The changes in the nucleic acid metabolism were studied in plants susceptible and resistant to 4-amino-3,5,6-trichloropicolinic acid (picloram). The total RNA and DNA content of the tissue correlated inversely with the herbicide resistance; the resistant plants were low in nucleic acids, whereas sensitive plants were high. The increase in the nucleic acids of the sensitive species 24 hr after picloram treatment appeared to be associated with lower levels of ribonuclease and deoxyribonuclease. The inability of the resistant species to make more RNA may be associated with high levels of nucleases in the tissue.

The relative volatilities of various technical and formulated ester and amine salts of 2,4-D [(2,4-dichlorophenoxy)acetic acid] were determined in a closed air-flow system consisting of a volatilization chamber and an ethylene glycol trap. The system was operated at a flow rate of 28.8 L/hr and a temperature of 30 C. Trapped herbicide vapor was analyzed by gas chromatography. The relative volatilities of technical grade iso-propyl, butyl, propylene glycol butyl ether, and iso-octyl esters, were 56, 25 to 38, 4, and 1, respectively. The relative volatilities of various formulated esters and amines were of the magnitude of 440:33:1 for the high volatile, low volatile, and amine salts, respectively.

Field studies were carried out in Laingsburg, MI, from 2002 to 2004 on Houghton muck soil to assess the impacts of cover crops and soil fertility regimes on weed populations and celery yield. The cover crops were oilseed radish, cereal rye, hairy vetch, and a bare ground control. The fertility rates were full (180, 90, and 450 kg ha−1 nitrogen [N], phosphorus pentoxide [P2O5], and potassium oxide [K2O], respectively), half (90, 45, and 225 kg ha−1 N, P2O5, and K2O, respectively), and low (90 kg ha−1 N). Each cover crop treatment was combined with the low or half rate of fertilizer. An additional treatment with bare ground plus the full rate of fertilizer was added as standard practice. Treatments were maintained in the same location for the duration of the study. Major weed species were common chickweed, prostrate pigweed, shepherd's-purse, common purslane, and yellow nutsedge. Each year, oilseed radish consistently produced the greatest biomass and provided over 98% early season weed biomass suppression. Hairy vetch and cereal rye provided about 70% weed suppression in early spring. Soil fertility level affected weed populations during the 2004 growing season. In 2004, weed biomass in treatments without cover crops or with vetch increased when greater amounts of fertilizer were applied. Within individual fertility levels, higher celery yields were recorded in the oilseed radish plots. For example, in the low fertility rate, celery yield was 34.8, 29.2, 23.9, and 24.4 ton ha−1 in the oilseed radish, cereal rye, hairy vetch, and control plots, respectively in 2003. Overall, the results of this experiment indicate that when included in a system where hoeing and hand-weeding are the only weed control methods, cover crops can successfully improve weed management and celery yield on muck soils, allowing reduced fertilizer inputs.

A 6-year study was conducted to compare the influence of 1,1′-dimethyl-4,4′-bipyridinium ion (paraquat), a mixture of paraquat and 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine), frequent mowing, hay mulch, and cultivation on growth, yield, and nutrition of peach (Prunus persica (L.) Batsch ‘Jerseyland’) trees. The growth and yield of trees which received two annual applications of paraquat or one application of paraquat plus simazine were comparable to those of trees maintained under cultivation or hay mulch. Hay mulched and cultivated trees responded comparably. Hay mulched trees had higher leaf potassium (K) and lower leaf magnesium (Mg) than the other trees. Where grass was controlled only by mowing, trees were generally lower in nitrogen (N), made less growth, and produced less fruit.

Experiments were conducted in a growth chamber to determine the effects of planting depth on the developmental sequence of velvetleaf seedlings and the effect of cyanazine placement relative to velvetleaf planting depth on cyanazine toxicity. Velvetleaf seedling emergence was delayed when seeds were planted 6 cm deep compared to the 2- and 4-cm planting depths. Lateral roots did not appear until after the cotyledons had emerged and expanded. This was followed by secondary root emergence from the primary root which occurred before adventitious roots appeared from the hypocotyl for the 2- and 4-cm planting depths. The first true leaf did not unfold until after the secondary root system was well developed. The quantity of adventitious roots on the hypocotyl increased with increasing planting depth. Velvetleaf adventitious roots appeared to be involved in cyanazine toxicity when cyanazine was placed above the seed. Increasing planting depth increased the proportion of seedling absorptive tissue above compared to below the seed. This resulted in increased cyanazine exposure for the deeper planted velvetleaf seeds when herbicide was placed above the seed.

Postdispersal processes play an important role in the regulation of weed population dynamics. Experiments were conducted at two locations in Arkansas to understand postdispersal loss of five arable weed species important to this region—barnyardgrass, johnsongrass, pitted morningglory, Palmer amaranth, and red rice—between seed dispersal in autumn and the production of fresh seeds the subsequent autumn. Total seed loss through predation, decay, germination (fatal or successful), and loss in viability was estimated, and the influences of residue level and seed burial depth (near ground vs. 5 cm deep) were also examined. On average, the active (i.e., viable) seedbank proportion in spring (5 mo after dispersal) ranged from 8 to 11% (barnyardgrass), 10 to 11% (johnsongrass), 20 to 23% (pitted morningglory), 4 to 6% (Palmer amaranth), and 5 to 10% (red rice) across the two locations. At 1 yr after dispersal, 0.7 to 1.5% of barnyardgrass, 7 to 8% of johnsongrass, 5 to 9% of pitted morningglory, about 1.5% of Palmer amaranth, and 0.2 to 0.7% of red rice were part of the active seedbank for the two locations. There was no evidence to suggest that establishing a vegetation cover (such as a rye cover crop) after harvest of the main crop could accelerate seed predation. Burial depth did not influence seed decay, but most (45 [pitted morningglory] to 99% [Palmer amaranth]) of the seeds retrieved from the predator feeding stations were found buried in the soil substrate, and thus, not available for most predator species. This suggests that practices that allow weed seeds to lie on the soil surface (such as no-till planting in autumn) are highly valuable in encouraging seed predation. The high levels of seed loss observed in this study indicate that seedbank management should be a vital component of integrated weed management strategies.

In a “closed system” nutrient solution experiment, shoot growth of downy brome (Bromus tectorum L.) was: (1) more rapid during early growth, (2) accelerated earlier by nitrogen, and (3) increased more by low to intermediate levels of nitrogen than that of crested wheatgrass (Agropyron desertorum Fisch. ex Link). Shoot growth of crested wheatgrass was increased more by high levels of nitrogen and at a more mature stage of growth than that of downy brome. Root growth of crested wheatgrass was greater at all nitrogen levels than that of downy brome and was increased by the high nitrogen levels. Downy brome removed more nitrogen from solution at the intermediate nitrogen level than did crested wheatgrass and more phosphorus at both the low and intermediate nitrogen levels. At the highest nitrogen level, crested wheatgrass removed more nitrogen from solution than did downy brome.

Wild oat (Avena fatua L.) seedlings from mature seeds emerged in silt loam from depths up to 17.5 cm in the greenhouse or 15 cm under field conditions. No emergence occurred from greater depths. Under field conditions, the greatest percentage of shoots emerged from seeds planted 5 cm deep. Wild oat in a space-planted nursery produced an average of 19 tillers, reached a maximum height of 79 cm, and produced an average of 1072 seeds. The number of tillers per plant, the number of seeds per panicle, and the total number of seeds per plant increased with an increase in annual precipitation. Wild oat seeds harvested as early as 3 days after anther extrusion germinated, but seeds harvested 6 or 7 days after anther extrusion germinated more rapidly.

Honey mesquite (Prosopis glandulosa Torr. # PRCJG) was treated with four herbicides at 1.1 kg ae/ha at 14 dates from April 30 through September 23 over a 2-yr period. Overall, clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) was the most effective herbicide, killing 80% or more of the honey mesquite at most dates. Picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid), triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid}, and 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid] were less effective than clopyralid. Honey mesquite control was correlated with Julian day of year sprayed, 15 plant, 11 environmental, and 8 herbicide concentration variables. Most effective control was negatively correlated with Julian day of year sprayed and new xylem ring thickness. Control was positively correlated with total daily apparent photosynthesis, upward methylene blue dye movement in the xylem, and herbicide concentration in the xylem 3 days after spraying. The only environmental variable of 11 measured that was significantly correlated with honey mesquite control was soil water and that was only with picloram. The concentration of herbicide in the stem was highest for clopyralid, followed by picloram, and then by 2,4,5-T and triclopyr, which were about equal. Variables most useful in regression equations for predicting control with herbicides were total daily apparent photosynthesis and rate of upward movement of methylene blue dye. Variables most incorporated into regression equations relating honey mesquite control with herbicide content were herbicide concentrations in the xylem 3 or 30 days after treatment.

The amount of C14-labeled triazine herbicides absorbed by the upper and lower surfaces of peanut (Arachis hypogea) leaves was found to be significant with the exception of 2-methoxy-4,6-bis(isopropylamino)-s-triazine (prometone). Almost always a surface active agent increased the absorption of triazine herbicides by the leaves. The surface active agent did not play any role in diffusion of these herbicides. Among the four herbicides tested 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine) and 2-methoxy-4,6-bis(isopropylamino)-s-triazine (prometone) were found to move both acropetally and basipetally, whereas 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) and 2-chloro-4,6-bis(isopropylamino)-s-triazine (propazine) moved only in the acropetal direction.

A high degree of control of annual weeds in cotton by the pre-emergence application of dinitro-secondary-butyl-phenol or its salts has led to appreciable commercial interest. Leonard, et al. showed that the above chemical was promising for controlling annual weeds in cotton, when applied to the soil as a pre-emergence herbicide, and there have been many reports on the method since that time. It was recognized that this treatment did not control perennial weeds such as nutgrass and Johnson grass, and that some additional treatment would be required to control these weeds. In January 1948, one of the authors had the opportunity to see sugarcane weeds in Puerto Rico controlled by the post-emergence application of fortified oil emulsions, and the procedure was included for study in the 1948 cotton tests. The primary interest of the authors was to find a spray that would control the perennial weeds and which, at the same time, would not be injurious to cotton. It was felt that the annual weeds could be controlled rather well by the pre-emergence treatment. The effect of fortified oils and oil emulsions and unfortified oils on cotton hypocotyls and on grass shoots showed that the fortifying chemicals (dinitro-secondary-butyl-phenol and pentachlorophenol) were quite injurious to cotton hypocotyls, but that some unfortified oils showed considerable promise for selectively weeding cotton.

In one study, twelve treatments consisting of 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine), 2-chloro-4,6-bis-(ethylamino)-6-isoproplyamino-s-triazine (simazine), 3-(p-chlorophenyl)-1,1-dimethylurea (monuron) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) applied alternately in a program pre- and postemergence to the weeds were evaluated on ten sugar cane plantations. Averaged data indicated that combinations including atrazine outperformed simazine in every instance and diuron was better than monuron on all plantations but one. Diuron was most effective on grasses, e.g., Digitaria spp. whereas atrazine was outstanding against Commelina diffusa Burm. In a second study on the same plantations, the initial treatment of 5 lb/A of atrazine followed by 3 lb/A of diuron gave outstanding weed control on five plantations. Using 3 lb/A of each material applied simultaneously had merit at times but some plantations required only a single application of either material. Net days of weed control obtained by diuron were inversely dependent on the amount of chemical adsorbed by the soil in both studies and total days control appeared to be dependent on ample rainfall in the second.

Glanded and non-glanded isogenic lines of cotton [Gossypium hirsutum L., var. Empire 61(WR)] were treated with 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) to determine the significance of lysigeneous glands in the tolerance of atrazine by cotton. The presence of lysigeneous glands did not increase atrazine tolerance by cotton or decrease the inhibition of photosynthesis by the herbicide. Detoxication of atrazine by N-dealkylation occurred in both glanded and non-glanded cotton. No significant difference in atrazine metabolism was observed between the two lines. Atrazine metabolism may be a more important factor than the presence of lysigeneous glands in the intermediate tolerance of cotton to atrazine.

Waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] is a troublesome weed occurring in cropping systems throughout the U.S. Midwest with an ability to rapidly evolve herbicide resistance that could be associated with competitive disadvantages. Little research has investigated the competitiveness of different A. tuberculatus populations under similar environmental conditions. The objectives of this study were to evaluate: (1) the interspecific competitiveness of three herbicide-resistant A. tuberculatus populations (2,4-D and atrazine resistant [2A-R], glyphosate and protoporphyrinogen oxidase [PPO]-inhibitor resistant [GP-R], and 2,4-D, atrazine, glyphosate, and PPO-inhibitor susceptible [2AGP-S]) with soybean [Glycine max (L.) Merr.]; and (2) the density-dependent response of each A. tuberculatus population within a constant soybean population in a greenhouse environment. Amaranthus tuberculatus competitiveness with soybean was evaluated across five target weed densities of 0, 2, 4, 8, and 16 plants pot−1 (equivalent to 0, 20, 40, 80, and 160 plants m−2) with 3 soybean plants pot−1 (equivalent to 300,000 plants ha−1). At the R1 soybean harvest time, no difference in soybean biomass was observed across A. tuberculatus populations. At A. tuberculatus densities <8 plants pot−1, the 2AGP-S population had the greatest biomass and stem diameter per plant. At the R7 harvest time, the 2AGP-S population caused the greatest loss in soybean biomass and number of pods compared with the other populations at densities of <16 plants pot−1. The 2AGP-S population had greater early-season biomass accumulation and stem diameter compared with the other A. tuberculatus populations, which resulted in greater late-season reduction in soybean biomass and number of pods. This research indicates there may be evidence of interspecific competitive fitness cost associated with the evolution of 2,4-D, atrazine, glyphosate, and PPO-inhibitor resistance in A. tuberculatus. Focus should be placed on effectively using cultural weed management practices to enhance crop competitiveness, especially early in the season, to increase suppression of herbicide-resistant A. tuberculatus.

Herbicide-resistant Echinochloa species are among the most problematic weeds in agricultural crops globally. Recurring herbicide selection pressure in the absence of diverse management practices has resulted in greater than 20% of sampled Echinochloa populations from rice (Oryza sativa L.) fields demonstrating multiple resistance to herbicides in Arkansas, USA. We assessed the resistance profile and potential mechanisms of resistance in a multiple herbicide–resistant junglerice [Echinochloa colona (L.) Link] (ECO-R) population. Whole-plant and laboratory bioassays were conducted to identify the potential mechanisms of non–target site resistance in this population. ECO-R was highly resistant to propanil (>37,800 g ha−1) and quinclorac (>17,920 g ha−1) and had elevated tolerance to cyhalofop (R/S = 1.9) and glufosinate (R/S = 1.2) compared to the susceptible standard. The addition of glufosinate (590 g ha−1) to cyhalofop (314 g ha−1), propanil (4,500 g ha−1), or quinclorac (560 g ha−1) controlled ECO-R 100%. However, cyhalofop applied with propanil (48% control) or quinclorac (15% control) was antagonistic. The application of the known metabolic enzyme inhibitors malathion, carbaryl, and piperonyl butoxide increased control of ECO-R with propanil (>75%) but not with other herbicides. Neither absorption nor translocation of [14C]cyhalofop or propanil was different between ECO-R and ECO-S. [14C]Quinclorac absorption was also similar between ECO-R and ECO-S; however, translocation of quinclorac into tissues above the treated leaf of ECO-R was >20% higher than that in ECO-S. The abundance of metabolites was higher (∼10%) in the treated leaves of ECO-R than in ECO-S beginning 48 h after treatment. The activity of β-cyanoalanine synthase, which detoxifies hydrogen cyanide, was not different between ECO-R and ECO-S following quinclorac treatment. Resistance to propanil was due to herbicide detoxification by metabolic enzymes. Resistance to quinclorac was due to a detoxification mechanism yet to be understood. The reduction in sensitivity to cyhalofop and glufosinate might be a secondary effect of the mechanisms conferring high resistance to propanil and quinclorac.

Galston and Baker have established that riboflavin is a photoreceptor in the destruction of auxin by visible light by in vivo and in vitro experiments. Carrol showed that riboflavin also reduced the activity of 2,4–dichlorophenoxyacetic acid (2,4–D). He believed that the reduction in activity occurred only in the presence of living plant material. The present studies using corn seedlings confirm the reduction in 2,4–D toxicity by riboflavin when the seedlings and substrate were illuminated. In addition, the inactivation of 2,4–D by riboflavin by light in vitro was also shown.

Winter wheat (Triticum aestivum L., var. Gaines) plots infested with seedling blue mustard (Chorispora tenella (Willd.) DC) were hand-weeded or treated in the fall with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) at 1 lb/A and 3,5-dibromo-4-hydroxybenzonitrile (bromoxynil) at ⅜ lb/A. Additional wheat plots were established in the fall with populations of one, three, and nine blue mustard seedlings per square foot, and the next spring these were either untreated, hand-weeded, or treated with (2,4-dichlorophenoxy)acetic acid (2,4-D) at 1 lb/A. All plots were harvested in July. Maximum wheat yields were obtained from plots hand-weeded or treated in the fall. Wheat production decreased significantly as weed population increased. Grain yield reduction, resulting from weed competition during the winter months, was significant and accounted for more than 50% of the total loss.

Experiments were conducted at Crawford and Kimball, Nebraska, during 1962, 1963, 1964, and 1965 to determine the effect of 13 herbicides on plains pricklypear (Opuntia polyacantha Haw.). The best control was obtained with 2-(2,4,5-trichlorophenoxy)propionic acid (silvex), 4-amino-3,5,6-trichloropicolinic acid (picloram), 3,6-dichloro-o-anisic acid (dicamba), and (2,4,5-trichlorophenoxy) acetic acid (2,4,5-T). Silvex and picloram were superior to dicamba and 2,4,5-T in controlling plains pricklypear, but picloram and dicamba injured the native grasses. Rotary hoeing plains pricklypear cladodes just prior to spraying improved herbicide performance. Simulated railing was not a consistent control measure. Evaluation of plains pricklypear control was made 1 and 2 years after herbicide application.

Fluorescence spectroscopy offers several advantages over other methods of detection and quantitation of chemical compounds. This technique has been underutilized in detection and quantitation of herbicides. The fluorescence properties of 39 herbicides representing several major types of chemistry were determined. The fluorescence of analytical standards was measured in acetonitrile, acetonitrile + water, and acetonitrile + water + strong acid. Fourteen of the 39 herbicides fluoresced to some extent, and seven (bentazon, chloramben, difenzoquat, fluometuron, imazaquin, MCPA, and norflurazon) were identified as good candidates for further method development. The technical advantages and disadvantages of fluorescence spectroscopy of herbicides are discussed.