Metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H)one] applied preemergence resulted in increased phytotoxicity with increasing soil pH. In the field, fall panicum (Panicum dichotomiflorum Michx.) control, number of dead corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] plants, and crop injury ratings increased, and plant height and grain yield decreased as the soil pH increased. In the greenhouse, corn and soybean grown in soil showed decreased dry weights due to metribuzin as pH increased. By contrast, the pH response to metribuzin was not evident in nutrient-sand culture, indicating that, in soil, pH affected the availability of metribuzin for plant absorption.

The adsorption of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] was measured on soil at water:soil ratio of 5:1 and 0.4:1. The adsorption isotherms for atrazine were described by the Freundlich equation. A decrease in water:soil ratio and in soil moisture content led to an increased adsorption of the herbicide. Increasing the concentration of electrolytes in the soil solution by adding solutions up to 0.1M CaCl2 increased atrazine adsorption. Greater amounts of atrazine were adsorbed at 30 than at 5 C on four soils at 0.1 bar moisture content. This indicates an endothermic reaction which was observed both before and after correction for differential atrazine solubility due to temperature. Thermodynamic quantities associated with the adsorption reaction were calculated to characterize the adsorption of atrazine on soil.

Crofton weed is a noxious invasive weed worldwide. However, the mechanisms underlying its invasiveness are still not well understood. We hypothesize that genetic differentiation and plasticity may help the plant invade heterogeneous habitats. To test this hypothesis, we compared the differences in seed morphometric and germination traits among 14 populations of the plant located at different elevations (640–2,430 m) in south Yunnan Province of southwest China. Germination capacity (GC) and index (GI) were markedly different among the 14 populations at the same temperature or water treatment. The results indicated genetic differentiations in these traits; maternal effects were not excluded. GC and GI were also different for each population across temperature or water treatments, showing phenotypic plasticity. Croton weed seed size and weight also responded genetically or plastically (or both) to different elevations. Seed width, weight, GC, and GI at each temperature increased with the increase of the elevation of origin, showing clinal variation patterns that reveal local adaptations to habitats encountered by the plant. The large seeds, high GC, and high GI could improve emergence, establishment, growth, and survival of seedlings at high elevation; the low GI could prevent seed germination before the onset of the rainy season at low elevation. On the basis of the results, genetic differentiation and plasticity in seed size and germination traits may help crofton weed acclimate to different elevations, facilitating its invasiveness.

The absorption and translocation of sethoxydim {2[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} as influenced by bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] were studied in goosegrass [Eleusine indica (L.) Gaertn. ♯3 ELEIN]. The presence of bentazon in the treatment emulsion decreased foliar absorption of 14C applied as 14C-sethoxydim by about half. Significantly less 14C was found in the treated leaf, shoot tissue above the treated leaf, shoot tissue below the treated leaf, and roots of plants treated with a mixture of 14C-sethoxydim and bentazon than with 14C-sethoxydim alone. After 6 h, 3.5% of applied 14C was recovered from plant parts other than the treated leaf in the absence of bentazon, compared to 1.1% of that applied in combination with bentazon. The difference in foliar absorption between the two treatments may account for the antagonistic interaction between these herbicides observed in the field.

Joint action of the effects of atrazine and mesotrione can lead to synergistic herbicidal activity in broadleaf weed species. The objective of these experiments was to determine if specific rates are required to provide synergistic joint activity between mesotrione and atrazine in both triazine-sensitive (TS) and triazine-resistant (TR) redroot pigweed. Herbicide rates were evaluated in TS and TR redroot pigweed in two experiments: a dose response of mesotrione alone and in mixture with a constant rate of atrazine and a dose response of atrazine alone and in mixture with a constant rate of mesotrione. Results from these experiments revealed that synergism was detected in the TS pigweed when 56 g ai ha−1 mesotrione was mixed with 126 g ai ha−1 atrazine. In the TR pigweed, synergism was detected when mesotrione rate at 10 to 56 g ha−1 was mixed with a constant rate of atrazine at 126 g ha−1. Additionally, when mesotrione was held constant at 10 g ha−1, synergism was detected in mixture with atrazine from 31 to 3556 g ha−1 in TR pigweed. Furthermore, in TR pigweed, analysis of slope deviation across the dose-response curves of mesotrione with and without atrazine revealed a divergence that increased in magnitude as the rate of mesotrione increased. In other words, increased synergism was observed with increased mesotrione rate in the TR pigweed, which was also supported by biomass reduction and atrazine-like injury to the leaves. An additional experiment investigated synergism between mesotrione and bromoxynil in both TS and TR pigweed. Mesotrione at 10 g ha−1 was synergistic when paired with bromoxynil from 70 to 210 g ha−1 in both the TS and the TR pigweed.

The impact of two weed management systems on the weed seed reserves of the soil, on the yearly weed problem, and on barley (Hordeum vulgare L. ‘Steptoe’), corn (Zea mays L. ‘Pioneer 3709′), and sugarbeet (Beta vulgaris L. ‘Mono Hy D2′) production was assessed where these crops were grown in rotation for 6 consecutive years. Weeds were controlled in each crop with a moderate (system I) or intensive (system II) level of herbicides, plus conventional tillage. Weed seeds from seven annual genera were identified, with redroot pigweed (Amaranthus retroflexus L. ♯3 AMARE) and Chenopodium comprising 56 and 30%, respectively, of the initial 1377 million weed seeds/ha that were present in the upper 25 cm of the soil profile. After the sixth cropping year, the overall decline in the total number of weed seeds in soil was 96% in system I and 97% in system II. Over the 6-yr period, about 1.3 times more weeds escaped control in system I than in system II; and within a crop, the fewest number of weeds escaped annually in sugarbeets, and the most in barley. Yields of barley grain, corn silage, and recoverable sucrose were similar each year in the two weed management systems.

Field studies were conducted to evaluate the effect of quackgrass [Agropyron repens (L.) Beauv. ♯ AGRRE] density and soil moisture on corn (Zea mays L.) growth and yield. Quackgrass densities ranging from 65 to 390 shoots/m2 reduced corn yield 12 to 16%. A quackgrass density of 745 shoots/m2 reduced corn yields an average of 37% and significantly reduced corn height, ear length, ear-fill length, kernels/row, rows/ear, and seed weight. In the soil moisture study, quackgrass was shorter than corn throughout the growing season, and analyses of corn leaf tissue indicated that quackgrass did not interfere with the nutrient status of the corn. In 1979, soil moisture was not limiting and corn yields were similar in all treatments regardless of irrigation or the presence of quackgrass. In 1980, soil moisture was limited and irrigation increased the yield of quackgrass-free corn. Irrigation also increased the yield of quackgrass-infested corn to a level similar to irrigated corn. When light and nutrients are not limiting factors, an adequate supply of soil moisture can eliminate the effects of quackgrass interference on the growth, development, and yield of corn.

In Europe, 18 weedy grass species had been confirmed to have biotypes with resistance to herbicides. The most frequent is that of atrazine resistance, with nine resistant biotypes found. These biotypes are mainly resistant because of changes in the D1 protein of photosystem II. All atrazine-resistant biotypes, except that of bristly foxtail, show cross-resistance to s-triazine and as-triazines. From an agriculture point of view, the most important cases of resistance are those found in blackgrass, wild oat, Italian ryegrass, rigid ryegrass, and barnyardgrass. In these species, cross- and multiple resistances were observed due to metabolism or changes in the target protein by genetic mutations or both. These biotypes are extremely difficult to control with alternative herbicides.

Benzyl isothiocyanate (BITC) was extracted from mature papaya (Carica papaya L.) seeds and applied to imbibed velvetleaf (Abutilon theophrasti Medic. ♯3 ABUTH) seeds. Complete inhibition of germination occurred at a concentration of 6 × 10-4 M BITC, and only 28% (compared to controls) of the seeds germinated after 4 days exposure to the 4 × 10-4 M treatment. Corn (Zea mays L. ‘DeKalb XL-66′) was unaffected even at higher concentrations; however, 72% of imbibed soybeans [Glycine max (L.) Merr. ‘Tiger’] germinated in 6 × 10-4 M BITC. When this natural inhibitor was applied to etiolated velvetleaf seedlings at 4 × 10-4 M, 100% died in 2 days.

Solar heating (SH) of the soil by mulching it with transparent polyethylene (PE) during the hot season elevated the soil temperature by 10 to 18 C above that of the non-mulched soil. SH for 4 to 5 weeks resulted in effective control of most summer and winter annual weeds, the effect lasting for more than 5 months after PE removal. Melilotus sulcatus Desf., Astragalus boeticus L. and bull mallow (Malva nicaeensis All. # MALNI) were not controlled by SH. Perennial weeds which propagate from vegetative parts were only partially controlled with short SH, but mulching for 8 to 10 weeks improved control. Mulching the soil with perforated or shaded transparent PE or black PE resulted in a smaller increase of soil temperature and thus less efficient weed control. A combination of SH with soil-incorporated EPTC (S-ethyl dipropylthiocarbamate) or vernolate (S-propyl dipropylthiocarbamate) did not improve the weed control over SH alone, but significantly enhanced the disappearance of the herbicides from the soil. SH inhibited the disappearance of fluridone {1-methyl-3-phenyl-5-[3-(trifluoromethyl) phenyl]-4(1H)-pyridinone} but did not change the residual phytotoxicity of bromacil (5-bromo-3-sec-butyl-6-methyluracil). SH treatment improved plant growth and increased the yield of wheat (Triticum aestivum L. ‘895′) and turnip (Brassica rapa L. ‘Purple top’), but not of parsley (Petroselinum sativum Hoffm.).

No-tillage offers potential for improved soil quality, reduced erosion, and equal or increased crop yields. We hypothesized that, compared with conservation tillage (CT), no-tillage (NT) offers conditions more conducive to microbial decay of weed seed. In NT systems seed remain at or near the soil surface where crop residues, moisture, and lack of disturbance create an environment with greater soil microbial diversity. In late fall of 1998 and 1999, dormant seed of wild oat, either individually glued to plastic toothpicks or mixed with soil and placed in mesh bags, were buried (mean seed depth of 2.5 cm) in replicated field plots managed by NT or CT since 1982. Treatments including fungicide seed treatment (thiram + metalaxyl + captan) and soil fumigation (propylene oxide) provided estimates of the contribution of microorganisms to observed mortality. Seed were retrieved in May and August, 1999 and 2000. Contrary to our original hypothesis, the proportion of dead seed was generally similar in NT and CT systems. Lack of tillage system by seed or soil treatments affecting the proportion of dead or decayed seed suggests that the contribution of microorganisms to seed fate is similar in these tillage environments. However, the proportion of dormant seed was consistently lower in the NT compared with CT treatments; there was a corresponding increase in the proportion of germinated seed. Overall, more than half of the wild oat seed bank losses could be directly attributed to germination whereas losses due to decay were relatively minor by comparison. Despite favorable distribution of seed and improved quality of the surface-strata of soil in NT systems, this study fails to provide evidence that enhanced microbial decay will contribute to a “weed-suppressive” capacity in such cropping systems.

Soil solarization is a process of soil disinfestation that involves solar heating of moist soil covered with transparent polyethylene film. This nonchemical approach to controlling soil-borne pests is being investigated as an alternative to methyl bromide fumigation. Summer solarization controlled annual weed species and suppressed purple nutsedge. Although some nutsedge tubers sprouted despite the solarization treatment, the resulting shoots were almost always trapped under the clear solarization film. Conversely, in rows that were mulched with black film, nutsedge rhizomes punctured the film so that leaf expansion occurred above the film. In controlled pot experiments conducted in darkness, yellow nutsedge rhizomes readily penetrated 19- and 30-μm clear films as effectively as opaque films. Thicker clear films and bubble film reduced nutsedge penetration. In the greenhouse and laboratory, nutsedge penetration of transparent polyethylene film was inversely related to irradiance levels when the film was in direct contact with the soil. However, when there was a 5- to 10-mm space between the soil and the film, the lowest irradiance level (30 μmol m−2 s−1) was as effective as 320 μmol m−2 s−1 in reducing penetration by purple nutsedge. The film penetration by nutsedge rhizomes appears to be linked to a light-dependent morphological change from rhizome growth to leaf development, which occurs before film penetration with clear mulch and after film penetration with opaque mulch. The alternate sprouting and foliar scorching of nutsedge shoots trapped under clear films could potentially be exploited to deplete nutsedge tubers that occur at soil depths that do not develop lethal temperatures under soil solarization.

We studied the molecular genetics of dormancy-related traits in an F2 family of poplar (Populus) hybrids derived from a cross between a northern genotype of black cottonwood and a southern genotype of eastern cottonwood by mapping quantitative trait loci (QTLs) and candidate genes. Dormancy-related traits included timing of vegetative bud set, fall frost damage, chilling response, timing of vegetative bud flush, and winter survival under field conditions, as well as photoperiodic responses (timing of bud set and number of new leaves) in a warm greenhouse under either a uniform 8-h photoperiod or a naturally shortening photoperiod in the fall. QTL analyses were conducted using a linkage map consisting of AFLP, microsatellite, and candidate gene markers. The candidate genes were chosen because of their potential roles in either photoperiodic perception (PHYB1, PHYB2) or abscisic acid signal transduction (ABI1B, ABI1D, ABI3). Significant QTLs were detected for all dormancy-related traits, except for winter survival, which had a relatively low heritability compared with the other traits. Interestingly, half of the field bud set QTLs did not map near photoperiodic QTLs. This is consistent with the moderate genetic correlation between these traits (0.53 to 0.60) and suggests that genetic differences in photoperiodic responses play only a modest role in explaining genetic differences in the timing of bud set under field conditions. Except for ABI1D, each of the candidate genes tested mapped near one or more of the dormancy-related QTLs. We conclude that molecular markers and QTL analyses can be used to study the genetics of dormancy-related traits, to design more effective breeding programs, and to provide new insights into tree physiology.

Johnsongrass is one of the most troublesome weeds of the world and is listed as a noxious weed in Arkansas. Reduced johnsongrass control with the recommended application rate of glyphosate (840 g ae ha−1) was reported in a continuous soybean field near West Memphis, AR, in the fall of 2007. A greenhouse study was conducted (1) to confirm and characterize glyphosate resistance in the johnsongrass biotype from West Memphis and (2) to determine whether resistant and susceptible biotypes have differential glyphosate absorption or translocation. Dose–response studies revealed that the resistant biotype was five- to seven-fold less sensitive to glyphosate than the susceptible biotype. Glyphosate absorption was similar in resistant and susceptible biotypes at 72 h after treatment (HAT). However, the treated leaf of the resistant biotype retained 28 percentage points more absorbed 14C glyphosate compared to the susceptible biotype at 72 HAT. Additionally, the resistant biotype had less 14C glyphosate translocated to the aboveground tissue below the treated leaf and to roots compared to the susceptible biotype at 24 and 72 HAT. Reduced translocation and increased retention of glyphosate in treated leaves is a probable mechanism of resistance in this glyphosate-resistant johnsongrass biotype.

The morphological symptoms of 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine) and 3-(p-chlorophenyl)-1,1-dimethylurea (monuron) toxicity were expressed in the light but not in the dark. Various wavelengths of narrow band light were produced by the use of colored plastic filters. Oats, var Kanota, and red kidney beans were used. The degree of injury was a function of light intensity, the higher the light intensity the greater the injury. The action spectrum indicated that chlorophyll was the principal absorbing pigment involved in this injury.

With a corn (Zea mays L.) yield of 144 bu/A on weed-free plots, giant foxtail (Setaria faberii Herrm.) reduced corn yields 1, 2, 5, 7, and 18 bu/A, respectively, when removed when 3, 6, 9, and 12 inches high and at maturity. With a soybean (Glycine max L.) yield of 30 bu/A on weed-free plots, the same treatments reduced soybean yields 0, 0, 1, 2, and 18 bu/A, respectively. Giant foxtail competing with corn delayed tassel emergence but had little effect on moisture of grain or on shelling percent at harvest. The total dry matter produced per acre remained relatively constant whether the dry matter was made up of corn alone or corn and weeds. The greatest competitive effect of giant foxtail with soybeans occurred after the reproductive stage of soybeans began or after the weeds became sufficiently dense to reduce the light which soybeans received. Early-season competition had greater effect on corn than on soybeans. However, the taller growing corn competed better than soybeans with giant foxtail later in the season.

Interseeded cover crops have the potential to maintain and improve soil quality, reduce the incidence of insect pests, and suppress weeds in vegetable production systems. However, the successful use of interseeded cover crops has been limited by their tendency to either inadequately suppress weeds or suppress both weeds and the crop. We hypothesized that in irrigated broccoli production, winter rye could suppress annual weeds through rapid emergence and shading, without adversely affecting the taller transplanted broccoli crop. In field experiments conducted in New York from 1999–2001, broccoli was cultivated at 0, 10, or 10 and 20 d after broccoli transplanting (DAT), with or without rye at the final cultivation. Rye interseeded at 0 DAT suppressed weeds and improved yields relative to unweeded controls but resulted in broccoli yield losses relative to weed-free controls in 2 of 3 years. Rye seeded at either 10 or 20 DAT did not reduce broccoli yields but had little effect on weeds for a given level of cultivation and resulted in Powell amaranth seed production of up to 28,000 seeds m−2. Rye interseeded at 0 DAT reduced light availability to weeds in 2000 but not in 2001 when Powell amaranth avoided shading from rye through rapid emergence and vertical growth. In greenhouse pot experiments, low temperatures for 7 d after seeding delayed the emergence of Powell amaranth by 3 d relative to rye and increased the suppression of Powell amaranth by rye from 61 to 85%. Our results suggest that winter rye may be more successfully integrated into broccoli production (1) when sown at higher densities, (2) in locations or seasons (e.g., spring) with lower initial temperatures, and (3) in combination with other weed management tools.

Greenhouse and laboratory experiments showed that the biological activity of EPTC (S-ethyl dipropylthiocarbamate) + R-25788 (N,N-diallyl-2,2-dichloroacetamide) was lost more rapidly in soils if they had been previously treated with this herbicide. Treatments which reduced or eliminated the microbial activity in the soil restored the herbicide activity in ‘problem’ soils. A combination of 2 kg/ha of the insecticide fensulfothion (diethyl-4-methylsulphinyl phenyl phosphorothionate) with EPTC + R-25788 also restored the biological activity of the herbicide. Field trials confirmed the decreased biological activity with repeated use of EPTC + R-25788; other herbicides recommended for grass weed control in corn (Zea mays L.) performed satisfactorily in these ‘problem’ soils. The performance of EPTC + R-25788 was improved by a rotation away from it for one season (using another herbicide) but the level of activity was still below that obtained in a soil treated with EPTC + R-25788 for the first time. A new formulation of EPTC + R-25788 containing a herbicide extender R-33865 (O,O-diethyl-O-phenyl phosphorothioate) was not affected by the enhanced microbiological breakdown and provided effective season-long weed control in the ‘problem’ soils.

Adsorption capacities of Walla Walla silt loam soil for 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) were determined after each extraction with ethyl ether, ethyl alcohol, and hot water. The results suggested that the ether and alcohol-extractable components of the soil organic matter, i.e. fats, oils, waxes, and resins, had a negligible capacity to adsorb atrazine but that hot-water-extractable materials, i.e. polysaccharides, had a small adsorptive capacity. of the compounds chosen to be representative of some soil organic matter components, polysaccharide types had low affinities, a protein and a nucleic acid had intermediate affinities, and humic acid, lignin, and quinizarin had high affinities for atrazine. Comparison of adsorption isotherms of atrazine to lignin and humic acid at 25 and 62 C suggested that a weak chemical bond may contribute to retention of the herbicide by soil organic matter. Nearly quantitative desorption of atrazine from native and calcium saturated soils was affected by repeated water extraction.

C4 plants account for a small fraction of the total number of plant species (fewer than 1000 out of 250 000). A larger proportion of the world's weed species possess C4 physiology. There are 2000 species in 500 genera and 125 families of flowering plants listed in the WSSA composite list of weeds. of that number, 146 species in 53 genera and 10 families exhibit the C4 syndrome. This, as a percentage, is 17-fold greater than the percentage of C4 plants in the total world plant population. In this report, we have listed the C4 -weed species and provide specific information concerning various aspects of their Kranz anatomy and C4 physiology.