Four equations were evaluated as predictors of the rate of herbicide dissipation in soil. A biexponential equation was superior to the first-order equation for metribuzin and pendimethalin dissipation under five moisture levels and three temperatures in laboratory and field studies. The Hoerl function, adapted in the course of this work, is also a good descriptor. The first-order equation predicts slower initial and more rapid later dissipation than actually occurs and these deficiencies are not shared by the biexponential or Hoerl equations. The first-order equation ignores small residues remaining late in the dissipation process. These residues are important from an environmental point of view and the Hoerl and biexponential equations are more capable of dealing with them.

The influence of hot-dry and cool-moist environments on primisulfuron efficacy on giant foxtail and quackgrass was compared at the University of Wisconsin Biotron. Primisulfuron was applied POST at 20 and 40 g ai ha-1 with nonionic surfactant (NIS) or crop oil concentrate (COC) and with or without 28% nitrogen fertilizer (28% N). Giant foxtail control 3 wk after treatment was not affected by primisulfuron rates or adjuvants. Adding 28% N tended to improve quackgrass control with primisulfuron plus NIS or COC. Primisulfuron was more injurious to both giant foxtail and quackgrass in the cool-moist than the hot-dry environment. Absorption of 14C-primisulfuron into giant foxtail tissue 3 days after treatment (DAT) was greater in the cool-moist than the hot-dry environment Translocation of 14C out of treated leaf of quackgrass 3 DAT was greater in the cool-moist than the hot-dry environment. More 14C was absorbed and translocated in giant foxtail than quackgrass. Absorbed 14C was translocated equally to underground and above treated leaf sections of quackgrass, whereas more 14C was translocated above the treated leaf of giant foxtail.

Laboratory batch equilibrium studies were conducted to evaluate the sorption-desorption behavior of imazethapyr and its major plant metabolite, 5-hydroxyimazethapyr, in three Minnesota soils. Sorption of both compounds on all soils was low, and pH did not significantly influence sorption in the range of 4,8 to 7.1. Less 5-hydroxyimazethapyr was sorbed than imazethapyr on the same soil. Once sorbed, both compounds were only partially desorbable from all soils. Significant hysteresis and formation of nonextractable residues indicate that the small amount of chemical sorbed is bound to selective surfaces with strong bonds. Webster clay loam had greater irreversibility than Waukegen silt loam and Estherville sandy loam soil. The hysteresis observed in desorption may be responsible for the difference between mobility estimations made from laboratory sorption studies and the limited mobility observed in field experiments.

Field studies conducted at six locations in Georgia and one location in Virginia evaluated imazethapyr and imazethapyr mixtures for weed control, crop tolerance, and peanut yield. Imazethapyr applied early postemergence controlled bristly starbur, coffee senna, common cocklebur, Ipomoea species, jimsonweed, prickly sida, and smallflower morningglory at least 91% and controlled yellow and purple nutsedge 88 and 98%, respectively. Paraquat plus bentazon applied early postemergence did not control the aforementioned weeds as well as imazethapyr or imazethapyr mixtures. Paraquat applied with imazethapyr reduced bristly starbur control 15% compared to imazethapyr alone but did not influence control of the other species. Imazethapyr control of bristly starbur was not improved by the addition of bentazon. Sicklepod control was less than 24% with imazethapyr and was at least 58% with imazethapyr plus paraquat Imazethapyr plus paraquat controlled sicklepod better than paraquat plus bentazon at three of the four locations evaluated. Imazethapyr did not control Florida beggarweed, while imazethapyr plus paraquat controlled at least 53%. Peanut injury was minimal 30 d after application for all treatments.

Feasibility of supercritical CO2 fluid extraction of imazaquin from spiked soil (3.21 μmol kg-1) as an alternative to a conventional extraction method was investigated. The supercritical fluid extraction method involved single-step extraction of herbicide from soil with no further sample cleanup procedures. Extraction parameters were optimized for maximum herbicide recovery. Adding water as a modifier to air-dried soil significantly improved herbicide recovery. Extracting a 1-g soil sample with supercritical CO2 at 0.80 g ml-1 density and 3 ml min-1 flow rate, 80 C extraction temperature, 6 min static extraction followed by 25 min dynamic extraction, and analyte trap temperature of 40 C was optimum for maximum herbicide recovery. When optimum supercritical fluid extraction conditions were used, imazaquin recovery from three texturally different soils ranged from 55 to 64%, which was comparable to a conventional extraction method (63%). The supercritical fluid extraction method consumed 4 ml methanol and 75 ml supercritical CO2 and took approximately 1 h for sample extraction.

Field experiments were established to evaluate the effects of various band widths left open directly over the soybean row to improve soybean tolerance to AC 263,222 and chlorimuron applied POST, while maintaining weed control. Drop tubes with even fan tips extended below the spray boom to within 40 and 30 cm from the ground to obtain 5 and 10 cm open bands at the soybean terminals. AC 263,222 applied broadcast at 13, 18, and 36 g ai ha-1 injured soybean 14, 16, and 32%, respectively, 3 DAT. AC 263,222 at 36 g ha-1 applied with a 5-cm and 10-cm open band injured soybean 11 and 3%, respectively. Chlorimuron injury was 12% or less for all treatments. Metribuzin followed by AC 263,222 or chlorimuron was not more injurious to soybean than AC 263,222 or chlorimuron alone. Metribuzin PRE followed by 36 g ha-1 AC 263,222 applied in a 10-cm open band controlled sicklepod and pitted morningglory 86 and 92%, respectively, 6 WAT. Sicklepod control was similar with open-band treatments compared to the same rate applied broadcast; however, 18 g ha-1 AC 263,222 applied in a 10-cm open band provided less pitted morningglory control than the broadcast treatment. Soybean treated with herbicides in a band produced yields similar to or greater than soybean treated with the same herbicides broadcast. Metribuzin followed by 36 g ha-1 AC 263,222 with a 10-cm open band resulted in a soybean yield of 2840 kg ha-1, which was greater than any other herbicide treatment and equivalent to the weed-free check.

Laboratory and growth room experiments were conducted to determine effects of CGA-133205, oxabetrinil, and flurazole on grain sorghum hybrids ‘DeKalb 46’ and ‘DeKalb 48’ after the seed were treated with the herbicide safeners and stored for up to 24 wk at various relative humidity levels. Each safener treatment had an adverse effect on grain sorghum seed as reflected by fewer normal seedlings at 4 d when averaged over storage time and relative humidity levels. Flurazole and oxabetrinil increased the number of ungerminated seed of both hybrids, and 0.4 and 0.8 g ai kg–1 CGA-133205 increased ungerminated seed of DeKalb 46. At ≥ 8 wk after storage, a reduction in germination was observed for seed treated with flurazole or oxabetrinil compared to seed stored without safener. This reduction also occurred with 0.4 and 0.8 g kg–1 CGA-133205, but only at 24 wk after storage. Each safener treatment minimized reduction in seedling shoot fresh weight by herbicides compared to the no-safener check through 24 wk after storage.

The first occurrences of wild oat resistance to diclofop in the Willamette Valley of Oregon were reported in 1990. Among eight resistant biotypes, GR50 values for diclofop were 3 to 64 times greater than the GR50 for a susceptible wild oat biotype. GR50 values for other aryloxyphenoxypropionate herbicides varied from 1 to over 100 times greater than a susceptible biotype. Only one resistant biotype was resistant to cyclohexanedione herbicides, and this was only a three-fold increase in GR50. Except for one biotype that had a low level of resistance to pronamide, none of the wild oat biotypes were cross-resistant to any other commonly used wild oat herbicide. Levels of resistance and cross-resistance did not follow a consistent pattern among biotypes in this study, suggesting more than one resistance trait. There were significant differences in the light use efficiency, height, dry weight, leaf area, and extent and timing of tillering and flowering of four wild oat biotypes studied. These physiological and morphological differences suggest that these resistant biotypes were selected independently. The diversity of resistance patterns and the coevolution of resistance at several locations will add to the difficulty of resistance management

Previous efforts to model crop yield loss from multiple weed species constructed competitive indices based on yield loss from individual weed species. Our model uses a multispecies modification of Cousens’ rectangular hyperbolic yield function to estimate a nonlinear competitive index for weed-crop interference. Results from 13 Minnesota and Wisconsin data sets provide measures of the relative competitiveness of mixed green and yellow foxtails, common lambsquarters, redroot pigweed, velvetleaf, and several other weed species. Competition coefficient estimates are stable over years, but not locations.

Existing long-term cotton experiments established in 1976 with minimum and intensive herbicide programs were subdivided in 1985 and 1986 to determine the longevity of fluometuron and trifluralin soil residues after discontinuing herbicide application. In monoculture cotton, seed cotton yield increased when herbicide use was discontinued in 1985 and 1986 after 9 and 10 yr of continuous use, respectively, on a Sharkey silty clay and a Dundee silt loam soil. Yield increases coincided with reductions of fluometuron and trifluralin soil residues. Fluometuron dissipated from the Dundee silt loam soil by 10 mo after the last application but was present in the Sharkey silty clay soil at 0.20 ppmw after 28 mo. Trifluralin did not totally dissipate from either soil, and low levels were present in the Dundee (0.05 ppmw) and Sharkey (0.13 ppmw) soils 30 mo after the last application. Visual injury to fall-seeded wheat and vetch decreased as herbicide residues dissipated. Fall tillage had no significant effect on the rate of fluometuron or trifluralin dissipation from either soil but reduced seed cotton yields.

BAS 145 138, CGA 154 281, and 1,8 naphthalic anhydride were evaluated as seed treatments to safen 'Silver Xtra Sweet,’ ‘How Sweet It Is,’ ‘Zenith,’ and 'Sweetie 76’ sweet corn against nicosulfuron injury. Nicosulfuron POST injured sweet corn by causing slight leaf chlorosis and leaf base widening in Sweetie 76 and severe plant injury and total yield loss in Silver Xtra Sweet. Naphthalic anhydride or CGA 154 281 provided limited or no safening activity, respectively, and resulted in plant height reduction when applied alone. BAS 145 138 reduced nicosulfuron injury to Silver Xtra Sweet and prevented visible injury from nicosulfuron in How Sweet It Is, Zenith, and Sweetie 76.

Formation of 14C-atrazine degradation products and their distribution in the top 90 cm of soil was determined over 16 mo in a Webster clay loam in the field. After 16 mo, 64% of the applied 14C could still be accounted for in the 90-cm soil profile. At 1 mo after treatment (MAT), 14C moved to the 70- to 80-cm depth. Rapid movement of radioactivity could be attributed in part to preferential movement through vertical macropores. Atrazine accounted for 32% of the 14C applied 16 MAT and was the predominant 14C-compound in soil below 10 cm through 12 MAT. Hydroxyatrazine (HA) was the major degradation product in the top 10 cm of soil accounting for 9% of the 14C present 1 MAT and increasing to 24% within 6 MAT. Deethylatrazine (BEA) was the predominant degradation product at depths greater than 10 cm, accounting for 26% of the 14C in the 10- to 20-cm depth 16 MAT. Deisopropylatrazine (DIA) accounted for less than 10% of the 14C recovered at any soil depth. Deethyldeisopropylatrazine (DEDIA) and an unidentified product were detected in soil extracts 1 MAT indicating further degradation past primary metabolites. The proportion of DEA and DIA increased while the proportion of HA decreased as soil depth increased indicating that DEA and DIA are more mobile in soil than HA. The large amount of radioactivity remaining in the soil 16 MAT suggests that a large pool of atrazine and its degradation products are present in the soil for a long period of time, having the potential to move deeper in the soil and ultimately contaminate ground water.

Field experiments were conducted over a 4-yr period, 1989 to 1992, in southern Manitoba to investigate potato (5. tuberosum) response to simulated drift of the dimethylamine salt of dicamba, 2.8 to 22.2 g ai ha–1; alkanolamine salt of clopyralid, 4 to 32 g ai ha–1; and tribenuron, 0.15 to 1.2 g ai ha–1. Rate ranges investigated corresponded to 2 to 16% of the recommended field rate for each herbicide. Dicamba at 22.2 g ai ha–1, clopyralid at 32 g ai ha–1, and tribenuron at 1.2 g ai ha–1 reduced total tuber yields 40, 29, and 41%, respectively, and marketable yields 74, 75, and 53%, respectively. Average weight of marketable-size tubers was unaffected by any treatment in any year. No tuber malformations were observed. Tubers produced on treated plants were planted the year following treatment to investigate the effects of simulated drift on subsequent tuber regrowth. Injury symptoms were evident for each herbicide in 2 out of 3 yr, but neither total nor marketable yields were affected.

Membranes define the outer boundary of the living protoplast and the internal compartmentation of plant cells. From a structural point of view, membranes consist of a lipid bilayer and proteins essential for functions such as solute transport, signal transduction, and numerous metabolic reactions. While membranes can represent a significant barrier to the free movement of many solutes, those with sufficient lipid solubility may move across membranes by dissolving into the lipid bilayer. However, selective membrane transport is generally observed for hydrophilic solutes such as mineral nutrients and cell metabolites. Such selective transport requires an input of metabolic energy, and in plants this occurs via the production of proton electrochemical gradients across the membrane by substrate- (ex. ATP, PPi) driven proton pumps. Selective solute transport is then mediated by membrane-associated secondary transport systems which utilize the proton electrochemical gradient to drive the transport process. This review of membrane structure and transport system function provides a background for a further examination of herbicide interactions with plant membranes.

Movement and persistence of the herbicide metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] was assessed following two annual applications to a loamy sand soil under irrigated potato production in east-central Minnesota. Russet Burbank potato (Solanum tuberosum) was grown in plots in 1989 and 1990. Metribuzin was applied prior to potato emergence at rates of 0.56 and 1.1 kg ha-1. Water samples were collected weekly from suction samplers 90 and 150 cm deep. Bromide tracer and water balance data confirmed the movement of water to a depth of 150 cm. Metribuzin was not detected in soil samples below 45 cm during either year. Metribuzin was detected (mean concentration, 0.6 μg L-1) during 1990 in 17 of 473 (3.6%) water samples collected at the 90- and 150-cm depths in 8 of 16 plots. Averaged across rates and years, time for 50% dissipation of metribuzin was 30 d. At the end of the cropping seasons, an average of 17% of initial soil metribuzin remained in the 0- to 45-cm soil layer. Surface soil (0 to 15 cm) metribuzin adsorption coefficients, Kd and Koc, were 1.7 and 120, respectively. Metribuzin appeared to remain in the surface 15 cm of soil where it then degrades.

The high near-infrared reflectance (0.76 to 0.90 μm) of Big Bend loco and Wooton loco contributed significantly to their orange-red and red image tonal responses, respectively, on color-infrared aerial photographs making them distinguishable from associated vegetation and soil. Big Bend loco could also be distinguished on color-infrared and near-infrared black-and-white video imagery where it had distinct red and whitish tonal responses, respectively. Computer analyses of photographic and videographic images showed that Big Bend loco and Wooton loco populations could be quantified from other landscape features. A global positioning system was integrated with the video imagery that permitted latitude-longitude coordinates to appear on each image. The latitude-longitude data were integrated with a geographical information system to map Big Bend loco populations.

Relationships between soil sorption normalized to organic carbon (Koc) and molecular properties of 71 herbicides were examined. The Koc values were obtained from the literature. Various molecular properties were calculated by quantum mechanical methods using molecular modeling software. The quantitative structure activity relationship (QSAR) models based on four molecular properties, van der Waals volume (VDWv), molecular polarizability (α), dipole moment (μ), and energy of highest occupied molecular orbital (eHOMO), together accounted for 70% of the variation in Koc. Herbicides were broadly divided into six families based on structural similarities, and separate equations were established for each group. The three descriptors, VDWv, α, and μ, along with either energy of lowest unoccupied molecular orbital (eLUMO), or electrophilic superdelocalizability (SE), or eHOMO appeared to be determinants and accounted for 82 to 99% of the variation in Koc. Applicability of these models was tested for one herbicide analogue and 10 metabolites. The QSAR models appear to be specific to structurally similar chemicals. The QSAR models could be developed to predict Koc of structurally similar compounds even before they are synthesized or for some of the metabolites of existing herbicides. Models of this type can also be developed to create priority lists for testing, so that time, money, and efforts can be focused on the potentially most hazardous chemicals.

In most cases, a herbicide must traverse the cell wall, the plasma membrane, and organellar membranes of a plant cell to reach its site of action where accumulation causes phytotoxicity. The physicochemical characteristics of the herbicide molecule including lipophilicity and acidity, the plant cell membranes, and the electrochemical potential in the plant cell control herbicide absorption and accumulation. Most herbicides move across plant membranes via nonfacilitated diffusion because the membrane's lipid bilayer is permeable to neutral, lipophilic xenobiotics. Passive absorption of lipophilic, ionic herbicides or weak acids can be mediated by an ion-trapping mechanism where the less lipophilic, anionic form accumulates in alkaline compartments of the plant cell. A model that includes the pH and electrical gradients across plant cell membranes better predicts accumulation concentrations in plant cells of weak acid herbicides compared to a model that uses pH only. Herbicides also may accumulate in plant cells by conversion to nonphytotoxic metabolites, binding to cellular constituents, or partitioning into lipids. Evidence exists for herbicide transport across cell membranes via carrier-mediated processes where herbicide accumulation is energy dependent; absorption is saturable and slowed by metabolic inhibitors and compounds of similar structure.

Chlorimuron dissipation as determined by bioavailability in nine soils across the Southern United States was characterized by rapid initial loss followed by a slower dissipation rate following field application of the herbicide in soybean. The rate of bioavailable dissipation varied by year and soil The calculated time for 50% dissipation of bioavailable chlorimuron (DT50) ranged from 5 to 18 d in 1988, 5 to 20 d in 1989, and 5 to 11 d in 1990. In 2 of 3 yr, chlorimuron DT50 was positively correlated with temperature 24 h after application. Chlorimuron DT50 was positively correlated with precipitation in 1988 when regional rainfall was below normal and negatively correlated with precipitation 30 d after application in 1989 when regional rainfall was above normal.