Progression of crenate broomrape population density (PD, number of emerged plants m-2) in faba bean was studied over 8 yr in Spain. Spatial dispersion and effect of climatic conditions on parasite population growth rate (PGR) also were studied. With repeated cropping of faba bean, infestations of crenate broomrape increased from an initial PD of 0.15 to an average of 26, with a maximum of about 40 to 45. The average population growth rate (PGR, ratio between the PD of any 2 consecutive years) was approximately 3. However, this figure varied widely among localities and years, from 0.8 to 7.7. A highly significant relationship (P = 0.01) was found between PGR and rainfall and soil temperatures during December to February, months of crop vegetative growth. Dispersion of crenate broomrape infestations mainly followed direction of crop rows, most likely due to the effect of tillage and harvesting operations, which were the same direction as sowing.

Norflurazon [4-chloro-5-(methylamino)-2-(3-(trifluoromethyl)phenyl)-3(2H)-pyridazinone] persistence in five Georgia soils after application to the same plots in 1982 and 1983 was characterized by rapid initial degradation followed by slow loss of the herbicide. The rate of norflurazon loss at each location for each year was not affected by application rate (1.7 or 3.4 kg ai/ha). Relative rates of norflurazon loss were Dothan loamy sand ≥ Greenville sandy clay loam ≥ Rome gravelly clay loam = Appling coarse sanely loam > Bradson clay loam. The rate of dissipation was slower in 1983 than 1982 in the Greenville and Appling soils. Cool and/or dry environmental conditions combined with higher soil organic matter content caused slower herbicide loss. Norflurazon residue 1 yr after treatment in all soils was greater in 1983 than in 1982. Significant injury to grain sorghum [Sorghum bicolor (L.) Moench. ‘BR 64’) was observed in the spring of 1984 at all locations in all plots that were treated with norflurazon for the previous two seasons. The degree of injury corresponded to the concentration of norflurazon detected at that sampling date. Leaching did not appear to be an important method of norflurazon loss.

Differential sensitivity to paraquat was observed between cucumber cultivars and leaf age. Physiological responses to paraquat, including antioxidative enzyme activity, were investigated in leaf age classes of cucumber to identify mechanisms of paraquat tolerance. Leaf injury for ‘Naeseosamcheok’, ‘Daehandadagi’, ‘Baekgwangdadagi’, ‘Sangrokheukjinju’, and ‘Eunseongbaekdadagi’ cultivars was less than that of six other cultivars tested, averaged over leaf age and herbicide rate. The level of foliar injury caused by paraquat was Leaf 1 > 2 > 3 > 4 in seven of 11 cultivars used, where 4 was the youngest leaf. There was a positive correlation between leaf age and its relative susceptibility to paraquat, regardless of growth stage. Lipid peroxidation was less in the youngest leaf (Leaf 4) than in the older leaves at all herbicide concentrations. The youngest leaf had higher values for apparent photosynthesis than the oldest leaf. Differential leaf response to paraquat was partially correlated with the change in superoxide dismutase, ascorbate peroxidant, and glutathione reductase activities in treated leaves. Enzyme activity of ascorbate peroxidase (APX) was higher in Leaf 4 than in Leaves 1, 2, or 3 in untreated plants and after exposure to paraquat. APX isozymes were more abundant in treated than in untreated leaves and produced in higher amounts in younger than in older leaves. Application of ascorbate and glutathione before paraquat treatment protected cucumber leaves from paraquat injury.

Several weed-free and johnsongrass competition periods were established and maintained in cotton to evaluate their influence on cotton yield and reproduction of johnsongrass. When johnsongrass was sprayed with fluazifop in field plots 3, 6, 9, or 12 weeks after cotton emergence, only plots sprayed at 3 weeks yielded as much cotton as weed-free plots. Johnsongrass permitted to compete for 6, 9, 12, and 25 weeks reduced cotton yields 20, 60, 80, and 90%, respectively. When johnsongrass was transplanted into weed-free plots 3, 6, 9, and 12 weeks after cotton emergence, a weed-free period of 9 weeks was required to prevent significant cotton yield reduction. Cotton in plots weed free for 3 and 6 weeks yielded 81 and 89% as much seed cotton, respectively, as weed-free controls. Johnsongrass competing with cotton for 9 to 12 weeks produced 20 and 40% as many seeds and rhizomes, respectively, as weedy control plots. Some seeds and rhizomes were produced (1% of weedy check plots) in plots weed free for 9 to 12 weeks. Cotton grades were reduced in plots weed free for 3 and 6 weeks, and in plots where johnsongrass competed for 9 to 25 weeks.

Fall-applied herbicides were evaluated in field studies conducted in 1978 through 1980 in western Nebraska for selective weed control in nonirrigated, established alfalfa (Medicago sativa L. ‘Ranger’). All herbicides effectively controlled downy brome (Bromus tectorum L.). Broadleaf weeds were differentially controlled by the herbicides tested. All herbicide treatments increased forage production and protein content over that produced in the untreated check. Increasing the rate of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] and simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] above amounts needed for effective weed control caused an increase in the protein content of alfalfa forage above that resulting from normal use rates.

Common and giant ragweed are important weeds of soybeans in Indiana. These two weeds respond differently to imazethapyr POST treatments with common ragweed demonstrating more tolerance than giant ragweed. Both plants show initial susceptibility to imazethapyr, but common ragweed can regrow 10 to 14 days following herbicide application. Laboratory studies were conducted to determine the factors that contribute to the differential control of common and giant ragweed with imazethapyr. Differential absorption was observed at 72 h, with common ragweed absorbing 52% of the applied 14C-imazethapyr and giant ragweed absorbing 39%. The absorption of radioactivity was the same for both species by 672 h. Imazethapyr exhibited both xylem and phloem mobility by translocating both acropetally and basipetally from a treated leaf in giant and common ragweed. A higher percentage of the absorbed radioactivity accumulated in the lower foliage and roots of giant ragweed than common ragweed by 336 h. The rate of imazethapyr metabolism in common ragweed was greater than in giant ragweed. At 336 h, 81 and 68% of the identified radioactivity in the treated leaf was imazethapyr metabolites in common and giant ragweed, respectively. A higher level of the inactive glucose conjugate metabolite was found in the lower plant and root tissues of common ragweed than in giant ragweed. The differential control of common and giant ragweed with foliar applications of imazethapyr was attributed to differences in both translocation and metabolism.

This research examined the effect of mechanical soil disturbance (none or tilled) and legume crop residues (none or hairy vetch) on fluometuron dissipation for 2 yr from the top 0 to 8 cm of soil in a 10 yr field experiment. Soil pH in the upper 0 to 8 cm was ≤ 5.6, and soil organic matter was highest in plots not-tilled and plots which had a vetch cover crop. Calculated initial half-lives of fluometuron ranged from 19 to 38 d in the 2 yr. Neither tillage nor cover crop influenced early-season fluometuron dissipation. However, there were detectable amounts of fluometuron in all treatments 1 yr after application.

Velvetleaf was treated at the three and one-half leaf stage with one, three, or nine drops (1 μl each) of glyphosate and an amphoteric surfactant with the concentration of the droplets adjusted so that the total amount of active ingredient applied to each plant was constant. The phytotoxicity of glyphosate to velvetleaf was dependent on both the concentration of glyphosate and surfactant within the droplet A single concentrated droplet of glyphosate and surfactant was more effective in reducing the growth of velvetleaf than were more dilute droplets in greater number but with the same total quantity of herbicide. If the surfactant concentration was increased, the loss in efficacy was negated, provided the glyphosate concentration in the droplet was sufficiently high. Also, loss in efficiency was negated at increased glyphosate concentration and diluted surfactant level per droplet. The phytotoxic response of velvetleaf treated with one or three drops was greater at 45 than 90% relative humidity. This greater response at low relative humidity continued with increased surfactant concentration in each drop. However, the phytotoxic response was greater at 90 than 45% relative humidity when velvetleaf plants were treated with nine drops. When the surfactant concentration in each drop was increased, the plant's response at the two relative humidity levels did not differ.

Three herbaceous regimes were established, using herbicides, to examine the effects of interference on growth and biomass partitioning in loblolly pine (Pinus taeda L.). Trees were sampled near Auburn and Tallassee, AL. Trees at the Auburn site grown with low weed interference (LWI) had 4, 10, 10, 8, and 4 times greater total aboveground biomass than did trees with high weed interference (HWI) for ages one through five, respectively. Medium weed interference (MWI, Auburn site only) resulted in three times greater biomass the first 4 yr and two times greater total biomass by the fifth year compared to trees grown with HWI. Trees growing with LWI were 5, 8, 10, and 6 times larger than those with HWI for ages one through four, respectively, at the Tallassee site. At all levels of interference, the percentage of total biomass in foliage decreased, and stem and branch components increased, with increasing tree size at both sites. Trees growing with HWI had a lower percentage of total biomass in foliage and a greater percentage of total biomass in stem than those growing with LWI when compared over a common size. Growth efficiency per tree, expressed as annual increase in stem biomass per unit leaf area (g m−2), was slightly greater for trees growing with LWI compared to HWI when leaf area index (LAI3, total surface) was less than 0.2. For LAI values greater than 0.2 the relationship was reversed. The latter contradicts the idea that growth efficiency can be used as a measure of vigor for young loblolly pine. Changes in carbon partitioning to the development of leaf area are suggested to be driving the accelerated growth responses associated with a reduction of weed interference.

Weeds generally occur in patches in production fields. Are these patches spatially and temporally stable? Do management recommendations change on the basis of these data? The population density and location of annual grass weeds and common ragweed were examined in a 65-ha corn/soybean production field from 1995 to 2004. Yearly treatment recommendations were developed from field means, medians, and kriging grid cell densities, using the hyperbolic yield loss (YL) equation and published incremental YL values (I), maximum YL values (A), and YL limits of 5, 10, or 15%. Mean plant densities ranged from 12 to 131 annual grasses m−2 and < 1 to 37 common ragweed m−2. Median weed densities ranged from 0 to 40 annual grasses m−2 and were 0 for common ragweed. The grass I values used to estimate corn YL were 0.1 and 2% and treatment was recommended in only 1 yr when the high I value and either the mean or median density was used. The grass I values used for soybean were 0.7 and 10% and estimated YL was over 10% all years, regardless of I value. The common ragweed I values were 4.5 and 6% for corn and 5.1 and 15.6% for soybean. On the basis of mean densities, fieldwide treatment would have been recommended in 6 of 9 yr but in no years when the median density was used. Recommendations on the basis of grid cell weed density and kriging ranged from > 80% of the field treated for grass weeds in 3 of 4 yr in soybean to < 20% of the field treated for common ragweed in 2002 and 2004 (corn). Grass patches were more stable in time, space, and density than common ragweed patches. Population densities and spatial distribution generally were variable enough so that site-specific information within this field would improve weed management decisions.

Research was conducted to determine whether resistance to glyphosate among Palmer amaranth (Amaranthus palmeri S. Watson) populations within the U.S. state of Arkansas was due solely to increased EPSPS gene copy number and whether gene copy number is correlated with resistance level to glyphosate. One hundred and fifteen A. palmeri accessions were treated with 840 g ae ha−1 glyphosate. Twenty of these accessions, selected to represent a broad range of responses to glyphosate, underwent further testing. Seven of the accessions were controlled with this dose; the rest were resistant. The effective dose to cause 50% injury (ED50) for susceptible accessions ranged from 28 to 207 g ha−1. The glyphosate-resistant (GR) accessions had ED50 values ranging from 494 to 1,355 g ha−1, a 3- to 48-fold resistance level compared with the susceptible standard (SS). The 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene relative copy number was determined for 20 accessions, 4 plants accession−1. Resistant plants from five GR accessions (38% of resistant plants tested) did not have increased EPSPS gene copies. Resistant plants from the remaining eight GR accessions (62% of resistant plants tested) had 19 to 224 more EPSPS gene copies than the SS. Among the accessions tested, injury declined 4% with every additional EPSPS copy. ED50 values were directly correlated with EPSPS copy number. The highly resistant accession MIS11-B had an ED50 of 1,355 g ha−1 and 150 gene copies. Partial sequences of EPSPS from GR accessions without EPSPS amplification did not contain any of the known resistance-conferring mutations. Nearly 40% of GR accessions putatively harbor non–target site resistance mechanisms. Therefore, elevated EPSPS gene copy number is associated with glyphosate resistance among A. palmeri from Arkansas.

Glyphosate-resistant (GR) kochia is an increasing concern for growers across the U.S. Great Plains and Canadian prairies. Integrated strategies to mitigate resistance will require an improved understanding of the seed germination dynamics of GR kochia populations. Experiments were conducted to characterize the germination of GR vs. glyphosate-susceptible (GS) kochia populations under different constant (5 to 35 C) and alternating (5/10 to 30/35 C) temperatures. Seven GR and two GS populations were collected from wheat–fallow fields in northern Montana. Selected lines of GR and GS were obtained after three generations of recurrent group selection in the greenhouse. The GR-selected lines had 4.1 to 10.8 average EPSPS copies compared with a single EPSPS gene copy for the GS selected lines. Four out of seven GR selected lines had lower final germination (d parameter) and took more time to complete 50% cumulative germination (I50 values) under all constant and alternating temperatures, compared with the GS selected lines. Those GR selected lines also had a delayed germination initiation (I10 values), particularly at lower temperatures (5 to 10 C constant or 5/10 C alternating). In contrast, the final germination (d) of the other three GR selected lines did not differ from GS lines at a majority of temperatures tested. The I50 values of those GR lines were also comparable to GS lines under a majority of the temperatures. There was no significant correlation of observed percent cumulative germination and EPSPS gene copy number of selected kochia lines. The temperature-dependent dormancy and altered germination characteristics of the four GR kochia lines reflect the common selection of resistance and avoidance (glyphosate or other preseeding treatments) mechanisms. This is most likely attributed to long-term, intensive cropping practices and less diverse weed control methods, rather than a fitness cost or pleiotropic effect of multiple copies of the EPSPS gene.

Corn poppy is the most widespread broadleaf weed infesting winter cereals in Europe. Biotypes that are resistant (R) to both 2,4-D and tribenuron-methyl have evolved in recent decades, thus complicating their chemical control. In this study, field experiments at two locations over three seasons were conducted to evaluate the effects of different weed management strategies on corn poppy resistant to 2,4-D and tribenuron-methyl, including crop rotations, delayed sowing and different herbicide programs. After 3 yr, all integrated weed management (IWM) strategies reduced the initial density of corn poppy, although the most successful strategies were those which either included a suitable crop rotation (sunflower or field peas), or had a variation in the herbicide application timing (early POST or combining PRE or early POST and POST). The efficacy of IWM strategies differed between both locations, possibly due to different population dynamics and the genetic basis of herbicide resistance. Integrated management of multiple herbicide–resistant corn poppy is necessary in order to reduce selection pressure by herbicides, mitigate the evolution of new R biotypes, and reduce the weed density in highly infested fields.

Greenhouse and laboratory studies were conducted to examine the effects of site of plant exposure to glyphosate spray on efficacy, absorption, and translocation in pitted morningglory. Absorption of 14C-glyphosate in four-leaf pitted morningglory gradually increased with time from 19% at 1 h after treatment (HAT) to 44% at 192 HAT. The amount of 14C translocated with time ranged from 0.4% at 1 HAT to 25% at 192 HAT. Vining 1-m tall plants were controlled 75 to 100% when the top-, middle-, bottom one-third, or entire plant was treated with 1.38 or 2.76 kg ha−1 glyphosate, with control affected more by glyphosate rate than plant section exposed to glyphosate spray. Absorption of 14C-glyphosate at 96 HAT was similar whether it was applied to the top-, middle-, bottom one-third, or entire plant of 1-m tall pitted morningglory. The amount of 14C translocated out of the treated area (5 to 6%) did not differ whether it was applied to top-, middle-, or bottom one-third plant section. Results indicate that absorption and translocation of 14C-glyphosate in pitted morningglory was rapid and increased with time. Treating any one-third section of pitted morningglory plants was as effective as entire plant exposure, and control with glyphosate is more affected by rate than the degree of plant exposure to glyphosate.

Experiments were conducted to develop controls for cogongrass [Imperata cylindrica (L.) Beauv.] on noncrop areas. Single and multiple applications of herbicides were evaluated in field tests. All soil sterilants evaluated provided acceptable control. Single applications of TCA (trichloroacetic acid) at 56 kg/ha and dalapon (2,2-dichloro propionic acid) at 8.3 kg/ha controlled cogongrass without prolonged soil sterility. Multiple applications of amitrole (3-amino-s-triazole), paraquat (1,1′-dimethyl-4,4′-bipyridinium ion), glyphosate [N-(phosphonomethyl)glycine] and dalapon markedly reduced cogongrass stands 1 year after application.

Whenever moisture and temperature were favorable for germination, new seedlings of field dodder (Cuscuta campestris Yunck.) emerged each week for two years after seeding in the field or greenhouse. Many viable seeds probably remained in the soil, because 10% or less of the seeds produced seedlings.

Low-volatile and high-volatile esters of 2,4-dichlorophenoxyacetic acid (2,4-D) were sprayed on separate pastures at about double the usual rate. Milk from cows grazing these pastures contained from 0.01 to 0.09 ppm 2,4-D during the first 2 days after spraying and lower amounts thereafter. Residues in milk from cows put into the pastures 4 days after spraying were below 0.01 ppm, the practical limit of precision of the method used. Residues of 2,4-D, in or on forage, declined rapidly during the experiment. Almost all the 2,4-D in or on forage was hydrolyzed to the acid form in samples of forage taken within one-half hour after spraying with the butyl ester of 2,4-D, and about 75% after applying the 2-ethylhexyl ester.

Addition of surfactant (a polyoxyethylene thioether) to solutions of sodium 2,2-dichloropropionate (dalapon) increased the activity of both technical and formulated grades of dalapon on Johnsongrass (Sorghum halepense (L.) Pers.). The increase in activity was influenced by the rate and formulation of herbicide and in some instances by the volume of water in which treatments were applied. Varying the volume of diluent affected control with technical dalapon to a greater extent than that with formulated dalapon probably because of differences in surfactant concentrations. Plants arising from short rhizomes were more readily controlled by dalapon than those from longer rhizomes.

Field experiments were conducted in 1991 and 1992 to evaluate differential soybean cultivar tolerance to SAN 582H and metolachlor applied PRE, as influenced by soil moisture. Eighteen d after planting, a 3× rate of SAN 582H (3.0 kg ai ha−1) reduced plant populations of ‘Wright,’ ‘Bedford,’ and ‘Stonewall’ and metolachlor did not. At leaf drop, neither herbicide reduced crop height with optimum moisture. With excessive moisture, 3× rates of SAN 582H reduced plant height of Wright, Bedford, ‘Walters,’ ‘Young,’ ‘Brim,’ and ‘Bryan,’ and metolachlor reduced height of ‘Sharkey.’ The greatest yield reductions were due to excessive moisture, regardless of herbicide. Neither herbicide reduced yields with optimum moisture nor did a 1× rate with excessive moisture. However, with excessive moisture, a 3× rate of metolachlor (5.0 kg ai ha−1) reduced yield of Bedford by 470 kg ha−1, and the 3× rate of SAN 582H reduced yield of ‘Rhodes,’ Walters, and ‘Centennial’ more than 500 kg ha−1. Overall, soybean tolerance to both herbicides was good, but if used above recommended rates and with excessive moisture, injury can occur.

Twenty-eight commercial soybean [Glycine max (L.) Merr.] cultivars of maturity classes V through VIII were evaluated for differences in response to metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] 0.125 ppm w/w in hydroponic culture. Top dry weight (TDW) of treated ‘FFR 666’ soybeans equaled that of the cultivar check and five other cultivars were not significantly different (P = 5%). ‘Semmes' was most sensitive to the herbicide with TDW 40% of cultivar check. ‘Tracy’ and ‘Coker 156’ were not significantly different (P = 5%) from Semmes. The other cultivars tested were intermediate in response to metribuzin.