Laboratory studies were conducted to investigate the processes of imazaquin and imazethapyr degradation in soil. Microbial degradation of 14C-imazaquin and 14C-imazethapyr was monitored by measuring 14CO2 evolution compared to nonsterile soil. 14CO2 evolution was greatest from carboxyl-labeled imazaquin and imazethapyr compared to ring-labeled imazaquin and imazethapyr. A corn root bioassay indicated nearly complete loss of herbicidal activity in nonsterile soil after 5 mo, but herbicide activity was reduced only 14% in sterile soil. 14CO2 evolution more than doubled when soil temperature increased from 15 to 30 C. Total CO2 production responded similarly. Degradation of imazaquin and imazethapyr increased as soil moisture increased from 15% (-2.4 MPa) to 75% of field capacity (-0.03 MPa).

Leaves of coca are the primary source of cocaine. There is interest in using Fusarium oxysporum as a mycoherbicide in addition to other measures to control coca production, but information on coca physiology, including the stress responses of coca leaves, is limited. Deleafing coca plants stimulates rapid production of new leaves, and young expanding leaves readily abscise if treated with ethylene. Commercial preparations of cell wall degrading enzymes, as well as a 24-kDa elicitor from Fusarium oxysporum, induced significant levels of ethylene production by coca leaves. Ethylene pretreatment of coca leaves enhanced the production of ethylene by coca leaves in response to the cell wall degrading enzyme preparation, Driselase, and the 24-kDa elicitor. However, ethylene pretreatment did not enhance the rate of necrosis induced in response to either Driselase or purified 24-kDa elicitor. Driselase failed to elicit levels of necrosis comparable to the 24-kDa elicitor even at 30-fold higher protein concentrations. The response of coca leaves to the 24-kDa elicitor saturated at 6.7 μg ml−1. Age of coca leaves influenced both the level of resulting necrosis and the amount of ethylene produced in response to protein. Very young leaves produced the highest levels of ethylene and necrosis in response to Driselase and the 24-kDa elicitor. The data suggest that responsiveness of coca leaves to control measures may be synchronized over the first few weeks following defoliation.

Efficacy of an indigenous fungus, Exserohilum monoceras, for the control of 3 Echinochloa species was evaluated and compared under both regulated greenhouse and screenhouse (field plots netted with 2 layers of metal mesh screening to exclude vertebrate pests) conditions. Under greenhouse conditions, an inoculum dose of 2.5 × 107 conidia m−2 killed all seedlings of both barnyardgrass and E. glabrescens, whereas an inoculum dose of 5.0 × 107 conidia m−2 was required to obtain 100% mortality of junglerice seedlings. The 1.5-leaf stage of all 3 Echinochloa species was the most susceptible. Increasing inoculum density increased weed control efficacy on younger or older Echinochloa seedlings. The highest level of control was observed for E. glabrescens, less for barnyardgrass, and least for junglerice. Under screenhouse conditions, Exserohilum monoceras caused more than 90% mortality of Echinochloa species when the inoculum was formulated as an oil emulsion or when applied as a dry powder.

Greenhouse and laboratory studies were conducted to determine the effects of dicamba, atrazine, and bentazon on efficacy, foliar absorption, and translocation of MON 12000 or CGA-152005 applied to velvetleaf. The efficacy of MON 12000, CGA-152005, and a combination of CGA-152005 plus primisulfuron applied at 4.5 g ai ha−1 was similar when applied alone or with 140 g ha−1 of dicamba. However, applying these herbicides in combination with 840 or 560 g ha−1 of atrazine or bentazon, respectively, reduced velvetleaf control. Increasing the rate of MON 12000, CGA-152005, or the combination of CGA-152005 plus primisulfuron to 9 g ai ha−1 or replacing crop oil concentrate (COC) with methylated seed oil (MSO) increased velvetleaf control of the atrazine and bentazon combinations but not to levels equal to these herbicides applied alone. Dicamba had no effect on the foliar absorption and translocation of 14C from MON 12000 or CGA-152005. Atrazine had little effect on foliar absorption of 14C from MON 12000 or CGA-152005, but bentazon reduced the foliar absorption of 14C from MON 12000. Replacing COC with MSO increased the foliar absorption of 14C from MON 12000 or CGA-152005 applied alone or with dicamba or atrazine, but not with bentazon. Translocation of 14C from MON 12000 or CGA-152005 out of the treated leaves was 11 and 12%, respectively, averaged across adjuvants and sampling times. These values were reduced to an average of 3 to 4% for both MON 12000 and CGA-152005 when applied in combination with atrazine or bentazon. The majority of 14C from MON 12000 or CGA-152005 was translocated acropetally. Atrazine and bentazon significantly reduced the acropetal translocation of 14C from MON 12000 at 24 and 72 h and for CGA-152005 at 12, 24, and 72 h. The physiological basis for the observed antagonism of MON 12000 and CGA-152005 by atrazine and bentazon appears to be due to reductions in acropetal translocation of MON 12000 and CGA-152005 to velvetleaf meristems.

In response to concern by the U.S. Environmental Protection Agency (EPA) about the health effects of farmworker exposure to 2,4-D and the phenoxy class of herbicides, the U.S. Department of Agriculture (USDA) initiated a benefits assessment in 1993 to determine the economic implications of a complete ban of these materials in agricultural and nonagricultural uses. About 55 million pounds of phenoxy herbicides are used annually in the United States, with 2,4-D comprising 86% of total use or about 47 million pounds of acid equivalent. The severest economic effects would be felt in major field crop applications and in alfalfa forage uses. The total loss of phenoxy herbicides in field crop applications could result in net societal losses, which combine producer and consumer effects of yield, cost, and price changes, approaching $1.2 billion. Greatest yield losses averaged over total planted acres under a total phenoxy herbicide loss scenario would be seen in peanut (13%), alfalfa (5.2%), barley (3.8%), sorghum (2.4%), and wheat (2.2%). Producers of orchard, vineyard, soft fruit, and nut crops could incur losses of $105 million. The estimated aggregate economic impact of losing phenoxy herbicides in the applications included in this paper is a loss of $1.3 billion. These estimates describe the yield and financial impacts of the initial production year after a cancellation. Subsequent years' losses and financial impacts would be less as farmers and markets adjust to the new production situation.

Imazaquin movement in a field site that had Karnak silty clay soil was evaluated during a 2-yr study to determine if leaching was a significant dissipation mode for imazaquin. Imazaquin in water samples collected from subsurface tile drainage was measured by immunoassay. Total imazaquin losses due to leaching in 1993 were 0.8% of the total applied and in 1994, practically zero. Imazaquin concentrations as high as 80 μg L−1 were observed in drain water when rainfall occurred shortly after herbicide application. Imazaquin concentrations were lower with each successive drain flow event. Higher concentrations of imazaquin were associated with higher drainage flow, shortly after application. However, 40 d after application, the levels of imazaquin were low (<7 μg L−1) even when high tile flow occurred. The time between application and the first rainfall is an important factor in imazaquin leaching. In the dry growing season of 1994, imazaquin concentration in drain water did not exceed 5 μg L−1.

Weed suppression in green pea was evaluated when green pea was planted after a fall planting of rapeseed, white mustard, rye, or wheat had been incorporated into the soil in spring. Tests were conducted at Mount Vernon, WA, in 1994 and 1995. Weed suppression in peas varied between different green manure crops. One month after planting, the highest weed population was in green pea following wheat, whereas the lowest was in green pea following rapeseed. Rye and white mustard suppressed early weeds relative to wheat by 25 and 30%, respectively. However, at harvest, weed density was similar in green pea planted after all green manure crops. Weed suppression improved when cultivation or metribuzin at 0.14 kg ha−1 was used in combination with white mustard, rapeseed, or rye. Pea population was not affected by green manure crops, except for rapeseed, which reduced pea population. In greenhouse experiments, white mustard added to the soil at 20 g per 400 g air dry soil reduced emergence of shepherd's-purse, kochia, and green foxtail by 97, 54, and 49%, respectively. Rapeseed suppressed emergence of shepherd's-purse, kochia, and green foxtail by 76, 25, and 25%, respectively.

Research was conducted from 1993 through 1995 to evaluate barnyardgrass control, rice yield, and estimated economic return with POST applications of propanil or propanil + molinate applied alone or with quinclorac. Herbicides were applied under a variety of water management practices and environmental conditions at rates ranging from 1.1 to 3.4, 1.7 to 5.6, and 0.17 to 0.40 kg ai ha−1 for propanil, propanil + molinate, and quinclorac, respectively. Reduced-rate combinations of propanil or propanil + molinate with reduced rates of quinclorac controlled small, actively growing barnyardgrass and provided yields and estimated economic returns similar to combinations of these herbicides at higher rates when irrigated. When herbicides were applied to larger barnyardgrass, propanil + molinate at 5.6 kg ha−1 was more effective than propanil at 3.4 kg ha−1 or quinclorac at 0.40 kg ha−1 applied alone. Propanil + molinate applied with quinclorac at 0.28 or 0.40 kg ha−1 controlled barnyardgrass more effectively and provided higher yields and greater estimated economic returns than propanil at 3.4 kg ha−1, propanil + molinate at 5.6 kg ha−1, quinclorac at 0.17, 0.28, or 0.40 kg ha−1, or combinations of propanil and quinclorac.

Levels of inter- and intrapopulation genetic variation were determined in five North American populations of leafy spurge using chloroplast DNA (cpDNA) RFLPs and RAPD markers. Thirteen plastome types were identified among 123 individuals collected from five geographically separated populations. Number of plastomes within a population ranged from one to seven, with four of the populations having a predominate type plus one or more rarer types. Some plastome types were shared by populations, but plastome distribution among populations was nonrandom. RAPD markers indicated greatest relatedness among individuals within a population. Relatedness among populations as established through RAPDs was greater for geographically closer populations; this relationship was not observed for cpDNA markers. Differences in the range of movement for pollen and seed may account for differences between results of the cpDNA and RAPD analyses. The high degree of genetic variability among North American leafy spurge suggests possible multiple introductions or a high degree of variability within leafy spurge populations in its native range.

The hydrolytic and dealkylation products of cyanazine have been detected in soils, but the sorption of these products in soil has not been well studied. We examined sorption characteristics of five cyanazine degradation products in relation to cyanazine in Norfolk loamy sand, Tunica silty clay, and Dundee silt loam soils. Sorption was determined using a batch equilibrium method. Air-dried soil (3 g) was shaken in 6 ml of solution containing cyanazine or one of its degradation products for 48 h at 4 C. Five concentrations (2.04 to 54.67 μmol L−1) of each chemical were evaluated. The cyanazine Freundlich coefficient (Kf) ranged from 0.64 in Norfolk soil to 4.75 in Dundee no-tillage (NT) soil, and was higher in Dundee NT than in Dundee conventional-tillage (CT) soil. The Freundlich exponent (N) values for cyanazine were less than 0.85 in all soils, indicating nonlinearity of the sorption isotherm. In general, cyanazine sorption among the soils increased in the order of Norfolk << Dundee CT < Tunica < Dundee NT. Cyanazine sorption among the soils was correlated with fine texture and higher organic carbon content. Sorption of cyanazine degradation products was less than cyanazine sorption in all soils. Isotherms were nonlinear, with sorption decreasing in the order of cyanazine > desmethylpropanenitrile cyanazine > hydroxyacid cyanazine > desethyl cyanazine > cyanazine amide >> chloroacid cyanazine. The Kf for chloroacid cyanazine ranged from 0.21 in Norfolk soil to 0.42 in Dundee NT soil. Sorption patterns of five degradation products among the soils were generally similar to that of cyanazine. Our data indicate that under field conditions, cyanazine degradation products (especially cyanazine amide and chloroacid cyanazine) are more likely to remain in the aqueous phase and thus have a greater potential to move with water compared to cyanazine.

A bioeconomic model was tested as a decision aid for weed control in soybean at Rosemount, MN, from 1991 to 1994. The model makes recommendations for preplant incorporated and preemergence control tactics based on the weed seed content of the soil and postemergence decisions based on weed seedling densities. Weed control, soybean yield, herbicide use, and economic return with model-generated treatments were compared to standard herbicide and mechanical control systems. Effects of these treatments on weed populations and corn yield the following year were also determined. In most cases, the model-generated treatments controlled weeds as well as a standard herbicide treatment. Averaged over the 3 yr, the quantity of herbicide active ingredient applied was decreased by 47% with the seedbank model and 93% with the seedling model compared with a standard soil-applied herbicide treatment. However, the frequency of herbicide application was not reduced. Soybean yields reflected differences in weed control and crop injury. Net economic return to weed control was increased 50% of the time using model-recommended treatments compared with a standard herbicide treatment. Weed control treatments the previous year affected weed density in the following corn crop but had little effect on weed control or corn yield. The bioeconomic model was responsive to differing weed populations, maintained weed control and soybean yield and often increased economic returns under the weed species and densities in this research.

This paper presents a framework in which the spatial and temporal domain of weed populations can be analyzed using geographically referenced information. The regional framework for analysis is based on the premise that the domain of a weed species or an assemblage of species can be described in terms of the space and time in which they survive. Published maps of the spatial distribution of individual weed species were converted to digital records defining their geographic domain. Digital records were imported into a geographically referenced data system. Assemblage maps were produced by intersecting domains of individual species. The assemblage maps show areas with similar intensity of the occurrence of species selected for analysis. The most intense occurrence of summer annual species, for example, exists in a relatively small area of the Midwest. The framework presented in this paper is a concept for a tool to evaluate, manage, and manipulate weed distribution data at the regional scale. Analysis at the regional scale is a perspective substantially different from the plot or field-scale analysis traditionally used to study and manage weed populations. Regional analysis has application to weed science by increasing the understanding of regional patterns of weed infestation and the factors that regulate them. This information will be useful to the herbicide industry, plant ecologists, resource managers, and agricultural policy makers.