The distribution of simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] in whole plants and the effect of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] on isolated leaf cells and chloroplasts of two biotypes of common groundsel (Senecio vulgaris L.) were studied. Both biotypes accumulated 14C at the leaf margins 60 hr after exposure of roots to 14C-simazine. No difference in herbicide distribution between the two biotypes was observed. Atrazine (10 μM) inhibited photosynthesis, RNA synthesis, and lipid synthesis in isolated susceptible (S) cells but not resistant (R) cells. Lipid synthesis in S cells but not R cells was enhanced by exposure to 1 and 0.1 μM atrazine for 1 hr. Photochemical activity of R chloroplasts was not inhibited by atrazine but S chloroplasts were severely inhibited.

Greenhouse and field experiments were conducted to determine the effects of nitrogen (N) supply and purple nutsedge population densities on the yield of radish. In the greenhouse studies, additive series with purple nutsedge densities of 0, 50, 100, 200 or 350 plants m−2 were established. Nitrogen rates of 0, 110, 220, or 330 kg ha−1 were provided to the potting medium. A significant density by N interaction was found for radish fresh weight. Within a given nutsedge density, radish yield decreased as N rate increased. In field studies, additive series of 0, 50, 100, 150, or 200 nutsedge plants m−2 were established the same day radish was sown. Nitrogen rates were 100 or 200 kg ha−1. Marketable radish yield losses and nutsedge shoot dry weight and height were determined 30 d after seeding the crop. Nutsedge densities and N rates interactively influenced radish root yield. Radish yield loss reached 100% at nutsedge densities of 75 and 125 plants m−2 at 200 and 100 kg N ha−1, respectively. Purple nutsedge produced larger shoot biomass as N increased from 100 to 200 kg ha−1. Results of both greenhouse and field studies showed that as N increased, the negative effect of the weed on the crop was enhanced.

Insensitivity to atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] and certain other herbicides was found in plants of horseweed (Conyza canadensis (L.) Crong. ♯4 ERICA) collected from vineyards where atrazine had been applied extensively. There were no differences in the uptake, translocation, or accumulation of radiolabeled atrazine between plants of the susceptible and resistant biotypes. However, atrazine at 1 × 10-4 M caused no inhibition of photosynthetic electron transport in resistant leaves. Photosynthetic characteristics of leaves of susceptible and resistant plants showed different sensitivities to atrazine, but similar sensitivities to diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] and ioxynil (4-hydroxy-3,5-diiodobenzonitrile). Isolated chloroplast membranes from resistant plants exhibited a resistance factor of 1000 to atrazine but only 10 to 30 to methylthio-s-triazines. DNOC (4,6-dinitro-o-cresol) was found to be more effective in susceptible chloroplasts than in resistant ones. A marked reduction in the galactolipid content, especially digalactosyl diglyceride, was found in resistant leaves. It is suggested that these alterations in lipid content and composition may contribute to atrazine resistance in horseweed.

The mechanism of glyphosate tolerance was investigated in nine birdsfoot trefoil selections that exhibited a threefold difference in glyphosate tolerance. Single-stemmed ramets (vegetative clones) established from the nine selections were used to evaluate tolerance to glyphosate, spray retention, and 14C-glyphosate absorption and translocation in growth chamber experiments. The tolerance of greenhouse-grown ramets correlated with the tolerance of field-grown plants, indicating that tolerance was not a function of plant size or affected by environment. The nine selections differed in spray retention and 14C-glyphosate translocation but not in glyphosate absorption. The differences in retention and translocation were not correlated with the level of glyphosate tolerance but could contribute to the tolerance of an individual plant. The specific activity of 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS) (EC 2.5.1.19) ranged from 1.3 to 3.5 nmol min−1 mg−1 protein among the nine selections assayed and was positively correlated with plant tolerance level. These results indicate that the primary mechanism of glyphosate tolerance in birdsfoot trefoil is based on the level of EPSPS activity.

Genetic variation of morphological and phenological characters of yellow nutsedge (Cyperus esculentus L.) from California was investigated and compared with isozyme data from the same populations. The importance of collection location, individual genotype, and isozyme genotype to phenotypic characteristics was evaluated. Analyses were conducted on 20 individuals collected from each of 10 widely separated populations in California. Replicate plants were started by tubers, grown in pots buried outdoors, and measured during the period from planting through flowering. Results showed that yellow nutsedge is variable across its range in California in all measured traits, including days to sprouting and flowering, height, rachis number and length, aboveground biomass, tuber weight, number of rays, ray length, number of spikelets per ray, as well as spikelet length and width. Population (collection location) was a larger variance component than genotype (individuals over all locations). As found in isozyme analysis, relatively more variation was found among than within populations, typical of clonally reproducing species. Overall, more variation was found in quantitative traits than in isozymes. The level of variation, based on coefficients of variation for each character and population, was similar within populations and not clearly related to cropping history or climate at each site. The two most common isozyme genotypes in the collection differed in four characters: date of emergence, weight of 10 tubers, rachis number, and mean ray length, which may represent different adaptive responses to cultural practices. These results indicate that isozymes do not reflect the high level of genetic and adaptive diversity of yellow nutsedge. Factors that are likely to determine the patterns of variation in this species include breeding system (vegetative reproduction), founder effects, polyploidy, and homogeneity of the agricultural environmental.

Soil characteristics, elevation, annual precipitation, potential evapotranspiration, length of frost-free season, and mean maximum July temperature were estimated for 116 established infestations of spotted knapweed (Centaurea maculosa Lam. # CENMA) in Montana using basic land resource maps. Areas potentially vulnerable to invasion by the plant were delineated on the basis of representative edaphic and climatic characteristics. No single environmental variable was an effective predictor of sites vulnerable to invasion by spotted knapweed. Only a combination of variables was effective, indicating that the factors that regulate adaptability of this plant are complex. This technique provides a first approximation map of the regions most similar environmentally to infested sites and, therefore, most vulnerable to further invasion. This weed migration prediction technique shows promise for predicting suitable habitats of other invader species.

A biotype of green foxtail found in Spain exhibited cross-resistance among acetyl-CoA carboxylase (ACCase)–inhibiting herbicides. Field doses that totally inhibited shoot fresh weight in the susceptible (S) biotype were determined for six aryloxyphenoxypropionates (clodinafop, diclofop, fenoxaprop-P, fluazifop-P, haloxyfop-P, and propaquizafop) and six cyclohexanediones (clefoxydim, clethodim, cycloxydim, sethoxydim, tepraloxydim, and tralkoxydim). The resistant (R) biotype showed cross-resistance to all herbicides except fenoxaprop-P, propaquizafop, clefoxydim, and tepraloxydim. There were no differences in the absorption, translocation, and metabolism of [14C]diclofop between the S and R biotypes. On the basis of herbicide dose that inhibited ACCase activity by 50% (I50 values), ACCase of the R biotype was 5.8-, 13.9-, 20.0-, 102.4-, 416.7-, and 625.0-fold less sensitive to clethodim, haloxyfop, diclofop, fluazifop, cycloxydim, and sethoxydim, respectively, than that of the S biotype. Two multifunctional ACCase isoforms (ACCase I and ACCase II) were purified partially and separated. ACCase II was highly resistant to diclofop acid in both biotypes, with I50 values ranging between 92 and 95 μM. However, the I50 values observed for ACCase I revealed that the R biotype was 30.8-fold less sensitive to diclofop than the S biotype. These results suggest the mechanism of resistance in green foxtail to diclofop relates to an altered ACCase I isoform.

A series of greenhouse studies examined the effectiveness of PRE- and POST-applied sulfentrazone in controlling purple and yellow nutsedge as influenced by selective tissue exposure. In addition, 14C-sulfentrazone was utilized to contrast absorption and translocation resulting from these exposures. Consistent control with preemergence applications to germinating tubers was obtained with a combined root and shoot zone exposure. Yellow nutsedge was more susceptible than purple nutsedge. Performance of the separate root and shoot zone exposure was soil pH- and nutsedge-species dependent. POST-foliar applications to established nutsedge were more effective when sulfentrazone was allowed to contact the soil surface. 14C-sulfentrazone was readily absorbed by the roots and translocated to the foliage of both species in hydroponic culture.

The effects of common cocklebur (Xanthium pensylvanicum Wallr.) on soybean [Glycine max (L.) Merr.] yields, grades, and net returns were determined by regression analysis of data from 450 research plots from 1971 through 1973. Common cocklebur control from 0 to 100% was achieved by various combinations of preemergence and postemergence practices. Foreign matter in soybean seed samples was 0.7% with total common cocklebur control and 5.1% with no control. At least 70% control of common cocklebur was required to avoid deductions from gross harvested weights due to seed moisture levels exceeding 13%. Small discounts for damaged kernels occurred when common cocklebur control was less than 40%. With 100% common cocklebur control, the estimated US soybean grade was 1.3. Failure to control common cocklebur resulted in a predicted grade of 3.9. When adjusted for various weight discounts, soybean yields were 1170 kg/ha without control of common cocklebur and 1890 kg/ha for 100% control. Soybean yields were increased about 6% for each 10% increase in common cocklebur control. Net returns to land, management, and general farm overhead were $63/ha when common cocklebur was not controlled, and $119/ha with 95% control.

Striga is a major constraint to food production in Africa. Most technologies developed for the eradication of Striga asiatica from the United States are not adaptable to Africa. Imazapyr and pyrithiobac coated imidazolinone-resistant (IR)-resistant maize seed prior to planting at rates of 30 to 45 g ha−1 provide near season long control of Striga and can increase maize yields three- to fourfold if supplied with fertilizer. Slow release seed coatings reduce maize injury when post-planting rains are sparse and improve Striga control when there is excessive rainfall early in the season. Models suggest that herbicide resistance may not be a significant threat in short season maize, but vigilance in removing flowering Striga plants that are not controlled is recommended due to the known risk of evolution of resistance to these herbicides. Stacking the IR gene with glyphosate resistance and using imazapyr treated seed and applying glyphosate mid-season would provide season long Striga control and delay the evolution of resistance to both herbicides. To date, adoption of this technology has been limited by a number of factors. However, it should be included as one component of a multi-factor approach to increasing maize productivity in areas of Africa where Striga is problematic.

Seed from six Australian near-isogenic lines of wild oat were germinated and grown in controlled-environment growth chambers under either ambient CO2 (357 parts per million by volume [ppmv]) or elevated CO2 (480 ppmv) at 20/16 C or 23/19 C. Three soil moisture treatments—−0.01 MPa (field capacity), −0.10 MPa, or −1.00 MPa—were imposed. Wild oat lines grown under elevated CO2 had higher seed production and greater plant dry weights, although the response of these variates involved a complex of interactions with temperature, soil moisture, and line. Plant height varied with wild oat line, and plants grown at 20/16 C were taller than those grown at 23/19 C. At 23/19 C, time taken to mature was reduced for some wild oat lines, and elevated CO2 reduced the time taken to maturity for some lines at 20/16 C. There was no significant difference in the level of dormancy developed in freshly harvested caryopses between the two CO2 treatments, but an effect was present in seed that had been after-ripened for 193 d. These results indicate that the main climate change variables ([CO2], soil moisture, and increased temperature) directly influence the growth and development of wild oat and are likely to affect the population dynamics of this species.

This investigation studied simazine (2-chloro-4,6-bis(ethylamino)-s-triazine), atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), propazine (2-chloro-4,6-bis(isopropylamino)-s-triazine), and prometone (2-methoxy-4,6-bis(isopropylamino)-s-triazine). Preemergence applications of granular formulations of simazine on attaclay or calcium sulfate of simazine at 1 to 2 ppmw controlled Potamogeton, Najas, Ceratophyllum, Heteranthera and Zannichellia. Higher rates were required to control filamentous algae (Cladophora and Pithophora) and chara (Chara vulgaris). Early postemergence applications of wettable powder to submersed aquatics and filamentous algae were often effective. Atrazine gave similar results. Concentrations of 0.5 to 1.0 were effective in controlling Cladophora and Pithophora and three species of Potamogeton in pond applications. Spray applications, generally, were more effective than broadcasting granular atrazine. Propazine and prometone did not control submersed species in concentrations up to 3 ppmw.

Simazine had a relatively high margin of safety with respect to acute toxicity to fish. It was less toxic than propazine, prometone or atrazine. Granular forms were less toxic than wettable powder or emulsifiable concentrates. Laboratory tests on bottom organisms gave an acute LD50 toxic dosage of 28 ppmw. Field observations did not demonstrate a serious reduction in the production of bottom organisms. However, the control of aquatic vegetation brought about ecological changes affecting bottom dwelling and weed clinging organisms. These changes would affect certain fishes with special food habits.

Annual grasses continue to be a problem. Some of the most prevalent annual grass weeds are members of the foxtail (Setaria) genus. Although these species can be controlled chemically, farmers have often had failures. One possible cause of these failures could be variability within the species.

In greenhouse experiments, soil-incorporated combinations of 0,0-diethyl S-[2-(ethylthio)-methyl]-phosphorodithioate (phorate) or 0,0-diethyl S-[2-(ethylthio)-ethyl]-phosphorodithioate (disulfoton) with a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) resulted in increased cotton (Gossypium hirsutum L.) seedling growth as compared to trifluralin used alone. This result apparently was due to the greater numbers of secondary roots in the zone of incorporation. Phorate was more effective than disulfoton in overcoming the inhibitory effect of trifluralin on secondary root development. A comparison of granular and emulsifiable phorate showed no advantage for either formulation. The combination of phorate and trifluralin in a 1-inch soil core, into which seeds of cotton were planted and surrounded by trifluralin-treated soil, also increased secondary root development as compared to using trifluralin alone.

Foxtail millet [Setaria italica (L.) Beauv. ‘Empire’] and proso millet (Panicum miliaceum L. ‘White’) seedlings were grown in glass units to expose selectively either roots or shoots to vapors arising from soil containing a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin). The root and shoot growth of both species was inhibited by trifluralin vapors. Suppression of root and shoot growth increased as trifluralin application rates increased. In shoot exposure, vapors arising from soil treated with 5 ppmw of trifluralin were lethal to seedlings of both species. In root exposure, root growth of both species was severely suppressed at 20 ppmw, but shoot growth was unaffected. Phytotoxic effects resulting from a given concentration of trifluralin were more severe as greater carrier volumes were used for application. Trifluralin vapors arising from soil 16 to 22 days after treatment were still sufficient to cause shoot growth inhibition.

In field and greenhouse studies, tank mixing 0.3 and 0.4 kg ai/ha of the butyl ester of fluazifop {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid} with 0.4 kg ai/ha of acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} reduced control of large crabgrass [Digitaria sanguinalis (L.) Scop. # DIGSA] and itchgrass (Rottboellia exaltata L.f. # ROOEX) compared to fluazifop applied alone in soybeans [Glycine max (L.) Merr.]. Less antagonism between the two herbicides was observed in a year when conditions were optimum for large crabgrass control with fluazifop. Application of acifluorfen 1 to 3 days before application of fluazifop decreased large crabgrass control. Antagonism between fluazifop and acifluorfen was avoided when fluazifop was applied 3 to 5 days before acifluorfen. No antagonism was observed when fluazifop at 0.3 or 0.4 kg/ha was tank mixed with acifluorfen at 0.4 kg/ha for control of itchgrass. Itchgrass was more susceptible to fluazifop than large crabgrass.

In greenhouse studies, the potential interactive effects of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] treatments combined with the air pollutant ozone (O3) or the antioxidants, piperonyl butoxide {α-[2-butoxyethyl) ethoxy]-4,5-dimethylenedioxy-2-propyltoluene} and n-propyl gallate, on the growth of sorghum [Sorghum bicolor (L.) Moench ‘Funk G522DR’] seedlings that were protected or unprotected with the antidote, CGA-43089 [α-(cyanomethoximino)-benzacetonitrile], were examined. Metolachlor was applied preplant incorporated at rates of 2.2, 3.9, and 5.6 kg/ha, and it was evaluated against fumigation with O3 at 0.2 and 0.3 ppmv or against 4.9, 6.7, and 9.0 kg/ha of each antioxidant applied preplant incorporated. In combination treatments, shoot dry weight at 30 days after planting was reduced more than expected by metolachlor in the presence of the protectant CGA-43089 and ozone or some rates of the two antioxidants, suggesting synergism. In the absence of the protectant, CGA-43089, growth responses of sorghum to combination treatments of metolachlor with ozone or propyl gallate suggested an additive effect, although some treatments of metolachlor combined with piperonyl butoxide interacted synergistically.

An open grassland at Sugarloaf Ridge State Park, Sonoma County, CA, was burned during three consecutive summers (1993–1995) to control yellow starthistle. By 1996, the yellow starthistle seedbank, seedling density, and mature vegetative cover were reduced by 99, 99, and 91%, respectively, and the plant community had greater diversity and species richness, particularly of native forbs. After the cessation of the prescribed burning after 1995, the community was monitored for 4 yr to determine if the reduced yellow starthistle population represented a stable state or if the population would quickly recover. The yellow starthistle seedbank rose dramatically over 4 yr. Seedling counts and summer vegetative cover also rose, though less rapidly. Total forb cover, particularly native species, total plant cover, and plant diversity decreased significantly after cessation of the burning. Grass cover did not show any strong trends, and year-to-year variation in the grass cover appeared to be more important than the treatment effects. In the absence of some overall changes in management, e.g., periodic prescribed burning, herbicide treatments, or revegetation, it may not be possible to establish and maintain a stable state with a low population of yellow starthistle in annual grasslands in California.

Germination of common mullein (Verbascum thapsus L.) in relation to constant and alternating temperatures, and light quality and duration was studied. In the dark, optimum germination occurred with 8-h warm periods of 30 to 40 C alternating with 16-h cold periods of 15 to 35 C. In the dark, no germination occurred below 10 C. With light during the 8-h warm periods, germination occurred at temperatures from 2 to 40 C, alternating with 0 C cold periods. The range of optimum temperatures for germination was also greatly expanded with light. Exposures to 24-h periods of red light, followed by incubation in the dark, produced the same germination response as 8 h of white light daily. Common mullein seeds germinated on the surface of the soil even though the seeds did not have mucilage.

The phase-out of methyl bromide requires alternative nutsedge management options in vegetable systems. Options that target tuber production, the primary means of reproduction, will be most beneficial. A study was conducted to evaluate the response of purple nutsedge and yellow nutsedge foliar growth and tuber production to a range of glyphosate rates. Glyphosate was applied at six rates between 0.41 and 2.57 kg ae ha−1 to 5-wk-old nutsedge plants with multiple shoots. The rate of glyphosate needed to reduce growth 50% (I50) was similar for purple nutsedge foliar growth (0.58 kg ha−1) and tuber biomass (0.55 kg ha−1). In contrast, I50 for yellow nutsedge foliar growth was 0.73 kg ha−1, which was greater than the I50 for tuber biomass (0.41 kg ha−1). First-order tubers, those directly attached to the initial tuber, had an I50 of 0.70 and 0.44 kg ha−1 of glyphosate for purple nutsedge and yellow nutsedge tuber biomass, respectively. For all higher-order tubers, I50 values ranged from 0.29 to 0.60 and 0.14 to 0.30 kg ha−1 of glyphosate for purple nutsedge and yellow nutsedge tuber biomass, respectively. Glyphosate at 0.74 kg ha−1 prevented fourth-order purple nutsedge and third-order yellow nutsedge tuber production (terminal tubers for yellow nutsedge). Fifth- and sixth-order purple nutsedge tuber production was eliminated by the lowest tested rate of glyphosate (0.41 kg ha−1). Effective nutsedge management options will require consistent control between spring and autumn crops. Glyphosate is economical, poses no herbicide carryover issues to vegetables, and minimizes nutsedge tuber production; therefore, it is a suitable candidate to manage nutsedges.