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.

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.

The joint action of metribuzin and tridiphane was investigated in metribuzin-tolerant (T) and metribuzin-susceptible (S) soybean and tomato cultivars within species, respectively, under growth room studies. Maple Arrow (T) and Maple Amber (S) exhibited similar tolerance to tridiphane applied at soybean emergence. Vision (T) tomato was more sensitive to tridiphane than was Springset (S) tomato, the reverse of the relative tolerance to metribuzin. A phytotoxic interaction was demonstrated following application of tridiphane and metribuzin at the respective rates (kg ai ha−1) of 0.1 and 1.1 in Maple Arrow (T) soybeans, 0.05 and 0.25 in Maple Amber (S) soybeans, and 0.25 and 0.2 in Springset (S) tomato. Tridiphane applied 1 or 4 h before metribuzin caused the greatest phytotoxicity in Maple Amber (S) soybeans. Soybean field results generally supported those of growth-room studies. Foliar spray pretreatment with tridiphane increased total radioactivity in Springset (S), decreased the total root radioactivity and increased total shoot radioactivity in both Vision (T) and Springset (S) and decreased metabolism of metribuzin to water soluble conjugates in Springset (S) roots over 24 h following 14C-metribuzin application to roots of intact tomato seedlings. The increased uptake, translocation to the shoots, and decreased root metabolism of metribuzin in Springset when pretreated with tridiphane could explain the phytotoxic interaction (which was unexpected, based only on the glucose detoxification pathway of metribuzin in tomato).

A three-piece soil-sampling device was constructed to collect soil cores in polyvinyl chloride tubing to a depth of 92 cm with no or minimum contamination. The device with tube is driven into the soil with a hydraulic post-driver and removed with a chain attached to the bucket of a front-end loader on a tractor. Various depths can be sampled for analysis by cutting off proportionate sections of the tubing containing the soil core. This procedure is more rapid and poses less chance of contamination than by taking progressively deeper soil samples.

Breeding herbicide tolerance into new cultivars can improve safety and weed control in turfgrass systems. The sensitivity to fluazifop-P-butyl of 27 zoysiagrass (Zoysia spp.) lines was screened under greenhouse conditions to identify potential tolerant germplasm for breeding programs. The herbicide rate that caused 50% biomass reduction (GR50) and the rate that caused 50% injury (ID50) were calculated to select the three most-tolerant and the five most-susceptible lines for studying the physiological mechanisms responsible for fluazifop-P-butyl tolerance. The differences in GR50 and ID50 between susceptible and tolerant lines ranged from 4-fold to more than 10-fold. Cytochrome P450–mediated metabolism was not detected in fluazifop-P-butyl–tolerant lines. Sequencing of the ACCase gene confirmed that none of the seven previously reported mutations conferring resistance to acetyl-CoA carboxylase (ACCase)-inhibiting herbicides in other species were present in any of the tolerant or susceptible zoysiagrass lines studied. An Ala-2073-Thr substitution was identified in two tolerant lines, but this mutation did not completely explain the tolerant phenotype. No clear differences in absorption and translocation rates of 14C-radiolabeled fluazifop-P-butyl were observed among most lines, with the exception of a susceptible line that exhibited greater translocation than two of the tolerant lines. Metabolite profiles did not differ between tolerant and susceptible lines. Our results suggest that the diversity in tolerance to fluazifop-P-butyl in zoysiagrass germplasm is most likely the result of a combination of different, minor, additive non–target site mechanisms such as translocation rate and compartmentation after absorption.

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.

In a previous paper it was reported that there was no difference in the oxidation of ascorbic acid by homogenates from control and N–(3,4–dichlorophenyl)methacrylamide (referred to in this paper as DCMA) treated corn tissues until 3 to 4 days after treatment. At this time, there was a sudden increase in the oxidation of ascorbate by homogenates from treated tissue. The rate of oxidation continued to rise throughout the testing period (0 to 8 days after treatment). Since this appeared to be a significant finding, it was decided the reaction should be pursued further.

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.

It has long been recognized that one of the inherent problems in spray application, particularly with airplanes, is that of droplet size control. It is not generally known which droplet sizes or ranges of droplet sizes are the most effective biologically for the various types of treatments. It is known, however, that excessively large droplets result in a serious waste of material, and that very small droplets present a serious drift problem. For these reasons, spray droplet size and the range of sizes in a given spray have become important criterions in evaluating a spray distribution pattern.

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.

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.

At the Third British Weed Control Conference in 1956 I had the pleasure to discuss weed control research and development in Sweden (3). Some of the statements I made at that time will be used as a background to this paper. It was emphasized that there has been continuous change in weed control methods since World War I depending on the introduction of chemical weed killers but also on improved methods for cultivating and using the lands. Use of chemicals have brought with it changes in the weed flora meaning that certain weeds which were not serious in earlier days have now become a menace to the farmers. This brings with it a need for new chemicals with other weed killing properties. But it also brings with it a need for new cultivating methods and a better crop sequence. Cultivated plants are influenced by the chemicals. The effect on the cultivated plants depends on the stage of development of these plants. In uneven fields the harmful effect can be serious as there are always some plants that are sensitive at the time of spraying. The effect of the chemicals may depend on the purity of the acids. In MCPA, for example, the percentage of the 4–isomer, i.e. the 2–methyl–4–chlorophenoxyacetic acid, is important. It should be 95 per cent or slightly above as shown by åberg (1). The after effect of chemicals in the soils may be longer and more severe under northern conditions than under southern. For the choice of crops this must be considered. Thus there is no doubt that today, more than at any earlier period, weed control is a crop production problem. It can not be treated without considering crop production problems. Sometimes there is a feeling over in our part of the world that since the end of World War II chemicals and their properties have been emphasized too much in weed control discussions. Chemicals are, no doubt, important, but they can never get the place they are entitled to, unless our research work in weed control pays attention also to biology and ecology of those plants that we treat; cultivated plants as well as weeds. I believe we are in Europe thinking more about this particular end of the weed control work than you need to do over here. We usually have small farms with a number of different crops on each one of them. Thus we must learn to apply our chemical weed killers or use our cultivated methods in different ways for each one of these crops. For economical reasons and with regard to the supply of food and feed we need high yields per hectare. Every measure on a farm must be chosen with this in mind. Our problems, progress and organization of weed control must then be seen against the background I have now tried to give you.

Weeds in row crops of the Lower Coastal Plain in Alabama were sampled by line transects in both April and August. More than 100,000 feet of transects were assayed, with detailed questioning of producers in the area. The most important weeds of summer and fall and of winter and spring are listed, with per cent coverage and per cent frequency for weeds of row crops.

Annual weeds have always been a problem to the cranberry grower, but certain changes taking place in the bog environment have caused an increase in the population of annual weeds in some areas in recent years. The decline of soil acidity as a result of the long term use of alkaline water for frost flooding, coupled with the well drained bog culture recommended for Wisconsin growers, have produced an environment to which certain annual weeds are adapted. With some species, particularly common ragweed (Ambrosia artemisiifolia), the extensive use of petroleum products as herbicides has encouraged the invasion of annual weeds following the suppression of oil sensitive perennials.

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.

Horseweed [Conyza canadensis (L.) Cronquist] grows in one of two distinct growth phenotypes, “rosette” and “upright” growth types, and they have recently been observed co-occurring in Michigan fields. Previous research found that upright plants from two glyphosate-resistant populations were 3- and 4-fold less sensitive to glyphosate than their rosette siblings. Further experiments were conducted to investigate whether differential glyphosate sensitivity of the growth types was due to glyphosate retention, absorption, or translocation. The total amount of glyphosate retained on the C. canadensis leaf surface was similar for both growth types; however, on a per-weight and per-area bases, the upright growth type retained 21% and 18% less glyphosate, respectively. Glyphosate absorption was up to 85% at 168 h after treatment (HAT), and was not different between the rosette and upright growth types or between the susceptible (S) and resistant (R) biotypes. Additionally, there was no difference in translocation between the two growth types within each biotype at any time point. Interestingly, at 168 HAT, [14C]glyphosate translocation was higher in the S rosette compared with the two growth types from the R biotype; however, the S upright type was similar to both R growth types. Thus, glyphosate resistance in the R biotype may be due to an alternative mechanism rather than impaired translocation, which has been cited as the primary mechanism of glyphosate resistance in C. canadensis. These results suggest that reduced glyphosate retention on a per-weight and per-area bases of the upright growth type may contribute to increased glyphosate tolerance due to a diluted concentration of glyphosate in the plant. However, another factor is likely related to the mechanism of resistance within the R biotype, which is contributing to a 3-fold difference in glyphosate sensitivity between the two growth types, such as alterations in EPSPS gene expression or changes in undescribed metabolism genes.

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.

Black nightshade (Solanum nigrum L.) is one of the worst weeds in crop fields, and it spreads mainly by the dispersal of seeds. Temperature is one of the key environmental factors affecting seed germination. We investigated the seed germination response to temperature in six populations of S. nigrum from mid- to northern China and derived mathematical models from germination data. The results showed that S. nigrum seeds exhibit distinct germination responses to temperature within the range of 15 to 35 C. The optimum temperature for populations XJ1600, JL1697, and HLJ2134 was 30 C, and those for populations NMG1704, HN2160, and LN2209 were 25, 20, and 15 C, respectively. Based on the nonlinear fitting and thermal time models, the predicted base temperatures of the six populations ranged from 2.3 to 6.4 C, and the required accumulated growing degree days (GDD) ranged from 50.3 to 106.0 C·d. The base temperatures and the accumulated GDD for germination differed among populations, and there was a significant negative correlation. HLJ2134 population required a high base temperature and accumulated GDD for germination, indicating that it might be highly adapted to a warmer and moister environment. Based on the different germination responses of S. nigrum populations to temperature, the thermal time model reflects an innate relationship between base temperature and accumulated GDD required for initiation of seed germination, which provides a better basis for predicting seedling emergence and the timing for optimal control of S. nigrum under field conditions.

Public concern regarding the use of herbicides in urban areas (e.g., golf courses, parks, lawns) is increasing. Thus, there is a need for alternative methods for weed control that are safe for the public, effective against weeds, and yet selective to turfgrass and other desirable species. New molecular tools such as ribonucleic acid interference (RNAi) have the potential to meet all those requirements, but before these technologies can be implemented, it is critical to understand the perceptions of key stakeholders to facilitate adoption as well as regulatory processes. With this in mind, turfgrass system managers, such as golf course superintendents and lawn care providers, were surveyed to gain insight into the perception and potential adoption of RNAi technology for weed management. Based on survey results, turfgrass managers believe that cost of weed management and time spent managing weeds are the main challenges faced in their fields. When considering new weed management tools, survey respondents were most concerned about cost, efficacy, and efficiency of a new product. Survey respondents were also optimistic toward RNAi for weed management and would either use this technology in their own fields or be willing to conduct research to develop RNAi herbicides. Although respondents believed that the general public would have some concerns about this technology, they did not believe this to be the most important factor for them when choosing new weed management tools. The need for new herbicides to balance weed control challenges and public demands is a central factor for turfgrass managers’ willingness to use RNAi-based weed control in turfgrass systems. They believe their clientele will be accepting of RNAi tools, although further research is needed to investigate how a wider range of stakeholders perceive RNAi tools for turfgrass management more broadly.