Experiments were conducted to determine adaptation characteristics to reduced irradiance of velvetleaf, common lambsquarters, eastern black nightshade, tumble pigweed, and soybean. Plants were grown to the 5- to 8-leaf stage in the greenhouse at ambient radiation (850 μE·m–2·s–1), and 26 and 13% of ambient radiation. Tumble pigweed, a C4 plant, had the highest light-saturated photosynthetic rates at all growth irradiances, while common lambsquarters had the highest rates of the four C3 species. All species adjusted to reduced irradiance by decreasing light-saturated photosynthesis, leaf respiration rates, root:shoot ratios, and leaf densities, while increasing leaf area ratios (LAR)3. As growth irradiance was reduced, support tissues (roots, stems, and petioles):leaf ratios did not change for common lambsquarters or velvetleaf, increased for soybean, and decreased for eastern black nightshade and tumble pigweed, indicating superior adaptation of the latter two species for efficient light harvesting at reduced growth irradiances. Of these five species, eastern black nightshade had the lowest respiration rate, the highest LAR, and the lowest support:leaf ratio for optimum adaptation to shaded environments.

Field and laboratory studies were conducted in 1987 and 1988 on several Arkansas soils to determine the effects of residual levels of imazaquin on cotton. Imazaquin concentrations ranging from 0.007 to 0.024 μg/g and 0.00 to 0.015 μg/g were found in soil samples taken 12 and 24 months, respectively, following preplant-incorporated applications at rates of 140 g/ha. Cotton yields were reduced from 7 to 42% in 1987 as the soil concentration of imazaquin increased from 0.007 to 0.024 μg/g. Imazaquin persistence was greater in clay soils than loams or sandy loams. Imazaquin was weakly adsorbed to a clay, loam, and sandy loam soil in laboratory studies; the adsorption isotherms did not differ significantly by soil texture. A bioassay utilizing cotton as the indicator species was developed to determine imazaquin concentrations in soils ranging from 0 to 0.06 μg/g. Imazaquin concentrations determined by the bioassay method were found to be 75, 77, and 80% of those determined by chemical extractions for a clay, loam, and sandy loam soil, respectively.

The efficacy of glyphosate as affected by rate, time of application, and addition of surfactant3 was evaluated on blackberry, Japanese honeysuckle, poison ivy, sericea lespedeza, and trumpetcreeper. The addition of surfactant (0.5% v/v) to glyphosate had no effect on the control of the weeds studied. Glyphosate applied in mid-June to September at 1.1 or 2.2 kg/ha controlled blackberry. Mid-August glyphosate applications of 2.2 kg/ha controlled 83% of actively growing Japanese honeysuckle; there was less than 75% at one site due to moisture stress. Use of 2.2 kg/ha of glyphosate from mid-June through mid-August controlled 87% of poison ivy. Consistent commercially acceptable control of sericea lespedeza was obtained when glyphosate was applied at 1.1 or 2.2 kg/ha at the time of flowering. Applying glyphosate at 1.1 or 2.2 kg/ha from late July through early October controlled 50% or more of the trumpetcreeper.

Experiments conducted for two consecutive years indicated that barnyardgrass, large crabgrass, and giant foxtail interfered with the growth and development of container-grown ‘Gold Drop’ bush cinquefoil. Grass weed densities ranging from one to six weeds per container reduced Gold Drop bush cinquefoil growth. Bush cinquefoil shoot dry weight, averaged over all grass weeds, was decreased as much as 75% after 83 days of interference. The reduction of cinquefoil growth was independent of the number of grass weeds in the container.

Three grass selective herbicides, sethoxydim, fluazifop, and haloxyfop, applied to wild oat plants at the five-leaf stage inhibited growth and induced chlorosis in leaves. Young and actively growing tissues were affected first. Stem elongation in wild oat was inhibited within 2 days of treatment with sethoxydim and within 5 days of treatment with fluazifop or haloxyfop. At these same observation times, internodes that were elongating rapidly at the time of treatment were constricted at the base. These symptoms were followed by necrosis of the internode tissue. Histological examination of the affected internodes indicated that the herbicides inhibited cell division in very young internodes and inhibited both cell division and cell elongation in slightly older internodes. Initial injury occurred in the epidermal, cortical, and procambium cells of the peripheral regions of the stems located at the base of the affected internodes. Necrosis then progressed to the center of the stem tissue and all cells in the internodes were killed within 14 days of treatment. All three herbicides caused similar morphological and histological effects on developing wild oat stems.

Field studies were conducted in 1985 and 1986 to evaluate the stability of reciprocal interference relationships between common cocklebur and soybean under high and low soil moisture conditions. A significant soil moisture differential was established with portable rain exclusion shelters. Well-watered and drought-stressed common cocklebur reduced soybean yield 29 and 12%, respectively. Drought-stressed common cocklebur interfered with soybean over a shorter distance and the magnitude of the effect at a given distance was reduced. The reduced common cocklebur interference in drier soils was attributed to both common cocklebur and soybean growth responses to moisture stress. First, moisture stress caused greater reductions in common cocklebur canopy diameter, stem diameter, node number, and plant height than in soybean. Second, the soybean yield potential was reduced by moisture stress. The reduction in yield potential decreased the effect of the weed interference. Third, soybean canopy development was slowed, and canopy closure that occurred in about 12 weeks in well-watered soybeans never occurred in the moisture-stressed soybeans. This reduced the degree of light interference between both the common cocklebur and soybean and among the soybean plants. The results of this study indicate that the reciprocal interference relationships between common cocklebur and soybean are not stable across soil moisture conditions. The implications of unstable competitive parameters must be considered as threshold models are developed for various field crops.

Field experiments were conducted to evaluate the interactions of tillage systems with imazaquin and imazethapyr on weed control and soybean injury and yield. Control of jimsonweed, common cocklebur, ivyleaf morningglory, velvetleaf, and giant foxtail from imazaquin and imazethapyr in conventional tillage was generally equal to or greater than control in no-tillage. However, under limited rainfall, weed control in no-tillage was generally equal to or greater than control in conventional tillage. Reductions in soybean heights due to herbicide treatment were evident in both tillage systems in 1985 and 1986 but not in. Soybean yields were reduced in 1985 from imazaquin at 140, 210, and 250 g/ha and imazethapyr at 105 and 140 g/ha. Yields were not reduced in 1986 and. Imazaquin and imazethapyr appear to provide adequate control of jimsonweed, common cocklebur, ivyleaf morningglory, velvetleaf, and giant foxtail in conventional and no-till systems.

Field studies were conducted at three sites in Missouri in 1986 and 1987 to evaluate the performance of below-label rates of bentazon, acifluorfen, and chlorimuron tank mixed with sethoxydim and applied to soybeans 7 and 14 days after planting to evaluate broadleaf and grass weed control and weed seed production. Sequential applications of 0.25X-labeled rates of all three broadleaf herbicides tank mixed with 0.5X sethoxydim rates controlled giant foxtail, velvetleaf, and common cocklebur equivalent to one application of standard-labeled rates of the same tank mixes. Single applications of below-labeled rates of the postemergence herbicides did not control weeds, and soybean yields were not equal to sequential 0.25X or single full-rate treatments. A preemergence treatment of clomazone plus imazaquin applied at labeled rates controlled weeds, and soybean yields were equal to a handweeded check in both years at all test locations. Weeds survived and produced seed to reinfest the plots the following year unless nearly 100% control was achieved.

Scanning electron microscopy (SEM) was used to study alachlor microcapsule morphology and the effects of straw age and moisture on herbicide release. Microcapsule diameter in the formulation ranged from 2 to 15 μm. The polyurea encapsulating material was stable in water over time. Diffusion was suggested as the primary mode of alachlor release, with diffusion rate controlled by herbicide and salt concentration gradients between the microcapsule and the surrounding aqueous solution. Alachlor release was promoted by drying the microcapsules before addition to water. Microcapsule morphology was unchanged 48 h after application to dry straw, and microcapsules did not appear to adhere to the straw surface. Microcapsules applied to overwintered wet straw were shriveled at 48 h after treatment, indicating herbicide loss. Capsule walls were broken and appeared brittle after application to fresh wet straw. Organic constituents of the fresh straw cuticle may affect capsule wall integrity.

Whole plant and tissue culture experiments were conducted to determine the difference in phytotoxicity of 2,4-D and its metabolite, 2,4-DCP, to red clover. At the whole plant level, the mean concentration of 2,4-DCP (10 mM) required to cause 50% growth inhibition (I50) of shoot dry weight was 24 times greater than for 2,4-D (0.42 mM). Using callus tissue, the I50 value for 2,4-DCP (0.28 mM) was 22 times greater than for 2,4-D (0.013 mM) based on dry weights. The callus tissue was 36 and 32 times more sensitive to 2,4-DCP and 2,4-D than shoot tissue based on dry weights, respectively. These data indicate that 2,4-DCP was less phytotoxic than 2,4-D to red clover both in vitro and in vivo.

The effects of soybean row width and redroot pigweed density on growth of crop and weed were studied in field trials in 1983, 1984, and Structural relationships within the canopies of soybean and redroot pigweed in relation to row width and weed density were studied in 1984 and 1985 to assess canopy geometry in relation to intra- and interspecific competition. Early in the growing season, soybean biomass was reduced in the presence of both high and low densities of pigweed. Pigweed biomass was also reduced in the presence of soybeans, especially when grown in narrow rows (25 cm). By midseason, pigweed's contribution to total biomass had reached 43% in wide-row (76-cm) stands and 24% in narrow rows. Soybean produced two to four times more leaf area than pigweed during the first half of the growth season. Narrow-row planting favored soybean leaf area production. Soybean LAI values from weedy stands were reduced compared to those from weed-free stands. Pigweed's contribution to total leaf area averaged 29% in wide-row spacing and 15% in narrow rows. Pigweed leaf area was concentrated in the upper strata of the canopy and thus reduced light available to soybean leaves lower in the canopy. Leaf area distribution patterns suggested that soybean and pigweed were competing for light even though soybean had produced more leaf area than pigweed. These relations were consistent from year to year in spite of variable water conditions.

Phomopsis convolvulus Ormeno, a fungus, reduced growth and regeneration of field bindweed under greenhouse environments. Field bindweed seedlings at the cotyledon stage were severely injured and killed (95% mortality) with 108 conidia/m2. Three- to five-leaf seedlings (2 weeks old) were controlled when inoculated with 109 conidia/m2 (70% mortality; 98 and 89% reduction in dry weight of aboveground biomass and roots, respectively). This inoculum density reduced aboveground and root biomass, and adversely affected regeneration of 4-week-old seedlings and established plants, but few plants were killed. In controlled-environment studies, two inoculations were superior (P = 0.02) to one inoculation in reducing foliage aboveground of well-established seedlings (4 weeks old). However, new shoots produced between the first and second inoculation treatments were less diseased than expected.

In field studies, sprinkler irrigation application of the butyl ester of fluazifop-P, the methyl ester of haloxyfop, and the ethyl ester of quizalofop controlled large crabgrass as well as conventional spray applications. In greenhouse investigations, root uptake of the herbicides from sprinkler irrigation applications injured large crabgrass more than root uptake from conventional applications, but large crabgrass injury from shoot uptake was equal with sprinkler irrigation and conventional applications. Droplets with dilute concentrations of herbicide and crop oil, simulating sprinkler irrigation, were more active when applied to the whorl or second leaf than to the first leaf of large crabgrass. An increase in concentration of nonemulsified oil in the treatment solution increased herbicide deposition and retention.

The objectives of this research were to compare the weed control efficacy of liquid, granular, and microencapsulated formulations of preemergence herbicides in moldboard plow, chisel plow, ridge tillage, and no-tillage corn and soybean production systems, and to determine whether herbicide formulation can influence herbicide interception and retention on surface corn residue. Common lambsquarters populations were threefold higher in corn than in soybeans. A mixed population of giant foxtail and green foxtail was highest in the chisel plow and lowest in the ridge tillage system as were total weed numbers. Percent weed control was not influenced by tillage when considered across all herbicide treatments. Weed control was not influenced by herbicide formulation in the moldboard plow, chisel plow, or ridge tillage systems, but granular herbicide applications provided better weed control than liquid applications in the no-tillage system and across various rates of corn residue in an experiment with no tillage variables. Two- to threefold less granular-applied herbicide was intercepted by surface corn residue at the time of application compared to liquid-applied herbicide. Increasing amounts of postapplication rainfall decreased the difference among formulations with regard to both total soil reception of the herbicide and resultant weed control. There was no consistent advantage for the microencapsulated formulation over the other herbicide formulations. Surface corn residue controlled many weeds without the aid of a herbicide and actually contributed to overall weed control even where herbicides were applied. This suggests that the binding of preemergence herbicides on surface crop residue may not be the cause of weed control failures in reduced-tillage systems as is often assumed to be the case.

Research was conducted to determine the utility of a single, temperature-independent Weibull function for describing cumulative seed germination under several temperature regimes with 14 sets of weed and crop seed germination data. A modified cumulative Weibull function was derived to distribute germination times for individuals within the population and distributed the occurrence of germination given ample sample size and appropriate sample interval. The descriptive and predictive attributes of the stochastic model component are well suited for incorporation into seed germination models and are likely applicable to models to predict distribution of times for other developmental processes of plants.

The control of three summer annual grass weeds with herbicides during establishment of forage grasses was studied near Concord and Mead, NE, in 1984, 1985, and 1986. Three cool-season forage grasses, intermediate wheatgrass, tall fescue, and smooth bromegrass, and two warm-season grasses, big bluestem and switchgrass, were included. The control of three major summer annual grasses, green foxtail, barnyardgrass, and large crabgrass, was excellent with fenoxaprop at 0.22 kg ai/ha. Slight to moderate injury to cool-season forage grasses and severe injury to warm-season grasses were evident. Sethoxydim at 0.22 kg ai/ha and haloxyfop at 0.11 kg ai/ha controlled green foxtail and large crabgrass, but not barnyardgrass. Sulfometuron-treated big bluestem and switchgrass plots had the best forage stand frequencies and yields and, at the rate used, sulfometuron satisfactorily controlled green foxtail but only marginally controlled barnyardgrass and large crabgrass.

An elutriation system was developed to estimate weed seed densities in soil. The system provided reliable estimates of seed densities for weed species with a broad range of seed weights (0.06 to 9.80 mg) and had no effect on seed viability of three weed species tested. Elutriation of soil samples (up to 60 g) taken from a cultivated field took approximately 15 min. Separating, classifying, and counting seeds is time consuming (20 to 30 min per sample) but provides an accurate estimate of seed densities for weeds with a diameter of 0.5 mm or more.

In greenhouse experiments, sago pondweed plants were grown from three different size classes of tubers and treated with four levels of fluridone. Results indicate that although fluridone significantly reduced mean plant weight, the extent of the reduction in weight depended on the size of tuber from which the plant was grown. Mean plant weight was greater for plants exposed to fluridone (up to 0.5 mg/L) for 7 days, if the plants were initially from larger tubers (51 to 60 or 101 to 200 vs. 11 to 20 mg fresh weight). Mean plant length and the number of ramets/plant were affected in a similar manner. The results underscore the importance of specifying the size of tubers used in growth studies and contribute to understanding of the roles that biological factors play in regulating aquatic plant response to herbicide treatment.

Substances having anesthetic-like properties were examined for stimulation of redroot pigweed and witchgrass seed germination. Chemicals included the n-alcohols through C-5, 2-propanol, benzyl alcohol, diethyl ether, chloroform, and 2-bromo-2-chloro-1,1,1-trifluoroethanol. Redroot pigweed seeds required the far-red absorbing form of phytochrome (Pfr) for an anesthetic substance to increase germination, but in witchgrass seeds the active anesthetic substances stimulated germination without added Pfr. The correlation coefficient comparing relative activity (RA) versus the membrane/buffer partition coefficient (M/B) of the active substances was very poor (−0.37) for pigweed seeds but was −0.91 for witchgrass. Leakage of cellular materials, including electrolytes, substances absorbing at 280 nm, and amino acids from seeds treated with either active or inactive anesthetics, was not particularly indicative of effects on germination in both species. Application of increased pressure (0.965 MPa) during anesthetic treatment, which in animals prevents anesthesia, was found to act oppositely and increase seed response to ethanol in redroot pigweed seeds, but in witchgrass seeds pressure suppressed the stimulatory action of ethanol. In both species, pressure appeared to have an action of its own, but for this action to be expressed the presence of a stimulatory anesthetic was also required. The results do not appear to support a good relationship of anesthetic activity with seed membrane lipophilic components in redroot pigweed seeds, but the relationship seems likely for witchgrass seeds.