Experiments were conducted on Sharkey clay and Gigger silt loam soils in northeast Louisiana from 1990 through 1993 to compare season-long barnyardgrass control, rice yield, and estimated economic returns with single and multiple applications of propanil alone or with molinate or thiobencarb. Multiple herbicide applications were more effective than single applications in controlling barnyardgrass and provided the greatest rice yields and economic returns. Applying residual herbicides with propanil improved barnyardgrass control in five of six experiments and increased rice yield and economic returns in three of six experiments.

Field studies were conducted from 1991 through 1993 to compare Weed control, peanut tolerance, yield, and net return from imazethapyr applied alone or in combination with paraquat. Sicklepod and Florida beggarweed were controlled with paraquat early POST followed by a POST application of either paraquat with 2,4-DB or paraquat with 2,4-DB and bentazon. Imazethapyr-based early POST treatments offered no improvement. An early POST application of paraquat with bentazon or imazethapyr was required for maximum control of bristly starbur. Imazethapyr applied alone early POST, with no further treatment, provided optimum yellow nutsedge control. Maximum yield and net return were associated with any paraquat-containing early POST-applied treatment followed by one of the tank mixed POST options.

Field experiments conducted in Marianna, Trenton, and Archer, FL in 1990 and 1991 investigated the effect of time of application of nicosulfuron and nicosulfuron mixtures on ‘Sunrunner’ peanut. Nicosulfuron was applied alone at 36 g ai/ha and in mixture with acifluorfen, bentazon, chlorimuron, chlorothalonil, fluazifop-P, imazethapyr, lactofen, or pyridate at 5, 7, or 9 weeks after planting (WAP). Applying nicosulfuron with chlorimuron, pyridate, or imazethapyr increased visible peanut injury and reduced yields more than nicosulfuron alone. Conversely, nicosulfuron applied with acifluorfen or lactofen was less injurious than nicosulfuron applied alone. Bentazon, chlorothalonil, and fluazifop-P generally did not influence phytotoxicity from nicosulfuron. Peanut tolerance to nicosulfuron increased with age.

Propanil-resistant barnyardgrass was reported in Poinsett County, AR, in 1990. Greenhouse studies were initiated to determine the distribution of propanil-resistant barnyardgrass in the state and to characterize the resistance. Barnyardgrass seeds were obtained in 1991 and 1992 from fields in 19 of the 38 rice producing counties in Arkansas where propanil treatment at recommended rates gave unsatisfactory barnyardgrass control. Barnyardgrass seedlings from the various sources were treated with propanil at 4.5 kg ai/ha and seedling injury response was compared to the response of seedlings collected from known resistant and susceptible barnyardgrass populations. Propanil-resistance of varying levels was confirmed in 115 (16 counties) out of the 138 Arkansas barnyardgrass seed sources. Propanil I50 values (rate of herbicide required to provide 50% injury/control) were determined to be 14, 20, and 39 kg/ha for slightly, moderately, and highly resistant barnyardgrass, respectively. A resistance factor of 20® was found in the highly resistant barnyardgrass category. Development of resistance was highly correlated with crop rotations where rice was grown one out of two, or two out of three years.

Field experiments were established in 1991 and 1992 on silty clay and sandy loam soils to evaluate combinations of non-selective and selective herbicides for stale seedbed soybean weed control. Metribuzin PRE controlled sicklepod and pitted morningglory more consistently than other treatments. At 9 wk after planting, antagonism occurred in most cases on sicklepod control when metribuzin was tank-mixed with a non-selective herbicide. Other selective herbicides required addition of a non-selective herbicide at planting to effectively control sicklepod and pitted morningglory. Sicklepod and pitted morningglory control was better with POST selective herbicides when following glufosinate or paraquat than when following glyphosate or SC-0224. In most instances a follow-up POST treatment was needed to maintain weed control from non-selective herbicides applied PRE. Metribuzin and metribuzin plus chlorimuron increased soybean yields when tank-mixed with paraquat compared with yields obtained with paraquat alone.

Five sulfonylurea herbicides were applied to Typic Boroll soil with pH 8.0 and 2% organic matter in the field and growth chamber to assess their residual effects on following crops. Under irrigated conditions, triasulfuron at 22 g/ha reduced the growth of alfalfa, canola, corn, lentil, pea, potato, and sugarbeet the year after application but caused no injury after two years. Alfalfa GR50 (rate required to reduce growth by 50%) values immediately after application of chlorsulfuron, triasulfuron, and metsulfuron were < 2.2 g/ha and similar in the growth chamber and field. Alfalfa and canola GR50 values immediately after thifensulfuron application in the growth chamber were ≥ 90 g/ha. Alfalfa, canola, flax, and sugarbeet GR50 for tribenuron in the growth chamber were < 4 g/ha. Crop response curves developed by growth chamber experiments could replace many of the multiple site and crop field experiments now required to develop recropping guidelines for residual herbicides.

Field studies were conducted from 1991 through 1994 in several locations of Southern Spain to determine the efficacy of imazapyr as late postemergence application in sunflower for broomrape control and the crop response to this herbicide under weed-free conditions. Single applications of imazapyr at 10 to 15 g/ha on sunflower plants at about the V12 (12 leaves) to V19 (19 leaves) growth stages, infected with broomrape plants at the predominant growth stages “c” (small nodule with shoot bud visible) and “d” (shoot bud and roots well developed), resulted in effective control without damaging the crop. Double applications of imazapyr at 10 + 10 g/ha at about the same crop growth stages V12 to V19, at a time interval of 12 to 14 d between single applications, were also efficacious and well tolerated by the crop. The same herbicide treatments applied to weed-free sunflower did not adversely affect the biomass, head diameter or yield of the crop as compared with the nontreated check. As a result of the low rate of imazapyr required for broomrape control, the herbicide treatments are economical (about $3 to $6/ha), non-residual and not affected by the type of soil texture and rainfall conditions.

Field experiments were conducted at two locations in North Carolina during the summer of 1992 to ascertain the reasons for stunted growth of corn noticed occasionally in fields treated POST with nicosulfuron to control dense infestations of johnsongrass. Treatments consisted of nicosulfuron at 35 g ai/ha sprayed POST, either over-top or post-directed only to johnsongrass in a single early application or in sequential applications of 17.5 g/ha. Checks included a hand-weeded, paraquat post-directed to johnsongrass, and non-weeded. Nicosulfuron provided satisfactory control of johnsongrass irrespective of whether applied once or sequentially. Corn yield in nicosulfuron-treated plots was as much as three times greater than in non-treated plots depending on hybrid and location. In the absence of johnsongrass interference, nicosulfuron caused a corn yield reduction of 6 to 9%. However, in the presence of johnsongrass interference, an additional 16 to 26% reduction in yield occurred with a POST application of nicosulfuron. Reductions in corn plant height and dry matter accumulation also were noticed for nicosulfuron treatments compared to post-directed application of paraquat and hand removal of johnsongrass.

Field experiments conducted from 1990 through 1993 evaluated single and sequential applications of various postemergence graminicides for common bermudagrass control in peanut. UBI-1956 at 70 and 100 g/ha applied early POST (EP) or sequentially, controlled common bermudagrass equivalent to fluazifop-P applied EP or sequentially. Clethodim provided variable common bermudagrass control. Clethodim applied EP provided better control than sethoxydim while control from sequential clethodim applications were equivalent to fluazifop-P. Fenoxaprop-ethyl alone failed to control common bermudagrass while the addition of fluazifop-P to fenoxaprop-ethyl improved control at least 22% in two of three years. Quizalofop applied EP and fenoxaprop applied sequentially provided common bermudagrass control equivalent to fluazifop-P. Fenoxaprop-ethyl or sethoxydim applied once or twice controlled common bermudagrass similarly. Peanut yield increases from common bermudagrass control were erratic.

Broadleaf weed control ranged from 80% with clomazone at 0.14 kg/ha to 100% with clomazone at 1.12 kg/ha. Broadleaf weed control was higher with clomazone than naptalam or ethalfluralin. The combination of clomazone at 0.14 kg/ha and naptalam at 5 kg/ha or ethalfluralin at 1.25 kg/ha gave more than 90% broadleaf weed control. Clomazone caused chlorosis and bleaching on cucumber leaves but plants rapidly recovered. Cucumber yields were higher in plots treated with clomazone alone at 0.14 to 0.56 kg/ha than with ethalfluralin or naptalam alone. Clomazone dose response studies were conducted in weed-free plots with five cucumber varieties. The 0.28 kg/ha rate caused low levels of visible injury and did not decrease yields. Cultivar differences were more pronounced at higher clomazone rates. The five cucumber cultivars were, in order of increasing clomazone tolerance, ‘Sunre 3537,’ ‘Pioneer,’ ‘Quest,’ ‘Prince,’ and ‘Calypso.’ We concluded that clomazone is an effective and selective herbicide for broadleaf weed control in pickling cucumber.

Adjuvants were evaluated to determine the effect on increasing spray droplet size and reducing the amount of spray dispensed in small driftable size particles when applying water and paraffinic oil at ultralow volume. Spray solutions were applied with an air-assist system at liquid flow rates of 28 and 56 ml/min and atomized with 14, 28, 42, 56, and 84 kPa of air pressure. Water and paraffinic oil were applied alone and with two drift retardant adjuvants mixed individually in each. The two water soluble adjuvants were mixed at concentrations of 0.25, 0.50, 0.75, 1.0, and 2.0%; oil soluble adjuvants were applied at 0.125, 0.25, and 0.50%. Adjuvants used in water and oil were effective at increasing droplet size and reducing the amount of liquid dispensed in small driftable size particles. Effectiveness of the adjuvants decreased as air pressures increased, with water soluble adjuvants being more susceptible to air pressure. Volume median diameters > 200 μm with water could be achieved without adjuvants; whereas with oil, an adjuvant was required.

Sulfonylurea-tolerant soybean (STS soybean) tolerance to imazethapyr (70 g/ha) plus thifensulfuron (4.4 g/ha) tank mixture was evaluated in 1993 and 1994 at two locations in Illinois. Thifensulfuron injury was reduced from 27% in non-STS soybean to 1% in STS soybean 14 days after treatment (DAT). STS and non-STS soybean did not differ in response to imazethapyr alone with < 6% injury 14 DAT. Injury from the tank mix was reduced from 40% in non-STS soybean to 24% in STS soybean 14 DAT. However, a synergistic interaction between the herbicides exists in STS soybean. The yield of injured soybean was not reduced in either STS or non-STS soybean. Injury to STS soybean from the tank mix was decreased by reducing the rate of imazethapyr from 70 g/ha to 52.5 or 35 g/ha. Reducing the thifensulfuron rate mixed with imazethapyr at 70 g/ha did not decrease injury in STS soybean. Control of smooth pigweed, common lambsquarters, and common cocklebur was not affected by reducing the rate of either herbicide in the tank mixture.

Experiments were conducted in the greenhouse to determine nicosulfuron activity on large crabgrass as influenced by surfactant, ammonium salt adjuvants, and various calcium, sodium, and magnesium salts in the spray carrier. Nicosulfuron activity varied with surfactant. Ammonium salt adjuvants at 0.2 M (ammonia) and sodium, calcium, or magnesium salts at 0.02 M (cation) increased or decreased nicosulfuron activity dependent on the specific salt and surfactant. Spray carrier surface tension or pH of the spray solution did not relate to nicosulfuron activity. The specific ammonium component influenced nicosulfuron activity differently depending on the surfactant. Increases in ammonium nitrate concentration decreased, nicosulfuron activity with Pluronic® P85 surfactant and increased activity with X-77® surfactant. The response to ammonium nitrate was less pronounced at high surfactant concentrations. Sodium bicarbonate antagonism of nicosulfuron activity was overcome by ammonium sulfate and 28% aqueous nitrogen fertilizer in the presence of several oil and surfactant adjuvants. The results indicate the importance of surfactant selection for use with nicosulfuron when applied with nitrogen salts.

Broccoli cultivars were evaluated for differential response to oxyfluorfen applied postemergence (0.1 to 1.6 kg ai/ha) in the greenhouse and post-transplant (0.125 to 1.0 kg/ha) in a spring and a fall field study. One cultivar, ‘Green Goliath,’ was injured more by oxyfluorfen than other cultivars in all environments. ‘Pinnacle,’ was consistently less injured than others. ‘Early Dawn’ and ‘Emerald City’ were intermediate in response to oxyfluorfen. At application rates of 0.25 kg/ha or less, all cultivars recovered sufficiently from injury to produce broccoli heads of equal weight and quality as controls. Cultivars differed in their tolerance of oxyfluorfen at rates above 0.25 kg/ha. Late-maturing cultivars (i.e., Pinnacle) usually recovered from injury to yield similarly to hand-weeded plots while early-maturing cultivars were more likely to show yield losses. Broccoli cultivars possess sufficient tolerance to oxyfluorfen applied post-transplant for weed control. Appropriate choice of cultivar will enhance its utility.

California and Michigan strains of Xanthomonas campestris pv. poannua were compared as biocontrol agents for the annual and perennial subspecies of annual bluegrass. Additionally, inoculation frequency was evaluated for improved control of the perennial subspecies in New York. The two bacterial strains were similarly virulent to annual bluegrass in both growth chamber and field tests. When inoculated once per week for 4 wk, the annual and perennial subspecies were controlled 92 and 82% in growth chamber tests, but only 11 and 7% in field tests, respectively. Control of established annual bluegrass in the field was improved to a maximum of about 40% by increasing inoculation frequency to three times per wk for 4 wk, but recovery occurred 2 to 5 wk after inoculations were discontinued. Creeping bentgrass was not affected by the bacterium.

A carpeted roller applicator and broadcast sprayer were compared for application of glyphosate, metsulfuron, and picloram to control duncecap and tall larkspur on mountain range sites in southern Idaho and central Utah, respectively. Glyphosate was the most effective herbicide, controlling 96% of both larkspur species when applied by the sprayer and 78% when applied by the roller. There was no difference in larkspur control between application methods using metsulfuron or picloram. Application of herbicides by the carpeted roller was less detrimental to forbs compared to the sprayer, and the roller application of glyphosate did not reduce grass cover. Grass cover doubled following treatments with picloram and metsulfuron.

Problems with insecticide resistance have long plagued the field of economic entomology. Genetic, biochemical, and ecological information on insects has been used to develop strategies to slow the rate of insecticide resistance evolution. Documented cases of herbicide resistance have increased dramatically over the past 10 yr. This paper compares some aspects of insect and weed biology that can be used in determining whether or not resistance management strategies developed for insects are likely to be useful in combating herbicide resistance. Differences between insects and weeds in terms of genetic architecture, mating systems, and population structure lead to differences in the expected efficacy of some resistance management strategies. Because of the localized population structure of some weeds, it may be easier to get farmers to participate in herbicide resistance management programs and avoid a “tragedy of the commons.” A review of the herbicide resistance literature reveals a number of areas of basic research on ecology and genetics of weeds that could help in designing more appropriate resistance management programs.

Selected herbicide treatments from 25 winter annual broadleaf weed control experiments conducted across Oklahoma from 1979 to 1993 were examined for winter wheat yield response to weed control. All experiments contained one or two of the same three weeds (henbit, bushy wallflower, or cutleaf eveningprimrose) as the primary target(s). Herbicide treatments included chlorsulfuron at 8.8 or 17.6 g ai/ha, and/or 2,4-D at 420 g ae/ha, all broadcast POST to wheat. Most herbicide treatments controlled the weeds, but increased wheat grain yield at only one site, wherein bushy wallflower density was 830/m2. Correlation analysis found no relationship between weed densities and wheat yield when chlorsulfuron at 8.8 g/ha was applied for henbit or moderate levels of bushy wallflower or cutleaf eveningprimrose. There was a significant positive relationship favoring spraying of bushy wallflower when the correlation analysis included data from one site with a density of 830 plants/m2.

Soybean cultivars were grown in a hydroponic culture to evaluate tolerance to dimethenamid and metolachlor. Root length was reduced in 7 of the 32 cultivars with 50 ppb dimethenamid. ‘Asgrow 5403’ and ‘Bryan’ root lengths were reduced more than 20%. ‘Asgrow 6297’ was the only cultivar with greater than 12% root length reductions with 83 ppb metolachlor. The longest lateral root of all cultivars was shortened with dimethenamid. ‘NK-1460’ and Bryan expressed the most lateral root length reduction, at 73 and 62%, respectively. Asgrow 6297 was tolerant to dimethenamid, but lateral roots were reduced 16%. Metolachlor reduced the lateral root lengths of 12 cultivars, with Asgrow 5403, ‘Coker 6955,’ and Bryan reduced more than 20%. Bryan was the most sensitive cultivar to both herbicides, while ‘Brim’ was the most tolerant of cultivars evaluated.