Phytotoxic compounds from plants and microorganisms represent a wide range of chemistries and mechanisms of action that have potential in the design and development of new herbicides. Although several natural products of higher plants have been patented as herbicides, none have been developed commercially. Many microbial products have been patented as herbicides and several have been or are being developed, including bialophos {L-2-amino-4-[(hydroxy)(methyl)phosphinoyl] -butyryl-L-alanyl-L-alanine} and glufosinate [L-2-amino-4-(hydroxyl)(methyl)(phosphinoyl)-butyric acid]. The new tools of molecular biology and biotechnology are making natural products more attractive alternatives in herbicide discovery programs.

Both purple nutsedge (Cyperus rotundus L. # CYPRO) and yellow nutsedge (C. esculentus L. # CYPES) are problem weeds in crops in many parts of the world. Yellow nutsedge is found in all U.S. states. Purple nutsedge is confined to the southern region of the United States, ranging from North Carolina across southern Arkansas and into southern California.

Chloroacetamide and thiocarbamate herbicides have many properties in common: both herbicide classes are effective only as preemergence herbicides; they inhibit early seedling growth and cause similar injury symptoms in susceptible species; they are detoxified in plants by glutathione conjugation; they have a similar spectrum of selectivity; they can be applied safely in certain susceptible grass crops when applied with antidotes; and they can inhibit the synthesis of lipids, isoprenoids, and other metabolic processes requiring coenzyme A. It can be hypothesized that these similarities are due to the ability of the chloroacetamides and the sulfoxide of thiocarbamates to bind covalently to enzymes, coenzymes, or metabolic intermediates containing sulfhydryl (-SH) groups.

Studies of weed and crop competition can be used to predict yield losses from weed presence and to determine optimum levels or periods of weed control. However, competition is a complex phenomenon that is governed by various biological, environmental, and proximity factors. The factors of proximity include plant density, species proportion, and spatial arrangement among individuals. Several experimental methods have been developed that attach different levels of importance to proximity factors. These methods are described, and the advantages and disadvantages of each are discussed. Density, proportion, and arrangement of plants influence the outcome of competition experiments and should be incorporated into studies of crop-weed interference, since differing estimates for the effects of weeds on crop productivity can be obtained, depending upon the experimental method used.

Field research was conducted to evaluate the performance of chlorsulfuron {2-chloro-N-[[4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} applied to established Canada thistle plants [Cirsium arvense L. scop. # CIRAR] in the spring or fall to young vegetative shoots, and in June to plants in the bud growth stage. Canada thistle shoot densities measured in the spring following chlorsulfuron applications the previous year were lower when chlorsulfuron was applied in the bud and fall growth stage as compared to applications made in the spring to young vegetative shoots. Suppression of Canada thistle shoots from chlorsulfuron application during the bud and fall growth stage increased with rate, and was greatest at 269 g ai/ha. Chlorsulfuron applied at 269 g/ha at the fall growth stage only partially controlled the root system of Canada thistle.

Predicting crop yield reductions from weeds continues to challenge weed science. Useful methods based upon weed densities and upon area weeds influence have been developed. Results vary greatly from year to year and location to location for several reasons which are reviewed: 1) the shift in crop yield from a weed density-dependent to a weed density-independent relationship; 2) the effect of density on essential growth factor competed for; 3) differences among weed species in relative competitiveness for essential growth factors; 4) the differential effect of environmental conditions on the competitiveness of weed species; and 5) the effect of time of emergence on competition. To reduce the variability in prediction methods will necessitate developing a method based upon the factor(s) responsible for yield reduction.

Soybean-based [Glycine max (L.) Merr.] and petroleum-based crop oil concentrates were equally effective as spray adjuvants for control of quackgrass [Agropyron repens (L.) Beauv. #3 AGRRE] with the butyl ester of fluazifop {[±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} and sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one}. With favorable growing conditions at application in 1983 and 1985, an adjuvant concentration of 1% (v/v) with these herbicides was adequate for quackgrass control. Unfavorable growing conditions at application in 1984 resulted in unacceptable quackgrass control, and spray volume did not affect control. However, increasing the adjuvant concentration to 4% (v/v) with either herbicide applied in a spray volume of 70 L/ha nearly doubled quackgrass control. In 1983 and 1985, equal or greater quackgrass control with each herbicide was achieved with spray volumes as low as 35 L/ha compared to 280 L/ha.

Field experiments were conducted to determine the influence of growth stage on tolerance of seeded tomatoes (Lycopersicon esculentum Mill.) and of eastern black nightshade (Solanum ptycanthum Dun. #3 SOLPT) to the sodium salt of acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} at low use rates. Tomatoes as young as the 2 true-leaf stage exhibited some tolerance to acifluorfen at rates of 0.14 kg ae/ha and less. Rates of 0.28 and 0.56 kg/ha caused increased phytotoxicity; however, yield was not greatly reduced. At the 8-leaf stage, foliar injury was slight and existed only at the highest rates tested; however, tomato yields were reduced. Control of black nightshade at the 2-leaf stage was acceptable at rates as low as 0.14 kg/ha. When nightshade was at the 5-leaf stage, 0.28 and 0.56 kg/ha rates provided acceptable control − 87% and 96%, respectively. Nightshade in the 10-leaf stage was not controlled effectively at rates below 0.56 kg/ha.

Research was conducted from 1983 through 1985 in east central Texas to determine the effect of grass-specific herbicides, herbicide rates, and weed height on johnsongrass [Sorghum halepense (L.) Pers. #3 SORHA] control and cotton (Gossypium hirsutum L. 'Stoneville 825’) yield. Seven herbicides were applied to johnsongrass less than 30 cm tall with 2 to 5 leaves approximately 4 weeks after cotton emergence. Good-to-excellent johnsongrass control was observed for each herbicide except cloproxydim {(E,E)-2-[l-[[(3-chloro-2-propenyl)oxy]imino] butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} at 0.56 kg ai/ha and sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} at 0.28 kg ai/ha. Few significant differences were found in seed cotton yields even though johnsongrass control ranged from 43 to 99% 26 days after treatment (DAT). These data suggest that johnsongrass regrowth and/or reinfestation following initial control of 75% or higher during the first 4 to 6 weeks after cotton emergence has a minimum negative effect on cotton yield. Johnsongrass control was highest when either fluazifop-P {(R)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid} or haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy] propanoic acid} was applied to 15 to 30 cm tall johnsongrass.

Glyphosate [N-(phosphonomethyl)glycine] toxicity to volunteer wheat (Triticum aestivum L.) and large crabgrass [Digitaria sanguinalis (L.) Scop. # DIGSA] was influenced more by carrier volume than application method. Glyphosate phytotoxicity increased when carrier volume was reduced with both application methods. When compared at equal carrier volumes, there was little difference in volunteer wheat or large crabgrass control when glyphosate was applied with a controlled droplet applicator or flat fan nozzles. Volunteer wheat and large crabgrass control with controlled droplet applicators was influenced by application factors. At a given carrier volume, application factors which yielded larger droplets generally resulted in superior vegetation control.

In 1983 and 1984, 20 commercial soybean [Glycine max (L.) Merr.] cultivars were field tested for their response to fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)benzenamine], metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-l,2,4-triazin-5(4H)-one], and vernoiate [S-propyl dipropylcarbamothioate]. Recommended rates for the herbicides were 1.1, 0.4, and 2.8 kg ai/ha, respectively. Each herbicide was applied at recommended (lx), 2x, and 4x rates in both years with an 8x rate added in 1984. Plant height, vigor, and injury were used to determine herbicide effects. Differences existed among cultivars, herbicides, and rates. Increased herbicide rates generally resulted in reduced plant height and vigor and increased plant injury. Cultivars tolerant to the three herbicides included Cumberland, Essex, FFR 560, NAPB 420, and Pixie.

The relatively new micro-encapsulated formulation of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] and diethatyl [N-(chloroacetyl)-N-(2,6-diethylphenyl)glycine] were soil-applied for yellow nutsedge (Cyperus esculentus L. #3 CYPES) control in cotton (Gossypium hirsutum L. ‘Acala SF-2’). Both herbicides at rates of 1.68 to 3.36 kg ai/ha provided 60 to 80% control of yellow nutsedge for 6 weeks. Although control declined to 50 to 70% by cotton harvest, plots treated with herbicides usually yielded more seed cotton than cultivated control plots. Yields were similar from herbicide-treated and hand-weeded plots. Fewer tubers were counted in herbicide-treated than in cultivated-control plots in the fall. Based on yellow nutsedge control and seed cotton yields, alachlor and diethatyl could be helpful in controlling nutsedge and protecting cotton yields.

Field studies were conducted for 3 yr to determine the foliar activity of acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} for control of common lambsquarters (Chenopodium album L. # CHEAL) in snap beans (Phaseolus vulgaris L. ‘Provider’ in 1983 and 1985 and ‘Green Crop’ in 1984). Control of 1 to 7 cm tall common lambsquarters varied between 75 and 100% with 0.28 kg ai/ha acifluorfen and frequently increased linearly with increases in acifluorfen rates to 0.84 kg/ha. Snap bean injury occurred each year and in 1985 was influenced by acifluorfen rate, stage of snap bean growth, and surfactant. Snap bean yields in 1983 were reduced linearly with increases in acifluorfen rates and in 1985 were reduced more from applications at the 1- to 2-trifoliolate leaf stage than at the 4- to 8-trifoliolate leaf stage. In the greenhouse, reductions in snap bean height from acifluorfen were related to application timing, surfactant and cultivar. Fresh weight reduction of snap beans was highest with the cultivar ‘Green Crop’ but was increased to both cultivars by early application timing and the addition of surfactant to the spray mix.

Several herbicides were evaluated for the control of velvet lupine (Lupinus leucophyllus Dougl. #3 LUPLE), a plant poisonous to sheep on western mountain ranges, and a secondary target species, mountain big sagebrush [Artemisia tridentata Nutt. # ARTTR]. Change in foliar cover of the two target species and associated vegetation was used to evaluate efficacy of the herbicides. Velvet lupine cover was reduced by greater than 50% in the 1983 trial by the butyl ester of 2,4-D [(2,4-dichlorophenoxy)acetic acid] at 2.2 and 4.5 kg ae/ha, the butoxyethanol ester of 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid] at 1.1 and 2.2 kg ae/ha, the dimethylamine salt of dicamba (3,6-dichloro-2-methoxybenzoic acid) at 2.2 kg ae/ha, and 2,4-D plus dicamba at 1.1 plus 0.6 kg/ha. Cover of velvet lupine and other forbs was reduced by drought in the spring of 1985 and obscured the comparison among herbicides in the 1984 trial. Cover of mountain big sagebrush was consistently reduced (>88%) by 2,4-D at 4.5 kg/ha in both trials, and by three rates of the butoxyethyl ester of triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid} (>67%) in the 1984 trial. Grass cover increased in plots where herbicides effectively reduced velvet lupine, forbs, and mountain big sagebrush.

Field experiments were conducted to examine the effect of tillage on atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine] persistence in the soil and soybean [Glycine max (L.) Merr.] injury. Tillage systems evaluated were no-tillage, chisel plowing, and moldboard plowing. Reduced tillage systems, such as no-tillage or chisel plowing, resulted in greater soybean injury from atrazine residue than did moldboard plowing. Regardless of atrazine residue level, metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] resulted in the greatest injury to soybeans. Metribuzin application in combination with atrazine residue increased soybean injury under the chisel plowed system.

Experiments were conducted in the greenhouse and in the field to determine the influence of flax (Linum usitatissimum L.) seeding depth on tolerance to trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine]. Flax stand and fresh weight generally decreased as flax seeding depth increased within or below trifluralin-treated soil. Seeding flax below a trifluralin-treated soil layer did not increase tolerance to trifluralin. Although 0.8 kg ai/ha of trifluralin reduced stand when flax was seeded 3 to 6 cm deep in the field, seed yield remained constant.