bluuebunny/crossref_metadata_2025 huggingface dataset:
Method for predicting selective uprooting by mechanical weeders from plant anchorage forces
Authors: Kurstjens Dirk A. G. , Kropff Martin J. , Perdok Udo D.
Published: July 2006

Reliable mechanical weed control requires knowledge of the achievable levels of weed control and crop damage when using certain implements in specific conditions. Quantitative methods that use weed, crop, soil, and cultivator characteristics to predict weed control and crop damage need to be developed. To that end, the relative susceptibility of weeds and crop plants to mechanical weeding and the selective ability of cultivators need to be quantified separately. The method presented in this study uses measured plant anchorage forces to quantify crop and weed sensitivity to being uprooted by a weed harrow and predicts the relationship between weed and crop uprooting by mechanical weeding. Uprooting and anchorage force of young perennial ryegrass and garden cress plants were measured in laboratory harrowing experiments on sandy soil. A nonlinear equation was introduced to describe the relationship between weed uprooting and crop uprooting. The parameters representing the selective potential of the actual crop–weed condition (Kpot) and the implement selective ability (Kcult) did not depend on crop uprooting. The relationship between potential weed and crop uprooting that could theoretically be obtained by a perfectly selective implement (i.e., pulling each plant with equal force) was calculated from plant anchorage force distributions measured before harrowing. The observed uprooting percentages achieved by harrowing were lower than the potential uprooting percentages. WithKcultaccounting for imperfect weeder selective ability, prediction accuracy was satisfactory. Field validation is required to confirm whether this method improves comparison and prediction of weeding performance of different weeding implements in different crop–weed situations.
Effect of common waterhemp (Amaranthus rudis) emergence date on growth and fecundity in soybean
Authors: Hartzler Robert G. , Battles Bruce A. , Nordby Dawn
Published: July 2006

Field experiments were conducted in central Iowa to determine the growth of common waterhemp emerging after postemergence herbicide applications in soybean. Common waterhemp survival declined as emergence was delayed in relation to soybean. Ninety percent of plants emerging at approximately the same time as soybean survived, whereas only 13% of plants emerging approximately 50 d after planting (DAP) survived to maturity. Biomass accumulation declined rapidly as emergence was delayed in relation to soybean. Delaying emergence from 14 to 28 DAP resulted in a 50 to 80% reduction in shoot biomass. Common waterhemp emerging 50 DAP produced only 1 to 10% of the biomass of plants emerging at the same time as soybean. Plants emerging with soybean produced approximately 300,000 to 2.3 million seeds plant−1depending on the location. Fecundity of common waterhemp plants was closely related to biomass accumulation and declined rapidly with delayed emergence. Although common waterhemp emerging after the V4 stage of soybean (40 DAP) are unlikely to affect crop yield because of high mortality levels and reduced growth, these plants may contribute significant seeds to the soil seed bank.
Temperature effects on germination of nineAmaranthusspecies
Authors: Steckel Lawrence E. , Sprague Christy L. , Stoller Edward W. , Wax Loyd M.
Published: July 2006

Germination of weed seed and time of emergence are greatly affected by temperature. The effects of temperature on seed germination of tumble pigweed, prostrate pigweed, smooth pigweed, Palmer amaranth, Powell amaranth, spiny amaranth, redroot pigweed, common waterhemp, and tall waterhemp were examined under constant and alternating temperature regimens at 5, 10, 15, 20, 25, 30, and 35 C. Averaged over all temperatures, alternating temperature regimens increased total germination of all species, except Powell amaranth, which germinated similarly under both constant and alternating temperatures. In addition, Powell amaranth seed exhibited the highest total germination across all temperatures compared with the other amaranth species. Prostrate pigweed seed demonstrated the lowest total germination. Optimal temperatures for maximum germination were greater than 20 C for all species, except prostrate pigweed. The alternating temperature regimen centering at 30 C was used to compare the germination rates of the nine species. Palmer amaranth and smooth pigweed attained complete germination on the first day. The rate of germination for these species was much more rapid than the otherAmaranthusspp., which took 3 to 8 d to reach 50% germination.
High temperatures and durations of exposure reduce nutsedge (Cyperusspp.) tuber viability
Authors: Webster Theodore M.
Published: July 2006

Soil solarization has been proposed as an alternative to methyl bromide for controlling nutsedges. Little is known, however, about the relationship between soil solarization and nutsedge tuber viability. Combinations of elevated temperatures and durations of exposure were evaluated for their effect on purple nutsedge and yellow nutsedge tuber viability and new tuber production in growth chamber studies. Estimates of the duration of exposure at each temperature that reduced nutsedge growth parameters 50% (TT50) were supplied by log-logistic regression analysis. Nutsedge tuber viability was reduced when temperatures were ≥ 45 C. Relative to purple nutsedge, yellow nutsedge tuber viability had smaller TT50values for 45, 50, and 55 C. Tuber viability TT50at 60 C was similar for both nutsedges. The TT50for production of new purple nutsedge tubers at 50 C was larger than that for yellow nutsedge. However, there were no differences between species in TT50values for new tuber production at higher temperatures. With sufficient durations of exposure, both purple and yellow nutsedge tubers were killed at temperatures ≥ 50 C. However, application of these data to field situations in Georgia may be limited using present technology because the soil temperature cannot be raised to high enough levels for acceptable solarization effects.
Mechanism for the enhanced effect of mowing followed by glyphosate application to resprouts of perennial pepperweed (Lepidium latifolium)
Authors: Renz Mark J. , DiTomaso Joseph M.
Published: July 2006

Herbicides currently registered for use near water have been ineffective for control of perennial pepperweed. Previous research has demonstrated that mowing followed by an application of glyphosate at 3.33 kg ae ha−1to resprouting tissue can enhance the control of perennial pepperweed. The objectives of this study were to determine the mechanism(s) responsible for the enhanced effectiveness of glyphosate in combination with mowing. Mowing plants altered the leaf area distribution within the canopy. In mowed areas, the majority of leaf area was in the basal third of the canopy, whereas the bulk of the leaf area was in the top third of the canopy in unmowed plots. This change in plant architecture affected the deposition pattern of the spray solution. Unmowed plants retained 49 to 98% and 42 to 83% of a dye solution within the middle and top thirds of the canopy at the Colusa and Woodland sites, respectively, with only 1.9 to 6.0% dye deposited on the basal third of the canopy at both sites. In contrast, mowed plants had 18 to 34% and 26 to 70% of the dye retained in the basal third of the canopy at the Colusa and Woodland sites, respectively. Greenhouse studies showed that14C-glyphosate applied to basal leaves of mowed plants translocated significantly more to belowground tissue. Unmowed plants accumulated 0.37% of the applied14C-glyphosate in belowground tissue 48 h after labeling. In contrast, mowed plants accumulated 6.7%14C-glyphosate in the belowground tissue. In field studies, estimates of basipetal seasonal translocation rates using total nonstructural carbohydrate pools of roots indicate that mowing did not change the translocation rate. However, the delay in application timing to allow plants to resprout appeared to synchronize applications with maximal translocation of carbohydrates to belowground structures. We hypothesize that the change in the canopy structure of perennial pepperweed after mowing results in fewer aboveground sinks and greater deposition of herbicide to basal leaves where it can preferentially be translocated to the root system. Furthermore, the delay between mowing and resprouting synchronized maximal belowground translocation rates with herbicide application timing. These factors all appear to be involved in the observed enhanced control of perennial pepperweed when combining mowing and glyphosate.
Weed suppression in a broccoli–winter rye intercropping system
Authors: Brainard D. C. , Bellinder R. R.
Published: July 2006

Interseeded cover crops have the potential to maintain and improve soil quality, reduce the incidence of insect pests, and suppress weeds in vegetable production systems. However, the successful use of interseeded cover crops has been limited by their tendency to either inadequately suppress weeds or suppress both weeds and the crop. We hypothesized that in irrigated broccoli production, winter rye could suppress annual weeds through rapid emergence and shading, without adversely affecting the taller transplanted broccoli crop. In field experiments conducted in New York from 1999–2001, broccoli was cultivated at 0, 10, or 10 and 20 d after broccoli transplanting (DAT), with or without rye at the final cultivation. Rye interseeded at 0 DAT suppressed weeds and improved yields relative to unweeded controls but resulted in broccoli yield losses relative to weed-free controls in 2 of 3 years. Rye seeded at either 10 or 20 DAT did not reduce broccoli yields but had little effect on weeds for a given level of cultivation and resulted in Powell amaranth seed production of up to 28,000 seeds m−2. Rye interseeded at 0 DAT reduced light availability to weeds in 2000 but not in 2001 when Powell amaranth avoided shading from rye through rapid emergence and vertical growth. In greenhouse pot experiments, low temperatures for 7 d after seeding delayed the emergence of Powell amaranth by 3 d relative to rye and increased the suppression of Powell amaranth by rye from 61 to 85%. Our results suggest that winter rye may be more successfully integrated into broccoli production (1) when sown at higher densities, (2) in locations or seasons (e.g., spring) with lower initial temperatures, and (3) in combination with other weed management tools.
Gene flow from imidazolinone-resistant domesticated sunflower to wild relatives
Authors: Massinga Rafael A. , Al-Khatib Kassim , St. Amand Paul , Miller Jerry F.
Published: July 2006

Gene flow from imidazolinone (IMI)-resistant domestic sunflower to IMI-susceptible common sunflower and prairie sunflower was studied. Under greenhouse conditions, pollen from IMI-resistant domesticated sunflower was applied to flower heads of IMI-susceptible common and prairie sunflower. In addition, field studies were conducted in 2000 and 2001 near Manhattan, KS, to evaluate IMI-resistant gene flow from IMI-resistant domesticated sunflower to common and prairie sunflower under natural conditions. Common and prairie sunflower were planted in concentric circles at distances of 2.5, 5, 15, and 30 m around a densely planted IMI-resistant domesticated sunflower species. For both greenhouse and field studies, IMI-resistant gene flow was determined by treating the progeny of both wild species with 40 g ai ha−1of imazamox. Greenhouse crosses made by hand showed that 94% of common sunflower and 79% of prairie sunflower were resistant or moderately resistant. The resistant plants were allowed to grow in the greenhouse and were backcrossed with the corresponding susceptible wild parents. Progeny of the backcross showed a 1:1 ratio of resistant to susceptible plants. In the field, gene flow was detected up to 30 m from the pollen source for both species, and it decreased as distance from the pollen source increased. In 2000, 11 to 22% of the progeny were resistant at 2.5 m from the pollen source and 0.3 to 5% were resistant at 30 m. In 2001, the number of resistant progeny did not exceed 7 and 2% at 2.5 and 30 m from the pollen source, respectively. The results of this study showed that IMI-resistant domesticated sunflower outcrosses with common and prairie sunflower over distances typically encountered near production fields. Also, backcrosses of resistant hybrids with wild parents are successful, further increasing the potential for the spread of IMI-resistant feral sunflowers.
Pyricularia setariae: a potential bioherbicide agent for control of green foxtail (Setaria viridis)
Authors: Peng Gary , Byer K. N. , Bailey K. L.
Published: July 2006

One hundred and thirty-three fungal isolates, pathogenic to green foxtail, were evaluated for weed control potential under controlled conditions. To determine weed control efficacy, these pathogens were applied as spore or mycelial suspensions at approximately 105propagules ml−1to green foxtail at the three-leaf stage. One week after inoculation, most isolates caused only minor injury to the plants, but 15 isolates caused 50 to 100% disease. Among the most efficacious isolates, only those ofPyricularia setariaeexhibited strong host specificity to the target weed, revealing no significant pathogenicity on 28 other plant species tested, including many important crops such as wheat, barley, and oat. On green foxtail leaves, conidia of this fungus germinated readily at 14, 20, and 26 C, but the process of germination and appressorial formation was more rapid at the higher temperatures. The fungus applied at the concentration of 105spores ml−1reduced weed fresh weight by 34% 7 d after the treatment when compared with controls, whereas a concentration of 107spores ml−1reduced fresh weight by 87%. This efficacy was comparable with that of the herbicide sethoxydim. When applied to the weed at the one- to four-leaf stages, the fungus reduced green foxtail fresh weight by more than 80%. Efficacy was slightly lower on plants at the five-leaf stage or older. On the green foxtail biotype resistant to the herbicide sethoxydim,P. setariaecaused 80% fresh weight reduction compared with untreated controls, as opposed to 17% achieved with the herbicide. At 20 C, the fungus required a minimum of 6-h dew period to initiate infection, but a 10-h dew period was needed to cause severe damage to green foxtail.
Impact of composted swine manure and tillage on common waterhemp (Amaranthus rudis) competition with soybean
Authors: Menalled Fabián D. , Liebman Matt , Buhler Douglas D.
Published: July 2006

Use of composted swine manure produced in deep-bedded hoop structures is a promising approach for recycling farm waste products and improving soil fertility, but little is known about its effects on crop–weed interactions. A 2-yr study was conducted to evaluate the effect of compost amendments and tillage on soybean–common waterhemp competition. Experiments were conducted in no-tillage and chisel plow main plots with compost applied to one of two types of subplots. Common waterhemp and soybean growth was measured in sub-subplots accommodating weed-free soybean and soybean with common waterhemp sown at soybean planting, soybean emergence (VE), soybean second-node stage (V2), and soybean sixth-node stage (V6). Soybean heights were not influenced by compost or common waterhemp sowing time. Soybean stem diameters were influenced by year, tillage regime, and an interaction between compost and common waterhemp sowing time. In contrast, common waterhemp heights and basal diameters were greater when sown at planting and VE in compost-amended subplots than in compost-free subplots. Overall, there was a negative quadratic relationship between common waterhemp biomass and soybean yield (r2= 0.746). The extremely low common waterhemp emergence in V2 and V6 treatments suggested that early-season weed suppression was sufficient to protect soybean from common waterhemp competition. The sex determination of 2,557 common waterhemp plants showed a marginally higher male to female ratio in compost-amended treatments than in compost-free treatments (P = 0.0611). A linear-slope regression indicated that common waterhemp fecundity was positively related to individual plant biomass, with a change in slope occurring at 118.7 g. Under the conditions present in this experiment, compost did not enhance soybean yield but increased the competitive ability of waterhemp. Because composted swine manure can have a major influence on competition of common waterhemp with soybean, effective weed management practices should be in place when this soil amendment is used.
Influence of photoperiod and temperature on vegetative growth and development of Florida betony (Stachys floridana)
Authors: McElroy J. Scott , Yelverton Fred H. , Neal Joseph C. , Rufty Thomas W.
Published: July 2006

Experiments were conducted in environmental chambers to the evaluate effects of photoperiod and temperature on Florida betony growth and development. Plants were exposed to two photoperiods, short day (9 h) and long day (9 + 3 h night interruption), and three day/night temperature regimes, 18/14, 22/18, and 26/22 C. After 10 wk of growth, shoot length and weight were 3.4 and 3.5 times greater, respectively, in the long-day photoperiod and with the 26 and 22 than with the 22 and 18 C day and night temperature regime, respectively. Shoot number, however, was greatest in the short-day photoperiod and at a lower temperature of 22/18 C. Shoot number in long day 22/18 C and 26/22 C environments increased asymptotically. No difference in root weight was observed between long- and short-day environments, but root weight increased with increasing temperature. Flowering and tuber production only occurred in long-day environments, with greater production of both at higher temperatures. Results provide a general framework for understanding Florida betony growth and development characteristics in the field and provide insights that should be considered in developing control strategies.
Artificial and natural seed banks differ in seedling emergence patterns
Authors: Leon Ramon G. , Owen Micheal D. K.
Published: July 2006

Artificial weed seed banks are practical for studying seed bank depletion and weed seedling emergence because the number, depth, and species composition of seed banks can be managed. However, no studies have determined whether artificial seed banks are representative of natural seed banks. We compared the emergence of velvetleaf, giant foxtail, and common waterhemp in a natural seed bank, an artificial seed bank with stratified seeds, and an artificial seed bank with nonstratified seeds. Velvetleaf seedling emergence was higher in the nonstratified seed bank in 2001, but no differences were observed in 2002. The number of viable velvetleaf seeds at the end of the experiment was lower in the natural seed bank than in the artificial seed banks in 2002. Velvetleaf emergence occurred earlier in the natural seed bank than in the artificial seed banks. Giant foxtail emergence was higher in the artificial seed banks (58 to 82%) than in the natural seed bank (5 to 23%). Common waterhemp emergence ranged from 7 to 65% in the artificial seed banks and from 1 to 5% in the natural seed bank. In general, the distribution of emergence with time differed in the natural seed bank compared with the artificial seed banks. These differences were attributed to differences in soil temperature and soil bulk density between the natural and artificial seed banks. Artificial seed banks showed lower soil bulk density and greater temperature fluctuation than the natural seed bank. However, there was no consistent relationship between growing degree days and emergence timing in the three treatments for any of the species studied.
Biennial wormwood (Artemisia biennis) biomass allocation and seed production
Authors: Mahoney Kris J. , Kegode George O.
Published: July 2006

Biennial wormwood has become an important weed problem in the northern Great Plains, but little is known about its biology. Biennial wormwood seeds were collected from Fargo, ND, and Fergus Falls, MN, for field experiments in 1999 and 2000 to determine the influence of transplanting date on growth, biomass, and seed production. Seeds were seeded in a greenhouse every 2 wk, and seedling rosettes were transplanted to the field 2 wk after emergence from April 30 until September 15 to simulate season-long emergence. Weekly destructive subsampling started 2 wk after transplanting and ended on September 29 in both years. All seedlings that grew for at least 5 wk after transplanting produced flowers by mid- to late August of the same year. Late-transplanted seedlings with less than 5 wk of growth did not flower or survive the winter. Biennial wormwood biomass allocation patterns resemble those of an annual species, with about 15% of the total dry weight allocated to roots, 20% to stems, 25% to leaves, and 40% to flowers. Transplant date had a substantial influence on biomass partitioning. Seedlings transplanted early in the growing season produced more biomass and seed than late-season transplants. Biennial wormwood seedlings transplanted on April 30 produced over 435,000 seeds per plant, whereas seedlings transplanted on August 15 produced 500 to 3,000 seeds. Biennial wormwood was photoperiod sensitive and flowered when the day length was about 14 h or less, between August 18 and 25, in both years.
Exploratory data analysis to identify factors influencing spatial distributions of weed seed banks
Authors: Wiles L. , Brodahl M.
Published: July 2006

Comparing distributions among fields, species, and management practices will help us understand the spatial dynamics of weed seed banks, but analyzing observational data requires nontraditional statistical methods. We used cluster analysis and classification and regression tree analysis (CART) to investigate factors that influence spatial distributions of seed banks. CART is a method for developing predictive models, but it is also used to explain variation in a response variable from a set of possible explanatory variables. With cluster analysis, we identified patterns of variation with direction of the distance over which seed bank density was correlated (range of spatial dependence) with single-species seed banks in corn. Then we predicted patterns of the seed banks with CART using field and species characteristics and seed bank density as explanatory variables. Patterns differed by magnitude of variation in the range of spatial dependence (strength of anisotropy) and direction of the maximum range. Density and type of irrigation explained the most variation in pattern. Long ranges were associated with large seed banks and stronger anisotropy with furrow than center pivot irrigation. Pattern was also explained by seed size and longevity, characteristics for natural dispersal, species, soil texture, and whether the weed was a grass or broadleaf. Significance of these factors depended on density or type of irrigation, and some patterns were predicted for more than one combination of factors. Dispersal was identified as a primary process of spatial dynamics and pattern varied for seed spread by tillage, wind, or natural dispersal. However, demographic characteristics and density were more important in this research than in previous research. Impact of these factors may have been clearer because interactions were modeled. Lack of data will be the greatest obstacle to using comparative studies and CART to understand the spatial dynamics of weed seed banks.
Nitrogen and green foxtail (Setaria viridis) competition effects on corn growth and development
Authors: Cathcart R. Jason , Swanton Clarence J.
Published: July 2006

Agronomic research on the effects of nitrogen fertilizer and weed control in corn has focused primarily on maintaining or increasing yield. Few studies have examined the effect of nitrogen (N) fertilizer rate or weed competition (or both) on whole plant growth and development. The objectives of this research were to determine how N influences the growth and development of corn and to explore how green foxtail density affects this relationship. Field experiments were conducted on a sandy low organic matter soil from 1999 to 2001. The experiment was designed as a factorial with N rate ranging from 0 to 200 kg N ha−1and targeted green foxtail density ranging from 0 to 300 plants m−2. Under weed-free conditions, a higher rate of N fertilizer increased corn leaf and grain N content, leaf area index (LAI), plant height, and aboveground dry matter (DM) production, including kernel weight. However, in the presence of green foxtail, corn leaf N content, LAI, growth rate, plant height, and aboveground DM were reduced at each N level. Despite having significant main effects, there was no interaction between N rate and green foxtail density. Results indicate that in corn grown on a coarse-textured soil with low organic matter, the additional stress brought about by the presence of green foxtail exacerbated the effect of low N rates on corn growth and development. More intensive weed management may be required in corn if N fertilizer rates are reduced.
Effect of glyphosate spray droplets on leaf cytology in velvetleaf (Abutilon theophrasti)
Authors: Ryerse Jan S. , Downer Roger A. , Sammons R. Douglas , Feng Paul C. C.
Published: July 2006

Leaf cytoarchitecture was evaluated by light microscopy and scanning electron microscopy, and cell viability was monitored by fluorescence after treatment of velvetleaf with defined concentrations and droplet sizes of formulated glyphosate and blended tallowamine surfactant. In response to droplets of formulated glyphosate larger than in field sprays but useful for studying structural change, we observe that the leaf epidermis thins and flattens within 1.5 h, the epidermal, mesophyll, and vascular cells at the contact site exhibit localized cytolysis by 6 h, and cytolysis and pycnosis remain restricted to the contact site at 24 h. Using endogeneous fluorescence as a marker for nonviable cells, it was determined that cellular changes are directly correlated with droplet size and that the changes are minimal after exposure to spray sizes and concentrations of formulated glyphosate and blended tallowamine typically used in the field. The results show that, at field use concentrations, the effect of formulated glyphosate and blended tallowamine on leaf cytoarchitecture is modest and localized but sufficient to allow herbicide entry.
Detecting weed-free and weed-infested areas of a soybean field using near-infrared spectral data
Authors: Chang Jiyul , Clay Sharon A. , Clay David E. , Dalsted Kevin
Published: July 2006

Weed distribution maps can be developed from remotely sensed reflectance data if collected at appropriate times during the growing season. The research objectives were to determine if and when reflectance could be used to distinguish between weed-free and weed-infested (mixed species) areas in soybean and to determine the most useful wavebands to separate crop, weed, and soil reflectance differences. Treatments included no vegetation (bare soil), weed-free soybean, and weed-infested soybean and, in 1 yr, 80% corn residue cover. Reflectance was measured at several sampling times from May through September in 2001 and 2002 using a handheld multispectral radiometer equipped with band-limited optical interference filters (460 to 1,650 nm). The spatial resolution was 0.8 m2. The reflectance in the visible spectral range (460 to 700 nm) generally was similar among treatments. In the near-infrared (NIR) range (> 700 to 1,650 nm), differences among treatments were observed from soybean growth stage V-3 (about 4 wk after planting) until mid-July to early August depending on crop vigor and canopy closure (76-cm row spacing in 2001 and 19-cm row spacing in 2002). Reflectance rankings in the NIR range when treatments could be differentiated were consistent between years and, from lowest to highest reflectance, were soil < weed-free < weed-infested areas. Increased reflectance from weed-infested areas was most likely due to increased biomass and canopy cover. Residue masked differences between weed-free and weed-infested areas during the early stages of growth due to high reflectance from the residue and reduced weed numbers in these areas. These results suggest that NIR spectral reflectance collected before canopy closure can be used to distinguish weed-infested from weed-free areas.
Seed biology of jubatagrass (Cortaderia jubata)
Authors: Drewitz Jennifer J. , DiTomaso Joseph M.
Published: July 2006

Jubatagrass is one of the most invasive nonnative species along sensitive natural coastal sites of California. This study was designed to understand the biology of reproduction and seed longevity under field conditions. Jubatagrass can produce over 100,000 wind-dispersed seeds from a single inflorescence. Seeds are produced apomictically, and germination is directly related to seed size. Of the total seeds produced, only 20 to 30% were of ample size to readily germinate when exposed to light and under a temperature range similar to coastal environments. Seeds not exposed to light also germinated but at about 30% the level of light-exposed seeds. This suggests that exposed disturbed coastal sites with moderate temperatures have high potential for germination and establishment of jubatagrass. The percentages of germinable and viable seeds were not significantly different, indicating that jubatagrass does not have a primary dormancy. This was supported by field experiments demonstrating that seeds do not persist under natural conditions for more than 6 mo. These results indicate that an intensive 1-yr control program targeting established seedlings and mature plants should sufficiently manage existing populations. However, effective long-term management of jubatagrass must focus on anticipating environments susceptible to invasion, reducing new seed recruitment, and preventing subsequent seed germination and seedling establishment.
Detection of pitted morningglory (Ipomoea lacunosa) by hyperspectral remote sensing. I. Effects of tillage and cover crop residue
Authors: Koger Clifford H. , Shaw David R. , Reddy Krishna N. , Bruce Lori M.
Published: July 2006

Field experiments were conducted to evaluate the potential of hyperspectral reflectance data collected with a hand-held spectroradiometer to discriminate soybean intermixed with pitted morningglory and weed-free soybean in conventional till and no-till plots containing rye, hairy vetch, or no cover crop residue. Pitted morningglory was in the cotyledon to six-leaf growth stage. Seven 50-nm spectral bands (one ultraviolet, two visible, four near-infrared) derived from each hyperspectral reflectance measurement were used as discrimination variables. Pitted morningglory plant size had more influence on discriminant capabilities than tillage or cover crop residue systems. Across all tillage and residue systems, discrimination accuracy was 71 to 95%, depending on the size of pitted morningglory plants at the time of data acquisition. The versatility of the seven 50-nm bands was tested by using a discriminant model developed for one experiment location to test discriminant capabilities for the other experiment, with discrimination accuracy across all tillage and residue systems of 55 to 73%, depending on pitted morningglory plant size.
Small broomrape (Orobanche minor) germination and early development in response to plant species
Authors: Ross Kyle C. , Colquhoun Jed B. , Mallory-Smith Carol A.
Published: July 2006

Small broomrape, a holoparasitic weed, was recently introduced to the Pacific Northwest and contaminates a limited number of red clover fields in Oregon. Greenhouse and field studies were conducted to evaluate small broomrape response to common crop and weed species in the Pacific Northwest, and to evaluate a quick-screening method for plant species response to the parasite. In greenhouse studies, plants were grown in a hydroponic polyethylene bag system to allow for continuous visibility of their roots and monitoring of small broomrape seed germination and tubercle development. Results of the greenhouse study were validated in a field contaminated with small broomrape. In the greenhouse polyethylene bag study, small broomrape germinated and tubercles developed on alfalfa, arrowleaf clover, carrot, celery, crimson clover, lettuce, prickly lettuce, red clover, spotted catsear, subterranean clover, white clover, and wild carrot. Small broomrape germinated but did not develop tubercles when grown with barley, birdsfoot trefoil, common vetch, creeping bentgrass, cucumber, field corn, red fescue, flax, Italian ryegrass, nasturtium, oats, orchardgrass, perennial ryegrass, snap bean, sugar pea, sunflower, sweet corn, tall fescue, tomato, and wheat. In the field study, tubercles developed on alfalfa, arrowleaf clover, common vetch, crimson clover, red clover, subterranean clover, and white clover, but small broomrape flower stalks did not emerge in arrowleaf clover or crimson clover. Common vetch was a host for small broomrape in the field study but not in the hydroponic polyethylene bag system, thus, caution is warranted in using the quick screening method to elucidate potential small broomrape host species.
Soil characteristics and water potential effects on plant-available clomazone in rice
Authors: Lee Do-Jin , Senseman Scott A. , O'Barr John H. , Chandler James M. , Krutz L. Jason , McCauley Garry N. , Kuk Yong In
Published: July 2006

Clomazone has been successfully used for weed control in rice, but crop injury is a potential problem on light-textured soils. Experiments were conducted to determine the effect of soil characteristics and water potential on plant-available clomazone and rice injury. A centrifugal double-tube technique was used to determine plant-available concentration in soil solution (ACSS), total amount available in soil solution (TASS), and Kdvalues for clomazone on four soils at four water potentials. A rice bioassay was conducted parallel to the plant-available study to correlate biological availability to ACSS, TASS, and Kd. TASS was significantly different in all soils. The order of increasing TASS for the soils studied was Morey < Edna < Nada < Crowley, which correlated well with soil characteristics. The order of increasing TASS after equilibrium was − 90 < − 75 < − 33 < 0 kPa. TASS values at 0 kPa were greater than two times the TASS values at − 90 kPa. It appears that severe rice injury from clomazone on these soils could occur if TASS > 110 ng g−1and Kd< 1.1 ml g−1. We propose that the double-tube technique provides a more accurate estimate of available herbicide because the solution–soil ratios are < 0.33:1 and would be more representative of a plant root–herbicide relationship. This technique or some variation possibly could be further developed such that clomazone rates could be more clearly defined particularly on lighter-textured soils. TASS may be a better predictor of plant-available herbicide than ACSS when evaluating moderately to highly water-soluble herbicides in a nonsaturated soil environment.
Resistance to ACCase inhibitor herbicides in a green foxtail (Setaria viridis) biotype in Europe
Authors: De Prado Rafael , Osuna María Dolores , Fischer Albert J.
Published: July 2006

A biotype of green foxtail found in Spain exhibited cross-resistance among acetyl-CoA carboxylase (ACCase)–inhibiting herbicides. Field doses that totally inhibited shoot fresh weight in the susceptible (S) biotype were determined for six aryloxyphenoxypropionates (clodinafop, diclofop, fenoxaprop-P, fluazifop-P, haloxyfop-P, and propaquizafop) and six cyclohexanediones (clefoxydim, clethodim, cycloxydim, sethoxydim, tepraloxydim, and tralkoxydim). The resistant (R) biotype showed cross-resistance to all herbicides except fenoxaprop-P, propaquizafop, clefoxydim, and tepraloxydim. There were no differences in the absorption, translocation, and metabolism of [14C]diclofop between the S and R biotypes. On the basis of herbicide dose that inhibited ACCase activity by 50% (I50values), ACCase of the R biotype was 5.8-, 13.9-, 20.0-, 102.4-, 416.7-, and 625.0-fold less sensitive to clethodim, haloxyfop, diclofop, fluazifop, cycloxydim, and sethoxydim, respectively, than that of the S biotype. Two multifunctional ACCase isoforms (ACCase I and ACCase II) were purified partially and separated. ACCase II was highly resistant to diclofop acid in both biotypes, with I50values ranging between 92 and 95 μM. However, the I50values observed for ACCase I revealed that the R biotype was 30.8-fold less sensitive to diclofop than the S biotype. These results suggest the mechanism of resistance in green foxtail to diclofop relates to an altered ACCase I isoform.
Intraspecific variability of the acetolactate synthase gene
Authors: Tranel Patrick J. , Jiang Weilu , Patzoldt William L. , Wright Terry R.
Published: July 2006

Common ragweed and common cocklebur plants were collected at two sites each in Illinois, Minnesota, and Ohio to analyze intraspecific variability of the gene encoding acetolactate synthase (ALS). A 385-nucleotide fragment within the coding sequence ofALSwas compared among 24 plants of each of these two species from the six locations. Common ragweedALSwas highly variable, with polymorphisms observed at 48 (12.5%) of the 385 nucleotides among the 24 plants. Despite the numerous nucleotide polymorphisms, only two inferred amino acid polymorphisms were identified. No apparent population structure was suggested by theALSsequence data, indicating widespread gene flow consistent with the wind-pollinated nature of common ragweed. In contrast to common ragweed, noALSpolymorphisms were identified among the common cocklebur plants used in this study. As a basis for comparing the extremes observed between common ragweed and common cocklebur,ALSintraspecific variability also was investigated in 10 plants each of tall waterhemp and smooth pigweed. Normalized to the number of plants analyzed, the number of nucleotide polymorphisms for both tall waterhemp and smooth pigweed was greater than that in common cocklebur but less than that observed in common ragweed. Information on variability of herbicide target-site genes may be useful in predicting the likelihood for herbicide-resistance development. However, all four of the species investigated in this study have evolved resistance to ALS-inhibiting herbicides, despite the different levels ofALSvariability observed.
Response of paraquat-resistant and -susceptible horseweed (Conyza canadensis) to diquat, linuron, and oxyfluorfen
Authors: Weaver Susan , Downs Michael , Neufeld Brandi
Published: July 2006

Horseweed is a winter annual weed that has evolved resistance to multiple herbicide modes of action in 11 countries worldwide. A paraquat-resistant horseweed population in an Ontario orchard that was being managed by a rotation of herbicides began to show increased tolerance to the herbicide linuron. Experiments were conducted in the greenhouse to compare the response of this population and several paraquat-susceptible populations to linuron, diquat, and oxyfluorfen. Plants were sprayed with a range of doses of each herbicide when they were from 5 to 10 wk old, and the ED50, or dose at which shoot dry weight was reduced by 50%, was estimated. There was a sevenfold difference in the ED50values of the paraquat-resistant and -susceptible populations in response to diquat and a threefold difference in response to linuron. The response to oxyfluorfen was age dependent. The ratio of resistance to susceptible ED50values was estimated as 57 for 5-wk-old plants and 11 for 8-wk-old plants in response to oxyfluorfen. Ten-week-old plants from both populations showed no response to oxyfluorfen at rates up to 4.8 kg ai ha−1.
Nitrogen fertilizer timing and application method affect weed growth and competition with spring wheat
Authors: Blackshaw Robert E. , Molnar Louis J. , Janzen H. Henry
Published: July 2006

Managing crop fertilization may be an important component of integrated weed management systems that protect crop yield and reduce weed populations over time. A field study was conducted to determine the effects of various timings and application methods of nitrogen (N) fertilizer on weed growth and spring wheat yield. Nitrogen fertilizer was applied the previous fall (October) or at planting (May) at a dose of 50 kg ha−1. Nitrogen application treatments consisted of granular ammonium nitrate applied broadcast on the soil surface, banded 10 cm deep between every crop row, banded 10 cm deep between every second crop row, or point-injected liquid ammonium nitrate placed between every second crop row at 20-cm intervals and 10 cm deep. Treatments were applied in 4 consecutive yr to determine annual and cumulative effects over years. Density and biomass of wild oat, green foxtail, wild mustard, and common lambsquarters were sometimes lower with spring- than with fall-applied N. Spring wheat yield was never lower and was higher in 50% of the cases, when N was spring rather than fall applied. Nitrogen application method generally had larger and more consistent effects than application timing on weed growth and wheat yield. Shoot N concentration and biomass of weeds were often lower with subsurface banded or point-injected N than with surface broadcast N, and concurrent increases in spring wheat yield usually occurred with these N placement treatments. Depending on the weed species, the weed seedbank at the conclusion of the 4-yr study was reduced by 25 to 63% with point-injected compared with broadcast N. Information gained in this study will contribute to the development of more integrated and cost-effective weed management programs in wheat.
Inheritance of glyphosate resistance in goosegrass (Eleusine indica)
Authors: Ng Chin-Hong , Ratnam Wickneswary , Surif Salmijah , Ismail B. S.
Published: July 2006

The inheritance of glyphosate-resistant goosegrass was studied by making reciprocal crosses between resistant (R) and susceptible (S) biotypes. Eighty-four F1hybrids were confirmed using isozyme analyses. Reciprocal F1hybrids displayed uniform levels of resistance intermediate (I) to that of the parental types, with no indication of maternal inheritance. The F1hybrids were selfed to produce F2populations. F3populations were produced by selfing resistant and intermediate F2phenotypes. A segregation ratio of 1:2:1 (S:I:R) was observed in the F2and subsequently in the F3generations derived from selfing intermediate F2phenotypes. Individuals in F3families derived from selfing resistant F2phenotypes were resistant. Results from the present study show that glyphosate resistance in goosegrass is inherited as a single, nuclear, and incompletely dominant gene.
Chaff collection reduces seed dispersal of wild oat (Avena fatua) by a combine harvester
Authors: Shirtliffe Steven J. , Entz Martin H.
Published: July 2006

Combine harvesters have the potential to disperse weed seeds great distances. Reducing this dispersal may be important in an integrated weed management system. The objectives of this study were to determine the distance that wild oat seeds are dispersed by a combine harvester and the effect of chaff collection on combine harvester seed dispersal. This was measured by sampling wild oat seeds at varying distances behind a combine equipped with a removable chaff collection system after it passed through a wild oat patch. Chaff collection consistently reduced the amount and distance that wild oat seeds were dispersed. This occurred because more than 74% of the total wild oat seed that were ejected from the combine were in the chaff. Because most of the chaff falls in a row directly behind the combine, chaff collection only affected dispersal in this area. In 1996, chaff collection reduced wild oat seed dispersal past the wild oat patch to less than 10 seeds m−2at 45 m, whereas without chaff collection, there was greater than 10 seeds m−2up to 145 m. At distances beyond 145 m, chaff collection had no significant effect on seed dispersal. Chaff collection may be an important tool in an integrated weed management program because it may slow weed invasions and reduce the expansion of weed patches.
Cloransulam absorption, translocation, and efficacy on common broadleaf weed species
Authors: Barnes Jeff W. , Oliver Lawrence R.
Published: July 2006

Field and laboratory studies were conducted to evaluate herbicide efficacy, absorption, and translocation of cloransulam within broadleaf weeds. Control of morningglory species and velvetleaf with cloransulam was dependent upon application rate and timing. A reduced rate of cloransulam (9 g ai ha−1) was as effective as the labeled rate (18 g ha−1), when applications were targeted to small to midsize morningglory and velvetleaf. Prickly sida, hemp sesbania, and sicklepod were suppressed by cloransulam. Contour maps predicted accurately weed control for all species except tall morningglory. Cloransulam absorption and translocation provided some information about tolerance mechanisms. Susceptible species, entireleaf morningglory and velvetleaf, both rapidly absorbed14C-cloransulam. Absorption in the more tolerant velvetleaf was 20% lower than entireleaf morningglory at all harvest times. Absorption of14C-cloransulam in prickly sida was only 26% 6 h after treatment, and absorption did not increase with time. Differences in cloransulam absorption and translocation partially explained differences in susceptibility among some weed species but not others.
Light and growth rate effects on crop and weed responses to nitrogen
Authors: Harbur Matthew M. , Owen Micheal D. K.
Published: July 2006

The nitrogen (N) response of competing plants may be affected by photosynthetically active radiation (PAR) availability and maximum potential growth rate, which determine N requirements. The responses of two crop (corn and soybean) and six weed species (common lambsquarters, common waterhemp, giant foxtail, velvetleaf, wild mustard, and woolly cupgrass) in low and high (150 and 450 μmol m−2s−1) PAR levels to daily fertilization with either low or high (0.2 or 7.5 mM) NH4NO3levels were studied. Leaf area of all species responded positively to N by 8 d after emergence (DAE) when grown in high PAR; in low PAR, most species did not respond until 11 DAE. Dry weight and leaf area of all species at 18 DAE were greater with high than with low N. These responses to high N were also greater in high than in low PAR for all species. Dry weights with high N were up to 100% greater in low PAR and up to 700% greater in high PAR than dry weights with low N. These responses suggest that low PAR reduced the benefit of N to the plants. The regression of relative growth rate (RGR) with high N to RGR with low N had a slope that was less than unity (β = 0.79), indicating that species with a higher RGR with high N experienced greater decreases in RGR with low N. Similarly, the sensitivity (change in RGR) of plants grown with high and low N was positively related to RGR with high N. RGR differences among crop and weed species may be related to differences in N requirement that could be exploited for weed management. RGR and seed size were negatively correlated, which may explain previous observations that small-seeded weeds were more sensitive to environmental stress.