To improve understanding of relationships between crop diversity, weed management practices, and weed population dynamics, we used data from a field experiment and matrix models to examine how contrasting crop rotations affect velvetleaf. We compared a 2-yr rotation system (corn–soybean) managed with conventional rates of herbicides with a 4-yr rotation (corn–soybean–triticale + alfalfa–alfalfa) that received 82% less herbicide. In November 2002, a pulse of velvetleaf seeds (500 seeds m−2) was added to 7- by 7-m areas within replicate plots of each crop phase–rotation system combination. Velvetleaf seed, seedling, and reproductive adult population densities, seed production, and seed losses to predators were measured during the next year. Velvetleaf seed production was greater in the 4-yr rotation than in the 2-yr rotation (460 vs. 16 seeds m−2). Averaged over 12 sampling periods from late May to mid-November 2003, loss of velvetleaf seeds to predators also was greater in the 4-yr rotation than in the 2-yr rotation (32 vs. 17% per 2 d). Modeling analyses indicated that velvetleaf density in the 4-yr rotation should decline if cumulative losses of seeds produced in the soybean phase exceeded 40%. Achieving such a level of predation appears possible, given the observed rates of velvetleaf seed predation. In addition, no tillage occurs in the 4-yr rotation for 26 mo after soybean harvest, thus favoring seed exposure on the soil surface to predators. Models that included estimates of seed predation indicated that to prevent increases in velvetleaf density, weed control efficacy in soybean must be ≥ 93% in the 2-yr rotation, but could drop to 86% in the 4-yr rotation. These results support the hypothesis that diverse rotations that exploit multiple stress and mortality factors, including weed seed predation, can contribute to effective weed suppression with less reliance on herbicides.

To manage or control nonindigenous species (NIS), we need to know where they are located in the landscape. However, many natural areas are large, making it unfeasible to inventory the entire area and necessitating surveys to be performed on smaller areas. Provided appropriate survey methods are used, probability of occurrence predictions and maps can be generated for the species and area of interest. The probability maps can then be used to direct further sampling for new populations or patches and to select populations to monitor for the degree of invasiveness and effect of management. NIS occurrence (presence or absence) data were collected during 2001 to 2003 using transects stratified by proximity to rights-of-way in the northern range of Yellowstone National Park. In this study, we evaluate the use of environmental and remotely sensed (LANDSAT Enhanced Thematic Mapper +) data, separately and combined, for developing probability maps of three target NIS occurrence. Canada thistle, dalmation toadflax, and timothy were chosen for this study because of their different dispersal mechanisms and frequencies, 5, 3, and 23%, respectively, in the surveyed area. Data were analyzed using generalized linear regression with logit link, and the best models were selected using Akaike's Information Criterion. Probability of occurrence maps were generated for each target species, and the accuracies of the predictions were assessed with validation data excluded from the model fitting. Frequencies of occurrence of the validation data were calculated and compared with predicted probabilities. Agreement between the observed and predicted probabilities was reasonably accurate and consistent for timothy and dalmation toadflax but less so for Canada thistle.

Cover crop residues and other biologically based approaches often provide incomplete and inconsistent weed control. This research was conducted to evaluate interactions between hairy vetch residue on the surface of soil and the herbicide metolachlor. Herbicide was applied and incorporated with simulated rainfall before residue placement, residue was applied to the soil surface at precise rates, and potentially confounding variables such as nitrogen and soil moisture were controlled in a greenhouse experiment. Emphasis was placed on the use of suboptimal rates of both residue and metolachlor to explore the potential synergistic interactions between these factors. Deviation from a multiplicative model that included a quadratic response to hairy vetch residue and a log-logistic response to metolachlor was used to demonstrate the presence or absence of synergism. This model effectively showed that emergence of smooth pigweed, common lambsquarters, giant foxtail, and velvetleaf and early growth of smooth pigweed and common lambsquarters were reduced synergistically by the combination of hairy vetch residue and metolachlor. For example, smooth pigweed emergence was reduced 13% by 500 g m−2 of hairy vetch residue alone and was reduced 16% by 10 g ha−1 of metolachlor alone, but together, they reduced smooth pigweed emergence by 86%. This model could be used to determine synergistic interactions between any combination of a phytotoxin and a biologically based weed management approach that could be expressed in quantitative units.

Root traits and growth of spreading orach and common lambsquarters were compared in response to soil compaction, drought, and waterlogging under controlled environment conditions. On the basis of the typical habitats occupied, the hypothesis was that spreading orach would be more tolerant of compaction and waterlogging and common lambsquarters more tolerant of drought. When grown in buckets with two soil bulk densities (1.2 and 1.6 g cm−3) for 8 wk, the two species responded similarly to compaction, with the fraction of fine roots reduced by 10%, total root length by 70%, root and shoot dry weight and leaf area by 50 to 60%, and plant height by 30% at the high compared with the low bulk density. When grown for 6 wk in soil columns 1 m long, which were watered daily or allowed to dry, common lambsquarters was deeper rooted than spreading orach at both moisture levels and better able to sustain growth in the drying columns. The watering regime did not alter the rooting depth of either species. Total root length in successive 10-cm increments declined exponentially from the top to the bottom of the watered columns, but root proliferation was reduced in the upper 20 cm of the drying columns. The average root diameter of both species decreased with drought and increased with soil compaction. When grown in waterlogged soil at 10 or 20 C for 4 wk, seedlings of spreading orach survived with little reduction in growth, whereas survival and growth of common lambsquarters were drastically reduced, particularly under cool soil conditions.

Remote sensing technology is a tool for detecting invasive species affecting forest, rangeland, and pasture environments. This article provides a review of the technology, and algorithms used to process remotely sensed data when detecting weeds and a working example of the detection of spotted knapweed and babysbreath with a hyperspectral sensor. Spotted knapweed and babysbreath frequently invade semiarid rangeland and irrigated pastures of the western United States. Ground surveys to identify the extent of invasive species infestations should be more efficient with the use of classified images from remotely sensed data because dispersal of an invasive plant may have occurred before the discovery or treatment of an infestation. Remote sensing data were classified to determine if infestations of spotted knapweed and babysbreath were detectable in Swan Valley near Idaho Falls, ID. Hyperspectral images at 2-m spatial resolution and 400- to 953-nm spectral resolution with 12-nm increments were used to identify locations of spotted knapweed and babysbreath. Images were classified using the spectral angle mapper (SAM) algorithm at 1, 2, 3, 4, 5, and 10° angles. Ground validation of the classified images established that 57% of known spotted knapweed infestations and 97% of known babysbreath infestations were identified through the use of hyperspectral imagery and the SAM algorithm.

Competitive outcome between crops and weeds is affected by partitioning of new biomass to above- and belowground plant organs in response to nutrient supply. This study determined the fraction of biomass partitioned to roots vs. shoots in corn and velvetleaf in response to nitrogen (N) supply. Pots measuring 28 cm in diam and 60 cm deep were embedded in the ground and each contained one plant of either corn or velvetleaf. Each plant received one of three N treatments: 0, 1, or 3 g N applied as ammonium nitrate in 2001, and 0, 2, or 6 g N in 2002. Measurements of total above- and belowground biomass were made at 10 sampling dates during each growing season. The root:shoot ratio decreased over time for both corn and velvetleaf as a result of normal plant growth and as N supply increased. Root:shoot ratio was greater for corn than for velvetleaf at comparable stages of development and at all levels of N supply. Both corn and velvetleaf display true plasticity in biomass partitioning patterns in response to N supply. Velvetleaf root:shoot ratio increased by 46 to 82% when N was limiting in 2001 and 2002, respectively, whereas corn root:shoot ratio increased by only 29 to 45%. The greater increase in biomass partitioned to roots by velvetleaf might negatively impact its ability to compete with corn for light when N supply is limited.

Weed seedbanks have been studied intensively at local scales, but to date, there have been no regional-scale studies of weed seedbank persistence. Empirical and modeling studies indicate that reducing weed seedbank persistence can play an important role in integrated weed management. Annual seedbank persistence of 13 summer annual weed species was studied from 2001 through 2003 at eight locations in the north central United States and one location in the northwestern United States. Effects of seed depth placement, tillage, and abiotic environmental factors on seedbank persistence were examined through regression and multivariate ordinations. All species examined showed a negative relationship between hydrothermal time and seedbank persistence. Seedbank persistence was very similar between the two years of the study for common lambsquarters, giant foxtail, and velvetleaf when data were pooled over location, depth, and tillage. Seedbank persistence of common lambsquarters, giant foxtail, and velvetleaf from October 2001 through 2002 and October 2002 through 2003 was, respectively, 52.3% and 60.0%, 21.3% and 21.8%, and 57.5% and 57.2%. These results demonstrate that robust estimates of seedbank persistence are possible when many observations are averaged over numerous locations. Future studies are needed to develop methods of reducing seedbank persistence, especially for weed species with particularly long-lived seeds.

Development of more comprehensive and cost-effective integrated weed management systems is required to facilitate greater integrated weed management adoption by farmers. A field experiment was conducted at two locations to determine the combined effects of seed date (April or May), seed rate (recommended or 150% of recommended), fertilizer timing (applied in fall or spring), and in-crop herbicide dose (50% or 100% of recommended) on weed growth and crop yield. This factorial set of treatments was applied in four consecutive years within a spring wheat–spring canola–spring wheat–spring canola rotation in a zero-till production system. Both wheat and canola phases of the rotation were grown each year. Weed biomass was often lower with May than with April seeding because more weeds were controlled with preplant glyphosate. However, despite fewer weeds being present with May seeding, wheat yield was only greater in 1 of 7 site-years, and canola yield was never greater with May compared with April seeding. Higher crop seed rates had a consistently positive effect on reducing weed growth and the weed seedbank. Crop yield was sometimes greater, and never lower, with higher seed rates. Fertilizer timing did not have a large effect on crop yield, but applying N in the spring compared with fall was less favorable for weeds as indicated by lower weed biomass and a 20% decrease in the weed seedbank. In-crop herbicides applied at 50% compared with 100% doses often resulted in similar weed biomass and crop yield, especially when higher crop seed rates were used. Indeed, the weed seedbank at the conclusion of the 4-yr experiment was not greater with the 50% compared with 100% herbicide dose at one of two locations. This study demonstrates the combined merits of early seeding (April), higher crop seed rates, and spring-applied fertilizer in conjunction with timely but limited herbicide use to manage weeds and maintain high crop yields in rotations containing wheat and canola.

Field research was conducted in Tennessee at two locations over 3 yr under no-tillage conditions to examine the interaction of broadleaf signalgrass and corn yield. Broadleaf signalgrass was killed at various time intervals during the growing season by applying postemergent glyphosate. Corn injury was avoided by the use of a glyphosate-tolerant variety. Yield reductions because of broadleaf signalgrass interference occurred with weed densities > 150 plants m−2 and when the corn and weeds emerged simultaneously. Corn had the ability to withstand broadleaf signalgrass presence up to 28 d after planting with no yield loss at all locations.

Scientific understanding of multitactic weed management systems (MTS) is complicated by (1) the large number of potential combinations among tactics, (2) potentially noisy and complex system behavior because of individually more moderate mortality events, and (3) possible transient system behavior of unknown duration. Therefore, decomposing the relative performance of MTS components is much more difficult than it is for single-tactic strategies (STS). Attempting to accommodate the increased complexity of system behavior while maintaining the generality of results requires analytical methods capable of accomplishing these tasks. We provide two examples of statistical procedures that may help gain understanding of MTS systems using previously published weed demographic time-series data. First, we demonstrate the use of mixed-effects models capable of representing and removing factors contributing uncontrolled variation to system behavior. Model selection criteria are used to highlight the importance of the increased flexibility the mixed-model framework provides. Second, by explicitly modeling the probabilistic process presumed to be generating the data, we demonstrate how different components of the MTS can be compared and how the methodology can facilitate integration of such information into a decision-making application.

Since commercialization of Collego™ and Devine™ in the early 1980s, there has been a small but consistent research effort in the area of bioherbicides. The bioherbicide approach has promised effective weed management in cropping systems where the classical approach (using exotic natural enemies) is largely unsuitable. The overriding principle of the bioherbicide approach has been that a host-specific, coevolved natural enemy can be used as a bioherbicide when applied in simple formulations at inundative levels; however, two decades of research has effectively disproven this principle. Although research has revealed weaknesses in the bioherbicide approach, it has also revealed potential in a number of areas. A number of niche situations will remain in which host-specific plant pathogens can be developed as bioherbicides, such as for parasitic weeds and narcotic plants, but more research should be conducted with virulent, broad host range organisms, and more effort should be devoted to developing techniques for the cultural and genetic enhancement of bioherbicidal organisms.

Field experiments were conducted at the Rice Research and Extension Center at Stuttgart, AR, in 1997 and 1998 to evaluate the growth response of Stuttgart strawhull (Stgstraw) red rice to sowing densities of 0, 50, 100, and 150 kg ha−1 of ‘Kaybonnet,’ ‘Guichao,’ and ‘PI 312777’ rice cultivars. PI 312777 produced a greater leaf area index and tiller density than Kaybonnet when grown with red rice. In 1997, Stgstraw seed yields were lower when grown with PI 312777 and Guichao than with Kaybonnet. The increased weed population in 1998 did not increase seed yield production of red rice when grown with the three rice cultivars. The Stgstraw red rice seed yield was reduced when grown with 50 kg ha−1 rice when compared with its yield in monoculture and was reduced further when grown with 100 and 150 kg ha−1 rice. These results demonstrate that red rice was more competitive when compared with the tropical japonica Kaybonnet than the indica PI 312777. Despite its semidwarf stature, PI 312777 tended to suppress red rice more than did Guichao or Kaybonnet. The mechanisms responsible for this difference could be important keys to the effective use of weed-suppressive cultivars in reduced herbicide input systems.

Two studies investigated off-target exposure of soybean to plant growth regulator (PGR) herbicides and determined if simultaneous exposure to PGR herbicides and labeled soybean herbicides increase PGR injury. The PGR herbicides, 2,4-D, clopyralid, and dicamba, as well as dicamba plus the auxin transport inhibitor diflufenzopyr, were applied to glyphosate-resistant soybean at the V3, V7, and R2 soybean growth stages. Two rates were chosen from previous and preliminary research to approximate threshold rates that would cause a yield reduction so as to distinguish differences in sensitivity between growth stages. All four PGR herbicides caused significant soybean injury, height reduction, and yield loss at one or more application rates and growth stages. Relative to other PGR herbicides, dicamba reduced soybean yield at the lowest rate (a potential rate from residues remaining in improperly cleaned application equipment), followed by clopyralid, with 2,4-D requiring the highest rate to reduce soybean yield (a potential rate from a high level of spray drift). Dicamba and dicamba plus diflufenzopyr were applied at equal fractions of labeled use rates for corn to compare them directly at equivalent levels of off-target movement. Dicamba plus diflufenzopyr caused less injury and yield loss than dicamba applied alone. In a second study, the highest labeled soybean use rates of glyphosate, imazethapyr, imazamox, and fomesafen were applied alone and in combination with the highest rate of dicamba used in the first study (1% of a labeled use rate for corn) at the V3 and V7 stages. Dicamba demonstrated synergistic interactions with imazamox, imazethapyr, and fomesafen (but not with glyphosate) to further reduce yield under some circumstances, especially when applied at the V7 stage. Several treatments that included dicamba reduced soybean seed weight when applied at either the V3 or V7 stage and reduced the number of seeds per pod at the V7 stage.

Site-specific fertilizer application is of primary interest among those investigating site-specific crop management. Site-specific soil moisture levels may be associated with the relative success of certain weed species, and weed competition can be affected by fertilizer application. A field study was conducted to investigate how landscape position–based site-specific nitrogen fertilizer application would influence wild oat interference in spring wheat. Wild oat was allowed to grow in spring wheat at foot and knoll landscape positions in the presence or absence of 80 to 90 kg ha−1 of spring preseed broadcast nitrogen fertilizer. In all three site–years of the study, landscape position did not affect wild oat competitiveness in wheat or relative wild oat biomass, and there were no significant landscape position by nitrogen fertilizer interactions for these variables. The lack of landscape position effect may be attributed to the lack of substantive differences in soil characteristics between landscape positions at the sites used in this study and to normal to above-average seasonal precipitation levels in all three site–years. Wild oat competitiveness in wheat was significantly greater in the presence of nitrogen fertilizer for all three site–years. The results of this study suggest that, when there are not great differences in typical soil characteristics between landscape positions and precipitation levels are normal or above normal, landscape-based site-specific nitrogen fertilizer application does not affect wild oat competitiveness in spring wheat, but preseed spring broadcast nitrogen fertilizer does make wild oat more competitive in spring wheat.

Automated detection and identification of weeds in crop fields is the greatest obstacle to development of practical site-specific weed management systems. Research progress is summarized for two different approaches to the problem, remote sensing weed mapping and ground-based detection using digital cameras or nonimaging sensors. The general spectral and spatial limitations reported for each type of weed identification system are reviewed. Airborne remote sensing has been successful for detection of distinct weed patches when the patches are dense and uniform and have unique spectral characteristics. Identification of weeds is hampered by spectral mixing in the relatively large pixels (typically larger than 1 by 1 m) and will not be possible from imagery where weed seedlings are sparsely distributed among crop plants. The use of multispectral imaging sensors such as color digital cameras on a ground-based mobile platform shows more promise for weed identification in field crops. Spectral features plus spatial features such as leaf shape and texture and plant organization may be extracted from these images. However, there is a need for research in areas such as artificial lighting, spectral band requirements, image processing, multiple spatial resolution systems, and multiperspective images.

Field experiments were conducted at Ashland Bottoms in northeastern Kansas and at Hays in western Kansas in 2001, 2002, and 2003 to determine the response of soil microbial and nematode communities to different herbicides and tillage practices under a glyphosate-resistant cropping system. Conventional herbicide treatments were a tank mixture of cloransulam plus S-metolachlor plus sulfentrazone for soybean and a commercially available mixture of acetochlor and atrazine for corn. Glyphosate was applied at 1.12 kg ai ha−1 when weeds were 10 or 20 cm tall in both corn and soybean. Soil samples were collected monthly at Ashland Bottoms during the growing period for soil microbial biomass (SMB) carbon determination. In addition, substrate-induced respiration (SIR) and BIOLOG substrate utilization were determined at the end of the growing season each year at Ashland Bottoms, and nematode populations were determined at the beginning and the end of the growing season at both sites. Direct effects of glyphosate rates on soil microbial and nematode communities were also studied in a controlled environment. Values for SMB carbon, SIR, and BIOLOG substrate utilization were not altered by glyphosate. Nematode community response to the glyphosate treatment was similar under both conventional tillage and no-till environments. Total nematode densities were similar with the glyphosate and conventional herbicide treatments. SMB carbon and BIOLOG substrate utilization did not differ between tillage treatments. Nematode densities were greater under conventional tillage than in the no-till system. This study showed that soil health when glyphosate was applied in a glyphosate-resistant cropping system was similar to that of cropping systems that used conventional herbicides.

Any plant not sown from seed is often labeled a weed in improved pastures of New Zealand. Most improved pastures are a mix of perennial ryegrass and white clover but generally are infested with broadleaf weeds. Changes in forage production due to individual weeds were determined using measurements of perennial ryegrass and white clover before and after dairy cattle, beef cattle, or sheep grazing under, near, and far from individual plants of six rosette-forming weed species throughout a growing season. The larger weeds, bull thistle and musk thistle, reduced the amount of forage utilized 42 and 72%, respectively, in beef cattle– and sheep-grazed hill-country pastures. Forage production under and near Canada thistle, hedge mustard, broadleaf plantain, and hairy buttercup in a dairy pasture was greater (136, 140, 178, and 450%, respectively) than in the control areas. Although the dairy pasture was grazed following recommended grazing procedures, our results indicate that this grazing system was not maximizing forage yield potentials of perennial ryegrass and white clover and that these weeds served as an indicator that the pasture was being overgrazed.

Nascent research on the profitability of site-specific weed management has focused on reduced herbicide use, ignoring significant information costs for scouting, making treatment maps, and patch herbicide application. Including these information costs results in few, if any, studies, fully covering added costs with herbicide savings. Real-time, sensor-driven site-specific herbicide management promises to overcome many of the scouting and map-making costs so long as weed species recognition accuracy continues to improve and commercial capital costs to decline. Two means to increase revenues are possible from site-specific weed management: value-added crop products (e.g., herbicide-free, nontransgenic crops) and environmental stewardship payments from government programs. Site-specific ecological weed management offers a more fruitful means to garner both higher prices from “green” consumers and revenues for sound environmental stewardship. It entails using spatial tools to enhance habitat for weed predators and diseases and their vectors, as well as to grow crops that are better able to compete with weeds. The most economically promising areas for future weed management will draw upon both spatial and developmental management tools.