Weed-suppressive soils contain naturally occurring microorganisms that suppress a weed by inhibiting its growth, development, and reproductive potential. Increased knowledge of microbe–weed interactions in such soils could lead to the identification of management practices that create or enhance soil suppressiveness to weeds. Velvetleaf death and growth suppression was observed in a research field (fieldA) that was planted with high populations of velvetleaf, which may have developed via microbial mediated plant–soil feedback. Greenhouse studies were conducted with soil collected from fieldA (soilA) to determine if it was biologically suppressive to velvetleaf. In one study, mortality of velvetleaf grown for 8 wk in soilA was greatest (86%) and biomass was smallest (0.3 g plant−1) in comparison to soils collected from surrounding fields with similar structure and nutrient content, indicating that suppressiveness of soilA was not likely caused by physical or chemical factors. When soilA was autoclaved in another study, mortality of velvetleaf plants in the heat-treated soil was reduced to 4% compared to 55% for the untreated soil, thus suggesting that suppressiveness of soilA is biological in nature. A third set of experiments showed that suppressiveness to velvetleaf could be transferred to an autoclaved soil by amending the autoclaved soil with untreated soilA; this provided additional evidence for a biological basis for the effects of soilA. The suppressive condition in these greenhouse experiments was associated with high soil populations of fusaria. Fusarium lateritium was the most frequently isolated fungus from roots of diseased velvetleaf plants collected from fieldA, and also was the most virulent when inoculated onto velvetleaf seedlings. Results of this research indicate that velvetleaf suppression can occur naturally in the field and that F. lateritium is an important cause of velvetleaf mortality in fieldA.

Data obtained from two experiments were used to identify and describe factors affecting crop loss due to weeds in forage stands. Timothy (Phleum pratense L. ‘Champ′) was established in presence or absence of barley (Hordeum vulgare L. ‘Bruce′), red clover (Trifolium pratense L. ‘Florex′), broadleaf weeds, and grass weeds. Botanical composition, soil water, soil nutrients, light interception, and plant cover were determined 4 and 6 weeks after seeding, and crop yield was determined at periodic forage harvest dates. Principal components analyses permitted the extraction of 12 factors and regression analyses demonstrated that these factors can be used to describe crop yield. Each factor can be estimated either by obtaining the rotated factor scores or by selecting a representative variable that can be substituted for the rotated factor scores in the regression models. The approach described in this paper should be tested with other crops and other weed complexes. It is suggested to use weed density in combination with weed and crop cover data to predict crop loss.

Isopropyl N-phenyl carbamate (IPC) has been used to a limited extent during the past few years as a selective herbicide in the control of weedy grasses and certain broadleaf species. Some investigators have found IPC to be highly effective; others have found IPC to be ineffective. The present investigation and those of Freed (6) indicate that environmental conditions are extremely important in determining the extent of plant response. Shaw and Swanson (8) found that the herbicidal effectiveness of IPC was highly variable under greenhouse conditions.

The growth retardant activity of MBR 18337 {N-[4-(ethylthio)-2-(trifluoromethyl)phenyl]-methanesulfonamide} was determined by injection into bean plants (Phaseolus vulgaris L. ‘Black Valentine’), California privet seedlings (Ligustrum ovalifolium Hassk.), and saplings of white ash (Fraxinus americana L.) and red maple (Acer rubrum L.). Height and leaf expansion of bean were inhibited by 8 μg of MBR 18337 per plant in growth chamber studies. Sprout growth of California privet grown in the greenhouse was inhibited for 92 days by 80 μg of MBR 18337 per tree, and the growth in the field of ash and maple saplings was retarded by 3.2 mg of MBR 18337 per tree for an entire growing season with little injury. Radioassays for 14C-MBR 18337 indicated rapid translocation of 14C-activity in privet from the lower stem to the region of the apical shoot. More than 80% of the MBR 18337 in the xylem and phloem was oxidized within 14 days.

Losses of picloram (4-amino-3,5,6-trichloropicolinic acid) in water exposed to sunlight ranged from 50 to 80% after 1 days' exposure (14 h sunlight) and 95% or more after 4 days exposure (56 h of sunlight) at four locations in Southern Arizona. Samples exposed at the highest elevation (2800 m) consistently lost picloram more quickly than samples at the lower elevations. Dry, recrystallized picloram losses were 11, 47, and 67% after 1, 2, and 4 days of sunlight exposure, respectively. At the same times, picloram losses in water were 65, 89, and 99%, respectively. These results indicate that photodecomposition is an important factor for the use and residue longevity of picloram on high-altitude rangelands of the western United States.

Night applications of phenoxy herbicides on plains pricklypear (Opuntia polyacantha Haw.) resulted in more plants being killed than comparable day applications. Night applications gave rapid herbicidal responses that were negated by the second year if chemical concentrations were too high. With low herbicide concentrations (½ and 1 lb aehg) effectiveness increased with proportion of diesel oil in carrier. The ester forms of 2-(2,4,5-trichlorophenoxy)propionic acid (silvex) killed more plants than did the ester or amine forms of (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T) in diesel oil:water (1:4, v/v) carriers.

The dimethylamine salt and isopropyl and isooctyl ester formulations of 2,4-dichlorophenoxyacetic acid (2,4-D) were applied at 0, ½, 1, 2, and 4 lb/A 21, 14, and 7 days before harvesting Bison hard red winter wheat. Treatment effects on yield, chemical composition, or milling and baking properties of the grain were either nil or of no practical significance. When treated with ½ or 1 lb/A, rates generally recommended for farm use, 2,4-D residues in the grain ranged from 0.04 to 0.27 ppm. As rates of the three formulations of 2,4-D were increased from ½ to 4 lb/A, increasing amounts of 2,4-D residues occurred in the grain and reached a maximum of 1.8 ppm when the isopropyl ester of 2,4-D was applied at 4 lb/A 14 days before harvest. Grain from plots treated 21 days before harvest had less 2,4-D residue than grain from plots treated 14 and 7 days before harvest.

Canada thistle (Cirsium arvense (L.) Scop.) on roadsides was controlled by 4-amino-3,5,6-trichloropicolinic acid (picloram), (2,4-dichlorophenoxy)acetic acid (2,4-D), and (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T), alone and in combination at various rates. There was no synergistic or additive effect, as determined by Canada thistle response, due to the addition of 2,4-D or 2,4,5-T to picloram. Control with ¾ lb/A picloram was not significantly less than that obtained with higher rates 1½ and 2 lb/A. Picloram reduced Canada thistle stands 80 to 100% and 2,4-D generally reduced the plant population 50% or less during the year of application and the year after. Late summer appeared to be an opportune time for spraying Canada thistle with ¾ to 1 lb/A picloram.

Thirty variously substituted benzonitriles and derivatives applied at equimolar exponential rates were evaluated for their phytotoxic properties in nutrient solution. The order of activity of the variously substituted benzonitriles and derivatives was dissimilar to the benzoic acids and phenoxyalkylcarboxylic acids. Several of the materials evaluated were as active as, or more active than 2,6-dichlorobenzonitrile [dichlobenil]; namely, the 2,6-dibromobenzonitrile, 2,6-dichloro-β-benzaldoximeacetate, 2-bromo-6-chlorobenzonitrile, 2,3,4-trichlorobenzonitrile, and 2,3,5-trichlorobenzonitrile.

Eight technically pure herbicides were evaluated for their relative phytotoxicity to hydroponically grown seedlings of red maple (Acer rubrum L.) and white ash (Fraxinus americana L.). Six of the herbicides (acids) were prepared as the triethylamine salts to provide formulation uniformity. With four compounds, equi-mole dosages per plant produced different toxic responses depending upon whether the herbicide was applied to the shoot or root. The 2,4,5-trichlorinated phenoxyaliphatic acids 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), 2(2,4,5-trichlorophenoxy) propionic acid (2,4,5-TP), and 4(2,4,5-trichlorophenoxy) butyric acid (2,4,5-TB) were consistently more toxic on a mole basis when applied to the roots of both species than when applied to the shoots. The opposite effect was observed with ammonium sulfamate (AMS) where shoot treatments were always more toxic. Shoot and root treatments were equally effective for the compounds 2,4-dichlorophenoxyacetic acid (2,4-D), 2-methoxy-3,6-dichlorobenzoic acid (dicamba), 4-amino-3,5,6-trichloropicolinic acid (picloram), and 3-amino-l,2,4-triazole (amitrole). Differential species susceptibilities to certain compounds were observed.

The present report marks the completion of a 24-month study. The early phases have been described in brief in a progress report (3). Data contained in the earlier report are included in the present discussion to make the entire experimental record available as a unit.

Cotton plants were treated with 2,4-D-2-C-14 (2,4-dichlorophenoxyacetic acid) applied with pentachlorophenol and arsenic acid. Seed kernels and regrowth leaves were extracted and various components of the extracts were applied to leaf surfaces and injected into the stems of cotton seedlings. Results indicated that 2,4-D itself and/or a metabolite of 2,4-D, were responsible for the persistence of the 2,4-D stimulus in cotton.

The addition of dicamba as a weed control option in soybean [Glycine max (L.) Merr.] is a valuable tool. However, this technology must be utilized with other herbicide sites of action (SOAs) to reduce selection pressure on weed communities and ensure its prolonged usefulness. A long-term trial was conducted for 7 yr in Indiana to evaluate weed community densities and species richness with four levels of dicamba selection pressure in a corn (Zea mays L.)–soybean rotation. Monocot densities and richness increased over time in the dicamba-reliant treatment. Dicot densities in the dicamba-reliant treatment declined over time, but dicot richness increased. The soil weed seedbank was affected by the varying herbicide strategies. The dicamba-reliant strategy had greater than 43% higher total weed density than all other treatments, primarily due to having a monocot density that was at least 71% higher than the other treatments. The fully diversified strategy with eight SOAs and residual herbicides used every year had the lowest total weed species richness in the soil seedbank, which supported the in-field observations.

Mathematical relationships are developed for a general system in which the concentrate chamber and diluent vessel contain the active material to be evaluated. The use of the system for increasing the effective half-dosage distance is illustrated. A technique is described for decreasing the half-dosage distance with a conventional system.

Mature plants of tomato (Lycopersicon esculentum Mill. ‘Super Sioux’), cucumber (Cucumis sativas L. ‘Early pickling’), and pinto bean (Phaseolus vulgaris L.), in small field plots, were exposed once to sprinkler or furrow irrigation water containing 0.11 kg/ha or 1.12 kg/ha MSMA (monosodium methanearsonate). Concentrations of MSMA in water were about 0.22 ppmw and 2.2 ppmw for the two rates, respectively. Samples of crops and soil were subsequently analyzed for elemental arsenic content by colorimetric and atomic absorption methods. Arsenic residues were higher, compared to controls, only in crops exposed to MSMA applied in sprinkler irrigation. Average arsenic levels were highest in tomatoes exposed to 2.2 ppmw MSMA by sprinkler irrigation, and after 30 and 45 days were 0.26 ppm and 0.35 ppm, respectively. Average arsenic levels in sprinkler-irrigated crops ranged from double to triple the arsenic levels in control (untreated) crops. Atomic absorption was more sensitive than the colorimetric method and could detect smaller differences in arsenic content of samples. The average arsenic residues in soil varied from 1 to 3 ppm, but the differences could not be attributed to treatment with MSMA.

Several herbicides were evaluated for control of spring parsley [Cymopterus watsonii (Coult. & Rose) Jones]. The propylene glycol butyl ether esters and dimethylamine salt of (2,4-dichlorophenoxy)acetic acid (2,4-D) applied at 2 lb/A were 99% effective. Ninety-nine percent control was achieved also with 0.5 lb/A of 4-amino-3,5,6-trichloropicolinic acid (picloram) and 2,4-D amine + picloram at 0.4 + 0.1 lb/A, 2,4-D amine + picloram at 0.33 + 0.17 lb/A, and (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T) + picloram at 0.25 + 0.25 lb/A.

Sodium 2,4–dichlorophenoxyethyl sulfate (hereafter designated as SES) has been widely tested for herbicidal use in pre- and post-emergence applications on seeded and vegetatively propagated crops. Several investigators have found this chemical to be particularly valuable for control of annual weeds in strawberries. Their reports indicate satisfactory weed control, but for the most part contain no critical evaluation of the effect of the chemical on the strawberry plants. Field tests conducted on crops other than strawberries have indicated some crop responses which may be characteristic of the general effects to be expected from application of SES to crop plants. More basic studies have shown no hormone response from application of SES to tomato foliage. In fact, the chemical per se was found to be herbicidally inactive, undergoing some change in the soil to attain its active form. The change from inactive to active form was found to occur in nonsterile soils over a range of pH from 3 to 7, and in sterile soils from pH 3 to 4. This change was postulated to be an acid hydrolysis of the ethyl sulfate form of the chemical to an active form, either in an acid soil or as a result of acid secretions of bacteria and fungi. Further research has indicated that SES is hydrolyzed to sodium 2,4–dichlorophenoxy ethanol and sodium bisulfite and that further reaction in the soil may lead to 2,4–dichlorophenoxy-acetic acid. The ethanol has been shown to be highly toxic to germinating seeds, but only 1/100 as toxic as 2,4–D when applied to tomato foliage.

Herbicidal rates (5 to 9 lb/A) of 2-chloroallyl diethyldithiocarbamate (CDEC) retarded emergence of head lettuce (Lactuca sativa L.) reduced both pre- and postthinning plant populations, caused injury symptoms in young plants, affected the subsequent rate of growth, date of maturity, weight of marketable heads, and yield. Decreased yields were attributed to a reduction in plant population, poor head solidity, and small head development. Reductions in yield were in proportion to dosage and of economic importance.

N–(3,4–dichlorophenyl)methacrylamide (hereafter referred to in this paper as DCMA) is a relatively new compound in the field of herbicides. However, several weed control workers (personal communication) have suggested that DCMA may be of value for control of crabgrass (Digitaria spp.), brachiaria (Brachiaria spp.), pigweed (Amaranthus spp.), morning glory (Ipomoea spp.), cocklebur (Xanthium spp.), and goosegrass (Eleusine indica), common weeds found in cotton.