Thirty herbicides were tested for possible effects upon the incorporation of radioactivity from malonic acid-2-14C into lipid by excised hypocotyls of hemp sesbania (Sesbania exaltata (Raf.) Cory). Concentrations of 1 to 20 mg/L were used. Inhibition was caused by 3-nitro-2,5-dichlorobenzoic acid (dinoben), 2-chloro-N,N-diallylacetamide (CDAA), 2-chloroallyl diethyldithiocarbamate (CDEC), 7-oxabicyclo(2.2.1)heptane-2,3-dicarboxylic acid (endothall), 2,6-dichlorobenzonitrile (dichlobenil), 3,5-diiodo-4-hydroxybenzonitrile (ioxynil), and pentachlorophenol (PCP). At 1 mg/L, stimulation was caused by 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), 4-amino-3,5,6-trichloropicolinic acid (picloram), and 3-amino-2,5-dichlorobenzoic acid (amiben).

The development of sustainable farming systems depends on our ability to predict and manage the response of weed communities to changes in cropping practices. A study was established at Normandin, Québec, Canada, to investigate the influence of liquid dairy manure and mineral fertilizer, as well as chisel and moldboard plow tillage systems, in a spring barley monoculture and a 3-yr spring barley-forage rotation that included red clover and timothy. Weed species richness (Margalef's DMG), evenness (Shannon's E), and diversity (Shannon's H') were examined in these treatments from 1992 to 1995. Nutrient source had no effect on any of the three diversity indices. Evenness values were extremely low in all years, suggesting dominance of a few weed species in most treatments. Weed species richness and diversity generally were greater in the barley-forage rotation compared with the monoculture. Tillage effects on richness and diversity varied with crop rotation. Margalef's DMG and Shannon's H' were greater in 1993 and 1995, but they were lower in 1994 when chisel was compared with moldboard plowing in the monoculture. In 1994, chickweed density was about five times greater in the chisel-plowed monoculture compared with other treatment combinations of rotation and tillage. In 1995, only one species with a density of six plants m−2 occurred in the moldboard-plowed monoculture compared with three to six species and densities of 51 to 832 plants m−2 in the other rotation by tillage treatments. Climatic conditions and herbicide use patterns in the different crop rotation treatments may have contributed to the more dynamic nature of weed species diversity in the barley monoculture. Reduced frequency of tillage and herbicide application; management of the forage stands, especially with regard to their termination; and improved soil resource availability likely explained the increased but more stable diversity of the weed communities in the barley-forage rotation.

The conditions causing the onset of secondary dormancy in seed of winter oilseed rape cultivars Falcon and Libravo were investigated in a series of laboratory experiments to provide practical guidance on how to minimize the persistence of volunteer oilseed rape seed. The germination of recently harvested oilseed rape seed in moist conditions at 12 C was close to 100% in both darkness and under white light. Imbibition in water stress with an osmotic potential (ψ) of −1,500 kPa at 12 C in far-red light (FR) or dark for more than 5 d induced secondary dormancy in both cultivars. Treatment with FR was more effective than darkness at inducing secondary dormancy. Libravo produced more dormant seed than Falcon. Imbibition in FR and absence of water stress (ψ = 0 kPa) decreased germinability especially at the low temperature (6 C). These conditions resulted in 19.4 and 27.2% dormant seed for Libravo and Falcon, respectively. The results suggest that phytochrome is involved in the induction of secondary dormancy in oilseed rape. In practice, much oilseed rape seed is buried by postharvest cultivation in late summer (low hydric conditions) and exposed to water stress and darkness for some time. To avoid soil seed persistence and subsequent volunteer oilseed rape, seed must be retained at the soil surface when there is adequate moisture available. Therefore, incorporation of seed by tillage should be avoided or delayed as long as possible.

Seed of common ragweed (Ambrosia artemisiifolia L.) ecotypes were collected from 12 locations in North America and were planted at Ithaca, New York. Plants from seed collected in the northern latitudes flowered earlier and produced less vegetative growth. The difference in growth and development of the ecotypes was caused by the day length or latitude where they had originated. Seedlings emerging in early July produced approximately one-third as much fresh or dry weight and about one-third as many viable seeds as those emerging in the middle of May. The latter produced more than 32,000 seed per plant.

The selectivity of {2-[4-(2′,4′-dichlorophenoxy) phenoxy] methyl propionate}, (hereinafter referred to as dichlofop methyl), among wheat (Triticum aestivum L. ‘Neepawa’), barley (Hordeum vulgare L. ‘Bonanza’), wild oat (Avena fatua L.), and green foxtail (Setaria viridis (L.) Beauv.) was investigated. On an ED50 basis, barley, wild oat, and green foxtail were 2, 190, and 1,090 times more sensitive, respectively, to foliar-applied dichlofop methyl at the two-leaf stage than was wheat. Selectivity decreased with increasing maturity of the plant material with the ratio of selectivity between barley and wild oat decreasing from 55 at the two-leaf stage to three at the four-leaf-plus-one-tiller stage. Greater spray retention and more rapid penetration of dichlofop methyl partially explained the susceptibility of green foxtail, but did not explain selectivity between wheat, wild oat, and barley. Root uptake of 14C-dichlofop methyl by the four species was proportional to the amount of solution absorbed during the treatment period and to the concentration of dichlofop methyl in the treatment solution but was not related to species sensitivity to this herbicide.

Effective in-season weed management options are limited for organic cereal farmers. Two alternatives to current farmer practices are improving efficacy of physical weed control through use of interrow cultivation or increasing the competitive ability of the crop through elevated seeding rates and more uniform spatial planting patterns. It is unknown how these two methods affect yield, quality, and economic returns. Field experiments were conducted in the northeast United States to determine whether the yield gain from increased weed control from these contrasting weed management strategies resulted in increased net returns and how these different systems affected grain quality. Wheat was planted at two seeding rates (400 and 600 plants m−2), in three row spacings (11, 18, and 23 cm). A fourth crop arrangement that approaches a more uniform spatial distribution through a combination of drilling and broadcasting seed was included. For weed control, treatments received tine harrowing. Wheat sown in wide rows also received interrow cultivation. Each system was sown in the presence and absence of condiment mustard, which was sown as a surrogate weed. Increased seeding rate reduced weed density 64% compared to a crop-free check and 30% compared to regional farmers' practices of 18-cm rows and 400 plants m−2. Increased seeding rates lowered grain protein 5% compared to standard seeding rates. Wide rows, in combination with interrow cultivation, reduced weed density 62%, increased yield 16%, and net returns 19% compared to regional organic practices. Significant increases in grain N were limited to weed-free plots. While increased seeding rates improved weed suppression, the high input cost of organic seed make this an unsatisfactory alternative to interrow cultivation and current farmer practices, as yield would need to be. 15 t ha−1 higher at elevated density to offset the extra cost of seed.

The diphenyl ether, oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene], exerts a very strong and rapid bleaching effect upon intact microalgae such as Scenedesmus acutus. Carotenoids, and subsequently chlorophylls, are destroyed concurrently with ethane formation and inhibition of photosynthetic oxygen evolution. These herbicidal effects are not observed before an activation time of approximately 2 h, during which photosynthetic electron transport is necessary. Diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] inhibits oxyfluorfen activity during this activation time, but not thereafter. Isolated spinach chloroplasts (Spinacia oleracea ‘Atlanta’) evolve ethane after a light-incubation phase in the presence of oxyfluorfen as well as paraquat (methylviologen, 1,1′-dimethyl-4,4′-bipyridinium ion). Depending on their chemical constitution, diphenyl ethers apparently act differently and multifunctionally. The effects described for oxyfluorfen are believed to represent the primary mode of action of bleaching diphenyl ethers.

Preplant-incorporated (PPI) applications of 280 g ai/ha of imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} had significantly greater persistence than preemergence surface (PES) applications throughout the growing season in 1985. In 1984 there was no statistical difference in imazaquin remaining between PPI and PES treatments. Very little imazaquin was detected below 10 cm in the soil profile for all sampling dates each year. Imazaquin dissipated rapidly both years during the first 30 days following PPI and PES applications. A subsequent decrease in the dissipation rate occurred in the next 120 days of the growing season. Spring tillage had no significant effect on injury to corn (Zea mays L.) planted into the field the year following imazaquin application. Corn planted in 1986 into imazaquin-treated plots of 1985 had no growth reduction when compared to corn grown in plots where zero imazaquin had been applied. However, corn planted in 1985 into imazaquin applications of 1984 had significant injury. Greatest injury occurred where imazaquin had been incorporated, and injury increased as imazaquin rate was increased.

A herbicide safener, CGA-43089 [α-(cyanomethoximino)-benzacetonitrile], has been discovered which will allow the safe use of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethylacetamide] in grain sorghum [Sorghum bicolor (L.) Moench]. Experiments revealed that sorghum is protected from injury by metolachlor when it is applied at rates as high as 4 kg/ha. Better protection was achieved when sorghum seed were treated directly with the safener than when it was mixed with the herbicide. A CGA-43089 rate of 1.25 to 1.5 g per kg of sorghum seed was adequate for protection. Sorghum tolerance to metolachlor was maintained with CGA-43089 when metolachlor was combined with several s-triazine herbicides. Metolachlor effectively controlled several annual grasses with an increase in sorghum yield.

The relative leaf area of weeds is a good predictor of the outcome of weed-crop competition. However, this variable has not been used in decision-making tools for integrated weed management because leaf area cannot be measured quickly. A powerful image analysis system for measuring leaf cover (the vertical projection of plant canopy on the ground) has been developed and validated. This research was conducted to compare the efficiency of weed relative leaf area and relative leaf cover in predicting corn yield loss. Field studies were conducted in 1996 and 1997 using varying densities of common lambsquarters, barnyardgrass, common lambsquarters plus barnyardgrass, and a natural weed community. Corn grain yield and biomass loss varied with weed infestation type and year. Values of the relative damage coefficient of weeds (q) were smaller in 1997 compared with 1996. For both years, the relative leaf area of weeds was an adequate predictor of corn yield loss (r 2 varied from 0.61 to 0.92). The precision of the predictions was not influenced by the leaf area sampling period (four- or eight-leaf stage of corn). In general, smaller values of q and m (predicted maximum yield loss) were obtained as a consequence of using the relative leaf cover of weeds in model fitting. However, percentages of variation explained by the model (from 0.67 to 0.90) were similar to values obtained with the relative leaf area. On the basis of the residual mean squares, neither of the variables could be declared superior to the other in yield loss prediction. The development of weed control decision-making tools using the relative leaf cover of weeds may require improvements prior to being used in weed management systems. Such improvements would include use of appropriate sampling and image-processing techniques, development and validation of empirical models specific to individual situations, and proper identification of the crop growth stage at which leaf cover must be assessed.

The introduction of Clearfield (CL) rice cultivars resistant to imidazolinone herbicides, acetolactate synthase (ALS) inhibitors, has raised concerns of gene flow to weedy rice genotypes collectively called “red rice” that infest rice-growing areas in the southern United States. This experiment was conducted to study hybridization between CL rice and red rice using simple sequence repeats (SSR) markers, identify mutations in the ALS gene of imazethapyr-resistant red rice, and to detect the introgression of the ALS-resistant gene from CL rice into red rice. Natural outcrossing experiments between CL rice and strawhull (SH) red rice were set up in Stuttgart, AR, in 2002 and 2003. Putative red rice hybrids were detected among volunteer plants in the following year. Hybridization was confirmed using SSR markers, and introgression of the resistant ALS gene from CL rice to red rice was detected by ALS gene sequencing. The ALS gene sequences of U.S. rice cultivars ‘Bengal’ and ‘Cypress’, SH red rice, CL rice (CL161), and imazethapyr-resistant red rice/CL rice hybrids were compared. Nucleotide sequences of the ALS gene from the rice cultivars were identical. Three point mutations were present in the SH red rice ALS gene coding region relative to Bengal/Cypress. One of these resulted in the substitution of Asp630 for Glu630. The ALS gene sequences of confirmed hybrids were identical to that of the herbicide-resistant pollen source, CL161. We identified four ALS gene mutations in the herbicide-resistant red rice hybrids relative to the susceptible rice cultivars. One point mutation, resulting in a substitution of Ser653 with Asn, was linked to ALS resistance in callus tissue derived from a Kinmaze rice line from Japan. The other three mutations (Ser186—Pro, Lys416—Glu, and Leu662—Pro) are novel. This experiment confirmed that gene flow from imidazolinone-resistant rice resulted in herbicide-resistant red rice plants.

Recent reports indicate that manganese (Mn), applied as a foliar fertilizer in tank mixtures with glyphosate, has the potential to antagonize glyphosate efficacy and reduce weed control. It was hypothesized that Mn2+ complexed with glyphosate in a similar manner to Ca2+, forming salts that were not readily absorbed and, thereby, reducing glyphosate efficacy. This study was conducted to confirm the interaction of Mn2+ and glyphosate and to measure the effect of Mn on glyphosate absorption and translocation in velvetleaf. In aqueous solutions, Mn2+ binds with solvent molecules and with chelating agents to form hexacoordinate complexes. The distribution of paramagnetic species, both the free manganous ion ([Mn{H2O}6]2+) and the Mn2+–glyphosate complex, in Mn–glyphosate solutions at various pH values were analyzed using electron paramagnetic resonance (EPR) spectroscopy. Glyphosate interaction with Mn appeared to increase as the pH was increased from spray solution levels (2.8 to 4.5) to levels common in the plant symplast (7.5). Growth chamber bioassays were conducted to measure absorption and translocation of 14C-labeled glyphosate in solution with four Mn fertilizers: Mn-ethylaminoacetate (Mn-EAA), Mn-ethylenediaminetetraacetate (Mn-EDTA), Mn-lignin sulfonate (Mn-LS), and Mn-sulfate (MnSO4). Mn-EDTA did not interfere with glyphosate efficacy, absorption, or translocation. However, both MnSO4 and Mn-LS reduced glyphosate efficacy, absorption, and translocation. Mn-EAA severely antagonized glyphosate efficacy, and although glyphosate in tank mixtures with Mn-EAA was absorbed rapidly, little was translocated from the treated leaf. The Mn-EAA fertilizer contained approximately 0.5% iron (Fe) not reported on the fertilizer label. Iron is presumed to be partially responsible for the very limited translocation of glyphosate from the treated leaf in Mn-EAA tank mixtures. Adding ammonium sulfate increased the efficacy, absorption, and translocation of glyphosate for each Mn fertilizer tank mixture.

There are various reasons for using statistics, but perhaps the most important is that the biological sciences are empirical sciences. There is always an element of variability that can only be dealt with by applying statistics. Essentially, statistics is a way to summarize the variability of data so that we can confidently say whether there is a difference among treatments or among regression parameters and tell others about the variability of the results. To that end, we must use the most appropriate statistics to get a “correct” picture of the experimental variability, and the best way of doing that is to report the size of the parameters or the means and their associated standard errors or confidence intervals. Simply declaring that the yields were 1 or 2 ton ha−1 does not mean anything without associated standard errors for those yields. Another driving force is that no journal will accept publications without the data having been subjected to some kind of statistical analysis.

Annual bluegrass is commonly controlled by acetolactate synthase (ALS)-inhibiting herbicides in managed turfgrass. An annual bluegrass population with suspected resistance to ALS-inhibiting herbicides was collected from Grand National Golf Course in Opelika, AL (GN population). Subsequent testing confirmed resistance of the GN population to foramsulfuron, trifloxysulfuron, bispyribac-sodium (bispyribac), and imazaquin when compared to a susceptible population collected locally at Auburn University (AU population). Sequencing of the ALS gene revealed a point mutation resulting in an amino acid substitution at Trp574. Cloning of the ALS gene surrounding the Trp574 region yielded two distinct ALS gene sequences: one producing Trp574 and one producing Leu574. Trp574 to Leu has been previously correlated with resistance to ALS-inhibiting herbicides. Both AU and GN gene sequences contained other similar silent and missense mutations. This research confirms resistance of annual bluegrass to ALS-inhibiting herbicides with Trp574 to Leu amino acid substitution being the most likely mode of resistance based on past literature.

Reductions in yields of processing tomatoes (Lycopersicon esculentum Mill. ‘H2653’ and ‘Earlirouge′) caused by interference from eastern black nightshade (Solanum ptycanthum Dun. # SOLPT) and hairy nighthsade (S. sarrachoides Sendt. # SOLSA) were estimated for transplanted and seeded tomatoes at two locations in southern Ontario. Tomato yield losses were significantly greater in seeded than in transplanted tomatoes. Stomatal conductance and transpiration rates of seeded tomatoes decreased more rapidly with increased nightshade density than did those of transplanted tomatoes. Percent yield losses also differed between sites. Seeded tomatoes grown at high density in twin rows (33 300 and 45 000 plants/ha) had higher yields and lower percent yield losses than did tomatoes grown at low density in single rows (12 500 to 22 500 plants/ha). Nightshade dry weight and seed production decreased per plant but increased per unit area with increasing nightshade density. Nightshade dry weights and seed production did not vary with site or method of tomato establishment. A hyperbolic model provided an excellent fit to data on both tomato yield losses and nightshade seed production as a function of nightshade density.

The inheritance and linkage of enhanced metabolism-based herbicide cross-resistance was examined in a multiple resistant population of rigid ryegrass. F1 hybrids between resistant and susceptible populations showed an intermediate response to all the four herbicides tested, with no indication of maternal inheritance. Segregation of F2 families fitted a single-gene model for resistance to simazine, chlorotoluron, and chlorsulfuron. But there was more than the expected mortality from the low dose of tralkoxydim. Segregating F2 populations were selected by high rates of each of the four herbicides to create selected F2 families. Analysis of the response of these families demonstrated that simazine resistance is linked to chlorotoluron resistance. No other herbicide resistances were linked. Therefore, in this population of rigid ryegrass, at least three separate genes are responsible for metabolism-based cross-resistance. This study shows that multiple herbicide resistance in rigid ryegrass is the result of accumulation of a number of different resistance genes.

The seed composition in the upper 15-cm soil horizon was determined and correlated with weed seedlings growing with fieldbeans (Phaseolus vulgaris L. ‘Valley’). The total seed reservoir averaged 250 seed/kg of soil, and 19 species were represented. Seed occurring with the most frequency were redroot pigweed (Amaranthus retroflexus L. ♯ AMARE), common lambsquarters (Chenopodium album L. ♯ CHEAL), and common purslane (Portulaca oleracea L. ♯ POROL). Seed from these plants accounted for over 85% of the seed found. The number of barnyardgrass [Echinochloa crus-galli (L.) Beauv. ♯ECHCG], buffalobur (Solanum rostratum Dunal ♯ SOLCU), common lambsquarters, common purslane, and common sunflower (Helianthus annuus L. ♯ HELAN) seed in the soil was correlated with the number of plants growing in the field with fieldbeans. A correlation occurred between redroot pigweed, yellow foxtail [Setaria lutescens (Weigel.) Hubb. ♯ SETLU], and barnyardgrass growing in corn (Zea mays L.) fields in the fall of the year and plants growing in the field with fieldbeans the following year.

Herbicide resistance has been reported in several Amaranthus species throughout the U.S. Because evidence exists of interspecies hybridization in some species of this genus, this study was conducted to determine whether acetolactate synthase (ALS)-inhibiting herbicide resistance could be transferred from Amaranthus palmeri to Amaranthus rudis through interspecific crosses. Plants of each species were grown in a growth chamber, and controlled interspecies crosses were made between ALS-resistant and -susceptible plants. A total of 15 putative hybrid plants were produced from an estimated 10,000 cross-pollinated flowers. Analysis of restriction enzyme digests of the ALS gene in which a single base substitution confers resistance inferred that herbicide resistance had been transferred from a resistant male A. rudis to the hybrid plant. Offspring of hybrid plants, backcrossed to the susceptible parent, survived herbicide treatment, demonstrating that herbicide resistance was transferred between species. DNA analysis also was performed using the amplified fragment length polymorphism (AFLP) technique between parental and putative hybrid plants. Several unique bands were found only in the hybrid.

Resistance to the methyl ester of diclofop, an aryloxyphenoxypropionate graminicide, was shown for a wild oat (Avena fatua) population from Western Australia, and marked resistance to a range of aryloxyphenoxypropionate and cyclohexanedione graminicides was detected in a winter wild oat (Avena sterilis ssp. ludoviciana) population from South Australia. The A. sterilis biotype exhibited high levels of resistance to the aryloxyphenoxypropionate herbicides diclofop, fluazifop, haloxyfop, fenoxaprop, quizalofop, propaquizafop, and quinfurop and low levels of resistance to the cyclohexanedione herbicides sethoxydim, tralkoxydim, and cycloxydim. Ratios of LD50 values for responses of resistant and susceptible A. sterilis to the aryloxyphenoxypropionate herbicides were between 20 for propaquizafop and > 1,000 for fluazifop, and were between 2.5 and 3 for the cyclohexanedione herbicides. The LD50 value for diclofop for the A. fatua biotype was 442 g ai ha-1 which was 2.7-fold that of a susceptible control. Thirty-three percent of the plants survived at the registered rate of application.

Inhibition and subsequent recovery of net CO2 exchange (NCE) by single leaves of 53 grass species of the subfamilies Festucoideae, Panicoideae, and Eragrostoideae was monitored following limited root uptake of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine]. Rates of NCE recovery were correlated with rates of 14C-atrazine metabolism in leaf sections of festucoid and panicoid grasses but not eragrostoid grasses. Rates of NCE recovery among the nine festucoid and 12 eragrostoid species tested did not exceed 0.2 and 1.1 mg CO2 per dm2 per hr/hr, respectively, whereas for the 31 panicoid species, it ranged from 0.2 to 2.4 mg CO2 per dm2 per hr/hr. Selected species exhibiting NCE recovery rates exceeding 1.2 mg CO2 per dm2 per hr/hr were tolerant to 1.0 kg/ha preemergence and 1.25 kg/ha postemergence atrazine applications. These panicoid species included a number of common weed species belonging to the genera Digitaria, Panicum, and Setaria, as well as species belonging to Bracharia, Pennisetum, Sorghum, and Zea. Unchanged 14C-atrazine accounted for 11.3 to 92.7% of the total 14C-radioactivity extracted from 14C-atrazine infiltrated leaf sections following an 8-hr incubation period. N-dealkylation, hydroxylation, and peptide conjugation occurred in all species, although rates of these metabolic pathways varied widely among species. Recovery of NCE was correlated with formation on the glutathione-atrazine conjugate, and conjugation was the major detoxification pathway in species exhibiting tolerance to atrazine.