Aquatic plants and physical-chemical data were collected during 1969 from 19 ponds and lakes. The fertility of these waters, as indicated by specific conductance, appeared to be a function of watershed area over the range 0 to 100 hectares. For watersheds greater than 100 hectares, differences in soil type and farming intensity were major causes of variability. Chara (Chara spp.) was found at specific conductances from 96 to 445 micromhos/sq cm (at 18 C) but was most abundant between 218 and 300. Chara also grew best in the upper half of the transparency range at Secchi disc readings varying from 2.4 to 5.8 m. Elodea (Elodea canadensis Michx.) flourished at specific conductances of 224 to 300 micromhos/sq cm and at transparencies from 2.7 to 5.8 m. Largeleaf pondweed (Potamogeton amplifolius Tuckerm.) was found only in the least fertile impoundment, whereas curlyleaf pondweed (P. crispus L.) occurred only in the most fertile impoundment. Sago pondweed (P. pectinatus L.) occurred at specific conductances of 200 to 349 micromhos/sq cm but was most prolific from 229 upward. This species also favored extensive shallow areas subject to a continuous summer flow.

The volatilities of iso and normal butyl esters of 2,4-D [(2,4-dichlorophenoxy)acetic acid] in a commercial formulation applied as thin films on pyrex and as aqueous droplets on pyrex and leaf surfaces increased directly with the available surface area/applied dose ratio (Q), and inversely with the adsorptive and absorptive characteristics of the surface, under the same conditions of temperature (39 ± 1.5 C), relative humidity (RH) (0%), geometry, airflow rate (750 ± 7 ml/min) and light. Partial pressures and rates of volatilization were computed. Herbicide in droplets below 200μ in diameter tended to volatilize faster than it penetrated, but the reverse occurred above this diameter.

Weed management can be a significant challenge in cropping systems, partly because the effects of tillage systems on weed seedbank and seedling population dynamics are not well understood. Field research was conducted from 1994 to 1996 in established tillage plots consisting of moldboard plow (MP), chisel plow (CP), and no-tillage (NT). The objectives were to determine the effects of long-term tillage systems on the timing and duration of Setaria spp. emergence and percentage cumulative emergence from the soil seedbank and to investigate the effect of tillage on Setaria spp. density and seed production following glyphosate application at Setaria spp. heights of 5, 10, and 15 cm. NT contained a greater number of Setaria spp. seed in the 0- to 1-, 1- to 3-, and 3- to 6-cm depths than MP or CP systems. There was little difference between the three tillage systems at depths greater than 6 cm. Setaria spp. emergence was greater in NT than in MP or CP in 1994 and 1996 and greater than in MP in 1995. There was a substantial increase in Setaria spp. emergence in NT between 3 and 4 weeks after planting (WAP) in 1994 and between 5 and 6 WAP in 1995 and 1996. Significant emergence did not occur past 5 to 6 WAP in 1994 and 1995 but continued over a longer period of time in 1996. Setaria spp. plants consistently reached targeted herbicide application heights 4 to 9 d earlier in NT than in CP and MP. In 1994, final Setaria spp. density was greater in NT compared to CP and MP at the 5- and 10-cm herbicide application timings. When glyphosate was applied to 15-cm-tall Setaria, very few weeds were present following application across all tillage systems. In 1995, NT resulted in greater Setaria spp. density than MP or CP across all application timings. There was no difference in final Setaria spp. density between MP and CP across all glyphosate timings in 1994 and 1995. Seed production was negligible in MP and CP, regardless of glyphosate timing. In NT, however, significant seed production occurred, especially with early application. Results indicate that the effectiveness of nonresidual herbicides for Setaria faberi Herrm. control is influenced by tillage system and the timing of application.

The effectiveness of postemergence applications of triallate [S-(2,3,3-trichloroallyl)diisopropylthiocarbamate] for wild oat (Avena fatua L.) control in spring wheat (Triticum aestivum L. ‘Waldron’) was evaluated in the field and greenhouse. The stage of the wild oat plants at the time of treatment did not appear to influence wild oat control with postemergence applications of triallate in the field; however, in the greenhouse, wild oat plants became more resistant with increased maturation. Granular triallate was more effective than the emulsifiable concentrate for postemergence wild oat control. Fair season-long postemergence wild oat control was obtained with the granular formulation of triallate at rates of 2.24 kg/ha or greater. However, some wheat injury was observed in the field with the 2.80 and 3.36 kg/ha rates of granular triallate. Moisture and temperature influenced postemergence wild oat control with triallate.

The basis for differential susceptibility of tolerant rice (Oryza sativa L.), susceptible wild rice (Zizania palustris L.), and susceptible giant burreed (Sparganium eurycarpum Engelm. # SPGEU) to foliar application of 1.1 kg ai/ha of bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] was investigated by evaluating herbicide absorption, translocation, and metabolism. Giant burreed and wild rice absorbed more bentazon than rice at similar growth stages. Less than 10% of the absorbed bentazon was translocated out of the treated leaf of any of the species. Differential tolerance of bentazon among the three species was due to differences in the rate of bentazon metabolism. Rice metabolized 98% of the bentazon retained in the treated leaf 1 day after treatment (DAT), while giant burreed and wild rice metabolized less than 2% of the bentazon retained in the treated leaf 5 DAT.

A population level, two-compartment, temperaturedependent model that predicts date of seedling johnsongrass flowering was formulated. The model consisted of a fourparameter poikilotherm rate equation to describe development rate as a function of temperature and a temperature-independent Weibull function to distribute flowering times for the population. Experiments were conducted to determine the effect of temperature, nitrogen availability, and water availability on development of seedling johnsongrass. Development was most sensitive to temperature while the effect of nitrogen concentration and water availability was minimum and inconsistent. The model was tested against three independent field data sets and provided accurate prediction of flowering dates for each data set.

Phenmedipham metabolism in leaf tissue of sugarbeet (tolerant) and rapeseed (sensitive) was compared. Sugarbeet leaf discs metabolized phenmedipham much more rapidly than rapeseed leaf discs, forming two metabolites of relatively low polarity. The less polar of these (metabolite 21) was a precursor to the other (metabolite 11), and its properties indicate derivation from phenmedipham by a single hydroxylation and monoglycosylation. Synthetic N-hydroxyphenmedipham was converted by both species into a compound that cochromatographs with metabolite 21. Purified metabolite 21 was much less inhibitory to light-driven oxygen evolution by isolated thylakoids of both species than was phenmedipham. Hydroxylation/glycosylation without prior carbamate hydrolysis appears to be a major factor in the tolerance of sugarbeet to phenmedipham.

White horehound (Marrubium vulgare L. # MAQVU) is one of the few perennial, herbaceous alien weeds to successfully colonize semiarid to arid rangelands in the Great Basin. Seeds of white horehound, collected from diverse habitats within the Lahontan Basin of northwestern Nevada, were incubated at constant or alternating temperatures ranging from 0 to 40 C. Germination was low and largely limited to alternating temperatures (10 to 25 C alternating with 35 to 40 C). At least a 15-C range in diurnal fluctuation was required for optimum germination (defined as not lower than the maximum observed minus one-half of the confidence interval with 1% probability). Cool-moist stratification at 2 C for 4 to 8 weeks enhanced germination at optimum temperature regimes by as much as 70% and increased overall average germination by 35 to 40%. Cool-moist stratification caused a shift in temperature optima for germination from widely fluctuating temperatures to constant 15 or 20 C. The magnitude of this shift varied among sources of seeds.

The selectivity of 2,4-dichlorophenyl p-nitrophenyl ether (nitrofen) among rape (Brassica campestris L., var. Echo) and two weed species, redroot pigweed (Amaranthus retroflexus L.) and green foxtail (Setaria viridis (L.) Beauv.), was determined quantitatively by a replicated dosage-response experiment. On an ED50 basis, green foxtail and redroot pigweed were, respectively, 5.8 and 63.3 times more susceptible than rape. Selectivity was divided into three parameters; viz., differential spray retention, differential penetration, and differential effects within the plant. Differences in retention were measured with the use of a water-soluble dye, while differences in penetration were determined with 14C-labelled nitrofen. Spray retention on green foxtail was 66% of that on the rape and 64% as much nitrofen penetrated redroot pigweed as penetrated rape. Under the conditions of these tests it was estimated that green foxtail and redroot pigweed were, respectively, 9 and 99 times more susceptible to nitrofen than was rape.

A method is presented to determine which of a set of sampling techniques is most nearly optimum for sampling weed abundance. To employ the method, one needs first to develop a set of sampling alternatives. A pilot survey is then conducted to determine time costs and sampling and measurement error variances associated with each technique. We used this method to determine which of three plot sizes (0.6 by 0.6 m, 0.6 by 1.5 m, 1.2 by 2.1 m) and two types of orchard scans is most nearly optimum for estimating weed abundance in apple [Malus domestica (Bork)] orchards. The results indicate that, for weed species that are fairly apparent, orchard scans are optimal when less than 8 h of sampling effort is expended, but when a high degree of precision is required, or when the weed species is more difficult to detect, plot samples may be the better choice. A size of plot ca. 0.6 by 1.5 m size was found to be most nearly optimum.

Johnsongrass seedlings were inoculated with conidia of four fungi, Exserohilum turcicum (Et), Colletotrichum graminicola (Cg), Gloeocercospora sorghi (Gs), and/or Bipolaris halepense (Bh). Leaf injury from Et was the most extensive. Johnsongrass was equally or more susceptible to these fungi 7 days after emergence (DAE) than at 14, 21, or 28 DAE. The greatest leaf injury, more than 90%, was induced by Et with 2 × 105 conidia/ml and a 24-h dew period. Sequential inoculation with Et at 15 and 20 DAE resulted in injury to more leaves than a single inoculation 15 DAE. No synergistic or antagonistic effects were observed in combinations of Et with Gs or Cg. Injury was limited to exposed leaves and sheaths, injured seedlings resumed growth and developed new leaves. Development of new leaves from inoculated plants was reduced by 30% or less at 14 days after inoculation (DAI) compared to uninoculated plants. Inoculation with Et or Bh was not as effective as paraquat or sethoxydim, since treatment with both herbicides resulted in seedling death. Seedling kill was not observed in any fungal tests.

Environmental stewardship refers to responsible use and protection of the natural environment through conservation and sustainable practices. Aldo Leopold (1887 to 1948) championed environmental stewardship based on a land ethic “dealing with man's relation to land and to the animals and plants that grow upon it.” Environmental stewardship as it relates to weed science has taken on varying roles as chemical weed control took hold in managing crops as a general practice soon after World War II, and became a well-known issue during the Vietnam War, with the extensive use of Agent Orange. As technologies in both chemistry and genetics have evolved, chemical weed control became safer with the advent of less toxicologically damaging materials. Combining toxicologically safe herbicides with genetic manipulation made it possible to apply chemicals that previously would have caused plant death, seemingly providing a magic bullet that simplified weed control for many producers during the mid to late 1990s. University scientists were guarded during the introduction of this technology; many understood that the magic bullet had flaws. By using predominately POST applications on weed species, genetic selection has given rise to substantial resistance, therefore presenting weed scientists with a grand challenge for the future. As new genetic technology is introduced for existing and future weed management problems, how will environmental stewardship be addressed and how can this technology be preserved? How can a producer afford it and how can they afford not to use it? When we have weeds present that used to be managed by herbicides and genetic technology, then society will be forced to deal with the same social, economic, agronomic, and environmental issues they dealt with prior to such technology. Herbicide-resistance technology and the concomitant herbicide-resistant weeds have provided a perfect case study to learn from if those in academia, extension, and industry will pay attention. Continuing education of the producer will be perhaps the biggest key in meeting the challenge to produce a safe and plentiful food supply for a growing population with minimal adverse effects of weeds while providing a desirable degree of environmental stewardship.

Rapid absorption and acropetal translocation of 14C from p-nitrophenyl-α,α,α-trifluoro-2-nitro-p-tolyl ether (fluorodifen) labeled with 14C at either the 1 position of the p-nitrophenyl ring (fluorodifen-1′-14C) or the trifluoromethyl carbon (fluorodifen-14CF3) were observed in cucumber (Cucumis sativus L. ‘Ashley’) seedlings after root treatment via nutrient solution. The major degradative products of fluorodifen-1′-14C were p-nitrophenol and Unknown I (possibly a conjugate of p-nitrophenol). Some p-nitrophenyl-α,α,α-trifluoro-2-amino-p-tolyl ether (2-amino-fluorodifen), p-aminophenyl-α,α,α-trifluoro-2-amino-p-tolyl ether (p,2-diamino-fluorodifen), p-aminophenyl-α,α,α-trifluoro-2-nitro-p-tolyl ether (p-amino-fluorodifen), and several minor unknowns also were detected. The major degradative product of fluorodifen−14CF3 was Unknown II (possibly a conjugate of 2-amino-4-trifluoromethylphenol). Also detected were some 2-amino-fluorodifen, 2-amino-4-trifluoromethylphenol, p-amino-fluorodifen, p2-diamino-fluorodifen, 2-nitro-4-trifluoromethylphenol, and several minor unknowns. The basic difference in susceptibility to fluorodifen of cucumber and peanut (Arachis hypogaea L. ‘Starr’) appears to be due to the rate of acropetal translocation (cucumber translocates more than peanut) and the rate of degradation (peanut degrades fluorodifen much more rapidly than cucumber).

This paper describes the communities of weeds in cereal and oilseed crops in Saskatchewan, using data collected in a 4-yr survey that sampled more than 400 fields. The survey data for the 40 most common weeds were analyzed in an attempt to distinguish natural groups of weed species and to compare the weed communities associated with the different crops and with different soils. The crops were barley (Hordeum vulgare L.), flax (Linum usitatissimum L.), oats (Avena sativa L.), rape (Brassica campestris L., Brassica napus L.), and wheat (Triticum aestivum L.). Phytosociological association and cluster analysis indicated that the associations of the weed species were more or less independent of the crop, although some differences existed and were determined more by soil or the associated climate. The 40 most common species were divided into three groups related to the soil and climatic subregions of the province.

Herbicides were applied postplant to field grown peanuts (Arachis hypogaea L.) and cotton (Gossypium hirsatum L.) in association with various cultivation devices. Rotary cultivations were compatible with single or multiple chemical treatments applied to soil. For eight preemergence herbicides commonly used on a nonincorporated basis, use of the multiple-application multiple-rotarycultivation technique, in comparison to the common method of use, increased the early-season weed control rating from 3.4 to 4.5 (scale 1.0 = no control, 5.0 = perfect), reduced weeds from 209 to 37 plants/50 sq ft, increased length of control from 36 to 50 days, and reduced the manual hoe time requirement from 24 to 10 hr/A. Incorporation of single or multiple chemical treatments decreased crop safety for some herbicides especially where cotton was the crop. The power driven rotary cultivator was superior to the standard sweep-equipped cultivator and the weeder for use in conjunction with multiple chemical applications. Herbicidal applications incorporated with the rotary cultivator at 0, 7, and 14; 7, 14, and 21; or 0, 14, and 28 days after planting provided equal weed control.

Hydrilla is one of the most serious aquatic weed problems in the United States, and fluridone is the only herbicide approved by the U.S. Environment Protection Agency that provides systemic control. Recently, hydrilla biotypes with varying levels of fluridone resistance have been documented in Florida. Hydrilla biotypes of varying fluridone resistance levels were maintained in 900-L tanks under natural atmospheric conditions from September 2004 to September 2005 in the absence of fluridone. Hydrilla shoot tips were collected from each biotype during September 2004 (at planting), December 2004 (3 mo after planting [MAP]), March 2005 (6 MAP), June 2005 (9 MAP), and September 2005 (12 MAP) and exposed to 5, 10, 15, 20, 30, and 50 µg L−1 fluridone to assess changes in susceptibility to this herbicide over time. Nonlinear regression analysis was used to calculate EC50 values for phytoene and β-carotene (effective fluridone concentration to increase/decrease the phytoene/β-carotene content in hydrilla plant tissue by 50% over the untreated control) at each time interval. EC50 values did not change in the susceptible hydrilla biotype over time. The EC50 values for phytoene and β-carotene for the susceptible biotype were 7.5 and 8.9 µg L−1, respectively, at planting and 7.6 and 9.4 µg L−1, respectively, at 12 MAP. Resistant hydrilla biotypes (R1–R5) also showed no change in EC50 phytoene values over time. Although, EC50 β-carotene values in resistant biotypes R1, R3, R4, and R5 did not change over time, R2 recorded a reduction in EC50 β-carotene at 12 MAP. Also, a 0.5-point decrease in resistance factor was observed for all resistant biotypes. Future long-term studies are needed to evaluate stability of resistant hydrilla biotypes in the absence of fluridone selection pressure.

Root applications of 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) at concentrations greater than 1.0 × 10-7M inhibited growth of parrotfeather (Myriophyllum brasiliense Camb.). Chromatographic analysis of exudate from detopped parrotfeather plants indicated that simazine was being tranlocated primarily as the intact molecule. A correlation coefficient of 0.985 was observed between the amount of simazine-14C removed from the treatment solution and transpiration during a 6-hr period. Absorption of simazine-14C by parrotfeather roots of intact plants was linear for the first 4 hr followed by a gradual decrease at the end of 6 hr. Simazine-14C at 1 and 2 hr after root applications was highest in the roots, intermediate in the lower part of the shoot, and lowest in the upper part of the shoot. After 192 hr, simazine-14C accumulated in the top of the shoot in a concentration higher than in the lower part of the shoot or the roots. A correlation coefficient of 0.988 was found between the amount of simazine-14C present in the tissue and the treatment concentration over a range of 1.2 × 10-6 to 5.0 × 10-6M.

Weedy rice (Oryza spp.) is one of the most competitive weeds in rice (Oryza sativa L.) production. Rapid growth, high tillering, enhanced ability to uptake fertilizers, asynchronous maturation, seed shattering, and high seedbank longevity make Oryza spp. more competitive than cultivated rice and highly persistent. Oryza spp. may be a source of useful traits for crop improvement such as herbicide tolerance. Greenhouse studies were conducted to evaluate the response of 54 Oryza spp. accessions collected between 2008 and 2009 from Arkansas to glyphosate, glufosinate, and flumioxazin applied at field rates. Rice cultivars ‘CL163’ and ‘REX’ were included for comparison. Accessions B20, B2, and S11 and B49, B51, and S59 showed reduced sensitivity to glyphosate and flumioxazin, respectively. These accessions had less than 40% injury 5 wk after treatment (WAT). Rice cultivars (CL163 and REX) were sensitive to both glyphosate and flumioxazin, with more than 95% plant mortality at 5 WAT. On average, blackhull accessions were more tolerant to glyphosate and flumioxazin than strawhull accessions. Dose–response analysis of B20, B2, and S11 confirmed 3- to 8-fold higher tolerance of these accessions to glyphosate. All Oryza spp. and cultivated rice were not affected by glufosinate applied at 874 g ai ha−1 (1X) and were controlled 100% by 1,311 g ai ha−1 (1.5X). Oryza spp. lines with reduced sensitivity to glyphosate and flumioxazin will be studied further for use in rice crop improvement.

Johnsongrass [Sorghum halepense (L.) Pers.] control in corn (Zea mays L.) was studied from 1970 to 1972 using consecutive annual applications of alachlor (2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide) at 3.4 kg/ha, butylate (S-ethyi diisobutylthiocarbamate) at 4.5, 9.0, and 13.4 kg/ha, and EPTC (S-ethyl dipropylthiocarbamate) at 3.4, 6.7, and 13.4 kg/ha. In 1970, control was less than 60% for all herbicides, and there was no significant rate response. In 1971 butylate and EPTC at all rates provided 79 to 93% control. Alachlor gave 49% control. In 1972 control was similar to 1971. Control with one cultivation was additive to these herbicidal effects. Seed treatment with 0.5% 1,8-naphthalic anhydride adequately protected the corn from herbicide injury. Corn yields were closely correlated with johnsongrass control.

Western ironweed (Vernonia baldwinii Torr.) and gray goldenrod (Solidago nemoralis Ait.) were mowed at various frequencies during two growing seasons in two experiments for each species. The initial mowings were made in late May or early June when rapid growth was taking place, presumably at the expense of carbohydrate reserves and also on June 15 and July 1. Subsequent mowings were made whenever the weed regrowth had reached 15 to 20 cm. The greatest reduction of western ironweed occurred with multiple mowing when the initial mowing was made on May 20. The degree of control was less in the second experiment than in the first. Multiple mowing was highly effective for reducing the stands of gray goldenrod; but in one of the two experiments, single mowings per season were almost as effective as multiple mowings.