Bentazon degradation in soil typically proceeds with development of bound residue. Low sorption of bentazon suggests that this residue consists of degradation products; however, there is little data on the sorption behavior of these products. This study was undertaken to determine the sorption of bentazon and the degradation products 2-amino-N-isopropyl benzamide, 2-aminobenzoic acid, and N-methyl bentazon in Dundee silt loam and Sharkey clay, two common agricultural soils of the Mississippi Delta. Greater sorption of bentazon and degradation products in the Sharkey soil was related to finer texture and higher organic C content. Isotherms were nonlinear, with sorption increasing in the order bentazon ≪ 2-amino-N-isopropyl benzamide < N- methyl bentazon. In general, methanol extraction indicated reversible sorption of these compounds. Therefore, it is unlikely that sorption of either 2-amino-N-isopropyl benzamide or N-methyl bentazon contributes to bound residue. In contrast, essentially all of the 14C sorbed from radiolabeled 2-aminobenzoic acid solution remained bound after methanol extraction. However, due to degradation of 2-aminobenzoic acid, it was not possible to conclude the extent to which this compound contributed to bound 14C. Sorption of 14C from 2-aminobenzoic acid exceeded that of N-methyl bentazon, and at low initial concentrations (≤ 20 μM) was nearly 1000-fold greater than bentazon sorption.

Herbicides used in containerized plant production have a potential to move off-site in irrigation runoff water. A granular formulation of isoxaben plus trifluralin was applied to a commercial container plant nursery bed. Herbicides were monitored in irrigation runoff and collection pond water over a period of 60 d. Greatest quantities of both herbicides were lost during the first runoff event following application. Approximately 9.2 and 0.7% of the applied isoxaben and trifluralin, respectively, moved from the application site in runoff water within 5 d after treatment. Herbicide concentrations in runoff water were highest (0.75 μg ml−1 isoxaben and 0.08 (μg ml−1 trifluralin in 1992) during the first 0.25 h following herbicide application. Concentrations in the runoff collection pond climaxed following the first runoff event and decreased to less than 1 ng ml−1 within 60 d after treatment. Photodegradation of isoxaben within the surface 2.5 cm of pond water was greater in light as compared to dark. These studies indicate that isoxaben and trifluralin move from the site of application, but that neither accumulate in the runoff water collection ponds.

Solid phase extraction (SPE) procedures were used to determine the recoveries of herbicides typically used in containerized ornamental plant production from water samples. Recoveries from C18 cartridges and disks were compared for each of 12 herbicides with variations in elution solvent and volume of elution solvent tested. Recoveries for nine of the herbicides from the cartridges and disks using acetone as an elution solvent were not affected by SPE matrix. Fluazifop recovery was greater with the disks, while napropamide and oxadiazon recoveries were greater with cartridges. Both cartridges and disks yielded low recoveries (23 to 47%) of benefin and prodiamine. Changing the elution solvent from acetone to acetonitrile resulted in 10% improvement for the recovery of benefin and a three- to four-fold increase in recovery of prodiamine. Acetonitrile decreased recoveries of napropamide, oryzalin, oxadiazon, oxyfluorfen, and pendimethalin from cartridges. For the disks, oxyfluorfen, prodiamine, and trifluralin had increased recovery, while fluazifop, oxadiazon, and simazine had decreased recovery with acetonitrile as the elution solvent. Increasing the amount of acetone eluting solvent increased the recovery of prodiamine and oxyfluorfen while decreasing the recovery of fluazifop, pendimethalin, simazine, and trifluralin. Binding capacities of oryzalin on cartridges and disks averaged 13.2 and 7.8 mg, respectively. The advantage of the disk lies in the greater volume of water that can be processed, while the higher cost and greater variability are disadvantages. Cartridge extraction yielded good recoveries with lower standard deviations, and used less organic solvent. Selection of an SPE extraction method depends upon the herbicides under evaluation, expected levels, and the water volume being processed. Both SPE techniques offer advantages over traditional liquid-liquid extraction methods such as reduced requirements for organic solvent and sample preparation.

Field studies indicated thiazopyr at 220 g ha−1 injured soybeans planted in no-till, and 340 g ha−1 injured soybean in 1 of 2 yr planted into tilled soil. Thiazopyr half-lives in three field studies ranged from 12 to 14 d, indicating minimal potential for carryover injury to sensitive rotational crops. Thiazopyr dissipation under controlled conditions was slower than under field conditions. Half-lives at 15 and 30 C were 133 and 43 d, respectively. This indicates that microbial degradation is an important mechanism in thiazopyr dissipation, but that other loss mechanisms also play a role. A method using methanol extraction of moist soil samples followed by gas chromatography using a nitrogen-phosphorous detector had > 90% recovery of thiazopyr.

Atrazine, metribuzin, and clomazone half-lives averaged over treatments and seasons were approximately 27, 22, and 55 d, respectively. Clomazone dissipation was not affected by the presence or absence of a soybean crop. Atrazine and metribuzin dissipation was not affected by crops in 1992, but was more rapid in no-crop plots than in cropped plots in 1993. The difference may have been the result of higher soil water content with no-crop (a few weeds present) in 1993 than either corn or soybean. Lower soil moisture may have slowed soil microbial activity, thus suppressing atrazine and metribuzin degradation in the 1993 growing season. Few significant correlations were found between herbicide half-life or herbicide concentration and cocklebur growth, although one would expect these to be an index of activity.

An intensive field survey of an eastern Nebraska corn and soybean field was conducted to characterize the spatial structure and temporal stability of broadleaf weed seedling populations over two growing seasons. Anisotropy, the effect of direction on the relationship between observations, is present in the semivariogram for the velvetleaf and common sunflower populations in 1992 and 1993. The directional trends in aggregation are visible in kriged maps as elliptical shapes oriented east to west across the study area. In addition, there are two distinct zones of aggregation from north to south. These two distinct areas of aggregation are reflected as a ‘plateau’ in the north-south semivariogram. The distance over which this plateau extends indicates that the shape or size of the patch is contracting in the north-south direction (perpendicular to the crop row). The slope of the semivariogram in the east-west direction (aligned with the crop row) remains consistent from 1992 to 1993 suggesting that the shape of the patch is not changing. For sunflower populations, the slope of the north-south empirical semivariogram changes at 20 m, similar to the velvetleaf population semivariograms. This change, however, is reflected as a downward trend in the empirical semivariogram. The distance over which this trend occurs increases from 1992 to 1993 suggesting that seedling patch size was smaller in 1993 compared to 1992. Weed seedling establishment resulting from seed dispersal, differential seed and seedling mortality, or emergence may have resulted in the observed patch dynamics.

Acifluorfen, alachlor, glufosinate, glyphosate, paraquat, and pendimethalin were evaluated for their effects on mycelial growth and sclerotia/microsclerotia production by Rhizoctonia solani AG-1 IA and IB in culture. All of these herbicides except glufosinate and glyphosate were evaluated for effects on severity of Rhizoctonia foliar blight of soybean in the field. In laboratory studies, all herbicides reduced colony radius of R. solani. Growth reductions for IB were greater than for IA in the presence of pendimethalin, alachlor, and acifluorfen, but glufosinate reduced growth of IA more than IB. Sclerotia production by both isolates was prevented by paraquat, greatly reduced by glufosinate, but markedly less affected by the other herbicides tested. In field studies, all tested herbicides influenced severity of Rhizoctonia foliar blight when disease pressure was low, but only paraquat reduced severity when disease pressure was high.

This paper gives an overview of computer assisted molecule design using quantitative structure activity relationships (QSAR). In deriving a QSAR for a set of compounds, one should be aware of possible properties one can use to correlate with the activity of this set of compounds. The paper describes some of the traditional and also more recently used properties that can be calculated. The traditionally used properties have been those calculated for the substituent groups that were varied on a particular scaffold, whereas the more recently used properties are those calculated on the molecule as a whole. One of the most recently used properties for comparative purposes are fields around the molecules. The technique is termed comparative molecular field analysis (CoMFA). The statistical techniques used to correlate the properties with the activity are also described and, finally, a case study is described. The case study shows the application of traditional and 3D QSAR (CoMFA) techniques on some photosystem II inhibitors.

This paper focuses primarily on the means by which biochemical information can be used to identify enzymes which, upon inhibition, produce lethal phenotypes and the enzyme inhibitor design strategies that have the highest probability of not only inhibiting the enzyme but also translating that inhibition into herbicidal efficacy. The identification of an exquisitely lethal target site is the key initial component to this approach and has often been one of the most difficult steps because the attributes of a lethal site have, at best, been ill-defined. An examination of the characteristics of known targets provides some insight as to the definition of a lethal target. Recently, antisense RNA suppression of enzyme translation has been used to determine the extent of inhibition required for toxicity and offers potential as a strategy for identifying lethal target sites. After identification of a lethal target, detailed knowledge of the enzyme's chemical and kinetic mechanism as well as the protein's structure may be used to design potent inhibitors. Various types of inhibitors may be designed for a given enzyme. The advantages and disadvantages of a given type with respect to in vivo efficacy as well as the probability of herbicide resistance development will be discussed.

A convergence of research effort in a number of scientific disciplines in the early 1980s resulted in a rapid expansion of knowledge of the structure and function of the photosynthetic reaction center in bacteria and higher plants. The structure of the reaction center from photosynthetic bacteria was determined by X-ray analysis. The herbicide binding protein (the D1 protein) was identified by photoaffinity labelling and found to be an integral part of the photosynthetic reaction center complex in higher plants. Studies using herbicide-resistant mutants enabled the location of the herbicide binding niche on D1 to be determined. Quantitative Structure Activity Relationships (QSAR) of families of inhibitors and their effect on photosynthetic electron transport helped elucidate the nature of the interaction between inhibitors and receptor. Binding appeared to be predominantly hydrophobic with hydrogen bonding also having an important role. Studies with a series of highly potent inhibitors, the 2-cyanoacrylates, identified certain steric constraints in the interaction of these molecules with the binding site. The activity of these inhibitors was particularly sensitive to minor structural change and they proved to be useful probes of receptor topography. The results of structure-activity studies of the 2-cyanoacrylates combined with a refined knowledge of the three-dimensional structure of the inhibitor binding site has enabled computer-based molecular modelling of interactions of these inhibitors with the receptor. The spatial arrangement of the inhibitor functional groups within the binding domain was shown to be a critical factor in determining binding affinity.

We have developed a mathematical model for the plant vascular system that enables the prediction of a compound's phloem systemicity as a function of its partition coefficients and acid dissociation constants. The mathematical model can account for the sensitivity of systemicity to plant parameters such as plant size and pH of the phloem sap. This paper reviews this model and demonstrates how it accounts for the phloem systemic properties of most herbicides as well as that of many endogenous substances such as plant hormones. The model also can be used to design phloem systemic pesticides as illustrated for a pronematicide that successfully controls nematodes when applied foliarly to transgenic tobacco plants capable of regenerating the parent nematicide in a root specific fashion.

Eight triazolopyrimidine sulfonanilides were tested for metabolic stability in a number of crop and weed species. These data, along with in vitro determinations of activity (I50) against acetolactate synthase, successfully described the in vivo activity of these compounds in a two-parameter model. Whole plant activity increased with increasing compound stability and decreasing I50 (r2 =.78, N = 36). The difficulty in obtaining metabolic stability data during a structure optimization program prompted a study with substituent parameters in models of in vivo activity. Models describing whole plant activity in jimsonweed were developed using a series of 5-methyl triazolopyrimidine sulfonanilides that differed only in ortho and meta substituents on the aniline ring. The I50 term and clogP were most important to jimsonweed activity. Hence, in vitro activity (I50) may be a useful component of whole plant structure activity models to aid in identification of barriers to in vivo performance.