The impact of either nicosulfuron or primisulfuron on maize dwarf mosaic virus (MDMV-A) severity in corn and corn susceptibility to MDMV-A infection were evaluated in greenhouse and laboratory studies. Neither herbicide influenced severity of MDMV-A in corn or corn susceptibility to the virus. Field experiments at five sites examined MDMV-A severity in corn as influenced by POST johnsongrass control with either nicosulfuron or primisulfuron applied at the fifth or eighth visible collar stage, no johnsongrass control, or johnsongrass control throughout the season with hoeing. Area under the cumulative virus curve (AUCVC) was reduced when either herbicide was applied at the fifth-leaf stage compared to the eighth-leaf stage, at four sites. Also, AUCVC was reduced when johnsongrass was controlled with a POST herbicide applied at the fifth or eighth collar stage compared to no control, at two sites. Increases in AUCVC were due to a greater number of infected plants rather than more severe MDMV-A infections.

The effect of oxyethylene (OE) chain length of three homologous series of nonionic surfactants (allinol, nonoxynol, octoxynol) on glyphosate uptake was markedly affected by the leaf surface fine-structure of sugarbeet and kohlrabi. Adaxial leaf surfaces of sugarbeet were covered with a layer of amorphous wax, whereas the adaxial surface of kohlrabi leaves was covered with fine crystalline wax. Foliar uptake of glyphosate (1 mM glyphosate, 20 mM glycine, pH 3.2) averaged 4% for sugarbeet without surfactant, but droplets were not retained by kohlrabi leaves in the absence of a surfactant. Glyphosate absorption with octoxynol (9 to 10 OE units, 0.5 g L−1) was rapid initially (0 to 2 h) and leveled off about 2 h after application in both species. Absorption by sugarbeet decreased from 12 to 3% as OE content of octoxynol was increased from 5 to 30 OE units. In contrast, surfactants of intermediate OE content (octoxynol, 16 OE units) induced the greatest uptake (17%) on kohlrabi. Leaf wetting was markedly affected by surfactant and leaf surface. As OE content of octoxynol increased from 5 to 30 OE units, droplet/leaf interface areas of 1-μl droplets decreased from 4 to 3 mm2 on the adaxial leaf surface of sugarbeet and from 61 to 2 mm2 on kohlrabi. Concurrently, the rate of droplet evaporation (1 μl) decreased from 1.0 to 0.7 nl s−1 on sugarbeet and 4.2 to 0.5 nl s−1 on kohlrabi leaves. The effect of OE content on enhancement of glyphosate uptake and wetting characteristics of spray solutions was similar within species for different hydrophobic moieties but differed markedly between species.

This field research was designed to compare the long-term effectiveness of late-September applications of several herbicides for reducing Canada thistle shoot density on noncropped, untilled abandoned farmland when reapplied annually for 3 yr. Clopyralid at 560 and 840 g ae ha−1 or picloram at 280 and 560 g ae ha−1 reduced Canada thistle shoot density as well as either glyphosate3 at 0.8 to 2.8 kg ae ha−1 or dicamba at 1.1 and 2.2 kg ae ha−1. These treatments were much more effective than 2,4-D at 1.1 and 2.2 kg ae ha−1, chlorsulfuron at 34 and 67 g ai ha−1, and metsulfuron at 34 and 67 g ha−1 for progressively reducing Canada thistle shoot density over three annual fall applications. Picloram and clopyralid greatly reduced and delayed shoot emergence from adventitious root buds in spring after two fall-applied treatments compared with nontreated checks.

The photosynthetic responses of rice (C3) and three Echinochloa species (C4), barnyardgrass, early watergrass, and late watergrass, to changes in CO2 intercellular partial pressure, light intensity, and leaf temperature were investigated under laboratory conditions. The three Echinochloa species exhibited photosynthetic responses characteristic of C4 plants. The three weedy species showed higher efficiency for CO2 utilization at low CO2 intercellular partial pressure (CO2i) than rice. Compensation and saturation of CO2i for photosynthesis were lower in the weedy species than in rice. The maximum photosynthetic rates at high light intensity were 33.5, 32.7, 30.5, and 21.5 μmol CO2 m-2s-1 for barnyardgrass, early watergrass, late watergrass, and rice, respectively. Photosynthesis temperature optimum was 35 to 37 C for the three Echinochloa species and 33 C for rice. Overall, under simulated summer conditions, the four taxa showed a photosynthetic ability hierarchy with regard to gas exchange performance as follows: barnyardgrass ≥ early watergrass > late watergrass > rice.

Five herbicides (cloproxydim, fluazifop, haloxyfop, quizalofop, and sethoxydim) were compared from 1984 to 1988 at 250 and 400 g ha−1 for controlling quackgrass within different crop competition and tillage conditions at the Lacombe Research Station. Crop competition usually augmented quackgrass control with the herbicides in conventional-tillage plots although direct statistical comparisons were not made. Without crop competition, haloxyfop and quizalofop at 250 g ha−1 were more effective than the other herbicides in conventional tillage 1 mo after treatment (MAT). In a conventional-tillage situation on a unit active ingredient basis, the herbicides ranked in order of decreasing activity as follows: quizalofop ≥ haloxyfop > fluazifop > cloproxydim > sethoxydim. In a zero-tillage situation, none of the herbicides reduced quackgrass shoot weights by 50% 3 MAT. However, haloxyfop and quizalofop were more effective in suppressing quackgrass shoot weight than the other herbicides in the zero-tillage experiments 3 MAT.

Reduced-tillage practices have allowed leafy spurge to infest cropland. The reduction in leafy spurge infestation and winterhardiness by fall tillage, N application, or herbicide treatment was evaluated. Cultivating leafy spurge twice each fall for 3 yr provided complete control. Cultivation followed by N application at 225 kg ha−1 reduced the stand by 85% after 3 yr, whereas N applied alone had little effect. Picloram plus 2,4-D at 0.6 plus 1 kg ha−1 reduced the infestation to 65% of the untreated control after 2 yr but then the infestation declined rapidly to 1% after a third application. Leafy spurge cold tolerance decreased as root depth increased. The GR50 and LT50, the temperatures required to reduce total dry weight and survival, respectively, by 50% varied by root depth and treatment. For example, the LT50 and GR50 for untreated leafy spurge averaged −19 and < −20 C for crowns, respectively, compared to the −16 and −18 C for roots from 0 to 15 cm deep and −13 and −12.5 C for roots from 15 to 30 cm deep. The GR50 and LT50 temperature after 2 yr of treatment was reduced to 0 C by two cultivations in fall or picloram plus 2,4-D. Carbohydrate content was not a good indicator of winterhardiness. Cultivation reduced the leafy spurge root system faster than herbicide treatment especially at the 15- to 30-cm depth.

Field studies in 1990 and 1991 evaluated treatment timings of nicosulfuron and bentazon alone and in mixture for weed control, peanut injury, and yield. The experimental areas were treated with a broadcast PPI application of benefin at 1.68 kg ai ha-1. Nicosulfuron was applied at 40 g ai ha-1 and bentazon at 560 g ai ha-1. Nicosulfuron mixed with bentazon applied 1 wk after crop emergence (WAE) controlled yellow nutsedge 81%. Bentazon or bentazon plus nicosulfuron applied 1 or 3 WAE controlled smallflower morningglory at least 94%. Nicosulfuron controlled sicklepod at least 76% in 1990. However, control of sicklepod was 71% from 1 WAE nicosulfuron application in 1991 and less than 26% for 3, 5, or 7 WAE application. Bentazon controlled Ipomoea spp. at least 70% but did not control Florida beggarweed and sicklepod. Nicosulfuron alone controlled Florida beggarweed and Ipomoea spp. Nicosulfuron reduced early-season peanut growth, but peanut recovered by mid-August. Bentazon reduced nicosulfuron control of Florida beggarweed when applied 1, 3, or 5 WAE, but not 7 WAE. Bentazon mixed with nicosulfuron applied 3 WAE in 1990 reduced weed-free peanut yields compared to nicosulfuron alone. Nicosulfuron and bentazon did not significantly reduce peanut yields under weed-free conditions in 1991.

Field and greenhouse experiments evaluated interactions of soil-applied insecticides and imazaquin and imazethapyr on growth and development of cotton. Imazaquin and imazethapyr were applied PPI at 0 to 6 and 0 to 16 g ae ha−1, respectively, in the greenhouse (plus a no-insecticide control), and 0 to 72 g ha−1 in the field in combination with aldicarb, disulfoton, and phorate (without a no-insecticide control) applied in the seed furrow. Cotton shoot fresh weight in the greenhouse experiment decreased linearly as herbicide rates increased. Greater reductions in shoot fresh weight were noted with imazaquin than with imazethapyr. Compared with no insecticide, the methylcarbamate insecticide aldicarb and the organophosphate insecticides disulfoton and phorate did not affect cotton response to either herbicide. In the field, cotton injury increased while stand, yield, and maturity decreased as herbicide rates increased. Delayed maturity was due to a lower percentage of bolls produced on sympodia from main stem nodes four to nine. Imazaquin caused greater injury, greater reductions in stand, greater delays in maturity, and lower yields than did imazethapyr. Earlier maturity was noted with aldicarb-treated cotton. Compared with aldicarb, disulfoton and phorate did not alter cotton response to imazaquin or imazethapyr.

Preemergence control of itchgrass in sugarcane with nonincorporated pendimethalin and prodiamine increased as rate for each herbicide increased from 2.2 to 3.4 kg ai ha-1. Itchgrass control at 3.4 kg ha-1 was similar for both herbicides, ranging from 78 to 94% (86% mean) for pendimethalin in six field experiments and 83 to 91% (86% mean) for prodiamine in three experiments. Incorporated trifluralin at 2.2 kg ha-1, the standard, gave 99% mean control of itchgrass. None of the nonincorporated treatments with pendimethalin or prodiamine consistently prevented a decrease in the sugarcane stalk population at harvest. Asulam applied postemergence at 3.7 kg ae ha-1 controlled itchgrass that survived the preemergence herbicides, and sugarcane stalk populations were maintained. Preemergence control of johnsongrass with nonincorporated treatments of pendimethalin or prodiamine at 2.8 kg ha-1, in a mixture with atrazine at 2.2 kg ai ha-1, ranged from 86 to 95% (89% mean) for pendimethalin and 80 to 95% (88% mean) for prodiamine. Metribuzin at 2.2 kg ha-1, the standard, gave 97% mean control of johnsongrass. Mixtures of pendimethalin with metribuzin or terbacil generally gave more effective control of johnsongrass than mixtures with atrazine. When used in conjunction with other herbicides, nonincorporated pendimethalin and prodiamine were effective alternatives to soil-incorporated treatments for weed control in sugarcane.

Common cocklebur is a very competitive and difficult to control weed species in soybean production. Field research was conducted at Rosemount, MN, from 1989 to 1991 to evaluate reduced rates of bentazon applied broadcast or banded over the crop row in combination with interrow cultivation for common cocklebur control in soybean. Bentazon at 0.6 kg ai ha−1 applied in a 38-cm-wide band over the soybean row followed by two cultivations controlled almost 90% of the common cocklebur when environmental conditions were favorable, and the majority of the common cocklebur emerged prior to bentazon application. However, when precipitation was below normal prior to bentazon application and a high proportion of the common cocklebur emerged after the initial bentazon application, sequential bentazon treatments controlled more common cocklebur and resulted in greater soybean yields than combinations of bentazon plus cultivation. Differences in soybean yield were attributed to differences in common cocklebur control rather than injury from cultivation or bentazon.

Concentrations of hexazinone and two metabolites in vegetation were determined for 2 yr after broadcast application of a 10% granular formulation of hexazinone at 2 and 4 kg ai ha−1 rates in August 1986 in a boreal forest. Prewinter concentrations of hexazinone in stems of trembling aspen, Saskatoon berry, and willow ranged from 0.02 to 0.05 μg−1 dry wt at 64 d after treatment (DAT). Absorption of hexazinone accelerated during spring thaw (1987), and residues in foliage of several woody and herbaceous species peaked during May to July. Patterns of accumulation of hexazinone and its metabolites varied with the species. Foliar concentrations diminished significantly by the end of the first growing season in 1987 (372 DAT) and were undetectable or extremely low at the end of the second growing season in 1988 (707 DAT). Based on the highest residue concentrations detected in several plant species, it is estimated that wildlife would ingest a maximum of 16, 28, and 24 mg of hexazinone, metabolite A, and metabolite B, respectively, for every kg of dry matter consumed. Reported LD50 values suggest that application of hexazinone at the 4 kg ai ha−1 rate or less poses no toxicological threat to wildlife.

Kochia growing on an industrial site where chlorsulfuron was applied repeatedly over several seasons was confirmed to be resistant to chlorsulfuron and several other acetolactate synthase (ALS) -inhibiting herbicides. In growth room experiments, resistant (R) plants were 2 to >180 times more resistant to five sulfonylurea herbicides and one imidazolinone herbicide (imazethapyr) than susceptible (S) plants, as measured by the ratio of dosages required to inhibit shoot dry matter accumulation by 50% (GR50 R/S). Similarly, in vitro assays of ALS activity indicated that from 3 to 30 times more herbicide was required to inhibit the enzyme from R plants than from S plants. Results of ALS enzyme assays indicated that R kochia was approximately equally resistant to metsulfuron, triasulfuron, and thifensulfuron, and 2.5 times more resistant to tribenuron than thifensulfuron. However, the response of R kochia growing in a spring wheat crop in the field was not consistent with results of the ALS enzyme assays. In field experiments, thifensulfuron at 32 g ai ha−1 had little effect on R kochia. In contrast, metsulfuron, triasulfuron, and tribenuron at 8 g ha−1 did not reduce R kochia seedling densities, but caused severe stunting such that 2 mo after treatment the shoot biomass of plants in untreated plots was four times greater than in sprayed plots. Herbicides with alternative modes of action including fluroxypyr, bromoxynil/MCPA ester, dichlorprop/2,4-D ester, and 2,4-D ester provided good control of R kochia in the field. Quinclorac did not reduce kochia densities, but surviving plants were stunted. To delay or avoid development of ALS inhibitor-resistant kochia populations, these alternative herbicides applied alone or in tank mixtures could be incorporated into a herbicide rotation.

Laboratory experiments were conducted to determine the effect of three phenolic acids, acetone, and riboflavin on aqueous photolysis of chlorimuron and imazaquin. The phenolic acids investigated were caffeic acid (CA), ferulic acid (FA), and p-coumaric acid (PCA). Treatment solutions were contained in quartz vessels and irradiated with 300 to 400 nm UV light in a photoreactor. The extrapolated photolysis half-life of chlorimuron in pure solution was 107 h, compared to a half-life of 0.42 h for pure aqueous imazaquin. Chlorimuron in solutions containing 10 ppmw riboflavin, acetone, CA, FA, or PCA exhibited half-lives of 9, 57, 58, 67, and 146 h, respectively. Imazaquin in solutions containing 10 ppmw riboflavin, acetone, CA, FA, or PCA had half-lives of 0.70, 0.55, 0.55, 0.48, and 0.55 h, respectively. The presence of PCA in aqueous media delayed chlorimuron photolysis, whereas all other compounds, especially riboflavin, sensitized chlorimuron photolysis. In contrast, imazaquin photolysis was delayed in the presence of the test compounds, with riboflavin having the greatest effect and causing in a 68% increase in imazaquin half-life over that of imazaquin alone. Quantum yields for sensitized photolysis of chlorimuron by riboflavin and for riboflavin by imazaquin were 0.1134 and 0.0477, respectively. These results suggest that some soluble and naturally occurring organic compounds may enhance chlorimuron photolysis yet delay imazaquin photolysis in surface waters.

A method of calculating confidence intervals of the “area of influence” of a weed plant, and of yield losses calculated from it, was developed. In a worked example using published data, the confidence intervals of the area of influence were found to be large. Yield losses calculated from this method were less precisely estimated than those from a more traditional additive density experiment. This limited evidence suggests that to give similar precision, the area of influence experiments may need to be at least double their present size. If this is indeed the case, published statements on the space, time, and effort advantages of the area of influence design will need to be treated with caution.

Overall efficacy of control practices for Johnsongrass was evaluated in soybean and sunflower cropping systems where timing of cultural and chemical control practices was adjusted to weed phenological stages by using visually estimated indices (stems 15 to 30 cm in height with 4 to 8 leaves) and with a thermal calendar model to forecast the crucial period for Johnsongrass propagation (when the population's rhizome mass is at a minimum). When visually estimated indices were used, weed infestation at the end of one growing season remained constant for the soybean cropping system and was reduced by about 70% in the sunflower crop. With the thermal calendar, weed infestation was reduced in both cropping systems by more than 90% in one growing season, and Johnsongrass was eradicated in two growing seasons. Although timing the control procedures to the crucial period severely hampered persistence of the weed in both cropping systems, the crop yield of sunflower was reduced by 35%. This yield reduction was caused by the delay in sowing date needed in order to adjust control procedures to the crucial period for propagation.

The purpose of this study was to quantify hysteresis during adsorption and desorption of atrazine as a function of incubation time for a Sharkey clay soil. Adsorption was carried out using one day batch equilibration and was followed by incubation periods ranging from 1 to 24 d. Incubation was subsequently followed by six consecutive desorption steps where each step represented 1 d. The Freundlich equation (S = K CN where S is the amount of atrazine retained, μg g-1; C is concentration, μg ml-1; K is the distribution coefficient, cm3g-1; and N is a dimensionless parameter) was used to describe batch results. Both adsorption and desorption isotherms were well described by the Freundlich model. Fitted K parameter values for desorption isotherms were consistently higher than those associated with adsorption. The opposite trend was observed for the exponential parameter N. The results revealed that desorption deviated significantly from adsorption data. The deviation, which is commonly referred to as hysteresis, was more pronounced as incubation time increased. Batch equilibration results also indicated that the extent of hysteresis was not influenced by soil sterilization. Attempts to quantify the extent of hysteresis using a simplified approach are presented. We found that, for a given batch data set, hysteresis can be quantified provided that Freundlich N from adsorption and desorption isotherms is known.

Shin oak is a deciduous shrub that forms dense stands of brush on sandy soils in rangeland areas of the Rolling and High Plains of Texas. Plant canopy reflectance measurements made on shin oak showed that it had both low visible (0.63- to 0.69-μm waveband) and nearinfrared (0.76- to 0.90-μm waveband) reflectance values, a characteristic generally not shared by associated plant species or mixtures of species. The low reflectance values of shin oak caused it to have dark-red, reddish-brown, or brown image tones on color-infrared photographic, videographic, and SPOT satellite images that made it distinguishable from associated vegetation and other land use features. The optimum time to remotely distinguish this noxious shrub is during the mature phenological stage from June to September. Computer-based image analyses of video and satellite images showed that shin oak populations could be quantified. This technique can permit “percent land area” estimates of shin oak on rangelands. The aerial imagery is useful for detecting shin oak on smaller rangeland areas, whereas the satellite imagery is applicable in mapping large areas of shin oak distribution.

Eighteen volatile monoterpenes, which are among the major constituents of essential oils from plants, were screened for phytotoxicity to several crop and weed species. Several monoterpenes containing an oxygen function severely inhibited germination of four different annual weeds while concurrently displaying little effect on soybean germination. Soybeans varied in their sensitivity to various monoterpenes (measured by seedling growth), with α-terpineol and geraniol selectivity inhibiting weed seed germination while having little or no effect on soybeans. All of the monoterpenes tested exhibited relatively high germination and growth inhibition to corn and wheat.

The persistence and movement of sethoxydim residues were determined in the top 45 cm of an Estherville sandy loam (sl), Port Byron silt loam (sil), and Webster clay loam (cl) in the field. Analysis of sethoxydim residues simultaneously quantified parent and eight metabolites by conversion to a common volatile end product, 3-[2(ethylsulfonyl)propyl]-pentanedioic acid dimethyl ester, followed by analysis using gas chromatography (GC) and a flame photometric detector. Recovery of residues from soil spiked with 100 μg kg-1 was 86 ± 21%. Residues remaining in soil 21 days after treatment (DAT) in 1989 were 37, 51, and 29% of the applied sethoxydim in the sl, sil, and cl, respectively, while in 1990, 86, 52, and 24% remained in the sl, sil, and cl, respectively. Alachlor remaining 21 DAT in 1989 was 66, 52, and 65% of that applied in the sl, sil, and cl, respectively, while in 1990, it was 16, 13, and 100% in the sl, sil, and cl, respectively. Atrazine had the greatest % of applied chemical remaining 21 DAT in 1989: 100, 71, and 73% in the sl, sil, and cl, respectively. of the atrazine applied in 1990, atrazine remaining 21 DAT was 87% in the sl, 42% in the sil, and 100% in the cl. Over all soils and years, the amount of total sethoxydim residues remaining 137 DAT was the least of the three herbicides. In terms of leaching, sethoxydim residues showed least movement, with minimal detections below 0 to 15 cm. Although sethoxydim appears to be advantageous over alachlor and atrazine in terms of potential impact on ground water quality, additional information regarding composition of the detected residues is needed to better assess its environmental impact.

Comparative spread of droplets of several different oils and water with different adjuvants on leaf surfaces was investigated. Spread was better on the lower surface of johnsongrass leaves than on upper leaf surfaces with nine of 14 oils studied; two spread best on upper leaf surfaces, and three were equal in spread on both surfaces. Differences in spread coefficients did not appear to be directly related to surface tension, viscosity, or mid-boiling point of the oils. Soybean or cottonseed oils did not spread as well as petroleum oils but methylated soybean and sunflower oils had high spread coefficients on both upper and lower leaf surfaces. All but one petroleum-base oil spread three to four times better on leaf surfaces than on oil-sensitive paper. Water mixtures of an organosilicone surfactant spread much better on water-sensitive paper and on johnsongrass leaves than water with conventional adjuvants, but spread of paraffinic oils exceeded that of any water-adjuvant mixture. Spread coefficients of most petroleum-base oils were better on lower than upper leaf surfaces. Spread usually increased as the age of leaves increased from 14 to 56 d. Water droplets with adjuvant had at least an 86% weight loss after 6 min, but low volatile paraffinic oil droplets had little weight loss 2 d after application.