Current weed demography models were reviewed to evaluate how the effects of cultural practices on weed dynamics were integrated into the models and to suggest possible ways to improve the simulation of cropping system effects. Several models were chosen to illustrate the interactions between cropping systems and weed dynamics. The first one described the structure of the weed life cycle. The second model integrated the effects of a wide set of cultural practices; the comparison of this example with other models suggested how the integration of cropping system effects could be improved. The last two models introduced the interactions of cultural practices with intraplot weed variability, either spatial variability of weed densities or genetic and phenotypic variability within weed populations. This review indicates some ways to make weed population models more comprehensive, robust, and accurate in order to improve their contribution to the evaluation and management of cropping systems.

Atrazine is the most widely used herbicide in the U.S. and has been detected in surface water and groundwater. Technologies are needed for onsite and in situ remediation of water and soil containing atrazine. We investigated the potential of using fine-grained, zero-valent iron (Fe0) to remove atrazine and promote its degradation in contaminated water and soil. Atrazine loss from aqueous solution increased with increasing Fe0 concentration (w/v). Agitating 20 μg 14C-ring-labeled atrazine L−1 with 10% Fe0 (w/v) removed 92% of the 14C from solution within 48 h. Only about 4% of the 14C lost from solution was extractable from the iron with 3 mM CaCl2 (readily available pool), 81% was extractable with CH3CN (potentially available pool), and 11% was unextractable residues. A companion experiment indicated that most of the 14C extracted from the iron with 3 mM CaCl2 after the 48-h Fe0 treatment was unaltered atrazine, while the CH3CN extract contained approximately 33% atrazine and 48% was unidentified atrazine transformation products. Treating a highly contaminated solution (20 mg atrazine L−1) with 20% Fe0 (w/v) removed 88% of the 14C (added as 14C-ring-labeled atrazine) from solution within 48 h. Deethylatrazine was the main atrazine transformation product detected in solution after treatment, but small amounts of deisopropylatrazine, didealkylatrazine, and hydroxyatrazine were also found. Treating Sharpsburg surface soil containing 1 mg atrazine kg−1 with Fe0 (2%, w/w) increased atrazine mineralization from 4.1 to 11.2% after 120 d. Pyrite (4% FeS2, w/w) also increased atrazine mineralization in surface soil, but was less effective in the presence of NO3− or SO42− (100 mg kg−1 soil). Adding 2% Fe0 (w/w) and 100 mg NO3− kg−1 to contaminated subsurface soil increased atrazine mineralization from 0.4 to 8.2% within 120 d, and unextractable residues increased from 4.6 to 9.8%. These results indicate iron can sorb atrazine and promote its transformation in water and soil.

Field studies were conducted at Goiânia, GO, Brazil, on an Oxisol (clayey, kaolinitic, isothermic, Typic Haplustox) and at Jussara, GO, Brazil, on an Oxisol (loamy sand, kaolinitic, isothermic, Typic Haplustox) during 1995 and 1996 to determine the carryover effect of fomesafen, imazamox, and acifluorfen, applied to edible bean, on rotational crops (maize, sorghum, rice, and millet) and to estimate the level of soil residues under Brazilian Savanna conditions. Averaged across locale, year, and rate, fomesafen dissipation time (DT50) (37.5 d) was longer than acifluorfen (27.5 d) and imazamox (25.9 d). For both locations, soil herbicide persistence (average of herbicides) was longer in 1995 than in 1996. This was due to higher soil moisture content in 1996. The sensitivity of rotational crops to fomesafen and imazamox residues was, in decreasing order: sorghum, corn, millet, and rice, and for acifluorfen: sorghum, corn, rice, and millet. The period between herbicide application and rotational crop planting (PAP) varied in agreement with the sensitivity of rotational crops to herbicide residues in soil and the persistence of the herbicide. Considering both location and year, the PAP for fomesafen (250 g ai ha−1) ranged from 69 to 132 d for corn, 114 to 179 d for sorghum, 29 to 95 d for rice, and 52 to 111 d for millet; the PAP for imazamox (40 g ai ha−1) ranged from 68 to 111 d for corn, 78 to 139 d for sorghum, 25 to 75 d for rice, and 40 to 102 d for millet; and the PAP for acifluorfen (170 g ai ha−1) ranged from 56 to 89 d for corn, 96 to 139 d for sorghum, 61 to 95 d for rice, and 43 to 82 d for millet.

Following 15 yr of successful use, glyphosate failed to control a population of the widespread grass weed rigid ryegrass in Australia. This population proved to be resistant to glyphosate in pot dose-response experiments conducted outdoors, exhibiting 7- to 11-fold resistance when compared to a susceptible population. Some cross-resistance to diclofop-methyl (about 2.5-fold) was also observed. Similar levels of control of the resistant and susceptible populations were obtained following application of amitrole, chlorsulfuron, fluazifop-P-butyl, paraquat, sethoxydim, sirnazine, or tralkoxydim. The presence of glyphosate resistance in a major weed species indicates a need for changes in glyphosate use patterns.

A new extraction method for the HPLC-UV determination of metsulfuron in soil was developed and compared with NaHCO3 extraction and a lentil bioassay technique. The new procedure consisted of metsulfuron extraction using anion exchange membranes followed by a dichloromethane concentration of the membrane eluates. Three soils representing different landscape positions were spiked with metsulfuron to yield concentrations in the range of 0 to 40 μg kg−1 dry soil and were analyzed by the above procedures. At a given spike rate, the efficiency of the anion exchange membrane extraction was highest for the soil from the upper slope position and lowest for the soil from the lower slope position, indicating that the amount of ion-exchangeable sulfonylurea was affected by soil properties. Similar trends in degree of root growth inhibition were observed for lentil response to metsulfuron presence in soil; percent root growth inhibition was related to the landscape position as the amount of membrane-extractable metsulfuron. The percent shoot growth inhibition was not soil dependent and did not differ among soils. The efficiency of NaHCO3 extraction for metsulfuron did not vary with soil type; in the NaHCO3 method, because of the buffered alkaline nature of the extraction, differences in soil properties, particularly soil pH, would have less effect on metsulfuron recovery. Because of similar trends for the results of the membrane extraction method and a lentil root bioassay, anion exchange membrane extraction may provide useful information on bioavailable fractions of sulfonylurea herbicides in soil.

Greenhouse studies were conducted to determine purple and yellow nutsedge response to selective placement of a 5-cm layer of pyrithiobac sodium-treated soil above, below, or above and below nutsedge tubers. Pyrithiobac sodium at 36 or 72 g ae ha−1 applied postemergence as foliar, soil, or foliar and soil treatments also was evaluated. Pyrithiobac sodium applied above, below, or above and below nutsedge tubers reduced yellow and purple nutsedge shoot number, shoot regrowth, and root-tuber dry weight at least 90%. Foliar-only treatment of pyrithiobac sodium was less effective at reducing emerged purple and yellow nutsedge numbers than application to soil only or to foliage and soil. Best reduction in yellow and purple nutsedge growth with pyrithiobac sodium was obtained with soil-incorporated treatments.

Sulfentrazone sorption kinetics, desorption, and mineralization were evaluated in surface 7.5 cm of soils collected from long-term conventional-till (CT) and no-till (NT) plots. The soils used were Miami silt loam and Drummer silty clay loam from Illinois and Dundee silt loam from Mississippi. Sulfentrazone sorption kinetics in Dundee silt loam CT and NT soils were adequately described by a simple two-site equilibrium/kinetic model. Rapid initial sorption (within 1 h) was followed by a slower sorption and equilibrium, largely achieved by 72 h of shaking, with a negligible increase in sorption thereafter. The sorption Kf ranged from 1.02 to 3.44 among the six CT and NT soils. The Kf values were greater for NT compared to their respective CT soils. Overall, Kf values were higher in Drummer silty clay loam followed by Dundee silt loam and Miami silt loam soil. The N values were less than unity in all soils indicating nonlinear sorption. Sulfentrazone desorption was hysteretic with a very low rate of desorption. The total amount desorbed in four desorptions ranged from 58 to 72% of that sorbed. Less than 2.1% of applied 14C-sulfentrazone was mineralized to 14CO2 in Dundee silt loam CT and NT soils during a 77–d incubation. Relatively low mineralization of sulfentrazone suggests poor adaptability of native microbial populations that have not been exposed to this herbicide. Higher sorption and lower desorption of sulfentrazone in NT soils compared to CT soils suggest that NT systems (which tend to increase plant residues) may prolong sulfentrazone residence time in soil.

A bioassay using red beet root length indicated an increase in imazethapyr bioactivity in Berryland sand (BLS) as soil pH increased from 3.7 to 6.5. Increasing pH above 6.5 had no effect on imazethapyr bioactivity. The lowest imazethapyr concentrations detected by the bioassay at pH 6.5 were 0.5, 1, 2.5, and 5 to 10 μg kg−1 in acidwashed quartz sand, BLS, Aura loamy sand, and muck soil, respectively. After application of 0.07 kg ae ha−1 with 12.5 mm of simulated rainfall at 3-d intervals, imazethapyr remained in the 0 to 15 cm soil in BLS soil columns. Under this rainfall regime, the herbicide moved 30 cm deep 5 mo after application, but residues decreased significantly in surface soil. Imazethapyr bioactivity was highest and mobility was lowest at low soil temperature (10 C) and low moisture (4 to 6% w/w). Imazethapyr movement was monitored successfully using the red beet root length bioassay, and an herbicide residue map was developed to show the pattern of imazethapyr bioactivity in soil columns.

The effect of herbicides applied to V3 soybeans on race 3 soybean cyst nematode (SCN) reproduction and glyceollin production in roots was measured. Soybeans were treated postemergence with 1 × and 2 × rates of herbicides plus adjuvants or with adjuvants alone, and SCN development was measured. Acifluorfen, bentazon, lactofen, crop oil concentrate (COC), and nonionic surfactant (NIS) applications reduced SCN egg population densities 50 to 60% compared with the untreated control 4 and 8 wk after application. The SCN reproduction on plants treated with fluazifop-P, sethoxydim, and imazethapyr was similar to the untreated control. Crop oil concentrate or NIS applications alone were as effective as acifluorfen, bentazon, or lactofen applications for reducing SCN reproduction. However, no additive effect of adjuvant-herbicide combinations was observed, nor did herbicide rate affect SCN reproduction. Treatments reduced SCN reproduction only when applied to soybeans and had no effect on SCN reproduction when applied directly to the soil. No treatment stimulated SCN reproduction relative to the untreated control. Soybeans treated with COC, NIS, acifluorfen, and bentazon also had more glyceollin detected than the untreated control. Herbicide-induced glyceollin production may have increased the resistance of soybean to SCN.

Herbicides have been used to control tall larkspur to prevent cattle deaths. There is some evidence that tall larkspur may become more toxic and more palatable during the desiccation process following herbicide treatment. A series of studies were conducted to measure toxic alkaloid concentration in tall larkspur following treatment with metsulfuron, picloram, and glyphosate. Herbicides were applied to larkspur plants grown in a greenhouse (1990), to field plots infested with larkspur (1993), and to individual larkspur plants in the field (1995). Leaves were harvested up to 14 or 28 d following treatment, and toxic alkaloids were measured. Metsulfuron increased toxic alkaloid concentration in the 1990 greenhouse study and in 1993 field plot studies, and increased both concentration and absolute amount of toxic alkaloids in 1995 individual plant treatments. Alkaloids in plants treated with picloram either remained unchanged or declined slightly in all three studies and were similar to control plants. In the 1995 study, glyphosate caused the relative concentration of alkaloids to increase as the plants desiccated, but the absolute amount of alkaloids was similar to control plants. Picloram and glyphosate did not reduce alkaloid concentration; thus, the risk of poisoning remains until plants desiccate. Metsulfuron increased alkaloid concentration, thus increasing the risk of poisoning. Cattle should not be allowed to graze sprayed areas until larkspur desiccates and withers.

Greenhouse experiments were conducted to determine the persistence of imazapyr and the tolerance of a wide variety of bioassay plants to imazapyr residues in central Texas soils. Eleven bioassay crops were planted in pots 1 d before and 2, 6, and 12 mo after imazapyr application at rates of 0, 0.02, 0.04, 0.07, 0.14, 0.28, 0.56, and 1.12 kg ai ha−1. Imazapyr was phytotoxic 12 mo after treatment to all crop plants in at least one rate. Forage grasses and herbs were highly susceptible to imazapyr residual carryover damage, as was corn and cabbage. Bean and squash were tolerant of imazapyr residues. No statistical differences occurred in imazapyr phytotoxicity to corn grown in leached vs. nonleached pots.

Field studies were conducted from 1993 through 1995 to determine potential reductions in herbicide use and associated cost savings by utilizing a weed-sensing sprayer, named Detectspray, to control weeds throughout the fallow season and to control perennial weeds after crop harvest. The Detectspray system gave comparable weed control to conventional broadcast spraying on 80% of the application dates and reduced glyphosate/dicamba use over the fallow season by 19 to 60%. This reduced herbicide use resulted in cost savings of $6 to $32 ha−1. A fallow treatment that combined two herbicide applications with the Detectsprayer plus one to two wide-blade tillage operations was less costly than conventional tillage at two of 11 sites and at all sites retained more surface crop residues to reduce the risk of erosion. Postharvest glyphosate use on quackgrass with the Detectsprayer was reduced 50 to 78% compared to broadcast applications and resulted in cost savings of $16 to $25 ha−1. Clopyralid use on Canada thistle with the Detectsprayer was reduced 71 to 80%, with cost savings of $44 to $50 ha−1. The Detectspray system is a useful tool to effectively manage weeds in conservation fallow and reduced tillage cropping systems.

Experiments were conducted in the field and greenhouse to evaluate the effects of cultivar and sulfentrazone rate on soybean injury and yield from soil-applied sulfentrazone and to determine soybean inheritance of sulfentrazone tolerance. Excessive rainfall and cool growing conditions in 1996 enhanced sulfentrazone injury and caused yield reduction in the susceptible cultivars ‘KS4895’ and ‘Hutcheson’ at 0.56 kg ai ha−1 and KS4895 at 0.42 kg ha−1. Yields of tolerant cultivars ‘Deltapine 3478’ and ‘Manokin’ were not reduced. Sulfentrazone at 0.42 kg ha−1 reduced soybean stand 17 and 35% for tolerant and susceptible cultivars, respectively. Field and greenhouse inheritance studies were conducted on plant families developed from two crosses: Manokin (tolerant) by ‘Asgrow A4715’ (susceptible) and ‘Northrup King S59–60’ (tolerant) by KS4895 (susceptible). The sulfentrazone tolerance of Manokin and Northrup King S59–60 appeared to be controlled by a single gene, with tolerance being dominant to susceptibility. The allelic relationship of tolerance from these two sources needs to be determined.

Biomass abundant in chlamydospores obtained by liquid fermentation of mycoherbicidal strains of Fusarium oxysporum was incorporated into alginate prills with various food substrates and granular formulations such as corn flour : starch, wheat flour : kaolin, rice : wheat flour, and rice : wheat gluten formulations. These fungal strains cause vascular wilts in coca (Erythroxylum coca var. coca) and in poppy (Papavar somniferum). Fungal strain, formulation method, and interaction of these two parameters significantly affected the shelf life of the formulations at room temperature as well as the ability to form secondary propagules on 1% water agar. In bioassays, there were no significant differences among F. oxysporum strain EN4-S formulations in their ability to colonize root tissue or the rhizosphere of E. coca seedlings. The study indicates that most of the formulations meet at least three of the four criteria important for biological control agents to be effective: low losses in viability during the formulation process, satisfactory shelf life at room temperature, abundant secondary chlamydospore formation, and rhizosphere colonization.