In greenhouse studies, the butyl ester of fluazifop {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid} applied preemergence at 0.035 kg ai/ha gave 91% control of goosegrass (Eleusine indica Gaertn. ♯ ELEIN), 79% control of large crabgrass [Digitaria sanguinalis (L.) Scop. ♯ DIGSA], and 73% control of giant foxtail (Setaria faberi Herrm. ♯ SETFA). In field studies, the butyl ester of fluazifop at 0.56 kg ai/ha applied preemergence gave 79% control of goosegrass, 76% control of large crabgrass, and 53% control of giant foxtail. The butyl ester of fluazifop at 0.07 kg/ha gave 79 to 85% control of each grass at the pretillering stage in field studies. The same rate applied at the early tillering stage gave 84% control of goosegrass but only 53 and 58% control of large crabgrass and giant foxtail, respectively. Relative species response was similar from spray and leaf-spot applications of the herbicide. The butyl ester of fluazifop at 4 μg/plant when spot-applied to leaves of the three grasses at the tillering stage resulted in 65% control of goosegrass but only 20 and 25% control of large crabgrass and giant foxtail, respectively.

The spatial and temporal pattern of wild oat emergence in eastern Washington is affected by the steep, rolling hills that dominate this landscape. The objective of this study was to assess the impact of landscape position and crop residue on the emergence phenology of wild oat. Emergence of a natural wild oat infestation was characterized over two growing seasons (2003 and 2004), at two wheat residue levels (0 and 500 g m−2), and at five landscape positions differing in slope, aspect, and elevation in a no-till winter wheat field. Wild oat emerged 1 to 2 wk earlier at south-facing landscape positions than at north-facing landscape positions. Crop residue delayed wild oat emergence by 7 to 13 d relative to bare soil at south-facing positions in 2003 and had a reduced effect on emergence at north-facing landscape positions. Therefore, preserving surface residues tended to synchronize emergence across the landscape and may facilitate better timing of weed control where residue is present. Emergence of wild oat was modeled as a function of thermal time adjusted by water potential using a Weibull function. Temperature explained more variation in the model than water potential. This model explained much of the variability in wild oat emergence among landscape positions over these 2 yr and may be useful as a tool to predict the timing of wild oat emergence. Results also indicate that site-specific modeling is a plausible approach to improving prediction of weed seedling emergence.

Germination studies were conducted on seed of wild poinsettia (Euphorbia heterophylla L.) collected in 1974 and 1975. Significant differences in germination existed between the seedlots. An alternating temperature regime of 25/35 C was optimum for germination, and no effect of light was noted at this temperature regime. However, light increased germination at temperatures of 25, or 35 C, and decreased it at 10/35 C. Storage of wild poinsettia seeds at 36 C for 2 to 12 weeks caused a significant decrease in dormancy as compared to corresponding storage at 5 C. Seed moisture levels below 7.7% did not seem to affect viability at 5 or 25 C after a storage period of 3 to 9 months. Seed viability decreased rapidly when seed were stored for 3 months at 25 C with a moisture content of 10.8% or at 5 or 25 C with a moisture content of 18.6%. Seeds buried in the fall at a depth of 5 cm germinated extensively in the field 9 months later. Field germination decreased as depth of burial increased.

The timing of weed seedling emergence relative to the crop is important in planning and optimizing the time of weed control, but very little work has been done to predict seedling emergence of tropical weed species, especially in low-input and small-scale farms. We developed a simple model based on hydrothermal time to predict seedling emergence of tropic ageratum. Hydrothermal time at 2-cm soil depth was calculated from soil moisture and soil temperature simulated from several micrometeorological and soil physical variables. The model was developed using 5 yr of field emergence data from a continuous corn–cassava production system in southwestern Nigeria. Percentage of cumulative seedling emergence from the 5-yr data set was fitted to cumulative soil hydrothermal time using a Weibull function. The predicted cumulative emergence curve significantly matched observed field emergence (r 2 = 0.83). Model predictions were evaluated with root mean square error (RMSE) using four field emergence data sets from southeastern Nigeria (RMSE ≤ 10.1) and Los Banos, Philippines (RMSE = 8.9). RMSE values ≤ 10 indicated that predictions represented observations well. With such models, extension personnel working on tropical soils, especially in West Africa, may be able to provide additional advice to farmers on the appropriate time for the management of tropic ageratum.

The partitioning coefficient is defined as the proportion of new dry matter partitioned among different plant parts. Partitioning coefficients can be used to model plant dry matter accumulation. In 1994 and 1995, field studies were conducted at two locations near Manhattan, KS, to determine the influence of density and relative time of emergence of redroot pigweed on dry matter partitioning to stem, leaves, and reproductive parts throughout the season. Redroot pigweed was grown with sorghum and in monoculture at densities of 2, 4, and 12 plants m−1 of row each year at each location. Dry matter partitioning during vegetative growth was not influenced by plant density. However, partition coefficients during the reproductive growth stage changed as a linear function of the time of pigweed emergence relative to the sorghum leaf stage. The later the emergence time relative to sorghum leaf stage, the higher the partitioning coefficient values for leaf (PCleaf) and stem (PCstem) and the lower the partitioning coefficient values for reproductive parts (PCrp). The observed differences in partitioning coefficients due to relative emergence time are valuable information to those interested in simulating growth of competing plant species, especially with reference to their seed production.

In a replacement series study, barley was more competitive than green foxtail and wild oat at 28/22 and 22/16 C. Wild oat was more competitive with green foxtail at 22/16 C than at 28/22 C. Maximum green foxtail dry weight and leaf area was produced at 28/22 C. As temperature increased, maximum dry weight and leaf area occurred earlier during plant growth. Similarly, maximum wild oat leaf area occurred earlier with increasing temperature, but the greatest leaf area was observed at 16/10 C. Leaf area ratio (LAR) and relative growth rate (RGR) of wild oat did not differ markedly between temperature regimes. Green foxtail LAR was higher while RGR was lower at 16/10 C than at 22/16 or 28/22 C. Under the 16/10 C regime green foxtail produced little mature seed. Greatest seed numbers were produced at 28/22 C. Wild oat produced mature seed under all temperature regimes with the greatest seed numbers produced at 22/16 C.

Effects of a 28% N solution containing urea and ammonium nitrate (28% UAN) on field efficacy of the methyl ester of thifensulfuron and combinations of thifensulfuron and the methyl ester of chlorimuron were investigated. Velvetleaf control and soybean injury increased as rates of thifensulfuron increased (0, 2.2, 4.4, and 5.8 g ai/ha) and as rates of chlorimuron increased (0, 4.4, and 8.8 g ai/ha). Treatments that included 28% UAN at 9 L/ha controlled velvetleaf 9% better, averaged across herbicides and rates 21 days after treatment (DAT), but also increased soybean injury. Thifensulfuron applications controlled common lambsquarters well, but control was not affected by additions of 28% UAN. In studies where 14C-thifensulfuron was applied without an additive to the second true leaf of growth-chamber-grown velvetleaf, less than 4% of that applied was absorbed 84 h after treatment (HAT), whereas treatments that included nonionic surfactant or 28% UAN absorbed 33 and 45%, respectively. When both nonionic surfactant and 28% UAN were added, plants absorbed 76% 84 HAT. Translocation 84 HAT was also greatest with both additives and the majority moved to tissues above treated leaves. The 28% UAN increased uptake of thiameturon regardless of soil fertility level.

Peak germination and emergence of common lambsquarters usually occur in early to mid-spring, but both processes can occur during summer and fall. Seeds produced by different common lambsquarters cohorts (seedlings that emerge at nearly the same time) may vary in dormancy status, response to environmental conditions, and response to management factors. Therefore, experiments were conducted to determine the influence of different cohorts on common lambsquarters demography. Field experiments determined plant density, biomass, and seed production of different common lambsquarters cohorts within a crop-free community of annual weed species that included redroot pigweed, giant foxtail, and velvetleaf. Common lambsquarters plant density and aboveground biomass were greater for a mid-May cohort than for early June, late June, mid-July, or early August cohorts, but seed production of the mid-May and early June cohorts did not differ (about 192,000 seeds m−2) and was greater than that of other cohorts (111,500 seeds m−2 or less). In the laboratory, percent germination prior to stratification (exposure of seeds to low temperatures) was less for seeds harvested from early May and late May cohorts (≥ 9%) than those of mid-June or early July cohorts (≤ 75%). After stratification in the field, percent emergence (seedlings per number of planted seeds) and mean emergence time were similar among early May, late May, mid-June, and early July cohort seed sources, and were not influenced by shallow burial in soil. These results suggest that recruitment from seeds produced by different common lambsquarters cohorts is similar, but proportional to the number of seeds produced by each cohort.

Two greenhouse experiments were conducted to study the influence of 12 soil-applied herbicides on the sprouting and mortality of yellow nutsedge (Cyperus esculentus L.) tubers. Alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide], butachlor [N-(butoxymethyl)-2-chloro-2′,6′-diethylacetanilide], cycloate (S-ethyl N-ethylthiocyclohexanecarbamate), EPTC (S-ethyl dipropylthiocarbamate), napropamide [2-(α-naphthoxy)-N,N-diethylpropionamide], and U-27267 (3,4,5-tribromo-N,N,α-trimethylpyrazole-1-acetamide) delayed sprouting of tubers and provided 6 to 12 weeks control, but failed to kill tubers. Tubers appeared to escape injury by failing to sprout until activity of the herbicides had substantially dissipated. Herbicides known to interfere with photosynthesis, atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], bromacil (5-bromo-3-sec-butyl-6-methyluracil), methazole [2-(3,4-dichlorophenyl)-4-methyl-1,2,4-oxadiazolidine-3,5-dione], prometryne [2,4-bis(isopropylamino)-6-(methylthio)-s-triazine], San-6706 [4-chloro-5-(dimethylamino)-2-(α,α,α-trifluoro-m-tolyl)-3(2H)-pyridazinone], and terbacil (3-tert-butyl-5-chloro-6-methyluracil) did not delay sprouting of tubers, but killed shoots after emergence. In addition to controlling vegetative growth, the photosynthetic inhibitors killed tubers by rapidly exhausting the food reserves of these storage organs.

Microarray analysis was used to identify changes in gene expression in corn leaves collected from plants at the V11–14 growth stage that resulted from competition with velvetleaf. The plants were grown in field plots under adequate N (addition of 220 kg N ha−1) and irrigation to minimize N and water stress. Consequently, only differences resulting from competition for micronutrients, light, and perhaps allelopathic stress were anticipated. Genes involved in carbon and nitrogen utilization, photosynthesis, growth and development, oxidative stress, signal transduction, responses to auxin and ethylene, and zinc transport were repressed in corn growing in competition with velvetleaf. Very few genes were induced because of competition with velvetleaf, and those that were provided little indication of the physiological response of corn. No differences were observed in genes responsive to water stress or sequestering/transporting micronutrients other than zinc, indicating that these stresses were not a major component of velvetleaf competition with corn at the developmental stage tested.

Variation in crop–weed interference relationships has been shown for a number of crop–weed mixtures and may have an important influence on weed management decision-making. Field experiments were conducted at seven locations over 2 yr to evaluate variation in common lambsquarters interference in field corn and whether a single set of model parameters could be used to estimate corn grain yield loss throughout the northcentral United States. Two coefficients (I and A) of a rectangular hyperbola were estimated for each data set using nonlinear regression analysis. The I coefficient represents corn yield loss as weed density approaches zero, and A represents maximum percent yield loss. Estimates of both coefficients varied between years at Wisconsin, and I varied between years at Michigan. When locations with similar sample variances were combined, estimates of both I and A varied. Common lambsquarters interference caused the greatest corn yield reduction in Michigan (100%) and had the least effect in Minnesota, Nebraska, and Indiana (0% yield loss). Variation in I and A parameters resulted in variation in estimates of a single-year economic threshold (0.32 to 4.17 plants m−1 of row). Results of this study fail to support the use of a common yield loss–weed density function for all locations.

Field bindweed is extremely susceptible to aminocyclopyrachlor compared to other weed species. Laboratory studies were conducted to determine if absorption, translocation, and metabolism of aminocyclopyrachlor in field bindweed differs from other, less susceptible species. Field bindweed plants were treated with 3.3 kBq 14C-aminocyclopyrachlor by spotting a single leaf mid-way up the stem with 10 µl of herbicide solution. Plants were then harvested at set intervals over 192 h after treatment (HAT). Aminocyclopyrachlor absorption reached a maximum of 48.3% of the applied radioactivity by 48 HAT. A translocation pattern of herbicide movement from the treated leaf into other plant tissues emerged, revealing a nearly equal aminocyclopyrachlor distribution between the treated leaf, aboveground tissue, and belowground tissue of 13, 14, and 14% of the applied radioactivity by 192 HAT. Over the time-course, no soluble aminocyclopyrachlor metabolites were observed, but there was an increase in radioactivity recovered bound in the nonsoluble fraction. These results suggest that aminocyclopyrachlor has greater translocation to belowground plant tissue in field bindweed compared with results from other studies with other herbicides and other weed species, which could explain the increased level of control observed in the field. The lack of soluble metabolites also suggests that very little metabolism occurred over the 192 h time course.

In field experiments, corn (Zea mays var. Dixie 18) and Johnsongrass (Sorghum halepense) seeds were planted in Norfolk sandy loam soil and treated preemergence with 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine) at 0, 2, 4, and 16 lb/A. In growth chamber studies, corn seeds were planted in flats of Independence loamy fine sand and treated pereemergence with atrazine at 0, 2, and 4 lb/A. In nutrient culture studies, corn was treated with 0, 2, 4, and 8 ppmw atrazine.

Treated plants of all species and rates tested usually were smaller than untreated plants and contained higher nitrogen percentages. Corn and Johnsongrass plants treated with high rates of atrazine always contained less nitrogen (mg/plant) than untreated plants. Atrazine-treated corn plants contained higher percentages of both 80% ethanol soluble and insoluble nitrogen than the checks. Percentage increases in both fractions were proportional to the rate of atrazine treatment. Nitrate percentages also were increased, but free ammonia content was not significantly affected.

Three thiocarbamate herbicides, S-ethyl dipropylthiocarbamate, S-propyl dipropylthiocarbamate, and S-propyl butylethylthiocarbamate, applied at a depth of 1.5 inches in Tifton loamy sand were much more toxic to yellow nutsedge (Cyperus esculentus L.), sicklepod (Cassia tora L.), and Texas millet (Panicum texanum Buckl.) than when applied at 0.75 inches. The optimum depth of application may vary with different soils. Optimum subsurface placement was more effective than either soil incorporation or preemergence surface application.

The target site of glyphosate [N-(phosphonomethyl)glycine] inhibition in plants and bacteria is 5-enolpyruvylshikimate 3-phosphate (EPSP) synthase. Our strategy for developing glyphosate-resistant crops has been to genetically engineer plants with a gene that codes for EPSP synthase with low sensitivity in glyphosate. We cloned such a gene from the aroA locus of a glyphosate-resistant mutagenized strain of Salmonella typhimurium. The enzyme encoded by this gene has a single amino acid change resulting in lower affinity for glyphosate and higher affinity for substrates than either plant or wild-type bacterial counterpart. A chimaeric gene containing the mutant aroA gene behind the octopine synthase promoter was constructed and integrated into Agrobacterium T-DNA vectors. Analysis of gall tissue from Brassica campestris L. (turnip rape) infected with A. tumefaciens K12 containing this chimaera showed mRNA and protein expressed from the bacterial gene; 50% of the total EPSP synthase activity present had kinetic properties of the mutant bacterial enzyme. Tobacco (Nicotiana tabacum L. ‘Xanthi′) plants have been regenerated from cocultivation with A. rhizogenes containing the same construct; analysis indicates expression of the gene and enhanced tolerance to glyphosate.

Rigid brome is a problematic weed of southern Australian cropping systems. Increased knowledge about the ecology of rigid brome and the influence of management strategies on its seedbank dynamics could facilitate development of more effective weed control programs. A field study was undertaken to investigate seedbank persistence and the influence of different management strategies on rigid brome control at Lock in South Australia during 2003 to 2005. Seeds of rigid brome were found to persist in the soil for up to 3 yr, with > 20% of the seedbank persisting from one season to the next. Therefore, a single year management program against this weed species is likely to be ineffective and could result in rapid buildup in weed infestations. However, management strategies that combined effective herbicides (ClearfieldTM technology) and crop competition over consecutive years provided effective control of this troublesome weed. Such cropping systems reduced rigid brome density (1 to 10 plants m−2) and seed production (8 to 160 seeds m−2) in the final crop of the 3-yr cropping sequence as compared to common grower practice of trifluralin and triasulfuron mixtures (138 plants m−2; 1,866 seeds m−2). These treatment combinations were able to deplete the initial seedbank (1,748 seeds m−2) to manageable levels (< 5 seeds m−2) within 3 yr. The results of this study should provide growers with confidence that severe rigid brome infestations can be managed effectively without compromising crop productivity.

Germination of tumble mustard (Sisymbrium altissimum L.), tansy mustard (Descurainia pinnata (Walt.) Britton), and yellowflower pepperweed (Lepidium perfoliatum L.) is partially dependent on total degree hours of incubation (temperature above 0 C times the duration of incubation). This relation was similar under constant or alternating temperatures. Alternating temperature regimes with −20 C inhibited germination, and greatly depressed subsequent germination at optimum temperatures. Tansy mustard and yellowflower pepperweed have temperature-dependent after-ripening requirements. Germination of all three species responded similarly to osmotic stress. The relation between tumble mustard germination under osmotic or soil moisture stress is dependent on the texture of the soil substrate. A mucilaginous coating greatly aids the germination of yellowflower pepperweed seeds on the surface of media under low (0.05 to 0.2 bars) moisture tension. Germination of seeds of the three species recovered in soil samples from the field is radically different from that of freshly harvested material. The seeds acquire a dormancy which is broken irregularly under ideal conditions for germination.

In field experiments during 1975 to 1977 the final yield of cucumbers (Cucumis sativus L. ‘Salty’) was unaffected when a naturally occurring weed infestation remained in the plots for the first 12 days after emergence in 1975 and for the first 24 days in 1976 and 1977, provided the crop was kept weed-free subsequently. If the cucumbers were kept weed-free for the first 12 days after emergence in 1975, 24 days after emergence in 1976, and 36 days after emergence in 1977, there was no loss of yield caused by weeds that emerged later in the season. The critical period of weed interference was therefore between 12 and 36 days after crop emergence. In other experiments, cucumber yields were reduced significantly when only 5% of the weeds were allowed to remain in the plots longer than 24 days after crop emergence. The percentage of various commercial grades, based on cucumber size, was not altered by weed interference and appeared to be dependent on harvesting frequency.

Common groundsel adapts readily to new environments and selection pressures and has been variably described as both a winter and summer annual. We characterized germination response to temperature in seeds from populations occurring at six sites along a 700-km north–south transect (Kentucky to Michigan). Seeds were collected in 2000 and 2002 from randomly selected plants (350 to 400), at each sampling site. Two germination patterns were observed: (1) seeds from the southern locations averaged 80 to 90% germination across the range of 5 to 25 C; and (2) seeds from northern locations had reduced germination when incubation temperatures were close to 5 or 25 C. When seed from all locations were grown in a common environment (14/10-h thermoperiod of 22/18 C), their progeny had a germination response that was similar across the temperature gradient, regardless of original location, suggesting germination of the parent seed was due to maternal environmental effects. In a subsequent experiment, common groundsel was grown in growth chambers with warm long days (22/15 C and 16 h of light), warm short days (8 h of light), cold long days (15/8 C and 16 h of light), and cold short days. Eighty percent of seeds from the warm environments germinated across the range from 5 to 25 C indicating that these maternal conditions had produced nondormant seeds. In contrast, 20% or fewer of the seeds from plants in the cold chambers germinated regardless of temperature, suggesting that dormancy had been induced by the cool maternal environment. Results also indicated that signaling of maternal environment varied with inflorescence development stages, meaning the earlier the inflorescences are exposed to cold conditions, the lower the percent germination in F1 seeds. Preventing seed maturation on common groundsel growing under cool conditions may reduce the formation of a persistent seed bank.

Crops that effectively compete with weeds may be more suitable in low-input agricultural systems. A 2-yr field experiment was conducted in northern Greece to assess the competitiveness of seven annual aromatic plants (anise, sweet fennel, sweet basil, dill, coriander, parsley, and lacy phacelia) on common purslane, common lambsquarters, black nightshade, and barnyardgrass. The phytotoxicity of the essential oils produced by these aromatic plants was also determined using a perlite-based bioassay with barnyardgrass. Separation, identification, and quantification of the volatile compounds of these essential oils were also performed. After the harvest of the aromatic plants (8 wk after planting), the greatest weed fresh weight reduction (94 to 100%) was recorded in lacy phacelia, whereas the least (0 to 30%) was recorded in parsley. Lacy phacelia and sweet fennel produced the greatest fresh biomass yield in weedy and weed-free treatments, whereas parsley, dill, and coriander produced the lowest. Biomass of sweet fennel and anise was reduced by only 9 to 11% by weed competition, whereas biomass of lacy phacelia was not significantly affected. The essential oils isolated from sweet fennel and sweet basil were the most phytotoxic on barnyardgrass, whereas those isolated from lacy phacelia and anise were the least phytotoxic. Conclusively, aromatic plants with great competitiveness such as lacy phacelia, anise, and sweet fennel provided great weed suppression and they could be cultivated with low inputs in herbicides. However, high competitiveness of aromatic plants may not always be correlated with high essential oil phytotoxicity.