The study of weed life cycles, reproductive strategies, and the soil seed bank is emphasized in the undergraduate weed science course at Ohio State University as central to an understanding of the survival of weeds in the environment. A laboratory exercise was conducted every spring and fall academic quarter from 1991 to 1993 to demonstrate the effects of long-term cropping and soil disturbance histories on weed seed banks and aboveground weed communities. Five sites with diverse histories of culture were sampled; these included a field cultivated in vegetables under continuous conventional tillage for 59 yr, a field cultivated in field corn under continuous no-tillage for 11 yr, a 24 yr-old turfgrass research farm, a 70 yr-old forest, and a section of the forest border. Students conducted a survey of the weeds growing at the sites and separated and identified seeds from soil samples over a 3-wk period in weekly 2-h laboratory periods. Students wrote reports interpreting the data based on their knowledge of the site histories, weed life cycles, and weed seed production and longevity characteristics. The data were consistent over academic quarters as well as with published research, indicating that the survey and soil sampling techniques provided a reasonably accurate representation of the weed flora and soil seed populations. Weeds found growing at the sites were primarily summer annuals at the vegetable site, and a mix of summer and winter annuals, biennials, and perennials at the remaining sites. Annual weeds dominated the seed banks of all sites with common lambsquarters, pigweed spp., and common purslane being the most commonly found seeds. The presence of most seeds in the soil could be explained by a combination of species seed production and seed longevity characteristics and species abundance in the standing community. Interpretation of the data required students to integrate and apply lecture material and provided an excellent thinking exercise.

A method was developed and tested for in vivo assay of acetolactate synthase (ALS). The method used foliar application of 1,1-cyclopropanedicarboxylic acid (CPCA) to inhibit ketol-acid reductoisomerase, the enzyme immediately following ALS in biosynthesis of branched-chain amino acids, thereby causing accumulation of acetolactate. Since the amount of acetolactate accumulation is a function of carbon flux through ALS, quantification of acetolactate accumulation determined ALS activity. Accumulation of acetolactate in soybean leaves resulted from CPCA rates as low as 15 g/ha and occurred within 1.5 h. Accumulation rates in soybean leaflets declined with leaf age from 84 μg/h/g tissue at 3 d to 17 μg/h/g tissue at 7 d. Foliar application of CPCA also caused acetolactate accumulation in corn, grain sorghum, velvetleaf, common cocklebur, and smooth pigweed. The ability of the in vivo assay to quantify the reduction in ALS activity following applications of ALS-inhibiting herbicides was validated by comparing ALS activity following thifensulfuron application to ‘Williams 82’ soybean, which has a sulfonylurea-sensitive ALS, and ‘Asgrow 3200’ soybean, which has a sulfonylurea-insensitive ALS. Thifensulfuron reduced ALS activity in Williams 82 soybean to 0, 0.8, 3.3, and 15.6% of the CPCA control at 6, 12, 24, and 48 HAT, but ALS activity in Asgrow 3200 soybean was reduced only to 34, 40, 57, and 88% of the CPCA control.

The pH of deionized water, initially adjusted to a pH of 6 to 9, all decreased to near pH 5 upon pressurization with CO2 in a backpack spray system. When deionized water contained bicarbonate (NaHCO3 or KHCO3) at 0 to 800 mg/L to buffer against a pH decrease from CO2 pressurization the return to their initial pH was more rapid than deionized water alone regardless of bicarbonate source. Acidification of spray solution following CO2 pressurization of 138, 276, or 414 kPa was similar and bicarbonate had a greater effect than spray pressure. Addition of acidifying buffer and bicarbonate to the solution resulted in an initial and post-CO2 pressurization pH range of less than one pH unit. All spray solution pH levels returned to near their initial pH following depressurization, indicating that the pH decrease was not permanent. Rate of recovery from acidification with CO2 increased with initial pH and bicarbonate concentration.

Field and greenhouse studies were conducted to evaluate selected PRE-applied herbicides for sprangletop control. In greenhouse studies, oxadiazon and dithiopyr provided excellent (> 89%) red sprangletop (L. filiformis) control. Pendimethalin and metolachlor + atrazine provided good (80–89%) to excellent control for 6 mo and 2 mo during studies 1 and 2, respectively. Isoxaben and atrazine provided poor (< 70%) control during both greenhouse studies. In field studies, good to excellent bearded sprangletop (L. fascicularis) control followed dithiopyr, pendimethalin, metolachlor, and metolachlor + atrazine treatments. Dithiopyr at 0.8 kg ai/ha provided best (> 95%) sprangletop control throughout the 6-mo testing period. Under field conditions, control was inconsistent following oxadiazon with good to excellent control during one study and poor control in another study. Better or equal control than metolachlor + atrazine followed dithiopyr, pendimethalin, and metolachlor alone treatments. Oxadiazon also provided similar or better control than metolachlor+atrazine in three of four studies. Over all studies, dithiopyr provided best sprangletop control, followed by metolachlor, metolachlor + atrazine, pendimethalin, and oxadiazon. Isoxaben and atrazine treatments provided poorest or inconsistent sprangletop control.

In field experiments conducted near Marianna, Trenton, and Archer, FL in 1990 and 1991, nicosulfuron applied 55 weeks after planting (WAP) at 18, 27, 36, or 54 g ai/ha injured peanut initially. When applied 7 WAP, only the two higher nicosulfuron rates injured peanut more than 10%. By 6 wk after application, crop injury was less than 8% for all treatments, except the highest (54 g/ha) rate. Nicosulfuron applied 5 or 7 WAP at 54 g/ha reduced peanut yield and sound mature kernels.

Field research was conducted to determine the effectiveness of DPX-79406 (a 1:1 mixture of nicosulfuron and rimsulfuron) for green foxtail control in field corn. Green foxtail control was similar when DPX-79406 was applied postemergence compared to preplant incorporated EPTC/dichlormid or metolachlor. DPX-79406 gave similar green foxtail control to that of cyanazine and better control than inter-row cultivation following soil-applied herbicides. Green foxtail control was greatest when DPX-79406 was applied at the one- to two-tiller stage compared to the one- to two-leaf stage, suggesting that green foxtail is more susceptible to DPX-79406 at later growth stages. DPX-79406 injured ‘Pioneer 3995’ corn in all trials but injured ‘Pride K020’ corn in only one treatment. DPX-79406 between 15 and 25 g ai/ha gave 85% control of green foxtail with minimal corn injury. Adjuvants tended to increase both corn injury and green foxtail control with Scoil and Merge increasing DPX-79406 activity the most. DPX-79406 provides an effective postemergence alternative for green foxtail control in field corn, for either preplant incorporated herbicides or postemergence cyanazine.

R230, an isolate of Rhizoctonia solani anastomosis group 5 from leafy spurge, caused root and crown rots on leafy spurge when inoculated via soil in greenhouse experiments. The fungus grew through soil over a wide range of matric potentials (–0.01 to −1 MPa). When applied to leafy spurge foliage, the fungus blighted young adventitious shoots, but not mature stems. Foliar infection required high relative humidity. Mycelial growth on plant surfaces stopped at below 92% relative humidity. Pathogenicity of R230 was not limited to leafy spurge. The fungus caused stem and foliar lesions on all crops tested (alfalfa, smooth bromegrass, corn, Kentucky bluegrass, soybean, tall fescue, and wheat) when plants were kept under high humidity. It reduced seedling emergence and survival in soybean, Kentucky bluegrass, and tall fescue. R230 also caused a foliar blight on spotted spurge. The potential for R230 to damage crops will limit application of the fungus to noncultivated areas and to sites where it is endemic.

Two furrow irrigated field experiments were conducted for two years at the Research and Extension Center, Powell, WY to determine the influence of various mixed densities and durations of wild oat and wild mustard interference in sugarbeet. Sugarbeet root yields were reduced by competition from all examined densities of wild oat and wild mustard, alone and in combination. Root yield reduction was less than additive with mixed densities of wild oat and wild mustard. Root yields decreased as the duration of interference after sugarbeet emergence from a mixed density of wild oat and wild mustard increased. Sucrose content of sugarbeet was not altered by competition. Based on regression analysis, the minimum time that a mixed density of 0.8 wild mustard and 1 wild oat/m of row can interfere with sugarbeet before causing an economic root yield loss was approximately 1.6 weeks after sugarbeet emergence.

Field studies were conducted in 1990 and 1991 to evaluate johnsongrass control and interference in corn using combinations of nicosulfuron, EPTC + dichlormid, and cultivation. Nicosulfuron applied to 30-cm-tall rhizome johnsongrass provided greater control and corn yield than a later application to 60-cm-tall johnsongrass. Johnsongrass control and corn yield within the later nicosulfuron treatment were improved by a preplant application of EPTC. EPTC and/or cultivation alone provided less johnsongrass control and lower corn yield than nicosulfuron or handweeding.

Sixteen sweet corn cultivars, representing a range of endosperm genotypes, were evaluated to determine cultivar tolerance to nicosulfuron plus rimsulfuron in greenhouse and field studies conducted over a two-year period. Response to nicosulfuron plus rimsulfuron varied widely, depending on cultivar and application rate. In the field, four cultivars were tolerant to applications of nicosulfuron plus rimsulfuron at rates of 50 g/ha. In the greenhouse, eight cultivars were tolerant to nicosulfuron plus rimsulfuron at 25 g/ha. ‘Merit’ and ‘Silver Extra Sweet’ were most sensitive, with applications of nicosulfuron plus rimsulfuron resulting in death of all plants. ‘Miracle,’ ‘Extra Early Super Sweet,’ ‘Bunker Hill,’ and ‘Sweet Belle’ were tolerant.

Corn (maize) genotypes which varied in days to maturity were evaluated for level of Striga hermonthica parasitism in western Kenya during three seasons. Fifteen genotypes, with a wide range in days to maturity, were grown in fields heavily infested with S. hermonthica. Early and late-season Striga numbers and corn grain yield were used to evaluate Striga parasitism. There was considerable variation in the timing and amount of Striga attachment among seasons. Days to silking and late-season Striga numbers were correlated (P ≤ 0.05) in all experiments although only weakly in one of the experiments. Grain yield was negatively correlated with days to silking in two of three experiments as were yield and early season Striga numbers. Although earlier-flowering genotypes consistently supported less Striga, they did not always yield more than the later-maturing genotypes due to confounding effects of yield potential and adaptation. There was considerable variation in Striga parasitism among genotypes within maturity groups, and the relative response of genotypes varied between seasons. These data suggest that in some seasons early maturing genotypes have potential to reduce Striga attack in heavily infested areas and yield more than later-maturing genotypes.

Field studies were conducted to determine rhizomatous johnsongrass and barnyardgrass control with clethodim, quizalofop-P-ethyl, fluazifop-P, sethoxydim, fenoxaprop-ethyl, and quizalofop-P-tefuryl applied alone and with lactofen, imazaquin, chlorimuron, and fomesafen. Graminicides applied alone controlled johnsongrass and barnyardgrass 83 to 99%. Of the graminicides evaluated, clethodim was the most susceptible to decreased grass control in mixture with broadleaf herbicides. Imazaquin and chlorimuron were most antagonistic of the broadleaf herbicides toward the activity of graminicides. Clethodim mixed with imazaquin reduced johnsongrass control as much as 64% and mixed with chlorimuron reduced barnyardgrass control as much as 52%. Quizalofop-P-tefuryl was least affected by broadleaf herbicides and fomesafen was least antagonistic in mixture with graminicides.

Field experiments were conducted to evaluate the hypothesis that johnsongrass control in corn causes increased maize dwarf mosaic virus (MDMV) and maize chlorotic dwarf virus (MCDV) disease severity because of increased movement of insect vectors from dying johnsongrass to the corn crop. Johnsongrass control treatments included 1) broadcast POST nicosulfuron, 2) directed POST imazethapyr, 3) mechanical control, and 4) no treatment. Disease severity in both a virus-susceptible and a virus-tolerant corn hybrid was evaluated. With the virus-susceptible hybrid, greater disease severity was observed where johnsongrass was controlled in the experimental area than where johnsongrass was not controlled. Increases in disease severity were independent of the method of johnsongrass control. Corollary studies conducted on the same site verified a double infection of corn with MDMV and MCDV and documented movement of blackfaced leafhoppers, the insect vector of MCDV, subsequent to treatment of johnsongrass.

Applications of both terbufos and nicosulfuron can result in a synergistic interaction that injures corn. Field studies were conducted at two Illinois locations in 1991 to determine the effect of insecticide formulation and application method on the serverity of this interaction. Terbufos, applied to the soil at planting, interacted with nicosulfuron applied postemergence to injure corn and reduce grain yield. In-furrow-applied terbufos caused a more injurious interaction to corn than terbufos placed in a surface band; the 15G formulation was more injurious than the 20CR formulation. Simulated rainfall timing experiments were conducted in 1992 and 1993 by treating corn with terbufos 15G and terbufos 20CR and applying rainfall (3.8 cm) 1, 7, or 14 d before nicosulfuron application. No visual injury or yield reductions were noted in corn treated with nicosulfuron alone. Terbufos 15G caused greater corn injury than did terbufos 20CR at each rainfall timing when sprayed with nicosulfuron. Corn injury and subsequent grain yield that can result from the interaction between nicosulfuron and terbufos was greater when simulated rainfall occurred 1 and 7 d before nicosulfuron application than when rainfall occurred 14 d before application.

A synthetic data set was created by incorporating results from previously published papers on antagonistic and synergistic herbicide interactions between two herbicides applied as a tank mixture or sequentially, and then analyzed on the basis of various properties of the herbicides and target plants. Generally, interactions between herbicides were antagonistic more frequently than synergistic. This trend held no matter whether the interacting herbicides were absorbed by the same or different parts of the plant, had the same or different translocating abilities, had the same or different modes of action, and regardless of whether the target plants were annual or perennial plants, or crops or weeds. Antagonistic interactions occurred much more frequently when the target plants were monocot than dicot, and in the Compositae, Gramineae, or Leguminosae than in the Chenopodiaceae or Convolvulaceae families.

Field experiments were conducted from 1986 to 1988 at Lacombe and Lethbridge, Alberta and Scott, Saskatchewan to determine growth and yield response of canola to mixtures of ethametsulfuron with specific grass herbicides. Ethametsulfuron did not usually cause canola injury when mixed with sethoxydim. However, ethametsulfuron mixtures with the following grass herbicides listed in decreasing order of injury potential, often caused canola injury and yield loss: haloxyfop > fluazifop > fluazifop-P > quizalofop > quizalofop-P. Canola yield losses were severe in some experiments, ranging from 59% with quizalofop mixtures to 97% with haloxyfop mixtures; in other experiments, the same mixtures did not cause significant yield losses. ‘Tobin,’ a Brassica rapa cultivar, tended to be more susceptible to injury than the B. napus cultivars ‘Pivot’ and ‘Westar.’ Canola injury symptoms were consistent with those expected from sulfonylurea herbicides. Therefore, we suggest that specific grass herbicides differentially impair the ability of canola to metabolize ethametsulfuron to inactive forms.

Increased crop densities and postplant tillage were evaluated as nonchemical methods to supplement metribuzin for improved broadleaf weed control in dry pea and lentil. The effects of 50, 100, and 150% of recommended 220 kg/ha pea and 67 kg/ha lentil seeding rates and two dates of rotary hoeing and harrowing on pea, lentil, and broadleaf weeds were studied with and without metribuzin for two years. Under favorable growing conditions, crop competition gave 72 and 99% weed control in pea and 33 and 70% weed control in lentil with the 50 and 150% seeding rates. Under less favorable conditions, control was 21 to 39% with the low and high pea and lentil seeding rates. At recommended seeding rates, metribuzin gave greater than 90% control in either crop or year. Postplant tillage 12 to 27 d after planting slightly reduced crop densities in three tillage treatments in one year, but not the second. Postplant tillage did not affect crop yield or improve weed control. In all studies, pea was similar to or more competitive than lentil in suppressing broadleaf weeds. Because neither non-chemical practice significantly improves weed control, changes are not recommended for weed management in pea and lentil.

The potential reduction in surface runoff loss of band-applied herbicides was studied in a rainfall simulation. Concentration and loss of atrazine, metribuzin, and metolachlor applied in a band 25, 50, or 75 cm wide on 1-m2 plots graded to < 0.5% slope were compared with that from a broadcast preemergence application. Herbicide concentration in runoff decreased with time after initiation of runoff and as application band width decreased. Band width reduced herbicide concentration in runoff to a greater extent than time after initiation of runoff. The 50-cm band treatment reduced atrazine and metribuzin loss 69 and 71%, respectively, from the broadcast treatment. Forty-nine percent less metolachlor was lost from the 50-cm band than from the broadcast treatment. Herbicide concentration or loss in surface runoff did not differ between the 50- and 25-cm band width treatments. Herbicide lost in runoff ranged from 4 to 7% of application with the smaller loss associated with narrower band widths. The greater reduction in atrazine and metribuzin loss in surface runoff than that associated with the reduction in input alone may be attributed to leaching or adsorption of herbicide by soil in the nonsprayed strip as runoff flowed from the treated to the nontreated areas. Band application of herbicides may provide an effective, consistent means to reduce herbicide loss in surface runoff.

Field experiments were conducted from 1989 to 1992 to determine whether no-tillage corn grown in 38-cm rows and a 2× population could improve weed control relative to 76-cm rows and 1× population under reduced-herbicide options. A standard treatment including 1.12 kg ai/ha of atrazine plus 2.24 kg ai/ha of metolachlor was compared with a treatment including the same herbicides applied at 25% of the standard rates. Both treatments included 0.56 kg ai/ha of paraquat which controlled annual weeds established at the time of application. Weed control was less in the 25%-herbicide treatment than in the standard treatment in two of four years when corn was grown in 76-cm rows with a l× population. The 25%-herbicide treatment provided weed control and grain yields similar to the standard treatment in each year when corn was grown in 38-cm rows with a 2× population. Weed control was poor and yield was reduced when no herbicides were applied regardless of row spacing or population. The leaf canopy of corn in the 38-cm row/2×-population treatment reduced light transmittance 1 wk earlier than corn in the 76-cm row/1×-population treatment.