A fungal pathogen, Colletotrichum gloeosporioides (Penz.) Sacc. f. sp. malvae, isolated from anthracnose symptoms of round-leaved mallow (Malva pusilla Sm.), was shown in greenhouse tests to be host specific to Malva spp. and velvetleaf (Abutilon theophrasti Medic. #3 ABUTH), and only with slight attack on hollyhock [Althaea rosea (L.) Cav. # ALGRO], Malope trifida Cav., and Venice mallow (Hibiscus trionum L. # HIBTR). Round-leaved mallow plants inoculated with a spore suspension of the fungus were killed after 17 to 20 days. It was less pathogenic on velvetleaf with 60 to 70% attack. The fungus can readily be cultured and field tests from 1982 to 1987 resulted in excellent control of round-leaved mallow under natural conditions. Therefore, it has good potential for biological control of round-leaved mallow in field crops.

Experiments were conducted in 1991 and 1992 to evaluate annual grass and broadleaf weed control with bromoxynil applied alone and with graminicides. Bromoxynil controlled velvetleaf better than entireleaf morningglory and controlled both species better than sicklepod. Mixing bromoxynil with fluazifop-P, quizalofop-P, and sethoxydim reduced control of barnyardgrass, green foxtail, large crabgrass, and shattercane. Control of broadleaf signalgrass with quizalofop-P was reduced by bromoxynil. Annual grass control with clethodim was not affected by bromoxynil.

Common cocklebur root and shoot interferences were investigated as factors causing soybean yield reductions in the field. Significant decreases in soybean yield resulted from common cocklebur shoot interference when the two were grown together with the common cocklebur roots confined in plastic bags. Soybean yield decreased further when common cocklebur roots were not confined, but this decrease resulted, in part, from increased common cocklebur shoot growth. Wire mesh cylinders of varying diameters, placed in gaps in the soybean rows and designed to spatially restrict soybean shoot growth, reduced yield more than did gaps alone. When similar cylinders lined with black plastic were placed next to the soybean rows to restrict both within-canopy light and space for soybean shoot growth, soybean yield was reduced most (44%) with concomitant changes in stem height, stem diameter, lodging, and branches, nodes, and pods/plant. Interference of common cocklebur with soybeans resulted primarily from shoot interference, and competition for light within the soybean canopy implicated as the major factor causing the soybean yield reduction.

Long term weed management programs for annual weeds should take into account seed production as well as yield losses from weeds. In this research economic optimum thresholds, determined with a simulation model, were compared to the economic thresholds derived from field studies with common sunflower and velvetleaf in soybean. An economic optimum threshold model, including a biological submodel and a submodel evaluating the financial benefit from controlling each weed species in soybean, was constructed to estimate the biological events and processes and economic costs of each weed. Long-term weed management programs were simulated in continuous soybean over a 15-yr period. The model optimized a POST treatment of bentazon by computing costs associated with weed interference and changes in estimated seedbank size. The economic optimum threshold populations were estimated to be 0.3 to 0.4 velvetleaf plants per 10 m2 and 0.4 common sunflower plants per 10 m2. The economic thresholds calculated to be 2.6 velvetleaf plants per 10 m2 and 1.5 common sunflower plants per 10 m2. The economic optimum threshold was calculated to be 7.5-fold and 3.6-fold lower than the economic thresholds for velvetleaf and common sunflower, respectively. The larger ratio for velvetleaf reflected the large quantity and longevity of seeds produced. The smaller ratio for common sunflower reflected the shorter persistence of the seeds and higher seed predation rates.

Barnyardgrass and many broadleaf weeds were severely stunted by POST applications of isoxaben. Most grasses were only moderately affected by isoxaben at rates up to 1 kg/ha. POST activity was enhanced by soil interception, but evident with foliar interception alone. Cell swelling in stems and in interveinal leaf lamina was observed after foliar application. A radiotracer study in redroot pigweed showed that only a small amount (3.3% of applied) of isoxaben entered the leaf following foliar application, and that little (0.08% of applied) isoxaben migrated beyond the point of application. The potential of isoxaben as a POST herbicide may be limited, in part, by poor absorption and translocation.

Soil-water profiles of cotton, several weeds, and bare soil were compared in research conducted in 1989 and 1990 near Perkins, OK. Neutron probe access tubes were installed prior to plant establishment to facilitate nondestructive volumetric water content determinations at selected depths throughout the season. Planting was in a double-circle pattern concentric around each neutron probe access tube. Phenological and soil-water data were collected weekly. As plants began to senesce, aboveground biomass yield data were collected. In 1989, rainfall frequently replenished soil-water in the upper soil profile. The plants used the recharge water to varying degrees, particularly late in the season. Common cocklebur and johnsongrass extracted soil-water from greater depths than cotton and velvetleaf throughout the season. In 1990, the largest differences detected between bare soil and weeds early in the season were measured for common cocklebur, devil's-claw, and silverleaf nightshade. Later in the season, however, there was little difference in soil-water content in the upper soil profiles for all species. Johnsongrass emerged later than the other species in 1990 which affected its relative soil-water extraction.

Velvetleaf, eastern black nightshade, wild mustard, common cocklebur, green foxtail, and yellow foxtail, grown for 4 weeks in nonaerated nutrient solution, developed satisfactorily compared to plants grown in soil. For experiments involving certain weed species grown in nutrient solution, aeration may be unnecessary.

The influence of various factors on the tolerance of corn and selected weed species to thifensulfuron was studied in greenhouse experiments. Corn fresh weight was reduced by postemergence application of thifensulfuron when applied at 180 g ai ha−1 without surfactant or at 18 g ha−1 with a nonionic surfactant. Corn was more susceptible to root exposure whereas velvetleaf was more susceptible to foliar exposure. Velvetleaf was most susceptible when plants were young, when a nonionic surfactant was added, and at 20 rather than 30 C. Simulated rainfall 8 h after application reduced velvetleaf injury by thifensulfuron at 18 g ha−1 without surfactant; however, with addition of a nonionic surfactant, velvetleaf injury was reduced only if rainfall occurred less than 2 h after application. Soil moisture level did not affect velvetleaf susceptibility.

The effect of sequential hexane, ethyl acetate, and aqueous methanol extracts of ‘Regal’ sweetpotato periderm on seed germination of sweetpotato, proso millet, and seven weed species was studied. The hexane extract, which contained the nonpolar components of the periderm tissue, was least inhibitory. It inhibited velvetleaf, proso millet, black nightshade, and redroot pigweed germination, and maximum inhibition was 56% for black nightshade at 200 mg of periderm extracted ml–1. The ethyl acetate fraction was inhibitory to proso millet, velvetleaf, black nightshade, goosegrass, tall morningglory, coffee senna, and redroot pigweed. The estimated I50 for ethyl acetate ranged from 17 mg periderm extracted ml–1 for black nightshade to 201 mg ml–1 for coffee senna. Sweetpotato, tall morningglory, and eclipta germination was not inhibited by this extract at the concentrations tested. The aqueous methanol extract was much more inhibitory than the hexane or ethyl acetate extracts, and there was considerable variation between species in response to this extract The I50 estimates for the aqueous methanol extract were 0.5, 0.6, 2.8, 4.4, 5.1, 9.6, 15.7, 21.0, and 25.8 mg ml–1 for velvetleaf, proso millet, black nightshade, goosegrass, sweetpotato, tall morningglory, eclipta, coffee senna, and pigweed, respectively.

Growth and morphological characteristics of four weed and two crop species were analyzed to determine how small-seeded weeds can compete with large-seeded crops despite the initial size disadvantage. Small-seeded weeds had higher relative growth rates because of increased percentages of biomass devoted to leaves. This morphological scheme was achieved primarily through a reduction in the percentage of biomass devoted to roots. Yet, small-seeded weeds were able to develop larger mean plant root lengths as a result of having both roots of smaller diameter and higher rates of root length increase.

The penetration, translocation, and metabolism of acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} in ivyleaf morningglory [Ipomoea hederacea (L.) Jacq. ♯3 IPOHE], velvetleaf (Abutilon theophrasti Medic. ♯ ABUTH), common cocklebur (Xanthium pensylvanicum Wallr. ♯ XANPE), and soybean [Glycine max (L.) Merr.] were studied. An application to plants of 0.3 kg ai/ha mefluidide {N-[2,4-dimethyl-5-[[(trifluoromethyl)sulfonyl] amino] phenyl] acetamide} 0, 3, 5, or 7 days prior to treatment with 14C-acifluorfen often altered one or more of the physiological processes under study. Pretreatment of ivyleaf morningglory with mefluidide 3, 5, or 7 days prior to application of 14C-acifluorfen increased penetration of 14C and decreased metabolism of acifluorfen, while translocation was unaffected. All mefluidide treatments increased penetration of 14C-acifluorfen into velvetleaf, while the 3-, 5-, and 7-day pretreatments decreased acifluorfen metabolism. Penetration of acifluorfen into common cocklebur was unaffected by pretreatment with mefluidide. However, the 7-day mefluidide pretreatment of common cocklebur increased translocation of 14C into the upper leaves and decreased acifluorfen metabolism. Penetration and translocation of the radiolabel from 14C-acifluorfen in soybean was unaffected by pretreatment with mefluidide. Metabolism of acifluorfen by soybean was decreased by the 0-day mefluidide treatment but was unaffected by the 3-, 5-, or 7-day mefluidide pretreatment.

The effects of herbicide–adjuvant combinations and times of application on weed density and corn yield were determined in a 3-yr field study. At a given rate and time of application of bentazon/atrazine, Assist and Merge were equally effective adjuvants. Assist would be the adjuvant of choice for bentazon/atrazine rather than Merge because of its currently lower cost. Weed density depended on the rate and time of application of bentazon/atrazine. In general, bentazon/atrazine applied at early stages (7 d after emergence) of the development of corn seedlings or at high or intermediate rate (1.6 or 0.8 kg ai/ha) maintained low weed densities, with a relatively small range of variation over years. Delay in time (14 or 21 d after emergence) or reduction in herbicide rate (0.4 kg ai/ha) increased the risk of high weed pressure, although it was not always associated with yield loss. The best time for the application of postemergence herbicide appears to be a few days prior to the onset of the critical period of weed control.

The influence of light, soil moisture, and temperature on establishment of selected species through hairy vetch residue on the soil surface was investigated under controlled conditions in the greenhouse. Hairy vetch residue at rates ranging from 0 to 616 g m−2 had no effect on corn, slightly reduced velvetleaf and green foxtail establishment, and severely inhibited common lambsquarters establishment under full sunlight conditions. The same rates of hairy vetch residue reduced velvetleaf, green foxtail, and common lambsquarters establishment more under a shade cloth with 9% light transmittance than under full sunlight. Day/night temperatures of 24/16 or 32/26 C had no effect and soil moistures of 50 or 133% field capacity had little effect on response of all species to residue rates. Weed establishment was similar under shade cloth without residue as under residue with an equivalent light transmittance, suggesting that light was more important than allelopathy or physical impedance for weed suppression by hairy vetch residue.

Greenhouse studies were conducted to investigate the effects of adjuvant and rainfall on bentazon spray retention, efficacy, and foliar washoff in hemp sesbania, sicklepod, smooth pigweed, and velvetleaf. Bentazon was applied at 0.28 to 2.24 kg ai/ha with Agri-Dex, a crop oil concentrate (COC) or Kinetic, an organiosilicone-nonionic surfactant blend (OSB) when weeds were at the three- to five-leaf stage. Plants were subjected to 2.5 cm simulated rainfall for 20 min at 1 and 24 h after application of bentazon. Shoot fresh weight reduction assessed 2 wk after treatment was similar with either adjuvant on velvetleaf and smooth pigweed. OSB enhanced bentazon efficacy in hemp sesbania and sicklepod as compared to COC. Rainfall at 1 h after application generally reduced bentazon activity in all weeds. OSB maintained bentazon activity in hemp sesbania when subjected to rainfall at 1 h after application as compared to COC. Overall, bentazon spray retention on plants was 9 to 550% higher with OSB as compared to COC among the species at 1 h after application. Amount of bentazon residue washed off from the foliage by rainfall within a weed species was relatively similar for both adjuvants except in smooth pigweed and ranged from 39 to 98% among the four weed species at 1 h after application. OSB exhibited specificity for certain weed species and the potential to minimize bentazon spray reaching the soil by increasing deposition.

Field experiments were conducted from 1993 to 1995 to compare weed control by the isopropylamine salt of glyphosate at 0.21, 0.42, 0.63, and 0.84 kg ae/ha applied at three stages of weed growth. Weed control by glyphosate applied at these rates alone or with ammonium sulfate at 2.8 kg/ha was also evaluated. In other experiments, potential interactions between glyphosate and acifluorfen, chlorimuron, and 2,4-DB were evaluated. Velvetleaf, prickly sida, sicklepod, pitted morningglory, entireleaf morningglory, palmleaf morningglory, and hemp sesbania were controlled more easily when weeds had one to three leaves compared with control when weeds had four or more leaves. Glyphosate controlled redroot pigweed, velvetleaf, prickly sida, sicklepod, and barnyardgrass more effectively than pitted morningglory, entireleaf morningglory, palmleaf morningglory, or hemp sesbania. Increasing the rate of glyphosate increased control, especially when glyphosate was applied to larger weeds. Greater variation in control was noted for pitted morningglory, palmleaf morningglory, prickly sida, and velvetleaf than for redroot pigweed, sicklepod, entireleaf morningglory, or hemp sesbania. Ammonium sulfate increased prickly sida and entireleaf morningglory control but did not influence sicklepod, hemp sesbania, or barnyardgrass control. Acifluorfen applied 3 d before glyphosate or in a mixture with glyphosate reduced barnyardgrass control compared with glyphosate applied alone. Chlorimuron did not reduce efficacy. Mixtures of glyphosate and 2,4-DB controlled sicklepod, entireleaf morningglory, and barnyardgrass similar to glyphosate alone.

Field experiments were conducted from 1985 through 1989 to evaluate herbicide selectivity and impact on seeding-year yields of spring oat and underseeded alfalfa, and carryover weed control benefits from increased legume-fixed N for second-year dryland no-till corn. PRE metolachlor, pendimethalin, and prodiamine controlled green foxtail and POST bromoxynil or 2,4-DB controlled broadleaf weeds. These herbicides caused 0 to 20% alfalfa injury and 0 to 17% oat injury, and increased oat yield one of three years but did not increase the yield of underseeded alfalfa. POST pyridate, thifensulfuron, and tribenuron were too injurious to either oat, alfalfa, or both crops. Forage yields of annual ‘Nitro’ and perennial ‘Wrangler’ alfalfa seeded alone were greater than when they were underseeded in oat, with herbicides applied in both systems. As a result of drought in 1988 and 1989, yield of second-year corn planted after one-year alfalfa was not increased from potentially greater legume-fixed N. Dryland corn yield following monoculture oat or corn was 254% higher than corn following alfalfa.

Isoxaben is an inhibitor of the synthesis of cellulose from glucose. Some dicot weed species are relatively insensitive to isoxaben inhibition. This study investigates mechanisms by which decreased sensitivity may occur in three dicot weed species: catchweed bedstraw, redroot pigweed, and velvetleaf. Dose response curves were generated to determine I50 values for the inhibition of glucose incorporation into cellulose by isoxaben and compared to that of a sensitive species, mouse-ear cress. Metabolic detoxification and uptake rates were measured and the degree of tolerance conferred by these mechanisms was calculated. In all cases, metabolic detoxification was negligible. Lower uptake rates were significant but minor components of tolerance in all species. It is suggested that the principal cause of isoxaben tolerance in these dicot weed species is decreased sensitivity at the target site.

A greenhouse study was designed to evaluate a series of silicone adjuvants for increasing efficacy and rainfastness of Na-acifluorfen, isopropylamine-glyphosate, and Na-bentazon on velvetleaf and common lambsquarters. Silicone adjuvants were identified that increased both herbicide efficacy and rainfastness at 0.9 to 1.2 L ha-1. Specificity of adjuvants for specific target weed species and herbicides was evident. Effective silicone adjuvants were identified that did not increase soybean injury by Na-acifluorfen or Na-bentazon beyond that observed with crop oil concentrate. The efficacy of the best silicone adjuvant did not appear to be solely a function of reduced surface tension of the spray solution.

Clomazone was evaluated preplant incorporated (PPI) or post-transplant (PT) for weed control in sweet potatoes. No crop injury by clomazone was detected. Clomazone controlled (≥90%). broadleaf signalgrass, large crabgrass, barnyardgrass, and prickly sida at all rates and methods of application (MOA). Common cocklebur was controlled (≥93%) by clomazone at 1.1 kg ai ha-1 applied PT or at 3.4 kg ai ha-1 PPI in 1988 and at all rates and MOA in 1989. Clomazone did not control carpetweed.

An experiment was initiated in 1970 and continued through 1979 by exhuming and germinating seed of 12 economic weed species buried beneath 23 cm of soil in eastern and western Nebraska. Loss in germination of exhumed seeds over years is mathematically characterized by the formula for the rectangular hyperbola, which represents many shapes of curves that have zero as their lower limit. Of the 12 weed species, only fall panicum (Panicum dichotomiflorum Michx.) and redroot pigweed (Amaranthus retroflexus L.) seed germination did not drop significantly over the 10-yr burial period. Germination of redroot pigweed seed was higher when buried in eastern Nebraska, but was higher for smooth groundcherry (Physalis subglabrata Mack&Bush.) and velvetleaf (Abutilon theophrasti Medic.) when buried in western Nebraska. Germination of the other nine species were not affected by burial location. The 12 weed species can be ranked as those showing most to least rapid loss of germination during burial for 10 yr as follows: honeyvine milkweed [Ampelamus albidus (Nutt.) Britt.], hemp dogbane (Apocynum cannabinum L.), kochia [Kochia scoparia (L.) Schrad.], sunflower (Helianthus annum L.), large crabgrass [Digitaria sanguinalis (L.) Scop.], common milkweed (Asclepias syriaca L.), musk thistle (Carduus nutans L.), velvetleaf, fall panicum, redroot pigweed, green foxtail [Setaria viridis (L.) Beauv.], and smooth groundcherry.