Acifluorfen tolerance in eastern black nightshade somaclones was characterized in two experiments. One experiment determined the involvement of absorption, translocation, and metabolism in acifluorfen tolerance. Less than 6% of the applied 14C-acifluorfen was absorbed. There were no differences in acifluorfen absorption between susceptible and tolerant somaclones. More 14C-acifluorfen was translocated in the susceptible than the tolerant somaclones. The susceptible somaclone did not metabolize acifluorfen while some somaclones (i.e., EBN-3A) metabolized 14C-acifluorfen. A second experiment determined the tolerance of the somaclones to oxyfluorfen, diquat, and paraquat Most acifluorfen-tolerant somaclones were tolerant to oxyfluorfen but were susceptible to diquat and paraquat One somaclone, EBN-3A, was extremely tolerant to acifluorfen, paraquat, and diquat.

The effect of black and eastern black nightshade on the amount of photosynthetically active radiation (PAR) intercepted by a processing tomato canopy was studied along with the correlation between PAR and tomato growth and yield. During 1989 and 1990, black and eastern black nightshade were established at densities of 0 to 4.8 m−2 within rows of transplanted, irrigated processing tomatoes. Increasing the density of either nightshade species decreased the number of tomato fruit; however, eastern black nightshade reduced tomato yield more than black nightshade. Eastern black nightshade was taller than the tomatoes, reducing PAR reaching the top of the tomato canopy. PAR reaching the top of the tomato canopy was positively correlated with yield and negatively correlated with eastern black nightshade density. Eastern black nightshade intraspecific competition decreased both stem and berry weight. Black nightshade was never taller than tomatoes and did not affect PAR reaching the top of the crop canopy. Increasing the density of black nightshade decreased berry dry weight but increased the weight of stems and leaves.

Growth and development of red rice (strawhull) and ‘Lemont’ and ‘Newbonnet’ rice were compared in 1987 and 1988 under noncompetitive field conditions at Stuttgart, AR. Growth and developmental differences were greater between red rice and rice than between rice cultivars. Compared to rice, red rice was taller, produced more culms m−2 and aboveground dry weight, had higher leaf area indices, and had a greater flag leaf area. Also, it had lower leaf to stem ratios late in the season, had greater crop growth rate early in the season but less late in the season, and produced a lower grain weight than rice. Compared to Newbonnet, Lemont plants were shorter at 60 d after emergence or later, produced more culms m−2, had a greater leaf area index, and produced higher grain weight than Newbonnet. Also, Lemont and Newbonnet produced comparable leaf to stem ratios, crop growth rates, and flag leaf areas.

Experiments were conducted to determine the effect of triglycerides, free fatty acids (FFA), and fatty acid methyl esters (FAME) on the foliar absorption, translocation, and phytotoxicity of quizalofop. Absorption, translocation, and phytotoxicity of quizalofop in oats were greater when quizalofop was applied with FFA or FAME than with their respective triglycerides. Triglycerides and FFA generally enhanced quizalofop absorption and translocation more when they contained unsaturated than saturated fatty acids. Methylation of the fatty acids reduced differences among fatty acids, but methyl stearate and methyl linolenate enhanced absorption of quizalofop less than the other FAME for oats and yellow foxtail. Quizalofop absorption and phytotoxicity to oats were greater when applied with sunflower oil, sunflower oil FFA, and sunflower oil FAME than with the corresponding linseed oil derivatives. Emulsifier generally reduced differences between linseed oil and sunflower oil derivatives in their enhancement of absorption, translocation, and phytotoxicity of quizalofop.

Greenhouse and field experiments were conducted to evaluate clopyralid formulations and triclopyr ester alone and in mixtures for control of honey mesquite. In the greenhouse the butoxyethyl ester of triclopyr enhanced activity of the monoethanolamine salt of clopyralid when applied as mixtures for total rates of 210 to 630 g ae ha-1. Greenhouse activity of triclopyr was not enhanced with the addition of clopyralid. In the field, 1989 applications of these same formulations, mixtures, and rates were usually synergistic, with each mixture killing more plants than the combined effect of either herbicide applied alone. For example, clopyralid plus triclopyr at 140 plus 140 g ae ha-1 killed 87% of the plants. When applied alone at 140 g ha-1 neither herbicide killed >27% of the plants. Honey mesquite response to the potassium or oleylamine salt of clopyralid was similar to the monoethanolamine salt when applied alone or in mixtures with triclopyr ester. Effectiveness of these mixtures in 1990 was similar to the 1988 and 1989 applications except that two to three times more herbicide was required for high mortality in 1990 because of unfavorable environment and plant growth prior to treatment.

Chloroplast DNA (cpDNA) restriction fragment length polymorphisms (RFLPs) were analyzed to assess genetic variation and relatedness among selections of North American and Eurasian leafy spurge. Leafy spurge accessions from Nebraska, Montana, Russia, Italy, and Austria were evaluated. Total DNA was extracted from young leaves and digested with the restriction endonuclease, EcoRI. CpDNA fragment patterns were determined by Southern blot analysis using mung bean cpDNA probes. Colinearity between the mung bean and leafy spurge chloroplast genomes was indicated by the observation that common fragments were hybridized by adjacent probes. Minimum estimates of chloroplast genome size for the five leafy spurge accessions, which ranged in kilobase size from 130 to 132, were within the size range of most terrestrial plants. Structural collinearity and reasonable estimates of chloroplast genome size provided evidence that the mung bean cpDNA library was suitable for characterizing leafy spurge cpDNA. Seven of the 13 mung bean probes hybridized to polymorphic leafy spurge cpDNA fragments. Based on number of polymorphisms unique to each Eurasian accession, the Austrian accession appeared to be most divergent followed by the Italian and Russian. The North American accessions seem to be most closely related to each other and to the Russian leafy spurge accession.

Field experiments were conducted in 1986, 1987, and 1988 to evaluate imazethapyr rate and time of application on postemergence control of 24 weed species. Contour graphs were developed that predicted imazethapyr rates required for various levels of weed control based upon weed leaf number at application. Rates below the labeled rate (70 g ha−1) provided 90% or greater control of common cocklebur, smallflower morningglory, and smooth pigweed if applied to 3 true-leaf or smaller weeds and of barnyardgrass, seedling johnsongrass, and Palmer amaranth if applied while weeds were in the cotyledon or 1 true-leaf stage. A rate of 70 g ha−1 provided 90% control of large crabgrass in the 1 true-leaf stage. Entireleaf morningglory, red rice, pitted morningglory, and velvetleaf are not susceptible enough to imazethapyr for 90% or greater control to be obtained with rates lower than 70 g ha−1 at the 1 true-leaf growth stage. These data demonstrate how control data can be used for developing effective reduced-rate herbicide recommendations based on weed leaf number.

Studies were conducted to determine the mechanism of acifluorfen tolerance within the Lycopersicon genus. Absorption of 14C-acifluorfen was not correlated with tolerance. There was a negative correlation (r = −0.57) between absorption 24 h after treatment and wax density. No other surface characteristic correlated with absorption. Less than 3% of absorbed 14C was translocated and there was no metabolism of acifluorfen. All genotypes were susceptible to paraquat, and acifluorfen-tolerant genotypes had lower levels of ascorbate than susceptible genotypes, implying that free radical protectant systems were not involved in tolerance. Genotypes varied in amounts of chlorophyll a, chlorophyll b, and total chlorophyll but the differences did not correlate to acifluorfen tolerance.

The objective of this field research was to compare relative effectiveness and profitability of alternative herbicides applied to the same plots for four consecutive years for controlling and reducing dense Canada thistle infestations in no-till spring wheat. Chlorsulfuron at 30 g ai ha−1 plus nonionic surfactant or clopyralid plus 2,4-D at 70 plus 280 g ae ha−1, respectively, applied annually for 4 yr controlled Canada thistle and was more effective for gradually reducing Canada thistle stands than 2,4-D at 560 g ae ha−1, MCPA plus bromoxynil at 280 plus 280 g ae ha−1, or tribenuron at 20 g ai ha−1. Chlorsulfuron and clopyralid plus 2,4-D also controlled Canada thistle ≥ 90% earlier (by 2 yr) than other treatments. Stochastic dominance analysis, a form of economic analysis, predicted that either chlorsulfuron or clopyralid plus 2,4-D would be preferred by farmers to the untreated check, MCPA plus bromoxynil, or 2,4-D treatments. Chlorsulfuron also would be preferred to clopyralid plus 2,4-D by risk-neutral farmers, whereas clopyralid plus 2,4-D would be preferred to chlorsulfuron by highly risk-averse farmers, those who are most likely to pick only consistently effective herbicides.

Individual droplets of 14C-2,4-D dimethylamine (DMA) were applied to oriental mustard seedlings to determine the effect of droplet size and herbicide concentration on absorption and translocation 1 or 3 d after application. Absorption of 14C-2,4-D was generally not affected by droplet size which ranged from 198 to 2760 μm. However, the percentage of absorbed 14C-2,4-D that was translocated away from the treated leaf increased as droplet size decreased. Absorption of 14C-2,4-D either increased slightly or was not affected by increased herbicide concentration, but translocation of absorbed 14C-2,4-D was reduced as herbicide concentration increased. Absorption and translocation of 14C-2,4-D DMA were reduced when applied after a drop of 21.3 mM commercially formulated 2,4-D had been applied at the same location on the leaf. These results indicate that large amounts of 2,4-D, either in the form of large droplets or a more concentrated herbicide solution, inhibit translocation of 2,4-D from oriental mustard leaves.

Shattercane interference in irrigated soybean was evaluated during 1987, 1988, and 1989 at Clay Center, NE, using ‘Rox’ forage sorghum to simulate shattercane. Soybean yield reduction did not occur if shattercane was removed by 2 wk after emergence in 1987 and 6 wk after emergence in 1988 and 1989. Shattercane interference with soybean began when shattercane height exceeded soybean height. Soybean yield was reduced up to 25% before the height differential reached 30 cm, the minimum difference required for selectively applying glyphosate with a wiper applicator. Soybean nodes per stem, pods per stem, and beans per pod decreased as duration of interference increased. A direct relationship between soybean yield loss and shattercane density fit a rectangular hyperbolic function. Yield loss per shattercane plant was highest at low shattercane densities. Soybean plant height, biomass, nodes per stem, pods per stem, pods per node, and beans per pod decreased as shattercane density increased. An interference model for estimation of soybean yield and economic loss based on shattercane density was developed.

The influence of nitrogen form on bentazon uptake and toxicity to soybean cell cultures and medium pH was examined. Soybean cell growth was completely inhibited at 40 μM bentazon when cells were cultured in a Murashige and Skoog medium containing 20 mM ammonium and 38 mM nitrate. However, bentazon, up to 100 μM, did not affect the growth of cells cultured in B5 medium (Gamborg B5 medium containing 2 mM ammonium and 25 mM nitrate). Twenty-minute 14C-bentazon uptake experiments on 48- and 96-h-old cells demonstrated that 14C uptake was nitrogen-type dependent, with cells cultured in AMM medium (B5 medium containing 38 mM nitrate and 12 mM ammonium) accumulating 5 to 10 times more 14C than those in B5 or NIT media (B5 containing 50 mM nitrate). Differential bentazon uptake between ammonium and nitrate-containing media appears to be attributable to the effect of nitrogen form on pH. From an initial pH of 5.5, the pH of AMM medium decreased to 4 by 48 h. The pH of NIT or B5 medium increased to approximately 7 from 5.5. Changes in pH are likely attributable to cellular maintenance of ionic balance; NH4+ and NO3− uptake results in H+ and OH- efflux, respectively. Methylamine at 12 mM did not result in a decrease in medium pH. Bentazon, a weak acid (pKa 3.3), would be expected to penetrate the plasma-membrane faster in the molecular form at low pH. Support for a nitrogen pH theory is provided by the finding that 14C-bentazon uptake was similar for ammonium and nitrate-containing media when media were buffered.

Field research was conducted at Hancock, WI, from 1985 through 1987 to evaluate effects of conventional tillage, chisel plow, ridge tillage, and no-tillage systems on population dynamics and control of annual weed species in corn grown continuously on a loamy sand soil without irrigation. In all years of the study, green foxtail densities were greater in chisel plow and no-tillage than in the conventional tillage system, while ridge tillage had densities lower than all other tillage systems. Common lambsquarters density in the chisel plow system reached nearly 500 plants m−2 compared to less than 75 plants m−2 in the other tillage systems when averaged over years. Average redroot pigweed densities in the no-tillage and chisel plow systems were 307 and 245 plants m−2 compared to less than 25 plants m−2 in the conventional and ridge tillage systems. Horseweed was observed only in no-tillage and ridge tillage plots. Green foxtail and redroot pigweed were more difficult to control in chisel plow and no-tillage than in the conventional and ridge tillage systems with several herbicide treatments. Corn yields were not affected by tillage systems under weed-free conditions. Corn yield differences among tillage systems when the same herbicide treatment was applied appeared to be due to differences in weed control.

Barnyardgrass inflorescence length ranged from less than 2 to over 20 cm. Inflorescences less than 4 cm long were a single first-order raceme, while the largest had 20 to 50 primary branch racemes. Inflorescences longer than about 12 cm and with more than about 12 racemes showed second-order branching. Air-dried structural biomass increased from less than 2 milligrams for the smallest to over 300 milligrams for the largest inflorescences. Inflorescence length and structural biomass were positively correlated (r2 = 0.95). Floret numbers increased from about 15 for smallest inflorescences to over 2000 for largest inflorescences and were positively correlated with inflorescence length (r2 = 0.94) and with inflorescence structural biomass (r2 = 0.94). Biomass of caryopses plus aborted florets had the same relationships as those for floret number. Measurement of frequency distribution of size and determination of numbers of inflorescences per plant should provide a means to predict fecundity in barnyardgrass.

Scanning electron micrographs of spray droplet residual on wheat, sunflower, and kochia indicated that salts and various adjuvants applied with glyphosate affected deposit crystal content, thickness, and contact with the leaf surface. Spray deposits of glyphosate applied with diammonium sulfate contained distinct crystals, which related to enhanced toxicity of glyphosate applied alone, or to overcoming antagonism of glyphosate toxicity by calcium chloride, when applied to wheat. In general, glyphosate applied with antagonistic calcium chloride salt formed spray deposits that were amorphous, thick, and without crystals. Glyphosate sprayed with nonantagonistic diammonium sulfate and ammonium chloride salts had deposits with crystals and an evenly spread residue beneath the crystals. The various micrographs indicate that the antagonism of glyphosate phytotoxicity by salts may be in part from physical entrapment of glyphosate in the spray deposit. Glyphosate spray droplet residue contact with wheat, sunflower, and kochia surfaces related to observed differences in glyphosate toxicity to these species.

Laboratory studies were conducted to determine the effect of various factors on germination of dogfennel and yankeeweed. Dogfennel seed were found to be strongly photoblastic with no germination in the dark. Yankeeweed seed were moderately photoblastic, with 12% germination occurring in the absence of light Germination for both species increased in response to red light (650 nm), indicating phytochrome regulation. At the soil surface, dogfennel and yankeeweed emergence was 40 and 48%, respectively, but declined rapidly with increasing soil depths where dogfennel germination was lower than that of yankeeweed. Yankeeweed germination was 60 to 75% from 10 to 30 C while dogfennel germination was 45 to 70% between 15 to 30 C. Both species germinated over a broad range of pH (6 to 10) with the highest germination occurring at pH 8. Yankeeweed and dogfennel were moderately tolerant to water stress, but yankeeweed tolerated higher water stress than dogfennel. Both species germinated over a wide range of conditions, possibly enhancing adaptation to many diverse environments.

In eight of nine trials spanning 5 yr, relative yield of semidwarf hard red spring wheat (yield expressed as a percent of estimated weed-free yield) decreased linearly as Canada thistle shoot density increased when measured in late July to early August in the northern Great Plains. Differences between yield loss assessment (YLA) equations could not be distinguished statistically between no-tillage and chisel-plowed production systems. Multiple linear regression equations of relative wheat yield versus wheat density plus Canada thistle shoot density accounted for more variability in YLA equations than simple linear regression equations of wheat yield versus Canada thistle shoot density alone. Estimated weed-free wheat yield and negative slope (b) for yield loss assessment equations increased as cumulative growing-season (April to August) rainfall increased. Thus, relative wheat yield was decreased more by increasing Canada thistle density (slope b became more negative) in years of greater growing-season rainfall.

Under field conditions, more photoassimilate moved to roots of Canada thistle at the bolt than at the bud, flower, or postflower stages. Similarly, greater photoassimilate accumulated in roots of Canada thistle in the greenhouse at the rosette and bolt than at the flower bud stage. Growth chamber experiments indicated that environmental conditions typical of fall, and possibly early spring, favored photoassimilate movement to the root and superseded growth stage control of assimilate partitioning. Allocation of assimilate within the root was strongly influenced by growth stage, with most assimilate being utilized for growth at the rosette stage and for fructan reserves in bolt and flower bud stages.

Cotton yield response was evaluated in 1987, 1988, and 1989 when weeds were managed with preemergence fluometuron [none (0%), band (50%), or broadcast (100%) surface coverage], cultivation (none, one, two, or three times), and postdirected fluometuron + MSMA. Weed densities (primarily prickly sida, morningglories, and hemp sesbania) varied widely among years and were directly related to early season rainfall. Postdirected herbicide application or cultivation(s) had little effect on weed density. The use of a banded fluometuron application reduced weed biomass 28 to 47%. A further decrease was observed when preemergence fluometuron was increased from banded to broadcast coverage. Seed cotton yields were low with no preemergence fluometuron. Banded fluometuron and at least one cultivation had yields similar to broadcast fluometuron only. Cotton yields were related to weed density and weed biomass in a hyperbolic relationship. Low weed densities caused more yield loss per unit weed density than higher densities.

The effect of different levels of tillage and weed management on population, distribution, and germination of weed seed was evaluated in three tillage systems at Arlington and Hancock, WI, in 1989 and 1990. Over 60% of all weed seed in the top 19 cm of soil were found in the top 1 cm in no-tillage at both sites. As depth increased, concentration of weed seed declined logarithmically in no-tillage. In chisel plowing, over 30% of seed were in the top 1 cm and seed concentration decreased linearly with depth. Moldboard plowing had uniform distribution of weed seed in the top 19 cm of soil. Preemergence metolachlor plus atrazine decreased weed seed population by 50% compared with no treatment over all tillage systems. One year of the herbicide treatment plus handweeding to assure weed-free conditions did not reduce seed numbers in chisel plowing or moldboard plowing compared to herbicide alone. Seed numbers with no-tillage and weed-free conditions decreased by 40% relative to herbicide alone. Common lambsquarters germination was 40% greater in moldboard plowing and chisel plowing compared with no-tillage. Germination was highest in seed taken from 9 to 19 cm deep in moldboard plowing and from 0 to 9 cm deep in chisel plowing.