Four postemergence herbicides were evaluated for control of horse purslane and barnyardgrass occurring alone and in combination in three separate field experiments at Haryana Agricultural University, Hisar (India). Haloxyfop, fluazifop, and sethoxydim controlled barnyardgrass. Fomesafen controlled horse purslane most effectively when applied at the two-branch stage of horse purslane in mung bean. Combinations of haloxyfop, fluazifop, or sethoxydim with fomesafen applied at the five-branch stage of horse purslane and two-leaf stage of barnyardgrass controlled both weeds. Mung bean treated with fomesafen at 250 g ae/ha plus haloxyfop at 190 or 250 g ae/ha, fluazifop at 750 or 1000 g ae/ha, or sethoxydim at 750 g or 1000 g ae/ha produced dry weights, pods/plant, and grain yield similar to weed-free mung bean.

Growth and morphology of two U.S. hydrilla biotypes grown under 10- and 16-h photoperiods in outdoor aquaria were investigated. The dioecious hydrilla biotype elongated extensively under both photoperiods and reached the water surface within 2 to 3 weeks after planting. In contrast, the monoecious biotype grew more densely near the sediment, primarily by producing a high number of horizontal stems, root crowns, and higher shoot densities. Monoecious hydrilla grown from tubers produced new tubers after 4 weeks of exposure to the 10-h photoperiod, and the number of tubers produced was five- to sevenfold higher in the monoecious than in the dioecious biotype. Furthermore, monoecious hydrilla produced tubers under both 10- and 16-h photoperiods, with tuber production much higher under the shorter photoperiod. Dioecious hydrilla grown from tubers under similar conditions produced new tubers after 8 weeks only under the 10-h photoperiod. No tubers were produced by the dioecious biotype throughout a 10-week exposure to the 16-h photoperiod.

Field infestations of a seed-feeding insect developed from overwintered populations reduced viability of velvetleaf seed to 17.5 and 15.5% at two locations in central Missouri, compared to 95.5 and 87.5% at insect-free sites. Insect feeding enhanced the proportion of seedborne microorganisms in seed up to 98% compared to the average fungal infection of 8% for seed not exposed to the insect. There was a strong negative correlation between fungal infection associated with insect feeding and percent velvetleaf seed viability. The insect transmits microorganisms externally just as pollen is carried by various other insect species and not by ingestion and regurgitation. The effectiveness of the insect on reducing seed viability and seed production in central Missouri is mainly limited by the time required to build up populations capable of significantly affecting early-season velvetleaf seed production.

Paraquat and diquat were used from 1963 until 1986 to control capeweed and other annual weeds on an alfalfa field at Elmhurst, Victoria, Australia. From 1983, control of capeweed with these bipyridyl herbicides was no longer satisfactory. In experiments conducted on this field in 1986 and 1987, 800 g ai/ha of diquat was required to equal the control of capeweed obtained by 100 g/ha diquat in an adjacent pasture field with no history of herbicide use. There were substantial numbers of a diquat-resistant biotype within the population in the alfalfa field. It is concluded that the diquat-resistant biotype of capeweed became dominant due to the strong selection pressure provided by over 20 yr of use of the bipyridyl herbicides.

Johnsongrass seedlings were inoculated with conidia of four fungi, Exserohilum turcicum (Et), Colletotrichum graminicola (Cg), Gloeocercospora sorghi (Gs), and/or Bipolaris halepense (Bh). Leaf injury from Et was the most extensive. Johnsongrass was equally or more susceptible to these fungi 7 days after emergence (DAE) than at 14, 21, or 28 DAE. The greatest leaf injury, more than 90%, was induced by Et with 2 × 105 conidia/ml and a 24-h dew period. Sequential inoculation with Et at 15 and 20 DAE resulted in injury to more leaves than a single inoculation 15 DAE. No synergistic or antagonistic effects were observed in combinations of Et with Gs or Cg. Injury was limited to exposed leaves and sheaths, injured seedlings resumed growth and developed new leaves. Development of new leaves from inoculated plants was reduced by 30% or less at 14 days after inoculation (DAI) compared to uninoculated plants. Inoculation with Et or Bh was not as effective as paraquat or sethoxydim, since treatment with both herbicides resulted in seedling death. Seedling kill was not observed in any fungal tests.

Field and laboratory studies were conducted in 1985 and 1986 to determine the effect of herbicide application timing and level of corn residue on metribuzin plus metolachlor for weed control in soybean. Herbicides were applied early preplant (35 days before planting), sequentially (early preplant and preemergence), or preemergence. Giant foxtail control was better with sequential application than early preplant herbicide application in both years. Control of velvetleaf, smooth pigweed, and jimsonweed was independent of application timing, except in 1986, when jimsonweed control was less from herbicides applied early preplant compared to sequentially. Metribuzin reaching the soil was less when applied to high levels of corn residue compared to low residue levels. However, corn residue level had no effect on weed control.

Longspine sandbur germinated readily after 3 months of storage at 0 to 4 C. Germination was 15 to 82 and 10 to 76% for seed retained by or that passed through a 1.7-mm-mesh screen, respectively, and was greatest in complete darkness near 30 C. Light inhibited seed germination at most of the temperatures tested. Incubating imbibed seed at 40 C and 40/10 C for 3 weeks reduced the viability of seed that did not germinate by more than 50%. Primary seed within the bur germinated 77% and secondary seed germinated 35% during 2 weeks at 35/10 C (16 light/8 dark h). Primary seed placed in polyethylene glycol (PEG) solutions ranging from −0.5 to −1.5 kPa water potential germinated as much or greater than seed placed in water, while secondary seed germination was inhibited by similar osmotic potentials. Seedlings emerged from depths to 11 cm but emerged most readily from 1- to 3-cm depths.

Barnyardgrass seeded in field plots at monthly intervals from March through October at Shatter, CA, began to emerge in March when soil temperatures reached 17 C at a depth of 5 cm. For all plantings, most of the seeds germinated and produced seedlings within 2 weeks. Increasing day lengths favored vegetative growth, with April, May, and June plantings producing the tallest plants, the most dry matter, and the largest number of tillers. July and August plantings, the first to flower, began flowering and producing seeds within 5 to 6 weeks after planting, but the greatest number of seeds (15 000 to 25 000/plant) was produced by the larger plants of the April, May, and June plantings. September plantings produced few seeds/plant (37), when compared to March (5900), July (9800), and August (9700) plantings. The earliest that seed production would be expected from seedlings emerging in March and April would be early June, and seedlings emerging in September could produce seeds before killing frosts.

Experiments were conducted in 1985 to 1987 to evaluate the effects of conventional and no-tillage systems on the weed control provided by clomazone applied preemergence in soybeans. The persistence of clomazone in soil of the two tillage systems was also determined. Increasing the clomazone rate from 0.8 to 1.4 kg/ha did not increase weed control. Clomazone controlled 80% or more of jimsonweed, velvetleaf, and giant foxtail. Common cocklebur control ranged from about 50 to 70% in no-till and from 80 to 90% in conventional tillage. Generally, soybean pods/plant and yields were lower from clomazone treatments than from handweeded treatments due to inadequate common cocklebur control. Over 40% of the clomazone applied did not reach the soil surface; it was either intercepted by wheat straw, volatilized, or both. Clomazone persisted longer in conventional tillage than in no-tillage in. However, in 1986, clomazone was equally persistent in the two tillage systems. The half-life of clomazone was 34 and 6 days in 1985 in conventional and no-tillage, respectively, and in 1986, 18 and 16 days in conventional and no-tillage, respectively. Significant clomazone concentrations were not found below 10 cm in the soil profile. Corn planted without tillage (no-till) approximately 1 yr after clomazone application was not injured and yields were not reduced due to prior clomazone use.

Eurasian watermilfoil, an aquatic flowering plant, was found to be a suitable bioassay plant for the detection of photosynthetic inhibitor herbicides in water and aqueous extracts of soil. Stock cultures of Eurasian watermilfoil were maintained in an algal-free medium under constant environmental conditions. Oxygen evolution from three lateral shoots 3 to 5 cm in length was measured before and after herbicide treatment. Inhibition of photosynthesis was detected within 10 min of treatment. Concentrations as low as 5.9 × 10–8M terbacil and 6.4 × 10–8M diuron were detected in water. Simazine, atrazine, and metribuzin were detected in water at a concentration of 10–7M. Eurasian watermilfoil bioassays in lake water spiked with various concentrations of terbacil, metribuzin, atrazine, and simazine indicated that the bioresidual activity of these chemicals can be estimated in samples of natural water without extraction or purification. Using aqueous extracts taken from herbicide-treated soil, the Eurasian watermilfoil bioassay detected minimum levels of 0.047 and 0.07 kg/ha terbacil and atrazine, respectively. Neither soybean nor oat showed visible injury symptoms at these concentrations. The Eurasian watermilfoil bioassay has the advantage of being more rapid and/or more sensitive than methods using algae, oat seedlings, or leaf discs.

In 1987, two applications of the methyl ester of diclofop failed to effectively control Italian ryegrass in some growers' fields. As a result of these observations, studies were initiated to determine if the surviving ryegrass is more tolerant to diclofop than previously untreated ryegrass. Growth chamber studies on five accessions of ryegrass demonstrated that collections from two growers' fields were highly tolerant of diclofop. Diclofop GR50 (rate required to reduce shoot weight by 50%) values of accessions A and B and commercial ryegrass, when diclofop was applied to seedlings at the two-leaf stage, were 2.9, 3.4, and 0.0056 kg ai/ha, respectively. Diclofop-tolerant biotypes were not more tolerant to postemergence barban or preemergence-incorporated triallate plus postemergence diuron or metribuzin. These herbicides are possible alternative treatments in fields where diclofop is no longer effective.

Greenhouse studies were conducted to determine the basis for reduced johnsongrass control when glyphosate was applied in mixtures with 2,4-D or dicamba. Glyphosate was applied to johnsongrass at 0.28, 0.56, 0.84, and 1.12 kg/ha alone and in combination with 2,4-D or dicamba at 0.14, 0.28, 0.14, or 0.56 kg/ha. Johnsongrass shoot and root fresh weights measured 4 weeks after treatment were higher when glyphosate was applied with 2,4-D (0.28 kg/ha glyphosate) or dicamba (0.28 kg/ha or 0.56 kg/ha glyphosate) compared to glyphosate applied alone at these rates. The antagonism of johnsongrass control was not observed with combinations of some of the higher glyphosate rates with 2,4-D (0.56 or 0.84 kg/ha glyphosate) or dicamba (0.84 or 1.12 kg/ha glyphosate). The reduction of glyphosate activity on johnsongrass occurred when any of four forms of 2,4-D or two forms of dicamba were added to the glyphosate spray mixture. Glyphosate uptake into johnsongrass leaves and subsequent translocation to the roots was reduced by the presence of 2,4-D or dicamba. The reduced glyphosate uptake and translocation could account for the decreased toxicity of glyphosate to johnsongrass when applied with 2,4-D or dicamba.

Gas exchange of jointed goatgrass leaves was affected by temperature, irradiance level, and soil matric potential. Net photosynthesis of leaves under saturating irradiance (PPFD3 = 1850 (μE·m–2·s−1) was optimum at about 20 C. At 25 C, net photosynthesis was nearly 90% of maximum at a PPFD of 800 μE·m–2·−1. Transpiration, and presumably water use, increased steadily with temperature from 10 to 40 C. Dark respiration rate and compensation points for light and for CO2 increased exponentially, or nearly so, from 10 to 40 C. Soil moisture deficits of −130 kPa reduced net photosynthesis and transpiration by about 30 and 55%, respectively, compared to well-watered plants.

The influence of depth (0 to 30 cm) and duration (1 to 24 months) of burial on the deterioration, germination, and viability of ripgut brome seed was studied in the field. Both surface-sown and buried ripgut brome seed were depleted within 15 months. Persistence of surface-sown seed declined relatively slowly during the first year, falling from 83 to 62 to 23% after 1, 9, and 12 months, respectively. Seed covered by soil, however, germinated more rapidly, with less than 10% of the initial population ungerminated after 1 month at all depths. The mode of seed disappearance was closely related to whether or not the seed was covered with soil. Seed loss at depths of 1 to 30 cm was primarily due to germination in situ, with little effect from viability loss or enforced or induced dormancy. In contrast, the persistence of surface-sown seed was due primarily to induced dormancy for up to 12 months, with viability loss and enforced dormancy becoming important thereafter. Tillage practices aimed at providing favorable germination conditions may reduce ripgut brome seed survival in the soil. Because seed is relatively short lived, seed supply in soil may be reduced by short-term rotation to a crop that allows for effective control of ripgut brome.

Field experiments were established to investigate the effects of tank-mixing sethoxydim or quizalofop with imazaquin, chlorimuron, or lactofen for barnyardgrass control in soybeans. Sequential applications, where the grass herbicide was applied 24 h before or after the broadleaf weed herbicide, were also evaluated. There was an antagonistic decrease in barnyardgrass control when sethoxydim or quizalofop was tank mixed with any of the broadleaf weed herbicides. Antagonism was also observed when either grass herbicide was applied 24 h after imazaquin or lactofen, but not with chlorimuron. Control was not affected when the grass herbicide was applied 24 h before the broadleaf weed herbicide. In greenhouse experiments, the effects of tank-mixing sethoxydim, quizalofop, fluazifop-P, haloxyfop, fenoxaprop, or clethodim with imazaquin, lactofen, chlorimuron, acifluorfen, fomesafen, bentazon, or 2,4-DB were also evaluated for barnyardgrass control. Although acifluorfen provided 45% barnyardgrass control, tank-mixing it with fluazifop-P, haloxyfop, or fenoxaprop was antagonistic. Antagonism also occurred when chlorimuron was tank mixed with any of the grass herbicides except fenoxaprop. Barnyardgrass control by all of the grass herbicides applied in the field or greenhouse was most severely antagonized by tank mixes containing imazaquin.

The extent of 14C-imazaquin and 14C-imazethapyr abiotic vs. biotic degradation in soil was investigated. Degradation was measured in an in vitro system which allowed 90% recovery of applied herbicide. Triallate biodegradation is well documented and therefore used as a standard. Herbicide degradation was compared in two soils, a Cisne silt loam and a Drummer silty clay loam. Herbicide degradation in gamma-irradiated soil was compared to fresh soil. Biomass quantities were measured for the duration of the experiments. 14CO2 evolution, extractable parent, metabolites, and unextractable residue were measured. After 12 weeks of incubation, 95% of the radioactivity could be extracted as parent from sterilized soil. In unsterilized soil, imazaquin and imazethapyr degraded at a similar rate which was dependent upon soil type. All herbicides degraded slower in the Drummer soil and triallate degraded two to three times faster than the imidazolinones in either soil. 14C-imazaquin degradation products included 14CO2 and unextractable residues. The major product from 14C-imazethapyr degradation was 14CO2. Evolution of 14CO2 from an imazethapyr-treated Cisne soil, containing a serial dilution of activated charcoal, demonstrated that adsorption of herbicide was negatively correlated with degradation. Therefore imidazolinone microbial degradation is regulated by the amount of herbicide in soil solution as determined by soil characteristics.

Postemergence control of broadleaf weeds in canola with DPX A7881 was studied under controlled-environment and field conditions. DPX A7881 controlled weeds of the Cruciferae family such as wild mustard, field pennycress, and flixweed, which are not controlled with existing herbicides used for weed control in canola. Weed control was better when DPX A7881 was applied at the two-leaf than at the six-leaf stage of the weeds. At the two-leaf stage, control of flixweed was achieved with 15 g ai/ha of DPX A7881; 20 to 30 g ai/ha was required for control of wild mustard and field pennycress. Control of redroot pigweed required 30 g ai/ha. Common lambsquarters was not controlled in this study with DPX A7881 up to 60 g ai/ha. Control of weeds with DPX A7881 increased canola yields in this study without altering the important quality components: oil content, 1000-kernel weight, and green seed content. DPX A7881 provides a postemergence alternative to existing herbicides for broadleaf weed control in canola.

Experiments were conducted in 1985 and 1986 to determine and compare the efficacy, phytotoxicity, and soil persistence of imazquin, imazethapyr, and clomazone in no-till double-crop soybeans. All herbicides controlled 93% or greater jimsonweed, velvetleaf, and giant foxtail. Imazaquin and imazethapyr controlled more common cocklebur and ivyleaf morningglory than clomazone. Soybeans were injured from imazaquin and imazethapyr applications in 1986 but yields were not reduced. First-order kinetics described the dissipation of each herbicide. Imazaquin and imazethapyr were more persistent in the soil than clomazone. Averaged over both years, half-lives of 10, 43, and 60 days were calculated for clomazone, imazaquin, and imazethapyr, respectively. Clomazone was not detected 10 to 20 cm in the soil profile. More imazethapyr was detected 10 to 20 cm in the soil profile than imazaquin in 1985. Slight corn injury (≤10%) was observed in 1987 following previous year applications of imazaquin to no-till double-crop soybeans but corn yields were not reduced.

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.

Adsorption of imazaquin and imazethapyr on soils, sediments, clays, and ion exchange resins was studied. Multivariate regression analysis of Freundlich K values and soil and sediment properties yielded a model for imazaquin adsorption which included soil pH, organic carbon, and clay as significant independent variables. Clay content and pH were significant independent variables for the imazethapyr adsorption model. Adsorption of imazaquin and imazethapyr was greater at pH below 6 than at a higher pH. Adsorption of both herbicides on Ca- and H/Al-kaolinite and Ca-montmorillonite was low, but increased on H/Al-montmorillonite. The results of adsorption studies with ion exchange resins indicated that binding of protonated herbicide at low pH could be due to physical forces, hydrogen bonding, or cationic binding. At soil pH levels above the pKa's of imazaquin and imazethapyr, ionic bonding of herbicide anions with positively charged soil components could also occur.