High levels of cover-crop residue can suppress weed emergence and also can intercept preemergence herbicides and potentially reduce their effectiveness. This research was conducted in continuous no-tillage corn to compare the effect of residue from a hairy vetch cover crop with that of background crop residue on the soil solution concentration of atrazine and metolachlor and on the emergence of weeds with and without herbicide treatment. In a 3-yr field experiment, 5-cm-deep soil samples were taken and the weed density measured in paired microplots with and without herbicide at approximately weekly intervals after application of atrazine and metolachlor. High levels of residue were present in both treatments; the percentage of soil covered by residue ranged from 91 to 99 in the no–cover-crop treatment and from 99 to 100 in the hairy vetch treatment. Initial metolachlor concentration was lower and degradation rate higher in two of the 3 yr with a hairy vetch cover crop than without a cover crop. Cover-crop treatment had little effect on atrazine concentration or degradation. Annual grass weeds (predominantly fall panicum) were the major species in this field. Hairy vetch alone reduced grass emergence by 50 to 90%, and preemergence herbicides alone reduced emergence by 72 to 93% compared with the treatment without cover crop and herbicide. The combination of preemergence herbicides with hairy vetch provided only 24 to 61% control of grass weeds compared with control by hairy vetch alone and 23 to 52% compared with control by herbicide alone, suggesting an antagonism probably resulting from reduced metolachlor concentration by hairy vetch residue. Metolachlor with hairy vetch delayed emergence of weeds and reduced the concentration of metolachlor required to prevent emergence initiation compared with metolachlor without a cover crop.

A greenhouse experiment used a replacement series design to compare the vegetative growth 6 wk after emergence in pure cultures and mixtures of winter wheat and Italian ryegrass, with phosphorus (P) levels recommended by soil testing. The planting proportions of wheat and Italian ryegrass were 100 and 0%, 75 and 25%, 50 and 50%, 25 and 75%, and 0 and 100%, respectively. There was no alleopathic interaction between the species. Both species in all pure and mixed cultures had substantially less growth in the low-P than in the recommended P treatment. However, the relative performance of the two species differed between P treatments. In the recommended P treatment in pure culture, Italian ryegrass had more tillers and greater root weight and length than wheat. Pure culture wheat in the low-P treatment exceeded pure culture Italian ryegrass in leaf area, weights of leaves, stems, and roots, and root length. Thus, the growth of wheat was inhibited less by P deficiency than the growth of Italian ryegrass in pure culture. In the 50:50 mixture of the recommended P treatment, wheat had greater leaf, stem, and root weights than Italian ryegrass. In the 50:50 mixture of the low-P treatment, the two species were very similar in growth, except that Italian ryegrass had about three times more tillers than did wheat. Whereas P deficiency limited the growth of wheat less than Italian ryegrass in pure culture, P deficiency did not affect the relative competitiveness of Italian ryegrass as much as wheat in mixed cultures. The ability of Italian ryegrass to compete with wheat when P was limiting may result from a difference in root growth. Italian ryegrass had a greater fresh root length to fresh root weight ratio than did wheat in the low-P treatment in pure culture and in the 50:50 mixture. The greater surface area of Italian ryegrass roots likely enhanced the competitiveness of Italian ryegrass relative to wheat under P-deficit conditions. Thus, the use of the recommended P nutrition from soil testing may be a key component to diminish Italian ryegrass competition in wheat fields.

Field research was conducted for 2 yr to determine the effects of reduced rates of bromoxynil on growth and yield of non–bromoxynil-resistant cotton. Rates of 4.5, 9, 17, 35, 70, and 140 g ha−1, representing 0.008, 0.016, 0.031, 0.063, 0.125, and 0.25 fractions of the maximum labeled use rate per application (560 g ha−1), were applied to cotton at the two-, five-, or nine-node growth stage. Visual injury was reduced because application timing was delayed from two- to five-node stage in all experiments and from five- to nine-node stage in two of three experiments. Although negatively affected at all application timings, plant height reduction response decreased with increasing cotton maturity. Plant dry weight was most negatively affected after application at the two-node stage. Bromoxynil application, based on the node above white flower number, did not result in maturity delays but did promote earlier maturity when applied at 140 g ha−1 to two- and five-node stage cotton in one of the three experiments. Final plant population was reduced only at the two- and five-node timings, with response more pronounced at the initial timing. Seedcotton yield after bromoxynil application at the highest rate to two-leaf cotton was reduced 34% compared with other rates and the nontreated control. Bromoxynil applied to five- or nine-node cotton did not significantly reduce yield.

Recent advances in our understanding of plant succession will provide new conceptual tools for wildland weed managers. Some of the conceptual advances should allow better linkages between the general management of wildlands and weed management specifically. For example, in the future we should be more capable of evaluating the impact of a management action on the risk of weed invasion than we have been in the past. Much of the discussion in this paper focuses on soil resource availability as a primary ecological factor in wildland weed management. Much of the research upon which this premise is based is less than 15 yr old; thus, we are in the testing stages of applying these concepts. Wider application of ecological principles to wildland weed management will require a coordinated education program and effective interaction among researchers and managers.

Field studies conducted at three locations in North Carolina in 1998 and 1999 evaluated crop tolerance, weed control, and yield with CGA-362622 alone and in combination with various weed management systems in transgenic and nontransgenic cotton systems. The herbicide systems used bromoxynil, CGA-362622, glyphosate, and pyrithiobac applied alone early postemergence (EPOST) or mixtures of CGA-362622 plus bromoxynil, glyphosate, or pyrithiobac applied EPOST. Trifluralin preplant incorporated followed by (fb) fluometuron preemergence (PRE) alone or fb a late POST–directed (LAYBY) treatment of prometryn plus MSMA controlled all the weed species present less than 90%. Herbicide systems that included soil-applied and LAYBY herbicides plus glyphosate EPOST or mixtures of CGA-362622 EPOST plus bromoxynil, glyphosate, or pyrithiobac controlled broadleaf signalgrass, entireleaf morningglory, large crabgrass, Palmer amaranth, prickly sida, sicklepod, and smooth pigweed at least 90%. Only cotton treated with these herbicide systems yielded equivalent to the weed-free check for each cultivar. Bromoxynil systems did not control Palmer amaranth and sicklepod, pyrithiobac systems did not control sicklepod, and CGA-362622 systems did not control prickly sida.

Cogongrass is widespread in the moist savanna and forest zones of West Africa, where recurrent fires, tillage, weeding, and other farm activities continuously disturb land. Field experiments were conducted in the forest–savanna transition zone of Nigeria from 1996 to 2000 to evaluate the potential of two cover crops (velvetbean and tropical kudzu) for reclaiming land that had been abandoned to cogongrass. Cover crops were grown on the same plots for 3 consecutive yr (1996 to 1998). The control was natural fallow dominated by cogongrass in 1996 to 1998. Corn was planted in all treatments in 1999 and 2000. Total dry matter of cogongrass before the treatments were imposed was 9,000 kg ha−1, and rhizomes contributed 49% of this. At all subsequent sampling dates, plots with cover crops had lower cogongrass shoot and rhizome dry matter than plots without cover crops. Shoot dry matter was reduced to zero 65 wk after planting in both cover crops; rhizome dry matter was reduced to zero after 97 wk in velvetbean plots and after 105 wk in tropical kudzu plots. Corn grain yield was 60% higher in plots with tropical kudzu and 102% higher in plots with velvetbean than in control plots without any previous cover crops. Further research is required to integrate use of cover crops with other control methods for improved cogongrass management.

Ecological management of invasive weeds will require substantial increases in the application of ecological knowledge and its integration with other forms of knowledge. To enable these increases, we call for purposeful development of knowledge networks in which new knowledge about a complex situation is created by the interaction of different forms of knowledge. We believe that invasive-plant management must be based on a fine-tuning of managed ecosystems, in which operations (e.g., farm activities) are comprehensively adjusted to confront invasives with a wide array of control measures. Land managers must have the primary role in this tuning process because of their holistic knowledge of the ecosystems they manage. Additionally, such ecological management of invasives will require support from new or improved practices in many relevant sectors, e.g., involving extension workers, farm advisors, and researchers of many sorts. Knowledge networks facilitate the creation, application, and integration of knowledge that will be needed to support ecological invasive-plant management. Worldwide, knowledge networks are under very active development as promising solutions to ecological-management challenges. To develop, networks require proactive organization and facilitation. We have developed an experimental knowledge network to facilitate ecological management of field-crop weeds on the basis of collaborative learning groups that help farmers and other professionals develop necessary knowledge. These groups have been favorably evaluated by most participants, and this article describes the results of our project, including our insights into development of such networks.

The effects of surfactants on retention, absorption, and efficacy were determined for bromoxynil, 2,4-D amine, and glyphosate on kochia and Russian thistle. Bromoxynil, 2,4-D amine, and glyphosate retention were similar for both species. Surfactants improved spray retention on kochia and Russian thistle compared with spray mixtures without surfactant. Herbicides mixed with allinol 810-60 surfactant were generally retained better than with MON 0818, oxysorbic 20, or R-11 surfactant. Bromoxynil phytotoxicity was not affected by surfactants, and all surfactants equally enhanced 2,4-D amine phytotoxicity. Glyphosate phytotoxicity to kochia was enhanced only by MON 0818 and oxysorbic 20, and phytotoxicity to Russian thistle was enhanced only by MON 0818. Bromoxynil, with or without surfactants, was absorbed similarly by kochia and Russian thistle. 2,4-D amine and glyphosate absorption were greater with surfactants than without. Kochia and Russian thistle leaves had visibly similar crystalline epicuticular wax structure when plants were grown at ≤ 40% or ≥ 80% relative humidity, which did influence absorption of these herbicides.

Sorption and desorption of cyanazine with three Mississippi Delta soils (two silt loams and one silty clay) were studied under laboratory conditions. Cyanazine sorption calculated using the Freundlich equation was greatest for the Sharkey silty clay soil. Partition coefficients (Kd values) for cyanazine sorption ranged from 1.67 to 1.82, 1.92 to 2.15, and 3.65 to 3.96 ml g−1 for the Bosket silt loam, Dubbs silt loam, and Sharkey silty clay soils, respectively. Differences in sorption and Kd values were attributed to clay content. At a given initial cyanazine concentration, cyanazine was desorbed more readily from the silt loam soils than from the Sharkey clay after the first 4-h desorption cycle. Desorption from the Sharkey clay continued for a longer period than that from the silt loam soils, with up to 6% cyanazine desorption from the Sharkey clay after a 16-h desorption cycle compared with 0% for the silt loam soils. Cyanazine losses increased with decreasing clay content, Dubbs = Bosket > Sharkey. This implies a potential relationship between cyanazine desorption and surface runoff losses of cyanazine.

Soil solarization has been proposed as an alternative to methyl bromide for controlling nutsedges. Little is known, however, about the relationship between soil solarization and nutsedge tuber viability. Combinations of elevated temperatures and durations of exposure were evaluated for their effect on purple nutsedge and yellow nutsedge tuber viability and new tuber production in growth chamber studies. Estimates of the duration of exposure at each temperature that reduced nutsedge growth parameters 50% (TT50) were supplied by log-logistic regression analysis. Nutsedge tuber viability was reduced when temperatures were ≥ 45 C. Relative to purple nutsedge, yellow nutsedge tuber viability had smaller TT50 values for 45, 50, and 55 C. Tuber viability TT50 at 60 C was similar for both nutsedges. The TT50 for production of new purple nutsedge tubers at 50 C was larger than that for yellow nutsedge. However, there were no differences between species in TT50 values for new tuber production at higher temperatures. With sufficient durations of exposure, both purple and yellow nutsedge tubers were killed at temperatures ≥ 50 C. However, application of these data to field situations in Georgia may be limited using present technology because the soil temperature cannot be raised to high enough levels for acceptable solarization effects.

Rigid ryegrass and wild radish dominate and coexist throughout southern Australian dryland cropping regions. Widespread herbicide resistance in these species has led to adoption of diverse and complex integrated weed management practices, which require evaluation of their impact on farming systems. Therefore, a multispecies version of the bioeconomic model resistance and integrated management (RIM) has been developed to compare long-term economic and weed population outcomes of various integrated management scenarios. We have extended the original single-species ryegrass RIM model to include wild radish biology and additional weed management practices used to control this species. The multispecies model can be used to evaluate weed management scenarios for coexisting herbicide-resistant species by investigating the implications of different crop–pasture rotational sequences and of varying herbicide availability. Multispecies RIM shows that economic differences between the scenarios are not due to differences in weed densities but to differences in total weed control costs.

In both reduced-tillage and tilled fields, dandelion can be a problem, but the effect of dandelion on canola yield is unknown. This study was conducted to investigate the effect of dandelion infestation on spring canola yield and the influence of tillage on interference and in-field distribution of dandelion. Dandelion distribution was not associated with tillage regimen. Dandelion distribution showed some relationship to past cropping history, for example, the presence of alfalfa in the rotation. The strength of correlation between measures of dandelion infestation level and canola yield loss was associated with tillage regimen. For tilled fields, there was no correlation between reduction in canola yield and any measurements of dandelion infestation level. For reduced-tillage fields, the most reliable measures of dandelion interference level were dandelion ground cover prespray, total dandelion rosette diameter prespray, relative dandelion ground cover prespray, and total dandelion root diameter at crop harvest.

The efficacies of bentazon and fomesafen in controlling annual weeds in dry and edible pod beans in New York State were investigated in greenhouse and field experiments. Dose responses to bentazon and fomesafen were studied for four weed species (ragweed, velvetleaf, eastern black nightshade, and hairy nightshade) under greenhouse conditions. Herbicides were applied at cotyledon to two-, two- to four-, and four- to six–true leaf stages, both with and without a crop oil concentrate (bentazon) or a nonionic surfactant (fomesafen). Field studies were conducted for 2 yr for all weed species except eastern black nightshade, for which no adequate field populations were found. Field studies confirmed greenhouse results, indicating that weed control could be improved by the use of an adjuvant, but there were exceptions. In general, adjuvant usage improved the efficacy of fomesafen more than it did with bentazon. The minimum rates of herbicide required for effective and consistent control was dependent on the particular combination of weed species, herbicide and its rate of application, growth stage at which the application was made, and adjuvant usage.

Research was conducted to determine the effects of management practices and precipitation on herbicide loss in surface water runoff. A field runoff experiment was conducted in 1999 and 2000 in Manhattan, KS. Some plots received only natural precipitation, whereas others received natural precipitation plus additional precipitation from a rainfall simulator. Atrazine was applied at 0.9 and 1.8 kg ha−1, S-metolachlor at 0.7 and 1.4 kg ha−1, and isoxaflutole at 0.05 and 0.11 kg ha−1 to field corn grown under conventional tillage and no-till. Runoff volumes and herbicide concentrations were determined for each runoff event. Across all precipitation, tillage, and placement variables, atrazine, S-metolachlor, and isoxaflutole and diketonitrile (DKN) (soil metabolite of isoxaflutole), hereafter referred to as isoxaflutole/DKN, losses were similar at 5.0, 4.1, and 4.1% of applied, respectively. Additional precipitation increased runoff 2.5-, 2.2-, and 3.4-fold for atrazine, S-metolachlor, and isoxaflutole/DKN, respectively. Preplant soil incorporation reduced atrazine, S-metolachlor, and isoxaflutole/DKN losses in runoff by 67, 69, and 57%, respectively, compared with soil surface applications. Lower preplant rainfall in 2000 resulted in sharply reduced runoff losses despite postplant precipitation similar to that in 1999. These findings suggest that the best management practices for atrazine can be used to manage S-metolachlor and isoxaflutole/DKN loss in surface water runoff.

The effect of droplet size on retention, absorption, and translocation of 14C-glyphosate was studied in glyphosate-resistant corn. Fine, medium, and coarse spray droplets were studied using a track-sprayer equipped with commercially available nozzles. Glyphosate-resistant corn was used to obtain measurements at field use rates in the absence of phytotoxicity. Spray retention on corn leaves was calculated based on recovered glyphosate per leaf area, and retention was higher with application of fine droplets (47%) than with application of coarse (38%) and medium (37%) droplets. Absorption in corn leaves was directly correlated with droplet size and reached a plateau 1 d after treatment (DAT) for all droplet sizes. Based on glyphosate recovered 3 DAT, coarse droplets showed the highest absorption (49%), followed by medium (35%) and fine (30%) droplets. Percentage of translocation also increased with droplet size, and translocation was primarily toward strong sink tissues such as roots and young leaves. Our results show that large droplets have slightly reduced retention in corn but have increased absorption resulting in increased translocation of glyphosate to growing sink tissues.

Field experiments were used to assess how distance mediates the nontarget effect on crepe myrtle by two chrysomelid beetles that were introduced to the United States in 1992 for biological control of purple loosestrife. Previous laboratory tests in Germany and concurrent tests in Oregon showed that although the control organisms can feed on crepe myrtle, they cannot complete development. Therefore, we predicted that negative effects on crepe myrtle would decrease with distance from the purple loosestrife stand. To test this prediction, cohorts of both plant species were transplanted at increasing distances (0, 5, 15, 30, and 50 m) from the colonization source. We found that leaf damage inflicted by the beetles was negatively correlated with increasing distance. Damage was significantly lower at each distance for crepe myrtle plants than for purple loosestrife plants, with a mean difference of 22% and a 95% confidence interval ranging from 12 to 31%. Extensive defoliation of crepe myrtle was limited to within 30 m of the edge of the loosestrife stand. Plant yield was negatively correlated with damage: the closer plants were to the purple loosestrife stand, the greater the suppression of biomass in both plant species. However, loosestrife biomass decreased significantly more quickly than crepe myrtle biomass, with a mean difference in slopes of 0.035 and a 95% confidence interval ranging from 0.022 to 0.048. Our results suggest that release of the Galerucella beetles in North America poses little risk to crepe myrtle. Beetles can feed but cannot complete their life cycle on crepe myrtle, and damage to crepe myrtle approaches zero approximately 50 m from the beetle colonization source.

Field experiments were conducted from 1996 to 2000 near Manhattan, KS, to determine the effects of application timing on atrazine loss in surface water runoff. In addition, Groundwater Loading Effects of Agricultural Management Systems (GLEAMS) was run to compare simulated loss with actual loss in the field. Atrazine treatments were fall plus preemergence (FALL + PRE), early preplant plus PRE (EPP + PRE), PRE at a low rate (PRE-LOW), and PRE at a full (recommended) rate (PRE-FULL). Ridge-till furrows served as mini watersheds for the collection of surface water runoff. Water runoff volumes and herbicide concentrations were determined for each runoff event. Across four sampling years, mean atrazine runoff loss was 1.7, 4.3, and 1.7% of applied for FALL + PRE, EPP + PRE, and the mean of the PRE treatments, respectively. Thus, actual average losses from FALL + PRE and EPP + PRE treatments were somewhat higher than that predicted by GLEAMS. For PRE treatments, actual average losses were significantly lower than that predicted by GLEAMS, with measured losses falling below the bottom of the graph in 3 of 4 yr. These findings suggest that in certain parts of the Great Plains, FALL + PRE split applications of atrazine offer acceptably low atrazine runoff loss potential; EPP + PRE is more vulnerable to loss than FALL + PRE; and the GLEAMS model may overestimate atrazine runoff potential for PRE applications.

Wind-erodible soil sediments are classified as aggregates and particles < 1 mm in diameter. Aggregate size has been reported to influence pesticide retention and behavior in the soil. Atrazine sorption and desorption isotherms were determined using batch equilibration methods for six aggregate sizes of Barnes loam (fine-loamy, mixed, superactive, frigid Calcic Haplodol) and Brandt silty clay loam (fine-silty, mixed, superactive, frigid Calcic Haplodol) soils. Aggregate and particle sizes used in this study ranged from < 0.11 to > 1.7 mm, and atrazine concentrations ranged from 0.65 to 39.2 μmol L−1. The Kf values for the isotherms were calculated using the Freundlich equation. In the Barnes loam, Kf values were strongly positively correlated to aggregate size, particle size, and organic carbon (OC) content (P = 0.01 for each parameter), whereas in the Brandt silty clay loam, Kf values were less correlated to size and OC content (P ≥ 0.1) but were better correlated to clay content and estimated specific surface area (P = 0.05 for each parameter). Desorption was hysteretic with about 18% more atrazine desorbed from smallest than from largest size fractions. At a concentration of 13.0 μmol L−1, the amount desorbed ranged from 9 to 13.7% of the initial amount of adsorbed atrazine after 5 d, whereas at 39.2 μmol L−1, the amount desorbed ranged from 10.3 to 16% of the amount adsorbed. These data indicate that wind-erodible size aggregates and particles could be a source of herbicide contamination of surface water.

Sulfentrazone is a herbicide that has been observed to injure crops in an unpredictable manner. Therefore, experiments were conducted to determine whether root absorption of sulfentrazone was dependent on the pH of the rooting medium. Studies were initiated to examine sulfentrazone uptake of whole plants from soil and hydroponic solution, as well of excised roots in solution. These experiments demonstrated that transpiration decreased as soil pH decreased and herbicide rate increased; it was our intention to use this measure as a description of herbicide injury. Likewise, plants grown for 24 h in 14C-sulfentrazone hydroponic solution accumulated a greater herbicide concentration in roots as solution pH decreased below 6.5. This trend of increased absorption with reduced solution pH was again demonstrated when excised cotton roots were placed for durations of 10 to 120 min in hydroponic solution containing 14C-sulfentrazone. However, when excised roots were placed in solution containing the weak acid herbicide glyphosate, no trend of increased absorption was observed with changes in solution pH. Therefore, it was concluded that the accompanying change in solubility, as sulfentrazone was converted from the ionic to the neutral form, was responsible for the increased absorption by plant roots. Localized differences in soil pH could be responsible for greater sulfentrazone uptake and explain the unpredictable patterns of injury that have been observed.