The effects of caffeic, vanillic, p-coumaric, chlorogenic, and ferulic acids, ragweed parthenium (Parthenium hysterophorus L. # PTNHY) residue and extracts on the growth, ion uptake, and chlorophyll content of 3-week-old tomatoes (Lycopersicon esculentum Mill. ‘Walter’) grown in the greenhouse were determined Vanillic, p-coumaric, chlorogenic, and ferulic acid at 10−3 M and parthenium extract at 0.5% (w/v) significantly reduced tomato root and shoot dry weight. Ragweed parthenium residue at 0.5% (w/w) and extract at 0.5% (w/v) significantly reduced tomato shoot and root dry weight. Leaf nitrogen content was reduced by phenolic acids (10−3 M) and ragweed parthenium extract (0.5%). Phosphorus content was reduced by all phenolic acids at 10−3 M and ragweed parthenium extract at 0.5% (w/v). In addition, p-coumaric and ferulic acids at 10−4 M reduced phosphorus content. No consistent relationship between chlorophyll content and tomato growth reduction was observed.

Several methods of evaluating plant responses to combinations of herbicides are discussed. They include: a direct-plot of percent response vs toxic units; determining ID50 (dosage that causes a 50% reduction in growth of the treated plant) for each herbicide and for each combination; percent response vs log-dosage; double reciprocal procedures; probit analysis, and calculation of expected values. A simplified version of the isobole method of Tammes (23) is presented as a method which can be readily applied by weed scientists to determine the validity of data on plant responses to herbicide combinations. Definitions are proposed which classify plant responses to herbicide interactions as additive, synergistic, antagonistic, or enhancement. Four points are listed that will improve the quality of data obtained from experiments on combinations.

Five crops were grown for three seasons following a single soil-incorporated application of MSMA (monosodium methanearsonate). Crop sensitivity to soil arsenic decreased as follows: snap beans (Phaseolus vulgaris L. ‘Early Gallatin’ and ‘Tenderette’); rice (Oryza sativa L. ‘Dawn’ and ‘Starbonnet’); soybeans [Glycine max (L.) Merr. ‘Lee 68′]; potatoes (Solarium tuberosum L. ‘Red LaSoda’ and ‘Red Pontiac’); and cotton (Gossypium hirsutum L. ‘Stoneville 7A’). Over the 3-yr period, yield of snap beans was reduced 8, 14, and 85% below the untreated check where MSMA had been incorporated initially at 22, 45, and 269 kg/ha, respectively. Similar reductions in yield of rice were 18, 25, and 55%. Soybean and potato yields were significantly reduced only at the highest rate of MSMA while cotton yields were unaffected. Arsenic content in the edible portion of the crops decreased as follows: rice, snap beans, potatoes, soybeans, and cotton. The arsenic content in crops from the highest MSMA plots seldom exceeded twice that from the untreated plots.

Sublethal amounts of 2,4-D [(2,4-dichlorophenoxy) acetic acid] were applied to several vegetable crops in order to determine effect on yield and quality in a simulated drift experiment. Tomato (Lycopersicon esculentum Mill.) and root crops were most sensitive to 2,4-D; as little as 2.1 g/ha of acid distorted tomato fruit shape and elongated radish (Raphanus sativus L.) roots. All root crops were rendered unmarketable by 10.4 g/ha and gross yields were reduced by exposure to 104 g/ha. Yield of peppers (Capsicum frutescens L. var. grossum) was increased by exposure to 2.1 g/ha and severely depressed by 104 g/ha. Cucumber (Cucumis sativus L.) fruit shape was distorted at 11 g/ha and yield was reduced slightly. Lettuce (Lactuca sativa L.), onion (Allium cepa L.), and cabbage (Brassica oleracea L. var. capitata) were least sensitive to 2,4-D. Exposure to 20.8 g/ha did not reduce yields of lettuce or cabbage. Onion yields were reduced by application of 104 g/ha. Bush bean (Phaseolus vulgaris L.) yield was decreased by exposure to 22 g/ha, but potato (Solarium tuberosum L.) yield increased at 16 g/ha. Herbicide residues in the crop foliage were proportional to the degree of exposure. Residue analysis combined with foliar symptoms may be useful in predicting crop damage following 2,4-D drift.

CGA-43089 [α-(cyanomethoximino)-benzacetonitrile] applied as a seed treatment enables sorghum [Sorghum bicolor (L.) Moench] to tolerate metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide]. Improved tolerance of alexandergrass [Brachiaria plantaginea, (Link) A. Hitchc.] Eleusine spp., proso millet (Panicum miliaceum L.) and rice (Oryza sativa L.) to metolachlor also was demonstrated. Seven sorghum cultivars, representing widely divergent types of germplasm, responded similarly to the safening effect of CGA-43089 when the cultivars were exposed to metolachlor. Treated sorghum seed may be stored more than 1 yr without a loss of safening activity. In these growth chamber experiments, neither extremes of soil moisture nor temperature reduced the effectiveness of the safener. The predominant site of uptake of CGA-43089 is the coleoptile of sorghum seedlings.

Weedy rice is a serious weed of cultivated rice in most of the rice-growing areas of the tropics and yet, despite its importance as a weed, limited information is available on its biology in Asia. A study was conducted to determine the seed characteristics and growth response to competition of weedy rice variants from five Asian countries (India [IWR], Malaysia [MWR], Philippines [PWR], Thailand [TWR], and Vietnam [VWR]) when grown alone or with either 4 or 12 cultivated rice (‘IR72’) plants. Seed characteristics including weight, length, width, and presence and length of awn differed between weedy rice variants. PWR had the heaviest seed (25 mg seed−1). Growth of weedy rice plants differed among variants and, generally, VWR grew the fastest. IR72 interference greatly reduced tiller and leaf numbers, leaf area, and shoot biomass of all weedy rice variants. At 8 wk after sowing, for example, shoot biomass of weedy rice variants in competition with 12 IR72 plants was 13 to 30% of that where plants were grown alone. TWR plants were shorter than those of other variants at all levels of crop interference. These results show that there are considerable differences among Asian weedy rice variants and yet, competition from the rice crop greatly suppresses weedy rice growth. This suggests that measures to ensure a competitive crop should be an important component of control strategies.

Degradation of 3-amino-l,2,4-triazole-5-C14 (amitrole-5-C14) in autoclaved, potassium azide, ethylene oxide, and dry-heat sterilized soil was compared with that observed in nonsterile soils by measuring evolved C14O2. Amitrole degradation occurred in azide and ethylene oxide-sterilized and nonsterile soils but not in autoclaved soils. Only slight degradation occurred in autoclaved soil re-inoculated with mixed cultures of soil microorganisms isolated from soil in which amitrole had been rapidly degraded. Addition of EDTA-Na to nonsterile and ethylene oxide sterilized soils reduced amitrole degradation. Addition of organic amendments to amitrole treated soil stimulated microbial activity but reduced amitrole degradation. Addition of specific metallic salts to certain soils increased the rate of amitrole degradation. Amitrole degradation occurred rapidly in several free radical generating chemical systems. These results and additional observations indicate that amitrole degradation, at least in three soils examined, was largely a chemical process, and that microbial involvement was only indirect.

Cross-resistance of the paraquat-resistant (R) hairy fleabane to other compounds that accept electrons from photosystem I (PSI) or produce toxic oxygen species was determined by chlorophyll loss, electron microscopy, and chlorophyll fluorescence suppression. Although the R bioype is approximately 100 x more resistant to paraquat than the susceptible (S) biotype based upon the assays for tissue damage, little or no cross-resistance was observed to a number of other PSI electron acceptors, including the bipyridilium herbicide morfamquat. A low level of resistance (approximately 10-fold) was noted to diquat and the singlet oxygen generator rose bengal. As measured by chlorophyll fluorescence suppression, the R biotype was about 100-fold resistant to paraquat, but only 10-fold resistant to diquat, and exhibited no resistance to morfamquat. Because differences observed with this protocol are direct measures of the ability of the herbicide to reach the active site and the results correlate with the level of resistance observed by chlorophyll bleaching or electron microscopy, these data suggest that compartmentalization is the major factor in paraquat resistance in hairy fleabane.

The first occurrences of wild oat resistance to diclofop in the Willamette Valley of Oregon were reported in 1990. Among eight resistant biotypes, GR50 values for diclofop were 3 to 64 times greater than the GR50 for a susceptible wild oat biotype. GR50 values for other aryloxyphenoxypropionate herbicides varied from 1 to over 100 times greater than a susceptible biotype. Only one resistant biotype was resistant to cyclohexanedione herbicides, and this was only a three-fold increase in GR50. Except for one biotype that had a low level of resistance to pronamide, none of the wild oat biotypes were cross-resistant to any other commonly used wild oat herbicide. Levels of resistance and cross-resistance did not follow a consistent pattern among biotypes in this study, suggesting more than one resistance trait. There were significant differences in the light use efficiency, height, dry weight, leaf area, and extent and timing of tillering and flowering of four wild oat biotypes studied. These physiological and morphological differences suggest that these resistant biotypes were selected independently. The diversity of resistance patterns and the coevolution of resistance at several locations will add to the difficulty of resistance management

Populations of horseweed and Virginia pepperweed resistant to the herbicide paraquat were found in several fruit orchards in Essex County, Ontario, Canada, where paraquat had been applied four to five times per year for at least 10 yr. Dose–response experiments were conducted in the greenhouse for each species, using 2-to 4-mo-old plants from populations that had received repeated exposure to paraquat and those that had not. The paraquat dose that reduced shoot dry weight by 50% (ED 50) was estimated for each population. Resistant populations of horseweed had ED 50 values 25 to 35 times higher than susceptible populations. A resistant population of Virginia pepperweed had an ED 50 10 times higher than susceptible populations.

Interference of common lambsquarters (Chenopodium album L.) in sugarbeets (Beta vulgaris L. ‘Mono Hy D2′) was determined in a 2-yr field experiment. Yield of sugarbeet roots and recoverable sucrose/ha decreased as the density of common lambsquarters increased. At densities of 6, 12, 18, and 24 common lambsquarters plants/30 m of row, root yields were reduced 13, 29, 38, and 48%, respectively, and recoverable sucrose yields were reduced 11, 27, 37, and 46%, respectively. The minimum number of common lambsquarters plants required per 30 m of row to reduce sugarbeet root yields was estimated to be six in 1980 and four in 1981.

Field experiments were conducted at Vegreville, Alberta, in 1990, 1991, and 1992 to investigate the influence of canola plant density and row spacing (1991 and 1992 only) on interference from Tartary buckwheat. Nonlinear regression equations incorporating both canola and Tartary buckwheat density provided good descriptions of the data. Parameter estimates for both weed and crop density were significant (P < 0.05) but did not differ between row spacings. Where no Tartary buckwheat plants were present, canola yield was little affected by canola plant density. At a given Tartary buckwheat density, canola seed yield increased as canola plant density increased, while Tartary buckwheat shoot weight and seed yield decreased. None of the factors significantly affected canola 1000-seed weight, or oil or protein levels in the canola seed. The results suggest that seeding canola to achieve a density of approximately 200 plants m–2 will significantly reduce the impact of weeds on canola yield, as well as reduce weed biomass and seed yield.

C14 ring-labeled atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) was applied in nutrient solutions. Soybeans absorbed more atrazine per g of fresh weight than corn or cotton. The amount of atrazine absorbed by the plants increased with increases in atrazine concentration, in water absorption, and in length of absorption. Less atrazine was absorbed in the second than in the first 24-period wtih occasional indications of loss of atrazine back into the solution. C14O2 evolution was not established for any plant although many different environmental conditions were tried. Degradation to C14O2 is not a significant process for these plants under these conditions.

The incidence of auxinic herbicide resistance in plants has increased worldwide. Auxinic herbicides were the first selective organic herbicides developed and have been used in agriculture for over 50 yr, primarily for the selective control of broadleaf weeds in cereal crops. However, the mode of action of auxinic herbicides and the molecular basis of auxinic herbicide resistance remain unknown, although an auxin-binding protein (ABP) is proposed to be the primary target site. Using auxinic herbicide-resistant (R) and -susceptible (S) biotypes of wild mustard as a model system, we have extensively studied the mode of action of auxinic herbicides and the resistance mechanisms at the physiological, biochemical, and molecular genetic levels. There are no differences in uptake, transport, and metabolism of auxinic herbicides between the R and S biotypes. Based on these results, as well as the studies on the role of auxin-enhanced ethylene biosynthesis and calcium in mediating the auxinic herbicide resistance, we hypothesize that resistance of the R biotype to auxinic herbicides is due to an altered target site, possibly an auxin receptor. We have identified and characterized a small ABP gene family as well as their cDNAs from both R and S of wild mustard. Amino acid changes were found in the ABP of the R biotype. Functional and mutational analyses of these genes are underway to determine the role of ABP in mediating auxinic herbicide resistance. In this review, we focus on the mode of action of auxinic herbicides and the molecular basis of auxinic herbicide resistance in wild mustard.

The proteolytic activity of the cotyledons of squash (Cucurbita maxima Duchesne) increases several fold during germination. The different herbicides varied widely in their ability to inhibit this increase. An attempt was made to correlate this inhibition with inhibition of growth. A direct correlation was evident only when the inhibition of proteolytic activity was severe or very slight to nonexistent.

Herbicide soil/solution distribution coefficients (Kd) are used in mathematical models to predict the movement of herbicides in soil and groundwater. Herbicides bind to various soil constituents to differing degrees. The universal soil colloid that binds most herbicides is organic matter (OM), however clay minerals (CM) and metallic hydrous oxides are more retentive for cationic, phosphoric, and arsenic acid compounds. Weakly basic herbicides bind to both organic and inorganic soil colloids. The soil organic carbon (OC) affinity coefficient (Koc) has become a common parameter for comparing herbicide binding in soil; however, because OM and OC determinations vary greatly between methods and laboratories, Koc values may vary greatly. This proposal discusses this issue and offers suggestions for obtaining the most accurate Kd, Freundlich constant (Kf), and Koc values for herbicides listed in the WSSA Herbicide Handbook and Supplement.

The decline in the natural soil population of seeds of wild mustard [Brassica kaber (DC.) L.C. Wheeler var. pinnatifida (Stokes) L.C. Wheeler ♯ SINAR] under nine cultural and chemical practice treatments was studied under field conditions. Wild mustard plants were not allowed to produce seeds in any treatment. After seven growing seasons, treatments in which the soil was not disturbed (continuous grass sod or continuous chemical fallow) had approximately 50% of the original population of wild mustard seeds remaining. In contrast, less than 3% of the original population of wild mustard seeds remained in a treatment that involved plowing three times a year with additional tillage throughout the growing season. This 3%, however, was equivalent to 2.4 million seeds/ha in the plow layer. Eradication of wild mustard from an infested field may be impractical with currently available techniques.

A simple technique is described for rapid determination of weed seed, rhizome, corm, or bulb populations in soil. Soil samples were placed in nylon mesh bags. The soil was washed from the bags by machine leaving only a small residue including the desired propagules. These were hand separated and the soil population of a given plant species calculated. The total manipulation time per sample was approximately 10 min.

Reports of kochia control failure with glyphosate in western Kansas increased dramatically in the years following confirmed presence of glyphosate-resistant (GR) populations in 2007. In this study, progeny from 8 of 16 geographically dispersed kochia populations in western Kansas (seed collected in 2010) were confirmed to be resistant to glyphosate by conducting whole-plant dose-response (in greenhouse and/or outdoor environments) and shikimate-accumulation assays. Additionally, the relationship between 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene copy number and glyphosate resistance levels was investigated. A known glyphosate-susceptible (GS) kochia population from Ellis County, Kansas was used for comparison in all studies. Based on the herbicide rate that caused 50% reduction in biomass compared to untreated control (GR50) values, the 8 GR kochia populations were 4 to 11 times more resistant to glyphosate compared to the GS population. The GR50 values of kochia populations were 1.58 to 1.85 times higher under an outdoor environment compared to when grown in the greenhouse. Glyphosate-treated leaf discs of the GS kochia plants accumulated consistently higher amounts of shikimate than those of the GR plants. Additionally, the GR plants with higher levels of resistance to glyphosate had higher EPSPS : acetolactate synthase (ALS) relative gene copy number compared to those with low levels of resistance.

A 9-yr (1990–1998) study was conducted at Woodstock, ON, Canada, to evaluate weed densities, crop yields, and gross returns in a modified no-tillage (no primary tillage) corn–soybean–winter wheat rotation under three weed management treatments: (1) minimum, preplant application of glyphosate followed by mechanical control; (2) integrated weed management (IWM), preplant application of glyphosate followed by band application of preemergence herbicides plus mechanical control; and (3) conventional, preplant application of glyphosate followed by broadcast application of preemergence herbicides in corn and soybean. In wheat the minimum and IWM treatments had no additional weed control measures other than the preplant application of glyphosate, whereas the conventional treatment had a broadcast application of a postemergence herbicide. Weed densities were assessed each year, (except in 1990) once during the growing season in corn and soybean and immediately after crop harvest in wheat. Adjusted gross return was calculated as the gross revenue minus the unique costs for weed control for each of the treatments. Weed densities were greater in the minimum treatment compared with the IWM or conventional treatment in all crops. Weed densities in the IWM and conventional treatments did not differ. There was no apparent “buildup” of weed density with time in the rotation resulting from weed escapes. Hence, these data challenge current thinking that weed densities increase with time if weed escapes are allowed to go to seed. Corn and soybean yields in the IWM and conventional treatments did not differ. However, the minimum treatment had the lowest corn and soybean yields. Winter wheat yield was not affected by the treatments. All weed management treatments provided similar gross returns for each crop and for the rotation. Thus, the minimum treatment consisting of glyphosate applied preplant followed by shallow interrow tillage appeared to be a viable option, especially if practiced in a farming system capable of ensuring adequate timing of cultivation operations.