Overreliance on tribenuron has resulted in resistance evolution in water starwort. This study investigates the resistance mechanisms to tribenuron in water starwort populations from China. The cytochrome P450 monooxygenase (P450) inhibitor malathion increased tribenuron sensitivity in all populations. The decrease in the amount of herbicide dose that causes 50% growth reduction (GR50) for the sensitive (S) population JS24 and the resistant (R) populations JS16 and JS17 were 2.3-, 2.5-, and 4.1-fold, respectively. However, the GR50 values for the R populations were still much higher than those of the S population. This observation indicates that P450-mediated enhanced metabolism is one mechanism for resistance in water starwort. The glutathione-S-transferase (GST) activity could be induced by tribenuron for all tested populations. In particular, the GST activity of JS16 is inherently greater and is more rapidly induced than that of JS17 or JS24. Resistance attributed to mutant acetolactate synthase (ALS) alleles was identified by sequence analysis for each population. Pro197Ser substitution was detected in JS16 and JS17. Molecular markers were also developed to rapidly identify resistance as well as individuals carrying the specific Pro197Ser mutation in water starwort populations. The resistance patterns experiment revealed that the R populations exhibited different levels of resistance to pyrithiobac sodium salt, florasulam, pyroxsulam, and flucarbazone-Na; however, R populations were sensitive to imazethapyr, fluroxypyr-meptyl, 2,4-D butylate, isoproturon, and diflufenican. This study establishes that either one or at least two resistance mechanisms are involved in herbicide resistance in water starwort. Moreover, these mechanisms might contribute to the different levels of resistance to tribenuron among water starwort populations.

In five field experiments conducted during 1972 and 1973, yellow nutsedge (Cyperus esculentus L.) was either undisturbed or removed from planting beds by hoeing 0, 2, 4, 6, and 8 weeks after cotton (Gossypium hirsutum L. ‘Acala SJ-1’) emergence. Yellow nutsedge undisturbed in beds increased from 23 plants/m of row at cotton emergence to 100 plants/m at harvest. Nutsedge competing with cotton for more than 4 weeks reduced yield of seed cotton. Nutsedge that competed all season reduced yields 34%, as compared with 20% when it competed for 6 and 8 weeks. Nutsedge that competed with cotton for more than 4 weeks reduced height and number of cotton plants and delayed maturity. Although nutsedge competition reduced yields and delayed maturity, it generally did not alter fiber properties. When nutsedge was removed at cotton emergence and was followed by 14 weekly hoeings, the final tuber population was reduced to 24% of the beginning population. When nutsedge removal was delayed for 6 weeks and was followed by nine weekly hoeings, a fourfold increase in tubers resulted. A further tenfold increase resulted when nutsedge was not removed.

The general appearance of plants was similar for 14 accessions of velvetleaf (Abutilon theophrasti Medic. ♯ ABUTH) obtained from Stoneville, MS (33° 25’ N), Rosemount, MN (44° 43’ N), and various locations in between. All accessions were equally susceptible to bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide]. Variation in maturity characteristics was found among the accessions in the field at Rosemount; southern accessions began seed capsule production later and continued it longer than did northern accessions. Velvetleaf accessions from Stoneville and Rosemount both produced seeds in the field at Weslaco, TX (26° 09’ N), but at Fairbanks, AK (64° 49’ N), neither accession produced seeds before being killed by frost.

Bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-(4) 3H-one 2, 2-dioxide] was applied postemergence to common cocklebur (Xanthium pensylvanicum Wallr.) and velvetleaf (Abutilon theophrasti Medic.) at various times of the day in growth chamber and field studies and to soybeans [Glycine max (L.) Merr.] in field studies. Soybeans were tolerant of bentazon regardless of time of day when treated. Results with common cocklebur and velvetleaf suggest that the time of day when bentazon is applied can be of practical importance. Poor control might occur following application in late evening, night, or early morning.

In field and greenhouse experiments, sicklepod (Cassia obtusifolia L. # CASOB) control with postemergence application of imazaquin {ammonium salt of 2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} was similar with spray volumes of 50, 185, and 360 L/ha. Application of DPX-F6025 {ethyl ester of 2-[[[[(4-chloro-6-methoxypyrimidin-2-yl) amino] carbonyl] amino] sulfonyl] benzoate} in 50 L/ha resulted in greater control than application in 185 or 360 L/ha in the greenhouse, but no effect of spray volume was noted in field studies. In the greenhouse, control was 25 and 63% less with imazaquin applied to three- and five-leaf plants, respectively, compared to one-leaf plants; control was 40 and 62% less with DPX-F6025 applied to three- and five-leaf plants, respectively, compared to one-leaf plants. In the field, initial control was greater with imazaquin and DPX-F6025 applied to one-leaf plants than to three- or five-leaf plants. Following cultivation, control was similar regardless of plant size at time of application. In the field, postemergence applications of imazaquin and DPX-F6025 were equally phytotoxic following preplant-incorporated application of vernolate (S-propyl dipropylcarbamothioate) or preemergence application of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide]. A synergistic interaction was observed in the greenhouse with imazaquin applied postemergence to sicklepod grown in vernolate- or alachlor-treated soil and with DPX-F6025 applied to sicklepod grown in alachlor-treated soil. Antagonism was observed with DPX-F6025 applied postemergence to sicklepod grown in vernolate-treated soil.

The efficacy of current wild oat herbicides and their high cost have resulted in use rates that are less than those recommended. While acceptable weed control may be attained at less cost, this practice does not consider the potential for increased wild oat seed production. The objective of this experiment was to determine the interaction of wild oat density and reduced imazamethabenz rates on wild oat seed production in spring barley. As wild oat densities increased from 8 to 1,100 plants m−2, wild oat seed production increased from 180 to 9,950 seed m−2 without herbicide, and from 0 to 2,810 seed m−2 using 0.53 kg ai ha−1 imazamethabenz. This general pattern was modeled using a cumulative logistic function. Estimates from this model indicated that < 1 wild oat seed m−2 was produced at population densities of ≤ 20 plants at any imazamethabenz rate. Imazamethabenz rates of 0.26 kg ha−1 or greater at wild oat densities of less than approximately 190 plants m−2 did not result in wild oat seed production above the initial population density. As wild oat density increased, however, imazamethabenz rates below 0.40 kg ha−1 resulted in substantially greater wild oat seed production compared to the recommended rate.

Field experiments were conducted in 1990 and 1991 at Lethbridge, Alberta, to determine if narrower row spacings and increased plant densities than commonly practiced would improve safflower's competitive ability with weeds. A factorial set of treatments of safflower at two row spacings of 11 and 22 cm and six densities ranging from 10 to 192 plants m−2 grown weed free or infested with green foxtail (Setaria viridis (L.) Beauv.). Decreasing safflower row spacing from 22 to 11 cm slightly improved competition with green foxtail, but increasing safflower density had a much greater effect. Weed-free safflower biomass and seed yield plateaued at 70 and 84 plants m−2 in 1990 and 1991, respectively. However, safflower infested with 500 plants m−2 of green foxtail increased in biomass and seed production up to 100 plants m−2 in 1990 and 156 plants m−2 in 1991. At these high densities, weedy safflower yielded less than weed-free safflower but three to four times more than achieved at lower stand densities. Concurrently, high safflower density reduced green foxtail biomass up to 72% and seed yield up to 85%. Dense safflower stands developed a closed canopy early in the season and shaded green foxtail more than thin stands.

Full-season interference of silverleaf nightshade (Solanum elaeagnifolium Cav. # SOLEL) with dryland and irrigated cotton (Gossypium hirsutum L. ‘Paymaster 145′) was evaluated in five field experiments during 1984 and 1985, Weed densities ranged from 0 to 32 plants/10 m of crop row. Dry weight of silverleaf nightshade increased from 0.08 to 0.39 kg/plot for each additional weed/10 m of row. Intraspecific competition among silverleaf nightshade plants was not evident. However, cotton height was reduced at weed densities of 4 plants/10 m of row or more. The densities at which initial lint yield reductions occurred ranged from 4 to 32 weed plants/10 m of row. Irrigated cotton more effectively competed with the weed than did dryland cotton, suggesting that soil water was a primary competition factor between the two species. Boll size was reduced at densities of 2 weeds/10 m of row and above. Silverleaf nightshade reduced mechanical harvest efficiency only at densities of 16 and 32 plants/10 m of row. Fiber properties were not affected. Linear regression predicted that lint yield would be reduced 1.54% for each silverleaf nightshade plant/10 m of cotton row. The distance silverleaf nightshade was established from the crop row did not affect the weed's interference with cotton within a range of 0 to 30 cm.

Greenhouse and laboratory experiments were conducted to determine the effect of DPX-PE350 on the herbicidal activity of fluazifop-P on large crabgrass. Greenhouse experiments indicated that fluazifop-P activity was antagonized by DPX-PE350. This antagonism was reduced by increasing fluazifop-P rates or by applying DPX-PE350 after fluazifop-P. Applying DPX-PE350 24 h prior to fluazifop-P reduced fluazifop-P activity more than applying it 10 min prior. The effect of DPX-PE350 application to large crabgrass on extractable acetyl-CoA carboxylase activity, and the effect of DPX-PE350 on acetyl-CoA carboxylase activity in vitro also was studied. Applying DPX-PE350 to large crabgrass prior to protein extraction did not reduce fluazifop-P inhibition of acetyl CoA carboxylase activity in vitro. The presence of DPX-PE350 in the enzyme assay mix did not affect fluazifop-P inhibition of acetyl-CoA carboxylase activity. The effect of DPX-PE350 on uptake, translocation, and metabolism of fluazifop-P was considered. DPX-PE350 did not affect uptake of fluazifop-P by large crabgrass. Applying DPX-PE350 24 h prior to fluazifop-P decreased translocation of fluazifop-P out of the treated leaf 44 to 53% whereas mixing DPX-PE350 with fluazifop-P decreased fluazifop-P translocation only 15 to 17%. Metabolism of fluazifop-P was not affected by applications of DPX-PE350. These studies suggest that the antagonism of fluazifop-P activity by DPX-PE350 is due to decreased translocation of fluazifop-P out of the treated leaf.

Differential selectivity was shown for 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), 2-chloro-4, 6-bis(isopropylamino)-s-triazine (propazine), and S-ethyl diisobutylthiocarbamate (butylate) on giant foxtail (Setaria faberi Herrm.) yellow foxtail (Setaria glauca (L.) Beauv.), giant green foxtail (Setaria viridis var. major (Gaud.) Posp.), robust white foxtail (Setaria viridis var. robusta-alba Schreiber), and robust purple foxtail (Setaria viridis var. robusta-purpurea Schreiber). The differentials obtained could explain the reports of uncontrolled Setaria species and the apparent increase of the latter three varieties. The two new varieties of green foxtail, robust white and robust purple foxtails, were the most tolerant varieties or species to atrazine and propazine with giant green foxtail intermediate between the robust foxtails and green, giant, and yellow foxtail. The ED50 values for the robust foxtails were also higher than for the other varieties and species studied. Yellow foxtail was the most tolerant to butylate. The effective rate, measured as percent of control, was directly related to soil organic matter content.

Recent interest in describing the spatial distribution of weeds and studying their association with site properties has increased the use of interpolation to estimate weed seedling density from spatially referenced data. In addition, farmers and consultants adopting elements of site-specific farming practices are using interpolation methods for mapping weed densities as well as soil properties. This study was conducted to compare the performance of four interpolation methods, namely inverse-distance weighting (IDW), ordinary point kriging (OPK), minimum surface curvature (MC), and multiquadric radial basis function (MUL), with respect to their ability to map weed-seedling densities. These methods were evaluated on data from four weed species, velvetleaf, hemp dogbane, common sunflower, and foxtail species, of contrasting biology and infestation levels in corn and soybean production fields in Nebraska. Mean absolute difference (MAD) and root mean square (RMS) between the observed point sample data and the estimated weed seedling density surfaces were used to evaluate the performance of the interpolation methods. Four neighborhood search types were compared within each interpolation method, and Search3 (12 to 16 neighboring locations) generated an interpolated surface with the smallest MAD and RMS indicating the highest precision. IDW with a power coefficient of p = 4 gave the smallest MAD and RMS, as did a test with an elliptical search and no anisotropy. The level of precision of all four interpolation methods was very poor for weed species with low infestation levels (> 75% of field weed-free; MAD ranged from 100 to 187% of the observed mean density), whereas precision was improved for weed species with high infestation levels (< 25% of field weed-free; MAD ranged from 45 to 85%). No single interpolator appears to be more precise than another. Implications of this study indicate that grid sample spacing and quadrat size are more important than the specific interpolation method chosen.

The volatilization of α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) from a Miller clay and commercial sand was determined by analysis of air samples above the soils. The method requires no extraction of soil or traps and is sensitive to approximately 8 ppm of trifluralin in air. The loss of trifluralin was influenced more by soil moisture than by soil type. Data indicated that trifluralin vapors in air are photolytically degraded. Similar studies were conducted with 4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline (nitralin), but volatilization was so slow that detectable concentrations of this herbicide did not occur.

Different herbicide-application strategies may serve as part of an integrated weed management (IWM) system. In 1987 and 1988, ten field trials were initiated in the south-central part of Sweden, with the objective of studying the long-term effects of herbicide-application strategies on the development of weed populations and productivity of spring-sown cereals. Each year until 1997, herbicides were applied at 25, 50, 75, or 100% of the full recommended dose. Treatments included herbicide exclusion every second year and herbicide application at 25 or 50% of a full dose during two out of three years, with a full dose applied in the third year. An untreated control was included. In 1998, weed densities in 25, 50, 75, and 100% of a full dose were reduced 43, 58, 64, and 67% compared with the control. At seven sites, the exclusion of herbicides every second year resulted in 43 to 178% higher weed densities than in 50% of a full dose, although the same amount of herbicide was applied when summarized over 2 yr. At four sites, herbicide application each year at 50, 75, or 100% of a full dose increased the density of difficult-to-control species by 24% compared with the untreated control. Averaged over sites, the untreated control contained 30% more weed species than herbicide application at 75 or 100% of a full dose. At three sites in 1996, the full dose resulted in 520 kg ha–1 (pooled over sites) higher barley yields than in the control. Neither in 1996 nor in 1997, was there any difference in crop yield between herbicide application at 25 and 100% of a full dose. This study demonstrates the potential of reducing the input of agrochemicals for weed control by using herbicides at reduced rates in competitive crops.

The role of corn (Zea mays L.) var. Dixie 18, sorghum (Sorghum vulgare) var. RS 610, and Johnsongrass (Sorghum halepense L.) in dissipation of 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine) from Norfolk sandy loam was studied in the field and in a pot-culture experiment in the greenhouse. Atrazine residues were determined by both chemical and bio-assay methods. Corn and Johnsongrass field plots had significantly less atrazine remaining in the 0- to 6-in depth than similarly treated fallow plots 1, 2, 3, and 6 months after planting, and the crops were equally effective in dissipating atrazine from soil. Soil in pots cropped with corn, sorghum, or Johnsongrass had significantly less atrazine remaining than in uncropped pots. Uptake by corn, Johnsongrass, and sorghum within 3 months in the pot-culture experiment accounted for losses of approximately 25, 25, and 20%, respectively, of the herbicide initially present. Bio- and chemical assays were equally sensitive methods of determining atrazine in the soil and gave similar results.

Chinese sprangletop [Leptochloa chinensis (L.) Nees] is one grass weed severely affecting rice (Oryza sativa L.) growth in paddies in China. Cyhalofop-butyl is the main herbicide used to control grass weeds in Chinese paddy fields, especially for controlling L. chinensis; however, L. chinensis has evolved resistance to cyhalofop-butyl due to continuous and extensive application. To investigate cyhalofop-butyl resistance levels and mechanisms in L. chinensis in some of the Chinese rice areas, 66 field populations were collected and treated with cyhalofop-butyl. Of these tested populations, 10 showed a high level of resistance to cyhalofop-butyl; the 50% effective dose ranged within 108.4 to 1,443.5 g ai ha−1 with resistance index values of 9.1 to 121.8 when compared with the susceptible population. Acetyl-coenzyme A carboxylase genes (ACCase) of susceptible and all 10 resistant populations were amplified and sequenced. Among them, Ile-1781-Leu, Trp-2027-Cys, Trp-2027-Ser, and Ile-2041-Asn mutations were found in five resistant populations. No known resistance-related mutations were found in the other five resistant populations, indicating that resistance to cyhalofop-butyl in these populations was likely to be endowed by non–target site resistance mechanisms. Notably, the Ile-1781-Leu and Trp-2027-Cys substitutions have previously been reported, but this is the first report of Trp-2027-Ser and Ile-2041-Asn mutations in L. chinensis. Furthermore, three derived cleaved amplified polymorphic sequence methods were developed to rapidly detect these mutations in L. chinensis.

Cover crop residue can act as a mulch that will suppress weeds, but as the residue degrades, weed suppression diminishes. Biomass of cover crop residue is positively correlated to weed suppression, but little research is available regarding the composition of cover crop residue and its effect on weed suppression. Field experiments were conducted to determine the impact of cover crop residue properties (i.e., total carbon, total nitrogen, lignin, cellulose, and hemicellulose) on summer annual weed suppression and cash crop yield. Cover crop monocultures and mixtures were planted in the fall and designed to provide a range of biomass and residue properties. Cover crops were followed by corn (Zea mays L.) or soybean [Glycine max (L.) Merr.]. At termination, cover crop biomass and residue components were determined. Biomass ranged from 3,640 to 8,750 kg ha−1, and the carbon-to-nitrogen (C:N) ratio ranged from 12:1 to 36:1. As both cover crop biomass and C:N ratio increased, weed suppression and duration of suppression increased. For example, a C:N ratio of 9:1 is needed to suppress redroot pigweed (Amaranthus retroflexus L.) 50% at 4 wk after termination (WAT), and that increases to 16:1 and 20:1 to have 50% suppression at 6 and 8 WAT, respectively. Similarly, with biomass, 2,800 kg ha−1 is needed for 50% A. retroflexus suppression at 4 WAT, which increases to 5,280 kg ha−1 and 6,610 kg ha−1 needed for 50% suppression at 6 and 8 WAT, respectively. In general, similar trends were observed for pitted morningglory (Ipomoea lacunosa L.) and large crabgrass [Digitaria sanguinalis (L.) Scop.]. Corn and soybean yield increased as both cover crop biomass and C:N ratio increased where no weed control measures were implemented beyond cover crop. The same trend was observed with cash crop yield in the weed-free subblocks, with one exception. This research indicates that cover crop residue composition is important for weed control in addition to biomass.

Coolatai grass is an invasive, perennial grass from Africa and the Middle East that has infested large areas of northern New South Wales, Australia, and also occurs in other Australian states. An understanding of the seed germination ecology of Coolatai grass can assist in predicting its potential distribution and developing effective management strategies. The effects of various environmental factors on Coolatai grass seed germination were investigated in a series of laboratory experiments. The effect of diurnal alternating temperatures (5–45 C) with a 12-h photoperiod were examined on a thermogradient plate. Seed germination occurred at almost all temperature combinations from 5 to 45 C. At moderate temperatures the speed of seed germination was very high; at 30/20 C more than 80% of seeds were germinated within 12 h. Germination was slightly enhanced by the presence of light, but length of photoperiod did not affect germination. Germination at neutral pH was over 90%, and declined to 65% at acidic (pH 4) and alkaline (pH 10) pH levels. Germination was completely inhibited at an osmotic stress of −0.55 MPa or greater and was reduced by 50% at −0.37 MPa. Greater germination in relatively dry soil conditions compared with native species may contribute to the establishment of this species in the field. Results indicate that Coolatai grass seeds can germinate over a wide range of diurnal temperatures, light regimes, pH levels, and under marginal water stress. These characteristics help explain the successful invasion of Coolatai grass and provide evidence that this species is capable of establishing in many parts of Australia.

Field studies were conducted in 1987 and 1988 to evaluate the vegetative response of soybeans to early postemergence applications of lactofen and to measure crop recovery. Soybean injury 4 d after treatment (DAT) averaged 29 and 34% with 0.22 and 0.44 kg ai ha−1 of lactofen, respectively. Severity of leaf injury depended on leaf size at the time of lactofen application. Leaf area of soybean trifoliates one through three were reduced 30 to 45% at 12 DAT but recovered by 26 DAT. Later emerging trifoliates were not affected. Fresh weights of lactofen-treated soybeans were less than those of untreated plants 4, 8, and 16 DAT, and dry weights were less at 16 and 26 DAT. Lactofen reduced stomatal conductance of expanded leaves but had no effect on unexpanded leaves. Soybean yields did not differ with lactofen treatment.

The preemergence herbicide cinmethylin is a benzyl ether derivative of the monoterpene 1,4-cineole. Other oxygenated monoterpenes (carvone, citronellol, fenchone, geraniol, and pulegone) were previously found to inhibit the germination and growth of several weed species while exhibiting low phytotoxicity to soybean. Benzyl ether derivatives of these oxygenated monoterpenes were synthesized and examined for preemergence and postemergence activity towards corn, soybean, wheat, and velvetleaf. Benzyl pulegyl ether exhibited the most preemergence activity when applied directly to the soil, completely inhibiting wheat and velvetleaf emergence at 1.0 kg ha−1 while reducing soybean emergence 80%. Several of the benzyl ethers were more inhibitory to velvetleaf radicle elongation relative to cinmethylin but less inhibitory to corn and wheat radicle elongation in petri dish bioassays. Several of the benzyl ethers exhibited limited postemergence activity when applied at 1.0 kg ha−1 to seedlings of the test species 10 d after emergence. The benzyl ether derivatives demonstrated altered selectivity and sensitivity as compared to the parent compounds and cinmethylin.

The effect of glyphosate [N-(phosphonomethyl) glycine] on barley (Hordeum vulgare L.) and corn (Zea mays L.) shoot δ-aminolevulinic acid (ALA) production was examined by monitoring ALA content in the tissue and measuring incorporation of 14C precursors into ALA and chlorophyll a. Barley shoot ALA content was significantly decreased by 1 mM glyphosate after 9, 11, and 15 h of illumination. ALA production by treated barley shoots was 30 nmoles•g fresh weight-1•h-1 at each interval tested, compared with 75 to 120 nmoles•g fresh weight-1•h-1 for the control. In corn shoots, ALA content was reduced 32, 45, and 58% by 0.1, 1.0, and 10.0 mM glyphosate, respectively, after 12 h illumination. Incorporation studies with 14C-glutamate, 14C-α-ketoglutarate, and 14C-glycine into ALA showed a 77, 92, and 91% inhibition, respectively, in barley shoots treated with 1 mM glyphosate. Incorporation of 14C-ALA into chlorophyll a was not affected by 1 mM glyphosate. Thus, the site of action of glyphosate may involve two enzyme pathways:one controlling the conversion of α-ketoglutarate to ALA, and the other controlling the condensation of glycine with succinyl CoA to form ALA and carbon dioxide. Inhibition of ALA synthesis blocks synthesis of chlorophyll, as well as all other porphyrin ring compounds found in higher plants. Thus, inhibition of ALA synthesis may be an integral component of the herbicidal mode of action of glyphosate.