Research was initiated to determine the periodicity of emergence, seasonal plant growth and reproductive potential, and response to selected preemergence (PRE) and postemergence (POST) herbicides for burcucumber. Under Indiana conditions, burcucumber germinated from late April to October and was stimulated by periodic rainfall. Relative growth rates of plants were greatest up to 10 wk after establishment and declined when flowering was initiated. Without competition, early spring (May)–established burcucumber plants attained a fresh weight of up to 86 kg and produced almost 80,000 seeds. With later establishment, less biomass and and a smaller number of seeds were produced. Seedlings emerging up to mid-August produced germinable seed prior to frost, thus indicating that season-long control strategies are needed to minimize reproduction. A PRE application of atrazine, metribuzin plus chlorimuron, or linuron plus chlorimuron provided greater than 90% visual control up to 8 wk after treatment (WAT). Greater than 80% visual control was obtained with POST applications of glyphosate or with combinations of glyphosate plus dicamba or glyphosate plus 2,4-D; chlorimuron, metribuzin plus chlorimuron; or paraquat. Both imazaquin and bentazon provided less than 70% control. Results indicate that burcucumber displays rapid development with periodic germination throughout the growing season and is capable of producing large amounts of plant biomass and seed.

Trials were carried out to investigate the effects of seed burial depth on seedling emergence rate of 20 weed species. Marked depth-mediated variation in emergence ability of the different species was observed, together with a general pattern of decreasing emergence with increasing soil depth. At 10 cm, only johnsongrass, velvetleaf, catchweed bedstraw, and cutleaf geranium emerged, albeit only in limited numbers. Species most severely inhibited by burial depth were buckhorn plantain, large crabgrass, common purslane, chickweed, and corn spurry, none of which emerged from beyond 6 cm. In all species, depth-mediated inhibition was found to be sigmoidal (polynomial regression). In addition, the number of seedlings and rate of seedling emergence decreased when depth of burial increased. The depth at which the number of emerged seedlings was halved varied by species and ranged from 3.6 cm for common purslane and chickweed to 7 cm for velvetleaf and catchweed bedstraw. Excessive burial depth generally induced dormancy (in roughly 85% of cases) rather than suicide germination. A close inverse relation (second-degree equation) between seed unit weight and depth-mediated inhibition was observed. The physiological involvement of depth inhibition in seed bank ecology is discussed.

Greenhouse studies determined dose responses of winter annual weeds and winter wheat to preemergence (PRE) and postemergence (POST) treatments of MKH 6561 and the residual effects on kochia. MKH 6561 at 11 to 45 g ha−1 did not affect wheat. MKH 6561 at 11, 34, and 45 g ha−1 reduced winter annual weed densities compared to an untreated control. Weed growth was inhibited as the rate of MKH 6561 was increased, but downy brome, cheat, and Japanese brome were three to six times more susceptible than jointed goatgrass. The GR70 values for cheat, downy brome, Japanese brome, and jointed goatgrass were 7, 11, 4, and >45 g ha−1, respectively. Dry weights of kochia seeded after removal of the winter annual grasses decreased as MKH 6561 rate increased, with average control being 77% compared to the nontreated control. Regression analysis indicated that kochia was controlled 80% with MKH 6561 at 28 g ha−1.

Field experiments were conducted at four locations in the Central Pacific region of Costa Rica between 1994 and 1996 to determine suitable tactics for integrated control of propanil-resistant junglerice in rain-fed rice. Stubble incorporation within 3 mo after rice harvest did not affect the density of junglerice that emerged with the crop at the beginning of the rainy season at any location. However, the elimination with glyphosate of the first junglerice seedling population emerging before rice planting consistently reduced the in-crop infestation of junglerice and resulted in increased grain yields. The positive effect of eliminating the first junglerice flush remained even after the in-crop treatments were applied and lasted after rice harvest. Substitution of the two customary applications of propanil (3.8 kg ha−1 each) with a single application of pendimethalin (0.75 to 1.5 kg ha−1), preemergence or early postemergence, also reduced junglerice infestation and improved grain yield. Both propanil, in mixture with the synergist piperophos, and quinclorac controlled propanil-resistant junglerice and increased grain yields. Control of the initial junglerice population and improved in-crop weed management can serve as the basis for integrated management of herbicide-resistant junglerice in rain-fed rice.

Pseudomonas fluorescens strain D7 (P. f. D7; NRRL B-18293) is a root-colonizing bacterium that inhibits downy brome (Bromus tectorum L. BROTE) growth. Before commercialization as a biological control agent, strain D7 must be tested for host plant specificity. Agar plate bioassays in the laboratory and plant–soil bioassays in a growth chamber were used to determine the influence of P. f. D7 on germination and root growth of 42 selected weed, cultivated or native plant species common in the western and midwestern United States. In the agar plate bioassay, all accessions of downy brome were inhibited by P. f. D7. Root growth of seven Bromus spp. was inhibited an average of 87% compared with that of controls in the agar plate bioassay. Root growth of non-Bromus monocots was reduced by 0 to 86%, and only 6 out of 17 plant species were inhibited 40% or greater. Among all plant species, only downy brome root growth from two accessions was significantly inhibited by P. f. D7 in plant–soil bioassays (42 and 64%). P. f. D7 inhibited root growth and germination in agar plate bioassays more than in plant–soil bioassays. Inhibition in plant–soil bioassays was limited to downy brome, indicating promise for P. f. D7 as a biocontrol agent that will not harm nontarget species.

In studies conducted from 1991 through 1994, researchers investigated the effects of several crop rotations and herbicide programs on crop yield and populations of common lambsquarters, common ragweed, Amaranthus spp., and jimsonweed at two sites. Crop rotations included continuous corn, continuous soybean, corn–soybean, and corn–tomato–soybean, and herbicide programs were the split-plots and included continuous use of acetolactate synthase (ALS)–inhibitor herbicides, continuous use of non–ALS-inhibitor herbicides, annual rotations between ALS- and non–ALS-inhibitor herbicides, combinations of ALS- and non–ALS-inhibitor herbicides in the same year, and no herbicide. Weed control and weed populations generally were affected by an interaction between crop rotations and herbicide programs. After 4 yr, common lambsquarters control was lowest, and populations were highest where fomesafen was used alone for four consecutive years or in rotation with other herbicides. Although common ragweed populations were low at site 2, control at both sites was generally lowest from treatments that included only ALS-inhibitor herbicides. Common ragweed populations were highest at site 1 in 1992 and 1993 following continuous applications of ALS-inhibitor herbicides. Jimsonweed populations were also low at site 2, but control at site 1 in tomato was low. Jimsonweed control from fomesafen and the combination of butylate plus atrazine in soybean and corn, respectively, was variable. Amaranthus spp. populations decreased as the study progressed, and in 1993, control was over 90% from all treatments, except in the case of the treatment combining butylate plus atrazine. Corn and soybean yields varied with year and site, and yields of these crops and tomato were related to rainfall and weed control.

The effect temperature, light intensity, time to initial light exposure, relative humidity, and the presence of dew have on CGA-248757 and flumiclorac efficacy was evaluated in laboratory trials. Increasing temperature from 10 to 40 C increased CGA-248757 and flumiclorac activity on common lambsquarters by 79 and 87%, respectively. Similarly, increasing temperature from 10 to 40 C increased CGA-248757 and flumiclorac activity on redroot pigweed by 68 and 60%, respectively. Increasing light intensity from 0 to 1,000 μmol m−2 s−1 increased CGA-248757 activity on common lambsquarters and redroot pigweed by 92 and 93%, while flumiclorac activity increased 91 and 99%. Time to initial light exposure and relative humidity did not affect CGA-248757 or flumiclorac activity on common lambsquarters and redroot pigweed. The presence of dew reduced herbicidal activity of both compounds on common lambsquarters by 5% and redroot pigweed control with CGA-248757 and flumiclorac by 21 and 20%, respectively. Field applications of CGA-248757 or flumiclorac at 6:00 A.M., 2:00 P.M., and 10:00 P.M. indicate environmental conditions at application strongly influence soybean tolerance and weed control with CGA-248757 and flumiclorac. The greatest soybean injury occurred from CGA-248757 or flumiclorac applications at 6:00 A.M. compared with applications at 2:00 P.M. or 10:00 P.M. Common lambsquarters control was greatest when CGA-248757 or flumiclorac was applied at 6:00 A.M. or 2:00 P.M. compared with 10:00 P.M. However, redroot pigweed control was greatest when CGA-248757 or flumiclorac was applied at 2:00 P.M. Application time of day did not affect velvetleaf control with either herbicide.

Improved fallow systems are needed to reduce the negative effects of increased soil erosion and reduced soil quality. Field experiments were conducted to determine weed suppression attained with yellow sweetclover grown as a green manure fallow replacement crop. Yellow sweetclover was undersown in field pea, flax, or Indian mustard and then killed in June of the following fallow year. Living yellow sweetclover competed strongly with weeds during the first fall and spring of fallow. Weed biomass accounted for <1 to 12% of the total plant biomass when yellow sweetclover was terminated in June. Yellow sweetclover residues remaining after termination of growth continued to provide excellent weed suppression. Weed densities in April before planting the succeeding wheat crop were 75 to 97% lower in yellow sweetclover than in untreated fallow treatments. Yellow sweetclover controlled the perennial weeds dandelion and perennial sowthistle, as well as the annuals kochia, flixweed, Russian thistle, and downy brome. Weed suppression was similar whether yellow sweetclover was harvested as hay or its residues were incorporated or left on the soil surface, suggesting that a portion of the weed suppression effect may be due to allelopathic compounds being released from decomposing yellow sweetclover. Results will be used to develop more sustainable agronomic practices in regions where fallow is still widely employed.

The adoption of precision technologies that spatially register measurements using global positioning systems (GPS) greatly facilitates conducting large-scale on-farm research by farmers. On-farm experiments that utilize producer equipment include variations in agronomic practices that occur in situations where we want to predict the effect of inputs on yield. The domain of inference for such on-farm studies therefore more closely matches that desired by researchers. To investigate the feasibility of on-farm research using GPS, a study was conducted to evaluate the potential benefit of site-specific weed management. The study utilized producer-maintained field-scale equipment on four Montana farms in dryland spring wheat production. Paired site-specific and whole-field herbicide treatment areas were established in 0.9 to 1.9-ha blocks using consultant weed maps and a geographic information system (GIS). Yield was unaffected by herbicide treatment strategy (site-specific or broadcast). Minimal detectable yield differences were evaluated for the experimental design (0.2 T ha−1). Net returns increased when the percentage of field infested by wild oat decreased. Visual ratings of wild oat density taken at harvest indicated no difference in wild oat control between treatments in two of four site-years. This research suggests that producer-owned equipment can be used to compare treatments, but the accuracy and subsequent power of such comparisons are likely to be low.

A single dominant mutation conferring resistance to aryloxyphenoxypropionate (AOPP) and cyclohexanedione (CHD) herbicides was incorporated into a quantitative model for the population development of wild oat. The model was used to predict the times required to develop field resistance in a number of different scenarios. Field resistance was defined as a threshold of four plants m−2 surviving herbicide treatment, and in most scenarios, a very large proportion of these plants were resistant. The model predicts that plow cultivation could delay the development of resistance relative to tine cultivation. With an initial seed bank of 100 seeds m−2 and annual use of AOPP/CHD herbicides, which kill 90% of susceptible but no resistant plants, field resistance develops in 15 yr with annual tine cultivation 10 cm deep but only after 23 yr with annual plowing 20 cm deep. The model predicts that herbicide rotation can dramatically increase the times required for field resistance to develop in a tine cultivation system. With annual use of AOPP/CHD herbicides, field resistance develops in 15 yr, whereas using alternative modes of action 1 in 2 yr delays field resistance to 28 yr. The model predicts that resistance can be delayed for at least 66 yr if three herbicides, each with a different mode of action, are rotated and each herbicide causes 90% mortality. The model predictions on the number of years required for field resistance to develop are not highly sensitive to the initial density of the seed bank (range modeled = 102 to 104), the mutation rate for resistance (10−4 to 10−7 per generation), the rate of outcrossing (0.1 to 100%) or the herbicide kill rate (80 to 95%).

Pyrithiobac control of Palmer amaranth on the Texas Southern High Plains was correlated previously with temperature at the time of application. In the present study, the thermal dependence of pyrithiobac efficacy was used to define a thermal application range (TAR) for postemergence pyrithiobac applications. Several years of temperature data from four cotton-growing regions of the United States were analyzed with respect to the TAR to determine the extent to which temperature limitations could affect pyrithiobac applications. Temperatures outside the TAR occurred in all years and regions analyzed. Analyses of four geographic regions utilizing 4 to 11 yr of data for each region indicated the following percentages of hours inside the TAR: Lubbock, TX, 54 to 94%; Maricopa, AZ, 27 to 33%; Raleigh-Durham, NC, 70 to 97%; and Jackson, MS, 81 to 99%. A detailed analysis of the frequency and duration of the TAR in Lubbock, TX, showed that, periodically, temperatures outside the TAR may limit the efficacy of postemergence pyrithiobac applications for several consecutive days. Finally, the TAR was shown to be useful as a postapplication diagnostic tool for evaluating herbicide applications that resulted in poor efficacy. These results suggest that long-term evaluation of historic temperatures with respect to the TAR for a given herbicide may provide insight into the potential limitations of herbicide efficacy and underscore the potential utility of developing TARs based on field and laboratory analyses of herbicide thermal dependence.

Soil organic carbon (OC), clay content, water content, and pH often influence the bioactivity of soil-applied herbicides, and these soil properties can vary greatly within fields. The purpose of this work was to determine the influence of within-field soil heterogeneity on the efficacy of RPA-201772 where corn, shattercane, and velvetleaf were seeded as bioassays. An experimental approach was developed to quantify RPA-201772 dose–response across a range of soil conditions in an agricultural field. Based on a logistic model, crop injury was quantified with the I20 parameter, the dose eliciting 20% greenness reduction, using a series of photographic standards. Weed biomass was quantified with the I80 parameter, the dose eliciting 80% biomass reduction, relative to the untreated control. Crop and weed responses varied by two orders of magnitude. Significant correlation, as high as 0.76, was observed between measures of plant response and soil properties, namely particle size and OC. Furthermore, native velvetleaf spatial distribution at the study site was heterogeneous, and seedlings were observed in plots where seeded velvetleaf biomass was high. Spatial heterogeneity of soil affinity for herbicide results in differential weed fitness and contributes to weed “patchiness.”

Understanding spatial distribution has become increasingly important in weed science. Seed dispersal, both between and within agricultural fields, is an important component of weed spatial distribution. Analysis of the effect of dispersal between fields has been relatively neglected in theoretical studies of weed population dynamics. In this paper, we present a simple landscape-level model of the influence of seed dispersal on winter wild oat population dynamics between fields. In the model, two fields are interconnected, with seeds being carried from one field to another as would occur when seeds are carried by field equipment or in irrigation water. The model is intended to characterize the effect of field-level weed management decisions on landscape-level weed population dynamics. Three scenarios were studied. The first employed no control measures in either field. The second employed annual application of herbicides in field 1 with field 2 receiving no treatment. In the third scenario, an herbicide application took place in field 1 only if the weed population in that field exceeded an action threshold. In the first scenario, the net result of the immigration and emigration processes determined the increase or decrease of the stable plant population. In the second scenario, weeds in the controlled field (field 1) were not driven to extinction as might be expected. The weed populations grew for practically all the dispersal parameter space. Each change in the parameter's values produced a new stable equilibrium. This situation might correspond to a multiplicity of stable states. The uncontrolled field (field 2) experienced an indirect control effect due to the use of control measures in field 1. In the third scenario, we observed an interesting behavior of the populations in both fields. The population in field 1 was not driven under the economic threshold, and both fields showed complex dynamics within defined combinations of migration and emigration values.

A population model of itchgrass was developed for a typical corn–based cropping system in the Pacific coastal region of Costa Rica. Field experiments were conducted to quantify density-dependent seedling mortality and fecundity. Additional information required for the model was obtained from the literature. Effect of control methods on itchgrass density—including a leguminous cover crop (velvetbean), a preemergence herbicide (pendimethalin), and classical biocontrol with the head smut—alone and in combination, were investigated using the model. According to model results, the cover crop planted at high and low densities between corn rows was highly efficient, reducing the initial itchgrass density from 54 plants m−2 to 4 and 17 plants m−2, respectively. Associating velvetbean with corn solely in the first crop each year resulted in predicted itchgrass densities of 33 and 36 plants m−2 (at high and low cover crop planting densities, respectively). The improvement in corn yield from preemergence herbicide or biocontrol in addition to the cover crop was only modest. This indicated that if, in practice, the cover crop is as effective as predicted, an inexpensive control tactic such as biological control (provided that an infection rate of at least 50% can be achieved) should be given priority to prevent income losses.

This study was conducted to determine the effects of three commonly applied soybean (Glycine max) herbicides (glyphosate, imazethapyr, and pendimethalin) on the mycelial growth, sclerotial production, and viability of Rhizoctonia solani isolates [anastomosis groups AG-1, AG-2-2, and AG-4] under controlled conditions. Pendimethalin significantly reduced mycelial growth of all three R. solani isolates investigated, whereas effects of the herbicides imazethapyr and glyphosate were not significant. Sclerotial production was affected differently by the three herbicides. Isolates AG-1 and AG-2-2 produced sclerotia both in vitro and in vivo, whereas isolate AG-4 did not produce sclerotia in vitro. In vitro AG-1 isolate showed a decrease, and AG-2-2 isolate showed an increase in sclerotial production in the presence of herbicide. In contrast, both AG-1 and AG-2-2 isolates showed reduction in sclerotial production in vivo compared to AG-4 isolate, which showed an increase in sclerotial production in the presence of herbicides. Sclerotial production was generally higher in vivo than in vitro. The number of sclerotia produced per unit sclerotial weight was often higher in the presence of herbicides. Viability of sclerotia produced in the presence of herbicides was not significantly different from the no-herbicide control.

The objectives of this study were to use a computer simulation model to predict the influence of herbicides and mechanical treatments on giant foxtail population dynamics, annualized net return (ANR), and the giant foxtail economic optimum threshold (EOT) in a corn–soybean rotation over 20 yr. Mechanical treatments were interrow cultivation in corn and rotary hoe in soybean. Herbicides at full (1 ×) and half (½ ×) rates applied alone reduced giant foxtail seedbank 95% within 4 and 8 yr, respectively. Predicted seedbank dynamics had more variability when managed with herbicides at ½ × than at 1 × rates applied alone. Mechanical treatments integrated with herbicide at ½ × rates resulted in giant foxtail seedbank and variability similar to herbicides at 1 × rates applied alone. ANR was maximized when herbicides were applied between ⅜ × and 9/16 × rates applied alone. As initial giant foxtail density increased from 100 to 10,000 seeds m−2, the herbicide rate that maximized ANR increased. Economic optimum thresholds (EOTs) did not vary when herbicides were applied at different rates, but integrating mechanical treatment with herbicides increased the EOT from 0.1 to 0.7 seedlings m−2. Sensitivity analysis determined that giant foxtail seedbank demographics, seedling survival, and seed production per plant had the most influence on model predictions. Model sensitivity varied little between 1 × and ½ × rates. Integrating herbicides and mechanical treatment decreased the sensitivity of the model to perturbations in parameter estimates. Herbicides at reduced rates were more profitable over the long term than 1 × rates, but risk of herbicide failure increased as rate decreased. Integration of herbicides applied at reduced rates with mechanical treatments increased ANR and minimized the risk of herbicide failure compared to herbicides applied at 1 × rates alone.

RPA 201772 is a preemergence herbicide that undergoes rapid conversion to a diketonitrile metabolite (DKN) in soil. The half-life of RPA 201772 is very short, but the half-life of DKN is much longer; hence, DKN remains for an extended time in soil. Sorption studies were conducted with five soils varying in physical and chemical properties using the batch equilibration technique. Analysis of 14C-ring–labeled DKN was performed using liquid scintillation counting, and sorption data were fitted to the Freundlich model. Isotherms of DKN were nonlinear in all the soils as depicted by the Freundlich exponent (n < 1.0), indicating differential distribution of site energies for sorption. Multiple regression of the sorption constants against selected soil properties indicated that soil organic matter content was the best single determining factor of DKN sorption (r 2 = 0.961) followed by soil pH (r 2 = 0.947). The Freundlich sorption coefficient (KF) decreased in the following order Chelsea, MI > Amherst, MA > Moorhead, MN > East Monroe, CO > South Deerfield, MA. The organic matter content of the soils decreased in the same order. Clay content had a minimal effect on the sorption of DKN, whereas the sorption of DKN increased with an increase in organic matter content and a decrease in soil pH. There was an increase in the sorption of DKN with an increase in Ca2+ concentration of the soil solution, whereas the net sorption constant (Kd) was correlated to the organic matter content of the soils. The site energy distribution of DKN sorption was governed mainly by the organic matter content of the soils.

Flumetsulam sorption and mobility studies were conducted on surface (0 to 15 cm) and subsurface (30 to 46 cm) soil of several southern soils. In a batch equilibrium study using a 1:1 ratio of soil–0.01 M CaCl2, flumetsulam adsorbed ranged from 2.9 to 48.7% and from 4.2 to 63.3% on surface and subsurface soils, respectively. Herbicide soil–solution distribution (K d) and organic carbon (K oc) coefficients ranged from 0.03 to 0.95 and from 5.1 to 77.1, respectively, in surface soils and from 0.04 to 1.72 and from 7.5 to 325.5, respectively, in subsurface soils. K d and K oc were correlated with humic and organic matter in surface soils. K d was correlated with extractable Fe, whereas K oc was inversely correlated to pH in subsurface soils. A desorption study using 0.01 M CaCl2 as an extractant on the Captina silt loam surface soil demonstrated that three to four washes were required to desorb more than 94% of the flumetsulam adsorbed over several equilibration times. Mobility studies on soil thin-layer chromatography plates demonstrated that flumetsulam and imazaquin had similar values, ranging from 0.50 to 0.90 and 0.59 to 0.90, respectively, in the surface soils, and both compounds had the same range of mobility in subsurface soils, with R f values between 0.60 and 0.93. At both soil depths, K d and K oc were inversely correlated with the R f of flumetsulam.

Subterranean turions (tubers) of hydrilla lose viability when desiccated. Experimental data showed that freshly collected tubers had a moisture content between 50 and 60% and more than 90% viability. When desiccated, there was an approximate 2% increase in tuber mortality with each percent decline in moisture content. However under field conditions, the tuber bank within the exposed sediments of a northern Texas reservoir showed no decline in number or tuber viability throughout a 12-mo continuous drawdown. Apparently, the buried tubers were never subject to sufficient dessication to damage them. Finally, an experimental pond with an extensive hydrilla tuber bank was manipulated through six flood/drawdown cycles to determine the effects of short-term drawdowns on tuber survival and quiescence. Initially, the pond had a tuber bank of about 676 and 305 tubers m−2 in the shallow and deep zones, respectively. Although the tuber number was reduced to fewer than 15 to 30 tubers m−2 by these repetitive drawdowns, hydrilla tubers were not eradicated from the pond.