The pre-emergence use of 3–(3,4–dichlorophenyl)–1,1–dimethylurea (diuron) offers a means of controlling weeds in cotton during the extremely critical period of stand establishment (1, 2, 6, 8, 9, 10, 11). The amount of diuron required to give effective weed control without injury to cotton has been observed to vary between locations. Factors such as temperature, moisture, light conditions, and physical and chemical properties of the soil may be involved in causing variation in the activity of this herbicide.

Enhancement of 3(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) toxicity to oat plants was not dependent on type—anionic, cationic, or nonionic—of surfactant. However, specific surfactant and concentration were important. Concentrations of surfactant above 0.1% were required for consistent phytotoxic effects. Concentration of diuron in solution at time of application was not an important factor in phytotoxicity. Lowering the temperature from 72 to 60 F slowed the expression of toxicity and increased the chances of plant recovery.

Biological weed control research at Arkansas has been with indigenous fungal pathogens applied as inundative inoculum to weed populations (9, 19, 22, 32, 34, 39). We term this concept the mycoherbicide approach to biological weed control (35). Our experience with this concept began in 1969 with the discovery by R. J. Smith, Jr., of a previously undescribed fungal pathogen on northern jointvetch, [Aeschynomene virginica (L.) B.S.P. # AESVI] (20, 22, 29). It was ultimately commercialized as COLLEGO™ in 1982 by the Upjohn Co. for use by Arkansas rice (Oryza sativa L.) and soybean [Glycine max (L.) Merr.] farmers. Nothing has been found nor can be foreseen from the experience during discovery, development, and deployment of COLLEGO™ that would preclude extension of the mycoherbicide concept to controlling other weeds in annual crops or noncrop areas (29). Nevertheless, the concept does have limitations. An overview of the COLLEGO™ effort plus a statement of some research interests engendered by it will be useful for assessment of the practical potential of this new approach to biological weed control.

Unshielded cultivation (during which soil is purposely moved into the drill row) and shielded cultivation (soil movement into drill row minimized) were compared in field plots of cotton [Gossypium hirsutum L. (var. DBES 8274)] which received no herbicide, a banded preemergence herbicide, one to three banded postemergence herbicide applications, or a combination of the banded pre- and postemergence herbicides. All plots were hoed and cultivated as needed to maintain reasonably good control of weeds. The study was a 3-year experiment replicated six times.

Unshielded cultivation was superior or equal to shielded cultivation as a weed control measure supplementary to hoeing or hoeing and herbicides as evaluated by effects on hoe-labor requirements, cotton yields, and weed infestations at harvest time. Replacing one or more postemergence herbicide applications with unshielded cultivation resulted in reduced control of weeds. Late season initiation of unshielded cultivation in plots previously shield cultivated had no detrimental effect on yield.

Significant increases in yield of cotton were obtained in plots treated with a preemergence herbicide as compared with plots receiving only hoeing and cultivation for weed control. Postemergence herbicide treatments (applied as a directed spray) did not result in significantly higher yields as compared with plots receiving hoeing and cultivation only. As measured by 3-year averages preemergence or postemergence treatments provided about 50 per cent control of weeds; combined they provided about 75 per cent control.

Smooth bromegrass (Bromus inermis Leyss.) treated in the fall tended to be more damaged than that treated in the spring, regardless of herbicide. Nitrogen fertilizer reduced the loss in yield resulting from herbicides but was least effective with 1 lb/A of 4-amino-3,4,6-trichloropicolinic acid (picloram). Laboratory studies showed that roots of smooth bromegrass were the primary site of picloram uptake. Root uptake explained the severe damage in curred from applications of picloram made in the fall. The order of increasing phytotoxicity of herbicides to smooth bromegrass studied in these experiments was 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 2,4-dichlorophenoxyacetic acid (2,4-D)<2-methoxy-3,6-dichlorobenzoic acid (dicamba) < picloram.

Metsulfuron is used for POST control of spotted spurge in many warm-season turfgrasses. A suspected resistant (R) biotype of spotted spurge was collected from turfgrass in Georgia with a history of exclusive metsulfuron use. Research was conducted to evaluate the resistance level of this biotype to metsulfuron, efficacy of other mechanisms of action for control, and the molecular basis for resistance. Compared with a susceptible (S) biotype, the R biotype required >90 and >135 times greater metsulfuron rates to reach 50% injury and reduce biomass 50% from the nontreated, respectively. The R biotype was also resistant to trifloxysulfuron but was injured equivalent to the S biotype from dicamba, glyphosate, and triclopyr. Gene sequencing of the R biotype revealed a Trp574 to Leu substitution that has conferred resistance to acetolactate synthase (ALS) inhibitors in previous research. This is the first report of ALS resistance in spotted spurge. More importantly, this is the first report of a herbicide-resistant broadleaf weed from a turfgrass system in the United States.

Diversity is key for sustainable weed management and can be achieved via both chemical and nonchemical control tactics. Genetically modified crops with two-way or three-way stacked herbicide-tolerant traits allow use of herbicide mixtures that would otherwise be phytotoxic to the crop. Early weed management (EWM) strategies promote the use of PRE herbicides with residual activity to keep the field free of weeds early in the season for successful crop establishment. To evaluate the respective sustainability and practicality of the two chemical-based management tactics (i.e., stacked traits and EWM), we used a population model of waterhemp, Amaranthus tuberculatus (Moq.) Sauer (syn. rudis), to simulate the evolution of resistance in this key weed species in midwestern U.S. soybean [Glycine max (L.) Merr.] agroecosystems. The model tested scenarios with a varying number of herbicide sites of action (SOAs), application timings (PRE and POST), and preexisting levels of resistance. Results showed that both tactics provided opportunity for controlling resistant A. tuberculatus populations. In general, each pass over the field should include at least two effective herbicide SOAs. Nevertheless, the potential evolution of cross-resistance may void the weed control programs embraced by stacked traits and diverse herbicide SOAs. Economic calculations suggested that the diversified programs could double long-term profitability when compared to the conventional system, because of improved yield and grain quality. Ultimately, the essence of a sustainable herbicide resistance management strategy is to be proactive. Although a herbicide-dominated approach to diversifying weed management has been prevalent, the increasing presence of weed populations with multiple resistance means that finding herbicides to which weed populations are still susceptible is becoming increasingly difficult, and thus the importance of reintroducing cultural and mechanical practices to support herbicides must be recognized.

The low-nitrogen status of highly weathered soils may offer a potential alternative for weed suppression in agricultural systems with N2-fixing crops. In this study, we used sicklepod as a model to evaluate weed response that might occur with managed reductions in nitrogen-soil fertility. A field study was conducted with the parental generation supplied 0, 112, 224, or 448 kg N ha−1. Decreased nitrogen fertility led to reduced shoot biomass, seed number, and total seed mass. Individual seed mass was lower, but seed % nitrogen was not affected. Analysis of seed-mass distribution confirmed that low parental fertility was associated with more small seeds as a proportion of total seeds produced. Additional experiments in hydroponics culture revealed slower growth rates of seedlings produced from small seeds when grown under low-nitrogen conditions. Competitiveness of plants from small (low nitrogen) and large (high nitrogen) seed classes was determined in a replacement-series experiment conducted in sand culture in a controlled environment at two densities and two levels of nitrogen nutrition. Plants produced from smaller seeds were less competitive in low-nitrogen fertility conditions, but plants from small and large seeds competed similarly when grown under high-nitrogen fertility. The results support the hypothesis that comprehensive management strategies to reduce nitrogen availability for weed growth in low-fertility conditions could decrease weed interference by decreasing growth and seed production of parental plants and through maternal effects that lower competitiveness of offspring.

Herbicide resistance is an increasing issue in many weed species, including rigid ryegrass (Lolium rigidum Gaudin); a major weed of winter cropping systems in southern Australia. Recently, this weed has also been found in summer crops in the southeastern region of Australia. Effective control of this herbicide-resistant weed across southeastern Australia requires alternative management strategies. These strategies can be informed by analyses on the interaction of germinable seeds with their regional environments and by identifying the differences between populations of varying herbicide-resistance levels. In this study, we explore how various environmental factors differentially affect the seed germination and seedling emergence of three L. rigidum populations, including one glyphosate-resistant population (GR), one glyphosate-susceptible population (GS), and one population of unknown resistance status (CC04). Germination was greater than 90% for all populations at each temperature regime, except 15/5 C. Populations germinated at a lower rate under 15/5 C, ranging from 74% to 87% germination. Salt stress had a similar effect on the germination of all populations, with 0% germination occurring at 250 mM salt stress. Population GS had greater tolerance to osmotic stress, with 65% germination at −0.4 MPa compared with 47% and 43% germination for CC04 and GR, respectively; however, germination was inhibited at −0.8 and −1.6 MPa for all populations. All populations had lower germination when placed in complete darkness as opposed to alternating light/dark. Germination in darkness was lower for CC04 (69%) than GR (83%) and GS (83%). Seedling emergence declined with increasing burial depth with the lowest emergence occuring at 8 cm (37%) when averaged over the populations. These results indicate that L. rigidum can survive under a range of environmental variables and that the extent of survival differs based on population; however, there was no difference based on herbicide-resistance status.

Lentil (Lens culinaris Medik.) is an important and expanding crop in southern Australia and a significant crop in western Canada. Currently, production in both countries is limited by an inability to effectively control weeds, due in part to a lack of registered safe and effective herbicides. Metribuzin is a broad-spectrum herbicide providing an alternative weed control option to the imidazolinones, but it has low crop safety in lentil. Two methods, germplasm screening using a hydroponic sand assay and field screening of a large mutated population of the Australian cultivar ‘PBA Flash’ were initially used to identify lines with putative metribuzin tolerance over current cultivars. Dose–response experiments showed the germplasm line SP1333 had GR50 (the rate required to reduce dry weight 50%) values up to four times higher than PBA Flash. However, the mutation selections M043 and M009 had GR50 values more than 25 times higher than PBA Flash. A field study in Canada, under conditions of induced shade and no shade 72 h before POST application of metribuzin, confirmed the intermediate level of tolerance in SP1333 and the high level in the two mutant lines compared with 20 Canadian and Australian genotypes. This relative increase in metribuzin tolerance of the two mutant lines over the parent cultivar is higher than all previous reports in a range of crop species. The development of large mutant populations combined with large M2 field screens was a successful method for developing high levels of metribuzin tolerance in lentil. The estimated mutation rate of the mutant lines was 9.4×10−8. All three lines are currently being used as parents in lentil breeding programs.

Some factors affecting the absorption and translocation of radioactive 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in blackjack oak (Quercus marilandica Muench.) were studied. Absorption of 2,4,5-T was less in plants treated late in the growing season than in those treated earlier. Absorption and translocation of 2,4,5-T were inversely related to soil moisture stress. Increases in the rate of application from 10 to 50 μg of 2,4,5-T per plant caused a decrease in absorption and translocation, but decrease of the latter did not occur until the rate was increased above 25 μg/plant. Translocation was highest in June and was higher from the leaves of mature plants than from young regrowth.

Aqueous solutions of sodium and calcium polysulfides convert labeled simazine, 2-chloro-4,6-bis(ethylamino)-s-triazine-2,4,6-C14, to a chloroform insoluble, charcoal adsorbable derivative which is thought to be hydroxysimazine, 2-hydroxy-4,6-bis(ethylamino)-s-triazine-2,4,6-C14. In soil, calcium polysulfide accelerates the breakdown of simazine as shown by oat bioassay. These results suggest the possibility of using polysulfides to remove unwanted residues of chlorotriazine herbicides from crop lands.

Several herbicides were tested in the greenhouse on ivyleaf morningglory (Ipomoea hederacea (L.) Jacq.), green foxtail (Setaria viridis (L.) Beauv.), purple nutsedge (Cyperus rotundus L.), and quackgrass (Agropyron repens (L.) Beauv.) to determine the degree of enhancement in activity that could be obtained with an isoparaffinic oil carrier applied at 140 L/ha. The enhancement varied with the herbicide and with the species, ranging from 16-fold enhancement with 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) and 2-sec-butyl-4,6-dinitrophenol (dinoseb) on ivyleaf morningglory to no enhancement of atrazine activity on purple nutsedge and quackgrass or (2,4-dichlorophenoxy)acetic acid (2,4-D) activity on quackgrass and ivyleaf morningglory. An oil adjuvant was less effective in enhancing dinoseb and 3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea (linuron) activity than was the isoparaffinic oil carrier. Also, the isoparaffinic oil carrier emulsified in water was less effective than the undiluted oil in enhancing dinoseb activity on green foxtail, even though equal volumes of the isoparaffinic oil were applied.

The phytotoxicity of barban (4-chloro-2-butynyl m-chlorocarbanilate) applied 14 days after emergence to wheat (Triticum aestivum L. ‘Waldron’) and wild oat (Avena fatua L.), at constant, fluctuating or alternating day-night temperatures was determined in controlled environmental chambers. Wheat and wild oat susceptibility to barban increased as post-treatment temperature decreased. Maximum barban selectivity for wild oat in wheat occurred with a constant 18 C or an alternating 18 C day and 10 C night post-treatment temperature. At least three days of 10 C immediately following barban application were necessary to cause wheat injury. However, two days at 29 C immediately after barban application in an otherwise 10 C post-treatment regime reduced wheat injury. Further, a night temperature lower than the day temperature after barban application increased wild oat control but did not correspondingly increase wheat injury compared to the constant temperature treatments.

The responses of established, spaced-planted strains of johnsongrass (Sorghum halepense (L.) Pers.) to repeated, foliar applications of herbicides were studied under irrigated desert conditions. Monosodium methanearsonate (MSMA) and disodium methanearsonate (DSMA) were the most effective herbicides. Six to 10 applications with 3 to 6 lb aihg of either herbicide (totaling 24 to 60 1b/A) killed johnsongrass in most experiments. Treatments initiated in the fall were more effective than similar treatments started in the spring. Higher rates of organic arsenicals were required to destroy topgrowth of johnsongrass in the spring and fall than during the summer. DSMA applications at 4 and 6-week intervals were more effective than applications at 8-week intervals. Six 1b/A of DSMA in 100 gpa of water applied broadcast was as effective as 400 gpa containing 6 lb aihg of DSMA applied as a spot treatment. Removing topgrowth of johnsongrass 4 or 24 hr after treatment did not reduce the effectiveness of DSMA. Combining or alternating organic arsenicals with 2,2-dichloropropionic acid (dalapon) reduced the effectiveness of the arsenicals. Susceptible strains were destroyed by five or six applications of DSMA and MSMA while 9 or 10 applications were needed to kill more resistant strains.

Various chemical and cultural treatments were evaluated for smutgrass [Sporobolus poiretii (R.&S.) Hitchc], control and their effects on pangolagrass (Digitaria decumbens Stent.) and bahiagrass (Paspalum notatum Flügge). Herbicide treatments were dalapon (2,2-dichloropropionic acid) at 1.7, 3.3, and 5.0 kg/ha with Sunoco HE spraying oil at 0, 9.4, 19.0 and 37.0 L/ha. Five weeks after herbicide treatment plots were clipped (clipped vs. nonclipped) and fertilized (0 vs. 112 kg/ha N). Increasing dalapon from 1.7 to 5.0 kg/ha increased smutgrass control and decreased the number of smutgrass plants/plot. Pangolagrass ground cover generally increased with fertilization, as the rate of dalapon increased up to 3.9 kg/ha. Bahiagrass ground cover decreased with rates of dalapon above 1.7 kg/ha. Increasing the rate of oil increased bahiagrass ground cover after 90 days. Nitrogen applied 5 weeks after dalapon treatment and semi-annually for 5 yr generally decreased ground cover of smutgrass and bahiagrass, but increased pangolagrass. Clipping treated smutgrass plants increased pangolagrass and bahiagrass ground cover while decreasing the ground cover of smutgrass.

Spring-applied herbicides were evaluated in the field in 1982 through 1984 in western Nebraska for selective weed control in irrigated seedling alfalfa (Medicago sativa L. ‘Appollo’). Weed densities were least in plots treated preplant with benefin [N-butyl-N-ethyl-2,6-dinitro-4-(trifluoromethyl)benzenamine] in combination with postemergence applications of 2,4-DB [4-(2,4-dichlorophenoxy) butyric acid]. Combinations of sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} with bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) and fluazifop-butyl {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} with bromoxynil provided less grass control than sethoxydim or fluazifop-butyl applied alone. Annual weeds did not reduce alfalfa stand density.

Greenhouse studies were conducted with soil columns to determine the influence of rainfall amount, intensity, and time of application on the bioactivity of 0.07 kg ae ha−1 imazethapyr in three soils of New Jersey: the Berryland sand (BLS) (sandy, siliceous, mesic Typic Haplaquod), Aura sandy loam (ASL) (mixed, mesic Typic Hapludult), and Quakertown silt loam (QSIL) (mixed, mesic Typic Hapludult). The Brockman-Duke simulated rainfall machine was used for rainfall application and smooth pigweed as an imazethapyr bioactivity indicator. As rainfall amount increased from low to high (75 to 300 mm mo−1 equivalent) imazethapyr bioactivity declined significantly in BLS and ASL but was unaffected in QSIL. Rainfall intensity from 6 to 25 mm h−1 had no effect on imazethapyr bioactivity in the three soils. A delay in rainfall application up to 14 d after herbicide application significantly reduced the bioactivity of imazethapyr in BLS but had no effect in ASL and QSIL.

Two months after 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone (pyrazon) application (2.7 kg/ha), 80 to 100% of the total residue (1.3 to 4.2 μg/g) was in the upper 5 cm of soil. Herbicide movement occurred in soils that contained 3.8% organic matter or less. Pyrazon did not move below 10 cm or accumulate in soil as a result of movement. Adsorption of pyrazon was related to soil organic matter content (r = 0.90) and accounted for differences in herbicide movement previously observed. Calculations of pyrazon movement, based on soil organic matter content, coincided with depths of movement observed under field conditions.

The impact of invasive weed management on plant community composition is highly dependent on location-specific factors. Therefore, treatment means from experiments conducted at a given set of locations will not reliably predict community response to weed management elsewhere. We developed a model that rescales treatment means to better match local conditions. The goal of this paper was to determine if this rescaling improves predictions. We used our model to predict leafy spurge stem length density and grass biomass data from field experiments. The experiments consisted of herbicide-treated plots, untreated controls, and, in some cases, grass seeding treatments. When herbicides suppressed leafy spurge, the model explained 21 to 48% more variation in grass response than did mean grass response to the same or similar herbicide treatments applied at other sites. When herbicides killed grass, the model explained 53% more variation in leafy spurge response than did mean leafy spurge response to the same herbicide treatment applied at other sites. We regressed model predictions against observed data and tested the null hypothesis that resulting slope terms were equal to 1.0. Because the null hypothesis was rejected in two of four tests, the model may systematically over- or underpredict in some situations. However, measurement error in the observed data, unintended herbicide injury, or an inaccurate allometric relationship may account for a major proportion of the systematic deviations, and these factors would not cause prediction error in some management applications. Because the model tends to be better than the means from experiments at predicting plant community composition, we conclude that the model could advance managers' ability to predict plant community responses to invasive weed management.