Several sequences of POST herbicides applied each year in fall alone, in spring alone, or both in fall and again in spring controlled Canada thistle stands in spring wheat by severely decreasing root biomass and the numbers of adventitious root buds to a depth of 50 cm over four years. These treatments included dicamba applied at 1.7 or 2.2 kg ae ha–1 for the first two successive falls followed in wheat by either chlorsulfuron at 30 g ai ha–1 plus nonionic surfactant, MCPA plus bromoxynil at 280 plus 280 g ha–1, or 2,4-D amine at 560 g ha–1 applied annually for each of four consecutive years from the start. Chlorsulfuron at 30 g ha–1 applied alone in spring for each of four years also reduced and prevented Canada thistle root growth as effectively as a sequence of fall-applied dicamba followed by spring-applied chlorsulfuron in spring wheat.

Weed science is responsible for reducing weed competition with crops, within constraints, allowing food production for a growing world population. We have been successful, but with an increasing reliance on the fantastic arsenal of herbicides. Heavy herbicide usage and the concomitant loss of control due to changing weed species spectra, as well as evolved herbicide resistance, along with cancellations of registrations, limit chemical options. Old “innovations” such as well-conceived rotations and various mechanical procedures partly alleviate chemical dependency. We must try innovative, meaningful herbicide mixtures, whether synergistic or additive, mixtures of crop varieties, more competitive varieties, including allelopathic varieties, which could all help to lower chemical dependency. Three cases where innovative genetic-engineering coupled with chemistry seem to be needed are: (a) for the control of parasitic higher plant weeds such as broomrapes (Orobanche spp.), dodders (Cuscuta spp.), witchweeds (Striga spp.) that cannot be controlled by mechanical or selective chemical means with a sufficient margin of user error; (b) for the control of weeds in wheat (Triticum spp.) that are evolving multiple and cross resistances to all wheat-selective herbicides; and (c) to replace the major s-triazine and chloroacetamide herbicides in corn (Zea mays L.) that are under attack and where many local restrictions and cancellations have been imposed. We need better replacements than the resistance-prone inhibitors of acetolactate-synthase and acetyl-CoA-carboxylase. Much of the long-term innovative planning and research must come from the public sector as no one else seems willing to fill the needs.

Foliar absorption of 14-chlorimuron at 6, 12, 24, and 48 h after application to common cocklebur was reduced when applied in combination with acifluorfen. A maximum reduction of 10% was observed at 48 h. 14C translocation to meristematic tissue above the treated leaves also was reduced by acifluorfen in common cocklebur at 24 and 48 h after treatment. Absorption and translocation of 14C-chlorimuron in sicklepod was not affected by acifluorfen. Thin layer chromatographic analysis of treated leaf extracts showed two metabolites in common cocklebur (Rf values of 0.0 and 0.42) and four metabolites in sicklepod (Rf values of 0.0, 0.16, 0.33, and 0.50). All metabolites were more polar than the parent compound (Rf value of 0.64). Addition of acifluorfen did not affect chlorimuron metabolism in common cocklebur but reduced metabolism in sicklepod. Absorption, translocation, and metabolism of acifluorfen in common cocklebur or sicklepod were not affected by the addition of chlorimuron.

The agricultural community is currently involved in a debate with other members of society concerning many of the tools and tactics used to grow food. The discipline of weed science represents a microcosm of this larger societal debate, and thus is indicative of many other applied agricultural disciplines. Weed science comprises six fundamental disciplines, which have been arranged to depict its three major areas of research: weed technology, weed biology, and the ethics of weed control. Insight into those components of the debate that involve weed science is gained by examining each area. It seems unlikely that solutions to the “neckriddle” can be found through traditional research because of the integrative nature of agriculture, which involves environmental, economic, and social components. It is hoped that movement toward more holistic approaches to research will lead to resolution of the debate between society and the agricultural community.

Two field experiments were established in 1988 and 1989 in southeast Missouri to evaluate several herbicides and herbicide combinations for giant ragweed control in soybean. In 1988, a timely rainfall was not received for soil-applied herbicides and giant ragweed control was less than 75%. However, in 1989 soil moisture was sufficient for uptake of soil-applied herbicides and early season giant ragweed control was generally greater than 80%. Chlorimuron, chlorimuron plus 2,4-DB, imazaquin plus 2,4-DB, acifluorfen followed by naptalam plus 2,4-DB, fomesafen, and imazethapyr applied to 2.5 to 5-cm giant ragweed controlled more than 85% in 1988. In 1989, all POST treatments except imazaquin controlled more than 81% of giant ragweed 2 wk after treatments. Imazethapyr controlled seedling giant ragweed at heights up to 12 to 25 cm. Giant ragweed regrowth and/or reinfestation and giant ragweed seed production occurred with all herbicide treatments.

Data from field studies at Fayetteville, AR, were used to predict the herbicide rate needed to provide 70, 80, or 90% control of a weed based upon weed age. Reduced herbicide rates generally needed to be applied within 6 to 12 d after emergence to control weeds 90%. Reduced rates (280 g ai ha–1 or less) of acifluorfen controlled hemp sesbania, smooth pigweed, Palmer amaranth, and pitted and entireleaf morningglory 90%. Bentazon at 350 to 650 g ai ha–1 controlled common cocklebur and prickly sida 90%. Common cocklebur, smooth pigweed, and pitted morningglory were controlled 90% with chlorimuron at 2 to 5 g ai ha–1 and imazaquin at 20 to 80 g ai ha–1. Prickly sida and hemp sesbania were controlled 90% with imazaquin at 70 g ha–1 and chlorimuron at 6 g ha–1, respectively. Barnyardgrass, large crabgrass, red rice, and sicklepod were not controlled with reduced herbicide rates.

An experiment was conducted at four locations during 1990 and 1991 to determine the response of cotton to 1.12 kg ai ha-1 of clomazone applied PPI in combination with in-furrow application of 0.50 kg ai ha-1 of aldicarb plus 0.14, 0.28, and 0.56 kg ai ha-1 of either disulfoton or phorate. Early-season cotton injury ranged from 15 to 63% with clomazone plus aldicarb. Stands were reduced 31 and 61% at two locations, and yield was reduced 50% at one location. Applied in combination with aldicarb, all three rates of disulfoton and 0.28 and 0.56 kg ha-1 of phorate protected cotton from clomazone injury. Phorate at 0.14 kg ha-1 partially protected cotton from clomazone injury.

Variables affecting ultra-low-volume (ULV) application of herbicides for postemergence grass control in soybeans were evaluated in field experiments from 1988 to 1990. Air-assist applications of clethodim at 2.3 and 4.7 L ha–1 were compared with 94 and 187 L ha–1 applications with a conventional hydraulic sprayer. Rate of herbicide, volume of diluent, type of diluent, air pressure, and four other herbicides were evaluated. Clethodim at 28, 56, and 112 g ha–1 applied in paraffinic oil at 2.3 L ha–1 controlled johnsongrass better 10 wk after treatment than equivalent rates applied at 2.3 or 187 L ha–1 in water. At 2.3 L ha–1, barnyardgrass control was improved by applying clethodim in paraffinic oil rather than water. Johnsongrass control 10 wk after treatment was better with clethodim at 28 g ha–1 applied in paraffinic oil than when applied in soybean oil, cottonseed oil, No. 2 diesel fuel, kerosene, or jet A fuel. Low air pressures of 14 or 28 kPa resulted in better control of johnsongrass and barnyardgrass than higher pressures of 56 and 112 kPa. Clethodim, fluazifop-P, haloxyfop, quizalofop, or sethoxydim were more effective on johnsongrass and barnyardgrass when applied in paraffinic oil than in water at 2.3 and 4.7 L ha–1 with an air-assist sprayer.

The acceptance of no-till crop production systems has been limited due to expected problems with weed management. Field experiments were established at two locations in Ontario in 1988 and one location in 1989. Band or broadcast applications of preemergence (PRE) combinations of high or low label rates of atrazine with or without metolachlor or inter-row cultivation, were evaluated for their effectiveness in controlling annual weeds in no-till corn. At each location, different herbicide and cultivation combinations were required to achieve adequate weed control. Corn grain yield was equivalent regardless of whether herbicides were applied as a band or broadcast treatment at all three sites. At two of the three sites, one cultivation combined with herbicides applied as a band was adequate to maintain weed control and corn grain yields. Selective application of herbicides in bands represented an approximate 60% reduction in total herbicide applied into the environment. The integration of a shallow post-plant inter-row cultivation combined with the soil conservation attributes of no-till, would enhance the sustainability of a modified no-till corn production system.

The influence of temperature and relative humidity on the activity of acifluorfen, fomesafen, lactofen, and acifluorfen plus bentazon on prickly sida, pitted and entireleaf morningglory, and common cocklebur was evaluated in a growth chamber. Reduced control of all species was observed at 50% relative humidity as compared to 85% relative humidity when temperatures were higher (32/55 C day/night). Similar response to relative humidity was observed at the lower temperature (25/15 C) when treatments were applied 14 days after emergence (DAE). Changes in temperature at the same relative humidity did not alter herbicidal activity. Delaying application timing from 7 to 14 DAE decreased control by all herbicides except lactofen applied at high relative humidity, which controlled prickly sida at both 7 and 14 DAE.

The influence of various nutrients on parasitism of tomato plants by Egyptian broomrape and on stimulation of broomrape seed germination by strigol analogs was investigated in greenhouse and laboratory experiments. Parasitism occurred most readily in potting media low in fertility. Addition of nitrogenous compounds to potting media resulted in inhibition of broomrape parasitism. Ammonium nitrate with potassium phosphate or ammonium phosphate alone was most effective in reducing parasitism and enhancing growth of tomato plants. In the laboratory, strigol analogs induced 55% germination in broomrape seeds preconditioned in distilled water. Seeds preconditioned in distilled water or various salt solutions, but not treated with strigol analogs, showed 18% germination. GR 24 (5 μM) induced higher germination in seeds preconditioned in 10 mM ammonium nitrate, with or without potassium phosphate, compared with seeds preconditioned in sodium chloride. Preconditioning of seeds in sodium nitrate (10 mM) and treating with GR 24 stimulated, whereas preconditioning of seeds in ammonium salts (1 mM ammonium sulfate or 10 mM urea) and treating with GR 24 inhibited germination significantly.

Imazaquin at 140, 280, and 420 g ai ha-1 did not negatively affect seedling emergence from high vigor (98% germination) soybean seed but, coupled with a 5.0-cm herbicide incorporation depth, reduced seedling emergence from low vigor (67% germination) seed at all rates. Imazaquin at 420 g ha-1 incorporated 5.0 cm deep reduced low and high vigor seedling height 36 and 28%, respectively. Imazaquin at all rates reduced root length from high vigor seed, whereas only the high rate reduced root length of seedlings from low vigor seed. Although 420 g ha-1 metribuzin did not affect seedling emergence or root length, emerged seedlings were severely damaged when herbicide was incorporated to 5.0 cm. Trifluralin at 840 and 1680 g ai ha-1 reduced emergence of low vigor seed when incorporated 5.0 cm deep. Trifluralin at 840 g ha-1 did not affect seedling height or root length, but 1680 g ha-1 reduced height of seedling from low and high vigor seed by 86 and 66% when incorporated 5.0 cm deep. Trifluralin at 1680 g ha-1 reduced shoot dry weight of both types of seedlings only at the 5.0-cm incorporation depth.

The effects of the imidazolinone herbicide AC 263,222 on crops that may be rotated with soybean were studied in the greenhouse and field. Corn, grain sorghum, cotton, rice, wheat, soybean, and Italian ryegrass were planted directly into treated soil after AC 263,222 was incorporated. In the field, AC 263,222 at rates as high as 35 g ai ha–1 did not reduce shoot weight or emergence of these species. Some visual injury occurred, but was 20% or less 28 d after planting for all crops. Crops were more sensitive in the greenhouse, where reductions in both shoot and root dry weight were observed. AC 263,222 at 13, 13, and 7 g ha–1 reduced corn, grain sorghum, and Italian ryegrass shoot weights, respectively. Cotton, rice, and wheat tolerated rates from 27 to 55 g ha–1. Biomass reduction occurred with all crops at 55 g ha–1 and above. Because AC 263,222 will be registered for use in soybean at 9 to 18 g ha–1, and potential rotational crops were planted immediately after soil treatment, problems with injury to these crops are not expected.

Fluazifop-P, applied in the spring or fall and pronamide, applied in the fall only, reduced quackgrass infestation in established alfalfa and increased the dry matter yield of alfalfa. Herbicides did not improve alfalfa in vitro digestibility or crude protein content. Average residues of fluazifop-P in alfalfa treated with rates of 0.25 or 0.5 kg ai ha–1, were 0.1 mg kg–1 when harvested 280 d after application, and 0.06 mg kg–1 when harvested 295 d after application. Average residues in alfalfa harvested 41 and 55 d after application were 0.13 mg kg–1 and 0.1 mg kg–1, respectively, with a 0.25 kg ha–1 treatment and 0.19 mg kg–1 and 0.05 mg kg–1, respectively, with a 0.5 kg ha–1 treatment.

Field studies were conducted in 1989 and 1990 to determine if corn seed treated with naphthalic anhydride (NA) at 0.5% (w/w) would result in a reduction in corn injury caused by clomazone applied preplant incorporated (PPI) or preemergence (PRE) at 0.56 and 0.70 kg a.i. ha–1. NA significantly reduced corn injury, especially where clomazone was incorporated. Plant population, height, ear number, and grain yield also were higher in plots where NA was used. In most instances, corn injury was less and plant population, height, ear number, and grain yield were higher in plots where clomazone was applied PRE compared with PPI.

Field experiments were conducted at two locations in 1989 and 1990 to investigate possible interactions between POST applications of primisulfuron and selected soil-applied carbamate and organophosphate corn insecticides. Insecticides were applied in-furrow at planting, at recommended use rates. Primisulfuron was applied POST to corn in the three- or five-leaf stage at 0, 20, 40, or 80 g ai ha-1. Combinations of primisulfuron with disulfoton, fonofos, isozophos, or terbufos resulted in foliar and root injury, plant height reductions, and yield losses. No foliar or root injury, plant height reductions, or yield losses were observed from combinations of primisulfuron and carbaryl, carbofuran, chlorpyrifos, or diazinon. Differences among and within the experiments are attributed to edaphic and environmental conditions at the time of primisulfuron application.