loss models were used to describe the interference of redroot pigweed and Powell amaranth populations with soybean. Data were obtained from field experiments conducted in 1992 and 1993. Pigweed densities of 0 to eight plants m−1 were established within the soybean row. Pigweed sowing dates were selected so that weed seedling emergence coincided with VE, VC, and V2 soybean growth stages within the time frame of the critical weed-free period. The model incorporating pigweed density and time of emergence gave the best description of soybean yield loss in comparison to the two relative leaf area models. This model was fit to a combined data set of percent yield loss because parameter estimates did not differ among locations and years. Estimated soybean yield losses decreased from 16.4 to 0.5% with delayed pigweed emergence from 0 to 20 degree days. Leaf area of pigweed relative to soybean encompassed pigweed density and time of emergence. Relationship between relative leaf area and soybean yield loss was best described by the one-parameter model estimating a relative damage coefficient ‘q’ than the two-parameter model that also estimated maximum expected yield loss. The relative damage coefficient ‘q’ decreased with later times of leaf area assessment but could be predicted with one leaf area observation. Empirical models that incorporate time of weed emergence represent a step toward improving predictions of yield loss. This is important for the selection of cost-effective weed control strategies.

The substituted 2,6-dinitroanilines were first reported as herbicides in 1960. At the present time, there are 14 2,6-dinitroaniline herbicides in various stages of product development. Literature on absorption, translocation, and mode of action of the dinitroaniline herbicides refers almost entirely to trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) and nitralin [4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline]. These herbicides do not directly inhibit the germination of seed. Inhibition of lateral root development is the most characteristic growth response. Swelling of the root tip is a universally recognized morphological effect caused by these compounds. The cells in this region are multinucleate, indicating that the mitotic process has been disrupted. Examination of these cells shows an increase in the percentage of cells in arrested metaphase due to the disruption of spindle microtubules. Inference is made that the dinitroaniline herbicides affect chromosomes in a manner similar to colchicine, but research with pig brain (Sus scrofa) microtubules has shown that effects of the herbicide on microtubular protein is different.

Injury to the top of plants is recognized by stunting, development of dark green color, and swelling and brittleness of the stem or hypocotyl. The major sites of uptake are the shoot of monocots and the hypocotyl or hypocotyl hook for dicots. Exposure of seedling roots to the dinitroaniline herbicides does not kill the plant, but exposure of the young shoot will result in death. Available data indicate that the dinitroaniline herbicides are either absorbed or adsorbed by the roots due to the proximity of the roots to the herbicide, but that translocation from the root to the top is minimal. When a dinitroaniline was found in the plant, the parent compound was the major product present with metabolites equal to 5% or less.

Insufficient information is available to accurately describe effects of these herbicides on plant carbohydrates, lipids, and nitrogenous compounds. The effect of trifluralin on RNA and DNA varies with the plant species, treatment time, concentration of herbicide, and plant part investigated, but differences observed do not correlate with dinitroaniline-susceptible and resistant species. Enzyme activity does not appear to be greatly inhibited by the dinitroaniline herbicides. However, the dinitroaniline herbicides interfere with photosynthesis and respiration in vivo and in vitro.

The phytotoxic action of trifluralin can be antidoted with the organophosphorus insecticides, phorate [0,0-diethyl-S-(ethylthiomethyl)-phosphorodithioate], disulfoton [0,0-diethyl-S-2-(ethylthioethyl)-phosphorodithioate], and externally applied lipids such as D-α-tocopherol. There is a strong correlation that seeds with a high lipid content are resistant to trifluralin and those with a low lipid content are susceptible. Despite all the work that has been conducted, the mode of action of the dinitroaniline herbicides is still unclear.

Weed seedling emergence is partially dependent on biotic and abiotic conditions directly surrounding the seed. When environmental conditions are appropriate, seed germination and emergence occurs. We studied the impact of seeding depth (surface, 1 to 2, 3 to 4, and 6 to 7 cm) and fluctuating soil moisture regimes (field capacity [FC]–1/3FC–FC; FC–1/6FC–FC) on percent weed emergence in a greenhouse. At FC, wild mustard and field pennycress had the greatest percent emergence when seeds were placed on or near the soil surface, whereas percent emergence of barnyardgrass and round-leaved mallow was unaffected by seeding depth. All the perennials tested had the greatest percent emergence at FC when seeds were placed near or on the soil surface, except for common milkweed which only emerged below the soil surface. When soil moisture levels fluctuated, surface seeds of barnyardgrass, catchweed bedstraw, green foxtail, wheat, and wild oat had less emergence than seeds below the soil surface; field pennycress had increased emergence when the seeds were placed on the surface; and round-leaved mallow and wild mustard emergence was unaffected by seeding depth. The emergence of curly dock, dandelion, and perennial sowthistle was unaffected by seeding depth, whereas foxtail barley and quackgrass emergence was reduced when seeds were placed on the surface and soil moisture fluctuated.

Knowing the interference potential of common waterhemp in corn could be beneficial in planning waterhemp management strategies. In 2000, 2001, and 2002, field studies were conducted to examine both early- and late-season common waterhemp interference in corn. Early-season interference was determined by removing common waterhemp at the VE (vegetative emergence), V4 (four visible leaf collars), V6, V8, V10, V12, and V14 growth stages of corn for the entire season, and late-season interference was determined by allowing common waterhemp to emerge and compete from the VE, V4, V6, V8, V10, V12, and V14 corn growth stages. The interference potential of common waterhemp varied between the year 2000 and the combined years of 2001–2002. This is probably due to differences in precipitation in May and June in these two environments (297 mm in 2000 compared with 198 mm in 2001–2002). An excess of 590 g m−2 of dry matter and 13,000 and 1,200 seeds per female plant were produced when common waterhemp emerged at V4 and V6 corn, respectively, the 2 yr that corn was drought stressed. When corn was not moisture stressed, common waterhemp that emerged at V4 and V6 corn produced less than 220 g m−2 and less than 500 seeds per female plant. Season-long common waterhemp interference reduced corn yield 74% in 2 yr of the study and 11% in the third. Early-season common waterhemp interference began at V6 corn, with a 4 and 23% yield loss in 2000 and 2001–2002, respectively. Common waterhemp interference from late-season emergence reduced corn yield when emergence occurred before the V8 corn growth stage. Taking into account early- and late-season common waterhemp interference. the critical common waterhemp–free period was around the V6 corn stage to optimize corn yield.

The effect of 4,6-dinitro-o-sec-butylphenol(DNBP) and several phenylureas, s-triazines, chlorinated phenoxyacetic acids and chlorinated benzoic acids on the photolytic activity (Hill reaction) of chloroplasts isolated from turnip greens (Brassica spp.) was studied. Photochemical activity was measured potentiometrically with ferricyanide as the electron acceptor.

For the various members of each herbicidal group close agreements were obtained between inhibitions expressed against the Hill reaction and their relative phytotoxicities, except for the chlorinated benzoic acids. Of the five phenylureas tested, 1-n-butyl-3-(3,4-dichlorophenyl)-1-methyrurea (neburon) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) were most inhibitory and 3-phenyl-1,1-dimethylurea (fenuron) was least inhibitory to the Hill reaction. 2-Chloro-4,6-bis(isopropylamino)-s-triazine (propazine) and 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) were the most inhibitory and 2-chloro-4,6-bis(diethylamino)-s-triazine (chlorazine) was the least inhibitory of seven s-triazines to the Hill reaction. DNBP inhibited the Hill reaction by 50 percent (I50) at 1.3 × 10-5 M. An I50 value of 5.1 × 10-4 M was obtained for 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) which was the most inhibitory of the chlorinated phenoxyacetic acids tested. 3,4,5-Trichlorobenzoic acid (3,4,5-TBA) was the most inhibitory chlorinated benzoic acid tested (I50 #x2A75; 2 × 10-3M).

I50 values were established and compared for chloroplasts isolated from turnip greens, corn and soybeans for diuron, simazine, DNBP, isopropyl N-(3,4-dichlorophenyl)carbamate (3,4-DCIPC), isopropyl N-(3-chlorophenyl)carbamate (CIPC), 2,4,5-T and 3,4,5-TBA. Chloroplasts from the three species were essentially equally sensitive to each of the seven herbicides. Hence, no evidence was obtained which would explain species tolerance or susceptibility.

When turnip-green chloroplasts were treated with inhibitory concentrations of diuron, simazine, DNBP, CIPC, 2,4,5-T and 3,4,5-TBA in the dark, the herbicides could be removed by washing without seriously impairing the photolytic activity of the chloroplasts. However, irreversible changes occurred when the chloroplasts were illuminated while in contact with the herbicides, i.e., photolytic activity was not completely restored when the chloroplasts were washed, except with simazine.

Substitutive (replacement) and partial additive experimental designs, with their underlying models, remain the two most popular techniques in weed-crop competition studies, despite considerable criticism of these approaches in the recent literature. We review standard designs for two-species competition experiments and demonstrate the advantages of a reciprocal yield model applied to data from an additive series experiment, using mixtures of Japanese millet and tomato. A traditional replacement series analysis failed to provide a general model of competition among these two species over several total plant densities, while an application of a reciprocal yield (inverse linear) model to the same data was successful. This technique allows evaluation of the influences of both weed on crop and crop on weed, as well as the partitioning of net competition effects into intra- and interspecific components. One Japanese millet plant was competitively equivalent to 3.7 tomato plants, as measured by effects on tomato biomass, while one tomato plant was equivalent to 0.14 Japanese millet plants, as measured by effects on millet biomass. Skewness of per plant biomass distribution is shown to be a result but not an unambiguous measure of competition. Expansion of a reciprocal yield model to mixtures of more than two species is illustrated using three species of duckweed. While some caution is recommended, the reciprocal yield analysis applied to data from appropriately designed experiments is a substantial improvement over more traditional methods.

Field experiments were conducted to measure the grain yield of wheat (Triticum aestivum L. ‘ANZA’) at various wild oat (Avena fatua L. ♯ AVEFA) and wheat plant densities. Wheat yield declined as wild oat plant density increased. Wheat yield increased in wild oat-infested plots as wheat plant density increased. Regression models were developed to describe the combined effect of wheat and wild oat plant densities on wheat grain yield. Wheat yields were best described by a nonlinear regression model using the relative density of wild oat in the weed-crop stand as the dependent variable. Inclusion of crop stand as a competitive factor significantly improved the fit of all regression models tested. Wild oat were more competitive against wheat in these experiments than in experiments reported by others. Possible reasons for differences in observed competitiveness are discussed.

The rates of degradation of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one], the isopropyl analog [4-ammo-6-isopropyl-3-(methylthio)-as-triazin-5(4H)-one], and the cyclohexyl analog [4-amino-6-cyclohexyl-3-(methylthio)-as-triazin-5(4H)-one] in soil were determined under laboratory and field conditions. First-order kinetics best described the rate of degradation of the three herbicides. At 5 and 20°C, metribuzin and the isopropyl analog degraded more slowly than the cyclohexyl analog. At 35°C, the rates of degradation for all three herbicides were similar. The calculated half lives were approximately 329, 44, and 16 days for the three herbicides at 5, 20, and 35°C, respectively. Rates of 1.12 and 2.24 kg/ha, applied in the field, remained in the upper 5 cm of the soil profile, and the pattern of herbicide degradation followed that found in the laboratory study at 20°C.

Resistance to imazethapyr was identified in a population of common sunflower that had been treated with imazethapyr for seven consecutive years. The imazethapyr-resistant biotype of common sunflower was approximately 170 times more resistant to imazethapyr than the susceptible biotype based on the rate required for 25% control. Resistance was due to altered acetolactate synthase (ALS) that is less sensitive to imazethapyr. The imazethapyr concentration required to inhibit in vitro ALS activity by 25% was 210–fold higher in the resistant biotype than in the susceptible biotype. Differences in absorption, translocation, and metabolism of imazethapyr in common sunflower biotypes were not sufficient to explain the resistance to imazethapyr.

Crenate broomrape is a major constraint for legume production in Mediterranean and East Asian countries. Resistance to this parasitic weed is scarce in many legumes but is common in chickpea germ plasm. A detailed in vitro study has shown that resistance in chickpea is the result of the combination of at least two mechanisms. First, and most importantly, the two chickpea lines studied have been identified with low rates of stimulant production. Once germination is induced by exogenous applications of the synthetic germination stimulant GR24, thus overcoming the primary resistance mechanism in these lines, a second resistance mechanism is apparent. This is evidenced by a darkening of host cell tissue in contact with the broomrape radicle, leading to failure of establishment, which was frequently observed in the chickpea accessions. Anatomical studies have shown that this apparently “hypersensitive” response does not correspond with the death of host cells in contact with the parasite cells but corresponds to blocking and death of the penetration structures of the parasite.

Common ragweed is a major problem in white bean production systems in Ontario. The influence of time of emergence and density of common ragweed on white bean growth and seed yield was examined in Ontario at Elora in 1990, and at Woodstock and Staffa in 1991 and 1992. Ragweed emerged with white bean seedlings (VE) and at the second trifoliate stage of white bean (V3). Time of ragweed emergence and weed density affected white bean yield at all locations. When 1.5 ragweed seedlings m−1 of row emerged at the VE stage of crop growth 10 to 22% seed yield loss occurred. Yield losses of 4 to 9% occurred when 1.5 ragweed seedlings m−1 of row emerged at the V3 crop stage. Yield loss parameter estimates, i.e., the predicted weed-free crop yield (YWF) and the maximum yield loss (A), varied among locations and with time of ragweed emergence, whereas the parameter for yield loss at low weed density (I) was more consistent across all locations and times of weed emergence. Although I values were relatively consistent across locations and times of ragweed emergence, the standard errors associated with each estimate were large. White bean leaf area index, above-ground biomass and pod number m−2 were affected most by ragweed interference. White bean density, number of seeds per pod, and seed weight per plant were not affected by ragweed interference. Ragweed emerging at VE and V3 produced a maximum of 6000 and 1000 seeds m−2, respectively. Time of ragweed emergence may be more important than weed density when evaluating weed control options.

The influence of soil pH on the uptake, degradation, and movement of chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} in soil was examined. Phytotoxicity decreased as pH increased in a silty clay loam with an adjusted pH range of 5.9 to 7.5. Fresh weights of sorghum [Sorghum bicolor (L.) Moench. ‘Funks G-499 GBR’] shoots grown in soil at pH 7.5 and containing 0.8 or 1.6 ppbw chlorsulfuron were not different from the controls while large differences occurred at pH 5.9. Uptake of 14C-chlorsulfuron by wheat (Triticum aestivum L. ‘Centurk’) in a silty clay loam with a pH of 5.9 was 67 to 100% greater than at pH 7.5. Soil pH strongly influenced the degradation rate of chlorsulfuron in a laboratory incubation study. Chlorsulfuron half-life in a silty clay loam was 1.9 weeks at pH 5.6 and 10 weeks at pH 7.5. Soil thin-layer chromatography indicated a high leaching potential for chlorsulfuron, and mobility increased as soil pH was increased.