Abutilon theophrasti is one of the worst agricultural weeds in North America, yet it has not reached that status in California in the 80 yr since it was first reported. The research reported here examined the distribution and modeled climatic requirements of A. theophrasti to determine whether it is likely to spread more widely in the state. Herbaria records and weed literature were surveyed to determine the historical occurrences of A. theophrasti in the state; current distribution was assessed through surveys sent to University of California personnel in each county. Combined results showed 42 counties out of 58 with A. theophrasti present historically or currently. A plot of the cumulative number of counties containing A. theophrasti by decade fit a logistic equation. The maximum rate of spread of this species occurred in 1962 and it is likely that its final distribution by county in California is leveling off and not likely to increase further. The climate-matching/mapping software CLIMEX® was used with observed and estimated parameters of environmental requirements of A. theophrasti to model its current distribution from India through China to Japan. The same model parameters were then used to map its potential distribution in California. Areas where A. theophrasti has been reported were predicted by CLIMEX to be poorly suited for its growth and development without added soil moisture in the form of irrigation. It appears that the Mediterranean climate is a deterrent to the integration of A. theophrasti into California. The climate-matching approach provided a biologically reasonable assessment of potential distribution of A. theophrasti in California. The approach also allowed assessment of the effects of common agricultural practices on potential distribution given the environmental requirements and limitations of A. theophrasti.

Tomatoes (Lycopersicon esculentum ‘Roma VF’) and four weeds were grown in various combinations in field plots in 1973 and 1974. Season-long interference by jimsonweed (Datura stramonium L.), tall morningglory [Ipomoea purpurea (L.) Roth], and common cocklebur (Xanthium pensylvanicum Wallr.) at densities of 11, 43, and 86/m2 reduced tomato yields in 1973. In 1974, tomato yields were reduced by these three broadleaf weeds at densities of 2.7, 5.4, 8.1, and 11/m2. Season-long interference by large crabgrass [Digitaria sanguinalis (L.) Scop.] reduced tomato yield at densities of 55, 215, and 430/m2 in 1973 and 11, 22, 33, and 55/m2 in 1974. The fresh weight of tomato shoots decreased with all weed densities in both years. Total weed shoot weight increased with density and individual weed weights decreased with increasing densities. Tomato fruit quality, as measured by soluble solids, acidity, and color, was not influenced by the various weeds and densities.

The potential losses from spray applications of glyphosate [N-(phosphonomethyl)glycine] with and without the inclusion of a polymer thickening agent were evaluated in field tests, applied under identified weather conditions with a boom-nozzle ground sprayer, helicopter, and fixed-wing aircraft. The downwind transport and diffusion of the spray loss was assessed by means of Mylar fallout sheets, high volume air samplers, and 4-leaf wheat (Triticum aestivum L. ‘Anza’) plants. Lowest drift losses occurred, under 0.01 g/ha on fallout sheets at 100 m, with deflector fan nozzles (KGF) operating at a low pressure (7 kPa) on the ground sprayer. Drift losses from a helicopter equipped with micro-jet nozzles were only slightly higher, less than 0.03 g/ha on fallout sheets at 100 m. Drift losses from 8003 nozzles mounted on a ground sprayer and D-6 jet nozzles directed back on fixed wing aircraft produced much higher drift deposits, from 0.2 to 1.0 g/ha on fallout sheets at 100 m. The highest levels of drift losses were obtained with D-4 jet nozzles directed down on a helicopter, 5 g/ha on fallout sheets at 100 m. Some evidence of reduced airborne drift was apparent where the polymer thickening agent was used with aircraft and ground equipment. The test results indicate that significant reduction in drift losses were obtained with proper application equipment; however, the use of a polymer thickening agent reduced drift only slightly for certain applications.

Prescribed burning and the herbicide clopyralid are very effective tools for the management of yellow starthistle. However, repeated use of either can be impractical or can present other problems. The potential solution is the development of an effective integrated weed management strategy using a combination of the two approaches. In small plot studies (0.2 ha), we tested one of five possible treatments: (1) untreated control, (2) 2 consecutive yr of clopyralid (0.105 kg ha−1), (3) 2 consecutive yr of prescribed summer burning, (4) first-year clopyralid followed by second-year prescribed burning, and (5) first-year prescribed burning followed by second-year clopyralid. Treatments were made in 1999 and 2000 at three study sites in California (San Benito, Yuba, and Siskiyou counties). In 2001, the year following the final treatment, 2 consecutive yr of clopyralid or first-year burning followed by second-year clopyralid consistently reduced yellow starthistle cover in the following year by 92 to 100%. However, at the Yuba site, clopyralid alone increased medusahead and ripgut brome cover. Although 2 consecutive yr of burning was effective in Yuba, very high levels of starthistle infestation in San Benito were not completely burned in the second year because of the lack of available consumable fuel. Clopyralid treatment the first year followed by prescribed burning in the second year stimulated yellow starthistle germination and did not reduce the infestation. In a large-scale study conducted at two sites (13 and 81 ha) in southern Monterey County, we used a first-year burn followed by either 2 yr of clopyralid (0.158 kg ha−1) or a single year of clopyralid (0.210 kg ha−1) and a subsequent burn. Results were in close agreement with those found in the small-scale studies. In the year following the final treatment, control of yellow starthistle was greater than 99% when the burn was followed by 2 yr of clopyralid. In contrast, when a prescribed burn was used in the last year of the program, the level of control was not as good, probably because of the increased germination of the remaining soil seedbank. These results indicate that a first-year prescribed burn followed by a second-year clopyralid treatment can provide consistently good control of yellow starthistle, as well as reduced levels of noxious annual grasses, including medusahead and ripgut brome.

Laboratory and greenhouse studies were conducted to determine the effect of temperature, soil moisture, light, planting depth, and rhizome water content on shoot emergence and vegetative growth of alligatorweed. Optimum shoot emergence and growth occurred at constant 30 C, and no shoot emergence was found below constant 5 C. A maximum shoot emergence of 93% occurred at constant soil moisture of 30% with temperatures of 10 to 35 C. Shoot emergence and growth decreased as rhizome water content decreased, and shoot emergence did not occur below a rhizome water content of 20%. Shoot emergence and growth decreased with burial depth; shoot emergence was above 90% when rhizomes were buried 0.5 to 1.0 cm deep compared to 16% when they were buried 18 cm deep. Alligatorweed shoot emergence and vegetative growth were not significantly affected by light. In the fields, shoot emergence began in late March and culminated in May and June. These data help explain why this species is most commonly found in crop fields, orchards, roadsides, rivers, lakes, ponds, and irrigation canals. This information may aid in the development of more effective management measures, such as bringing alligatorweed rhizomes to the surface or below 20 cm deep to restrain its emergence and growth at winter or summer plowing.

The influence of environmental factors on germination and emergence of aryloxyphenoxy propanoate herbicide-resistant (AR) and -susceptible (AS) Asia Minor bluegrass were studied in laboratory and greenhouse experiments. Seeds were collected from AR and AS plants cultivated in separate greenhouses under the same environmental conditions. The results revealed that optimum temperatures for the germination of AS biotype were 10 to 25 C or alternating temperature of 15/5 to 30/20 C and light was not necessary. However, maximum germination occurred at 10 C or 15/5 C, and no germination occurred above 15 C or 25/15 C for the AR biotype. The AS Asia Minor bluegrass was consistently more tolerant to environmental stress, as evidenced by their greater germination at same pH value, osmotic potential, and NaCl concentration at 15/5 C compared to the AR biotype. Higher emergence rates were obtained when seeds were sown on the surface of soil for both biotypes. Emergence percentage of the AR biotype was below 14% when buried, whereas the AS biotype had 20% emergence at 2.5 cm burial depth. It is concluded that several environmental factors affect the germination of Asia Minor bluegrass, and the AS biotype showed higher germination percentage and a wider adaptive range under same treatments compared with the AR biotype. Due to the reduced emergence at depth, deep tillage could be an effective management to reduce AR Asia Minor bluegrass infestation in the following crop.

Greenhouse experiments determined differences in imazaquin bioavailability over time under various soil moisture regimes. All soils were initially fortified with 63 ppb (w/w) of commercially formulated imazaquin. Treatments consisted of maintaining at field capacity for 15 wk (FC), maintaining air-dry for 15 wk (AD), soil that was maintained air-dry for 2 wk and then wet to field capacity at 2 wk intervals (2WAD), and soil that was wet to field capacity at the initiation of the experiment and at 2 wk intervals (2WFC). Little dissipation of imazaquin occurred in the AD soil over the 100 d of this study. Rapid dissipation over the first 35 d occurred for the other three treatments. Imazaquin half-life ranged from 17 to 18 d for the FC, 2WAD, and 2WFC soils. Imazaquin concentration increased or only slightly decreased in samples taken after the second watering in both the 2WAD and the 2WFC soils. Due to differences in adsorption/desorption of imazaquin with changing soil moisture levels, bioassays may not determine the dissipation of the actual concentration of imazaquin contained in soil.

The effect of moderate moisture stress on the activity of the methyl ester of diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid} applied to wild oat (Avena fatua L.) and the influence of morphological and physiological factors on the activity of the herbicide on stressed plants were investigated in growth-room experiments. Wild oat was grown in very fine sandy loam brought to a gravimetric soil moisture content (SMC) of 20% (water potential of -0.3 bars) or 10% (-6.5 bars) prior to herbicide treatment. The activity of diclofop on wild oat was slightly, but significantly, reduced when the SMC was increased from 10 to 20% immediately after spraying. Maintaining the SMC at 10% after spraying further reduced the activity of the herbicide. Wild oat plants grown at 20% SMC had 22% greater leaf area and retained 23% more herbicide than plants grown at 10% SMC. Penetration of the methyl ester of 14C-diclofop into wild oat grown at 20 or 10% SMC did not differ significantly 12, 24, or 48 h after application. Stressed and unstressed plants did not differ significantly in the metabolism of the methyl ester of 14C-diclofop. However, growth at a low SMC prior to treatment decreased the proportion of the radiolabeled herbicide recovered in the apex, third leaf, and tillers of wild oat. This may partially account for the reduced activity of this herbicide on wild oat subjected to moisture stress.

Green galenia is a South African woody prostrate perennial that was first recorded in Australia in the early 1900s and has since become a serious threat to indigenous temperate grasslands and surrounding agricultural areas. Laboratory and field based experiments were conducted to examine the effect of environmental factors on the germination and viability of green galenia seed. It was shown that green galenia was able to germinate over a broad range of temperatures, but short bursts (5 min) of high temperatures (80 C to 120 C replicating possible exposures to a fire) reduced seed germination. Seed germination was positively favored by light, declined rapidly in darkness, and decreased by > 80% at a depth of only 0.5 cm in soil. Water stress greatly reduced seed germination (45% germination at osmotic potentials below −0.2 MPa). Germination was completely inhibited at water potentials of −0.4 to −1.0 MPa. This species is moderately tolerant to salinity, with over 50% of seeds germinating at low levels of salinity (60 mM NaCl), and moderate germination (49%) occurring at 120 mM NaCl, it can germinate well in both alkaline (pH 10–83%) and acidic (pH 4–80%) conditions. The results of this study have contributed to our understanding of the germination and emergence of green galenia, and this will assist in developing tools and strategies for the long term management of this noxious weed in Victoria and other parts of Australia.

The adsorption of atrazine and alachlor was studied on samples of three horizons from soils with different textures and organic carbon contents. Soils were equilibrated with five concentrations of atrazine and alachlor using batch techniques. Adsorption affinity for atrazine and alachlor was approximated by the Freundlich constant (Kf), distribution coefficient (Kd), and the normalized Kd based on organic carbon (Koc). Adsorption was not significantly correlated with soil depth, clay content, or organic carbon. Atrazine adsorption was a linear function of equilibrium concentration for nearly all soil horizons but was nonlinear in most horizons for alachlor. The extent of atrazine adsorption was greater in all horizons of the fine-textured soils (Kd = 1.5 to 5.5) compared to coarse-textured soils (Kd = 0.40 to 0.87). The same general trends with texture were not apparent for alachlor. Conversion of Kd to Koc failed to reduce the variability in the linear adsorption coefficient for atrazine and alachlor in the different soils of this study.

Four forest watersheds (1 ha each) in the upper piedmont of Georgia were treated with hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H, 3H)-dione] pellets3 at a rate of 1.68 kg ai/ha. From the end of April, 1979, until May, 1980, 26 storms were monitored to determine movement of hexazinone and two of its metabolites [A: 3-(4-hydroxycyclohexyl)-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H, 3H)-dione, and B: 3-cyclohexyl-6-(methylamino)-1-methyl-1,3,5-triazine-2,4(1H, 3H)-dione] in runoff water. Residues in runoff peaked in the first storm after application (mean concentration of 442 ± 53 ppbw), and declined with subsequent storms in a power curve function: Conc. (ppbw) = 405 × rate × (1 + 0.44 × days)-1.1. Loss of hexazinone in storm runoff averaged 0.53% of the applied herbicide, with Storms 1 and 17 accounting for 59.3% of the chemical exported. Storm 1 had high residue concentrations and low runoff volume, while Storm 17 contained only low residue levels but a very large stormflow. Hexazinone was the predominant compound in the runoff of all 26 storms. Metabolites A and B occurred in runoff in low-to-trace concentrations (<23 ppbw) for up to 7 months after application. Subsurface movement of hexazinone appeared in streamflow 3 to 4 months after application and produced an additional loss of 0.05%. A second-order perennial stream below the treated watersheds periodically contained hexazinone residues of <44 ppbw.

Grower adoption of no-tillage (NT) approaches to organic soybean production has been limited, in part because of the perceived risks of ineffective cover crop management and lack of season-long weed suppression. We conducted research in 2008 and 2009 to assess those risks by quantifying the effects of winter rye cover-crop management (tilling, crimping, or mowing), soybean planting date (mid May or early June), and row width (19 or 76 cm) on weed recruitment, emergence patterns, season-long suppression, and late-season weed community composition in transitional organic production systems. The weed plant community consisted largely of summer annual species in each year, with velvetleaf or common lambsquarters as the most abundant species. Seedling recruitment from the soil seedbank varied between years, but velvetleaf recruitment was consistently greater in the tilled rye than in the NT rye treatments. Weed emergence tended to peak early in the season in the tilled rye treatment, but in the NT rye treatments, the peak occurred in mid or late season. More-diverse summer annual and perennial species were associated with the NT rye treatments. Even so, weed suppression (as measured by late-season weed shoot mass) was much greater in crimped or mowed rye NT treatments than it was in the tilled treatment. Weed suppression among NT rye treatments was greater in 19- than in 76-row spacing treatments in each year and was greater for mid May than it was for early June planted soybean in 2009. The NT planting of soybean into standing rye before termination (crimping or mowing) facilitated timely planting of soybean, as well as effective, season-long weed suppression, suggesting that those approaches to rye and weed management are of less risk than those typically perceived by growers. Our results suggest that NT systems in winter rye provide effective weed-management alternatives to the typical tillage-intensive approach for organic soybean production.

The growth responses of cogongrass [Imperata cylindrica (L.) Beauv.] were studied in a controlled-environment greenhouse with a day/night temperature of 29/23 C, under full available light and 56 and 11% of full light. The cogongrass plants were grown from stem and rhizome propagules originating from an interstate highway median, a pecan [Carya illinoensis (Wangenh.) K. Koch] plantation, and a pine (Pinus spp.) forest. After 89 days, the plants from all three populations produced, on average, three times as much total dry weight and leaf area in full available sunlight as in 56% full light and 20 times as much as in 11% full light. The distribution of plant biomass into rhizomes decreased with shading, whereas the distribution into leaves increased. The distribution of leaf biomass as leaf area also increased with shading, with the result that the plants grown in 11% full light had leaf area ratios about 2.5 times greater than those grown in full light. Reductions in dry matter production with shading were due to significant reductions in both net assimilation rate and leaf area duration or total amount of leaf area present. The plants from the shaded and exposed habitats generally did not differ significantly in their responses to shading. Thus, there is little evidence for the presence of sun and shade ecotypes in the populations of cogongrass studied.

As chemical management options for weeds become increasingly limited due to selection for herbicide resistance, investigation of additional nonchemical tools becomes necessary. Harvest weed seed control (HWSC) is a methodology of weed management that targets and destroys weed seeds that are otherwise dispersed by harvesters following threshing. It is not known whether problem weeds in western Canada retain their seeds in sufficient quantities until harvest at a height suitable for collection. A study was conducted at three sites over 2 yr to determine whether retention and height criteria were met by wild oat, false cleavers, and volunteer canola. Wild oat consistently shed seeds early, but seed retention was variable, averaging 56% at the time of wheat swathing, with continued losses until direct harvest of wheat and fababean. The majority of retained seeds were >45 cm above ground level, suitable for collection. Cleavers seed retention was highly variable by site-year, but generally greater than wild oat. The majority of seed was retained >15 cm above ground level and would be considered collectable. Canola seed typically had >95% retention, with the majority of seed retained >15 cm above ground level. The suitability ranking of the species for management with HWSC was canola>cleavers>wild oat. Efficacy of HWSC systems in western Canada will depend on the target species and site- and year-specific environmental conditions.

Toxicities of the isopropylamine salt of glyphosate [N-(phosphonomethyl)glycine], SC-0224 (trimethylsulfonium carboxymethylaminomethylphosphonate), and SC-0545 (trimethylsulfoxonium carboxymethylaminomethylphosphonate) were similar to hard red winter wheat (Triticum aestivum L. ‘Centurk 78′) at equal dosages, carrier volumes, carrier water qualities, and sulfuric acid concentrations in the greenhouse and field at Lincoln, NE. Toxicity of HOE-00661 [ammonium (3-amino-3-carboxypropyl)-methylphosphinate] was less than the other herbicides at equal dosages. Rate of development of injury to wheat was similar at 0.4 kg/ha for glyphosate, SC-0224, and SC-0545, but greater for HOE-00661 up to 4 days after application and less thereafter. Reductions in carrier volume from 190 to 96 L/ha resulted in improved phytotoxicities for glyphosate, SC-0224, and SC-0545 at 0.1 kg/ha and HOE-00661 at 0.4 kg/ha. Addition of 0.25% (v/v) concentrated sulfuric acid to 190 L/ha of well water carrier resulted in improved phytotoxicities for glyphosate, SC-0224, and SC-0545 at 0.1 kg/ha and HOE-00661 at 0.4 kg/ha. Carrier water with reduced levels of polyvalent cations resulted in increased phytotoxicities of glyphosate, SC-0224, and SC-0545 at 0.1 kg/ha in 190 L/ha. Phytotoxicity of HOE-00661 was not affected by carrier water quality. Glyphosate, SC-0224, and SC-0545 at equal rates and HOE-00661 at a 4 X rate showed similar toxicity to wheat.

A 2-yr field study was conducted to compare growth characteristics of 24 Palmer amaranth accessions collected from across the indigenous range of the species in the United States. Variation in growth and development of Palmer amaranth was noted among accessions based on leaf area ratio (LAR), specific leaf area (SLA), net assimilation rate (NAR), and stem leaf ratio (SLR), but only SLR varied across harvest intervals among accessions. Accessions collected across the range of Palmer amaranth in the United States displayed variation in growth and development based on differences in LAR, SLA, NAR, and SLR. Observed differences among accessions indicate the existence of Palmer amaranth ecotypes.

Cucumber (Cucumis sativus L.) accessions which had demonstrated allelopathy under controlled environmental conditions were evaluated in the field. Plant introduction (PI) 169391 suppressed proso millet (Panicum miliaceum L.) fresh weight and population 58 and 84%, respectively. Total overseeded and volunteer weed population was reduced 54% when grown in association with PI 169391. Plant introduction 169391 was approximately twice as effective as PI 285605 in suppressing proso millet growth. Inhibition of weed species did not occur consistently in the field. The allelopathic effect of cucumbers was suppressed during periods of increased rainfall. However, these tests demonstrated that allelopathic activity could be obtained under certain edaphic and environmental conditions.

Field experiments were conducted to determine the effects of tank-mixing fluometuron {N,N-dimethyl-N′-[3-(trifluoromethyl)phenyl] urea} or MSMA (monosodium salt of methylarsonic acid) with sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} or the butyl ester of fluazifop {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy] phenoxy] propanic acid} on cotton (Gossypium hirsutum L.) tolerance and large crabgrass [Digitaria sanguinalis (L.) Scop. # DIGSA] control. Postemergence (over-the-top) application of 1.7 kg ai/ha of fluometuron reduced cotton lint yield 17 to 23% in three of four tests. No yield reduction resulted from postemergence application of 1.1 kg ae/ha of MSMA. Tank-mixing 0.1 or 0.2 kg ai/ha of sethoxydim or 0.1 or 0.2 kg ae/ha of fluazifop with fluometuron or MSMA did not alter the effect of fluometuron or MSMA on cotton yield. Tank mixtures of 1.1 to 2.2 kg/ha of fluometuron plus sethoxydim or fluazifop were antagonistic. No antagonism was observed when fluometuron was applied 3 days before or 6 h following sethoxydim or fluazifop application. Tank-mixing 1.1 or 2.2 kg/ha of MSMA with sethoxydim or fluazifop initially reduced control of large crabgrass. Antagonism was not observed at 4 weeks after tank-mix application of MSMA plus sethoxydim, and was observed in only one of three experiments 4 weeks following tank-mix application of MSMA plus fluazifop. When antagonism was observed with tank mixtures of fluazifop plus MSMA, sequential application of MSMA 1 to 7 days before application of fluazifop did not overcome the antagonism. No antagonism occurred when MSMA was applied 6 or more hours after fluazifop application.