Red rice, which grows taller and produces more tillers than domestic rice and shatters most of its seeds early, is a major weed in many rice-growing areas of the world. Field experiments were conducted at Stuttgart, AR in 1997 and 1998 to evaluate the growth response of the Kaybonnet (KBNT) rice cultivar to various population densities of three red rice ecotypes. The ecotypes tested were Louisiana3 (LA3), Stuttgart strawhull (Stgstraw), and Katy red rice (KatyRR). Compared with KBNT alone, LA3, the tallest of the three red rice ecotypes, reduced tiller density of KBNT 51%, aboveground biomass at 91 d after emergence (DAE) 35%, and yield 80%. Stgstraw, a medium-height red rice, reduced KBNT tiller density 49%, aboveground biomass 26%, and yield 61%. KatyRR, the shortest red rice, reduced KBNT tiller density 30%, aboveground biomass 16%, and yield 21%. Tiller density of rice was reduced by 20 to 48% when red rice density increased from 25 to 51 plants m−2. Rice biomass at 91 DAE was reduced by 9 and 44% when red rice densities were 16 and 51 plants m−2. Rice yield was reduced by 60 and 70% at red rice densities of 25 and 51 plants m−2, respectively. These results demonstrate that low populations of red rice can greatly reduce rice growth and yield and that short-statured red rice types may affect rice growth less than taller ecotypes.

Tomato cultivation in the Mediterranean region is susceptible to infestation by the parasitic weed branched broomrape (Orobanche ramosa), and severe yield losses can result. The effectiveness of solarization, a soil disinfection technique that uses passive solar heating, to control the incidence of broomrape under greenhouse conditions was studied over two growing seasons. Solarization was accomplished by the application of clear polyethylene sheets to moist soil for 58 to 61 d during the hot season. The treatment increased maximum soil temperature by around 10 C, and at 5 cm below the soil surface, a temperature of more than 45 C was reached for 34 to 58 d, whereas this temperature was not reached at all in the first season and not for 20 d (second season) in unmulched soil. In solarized soil, no broomrape shoots emerged, and neither haustoria nor underground tubercles of the parasite were found on tomato roots. The treatment killed about 95% of buried viable seed, and induced secondary dormancy in the remaining 5%. In nonsolarized plots, broomrape shoots were present at a high density, decreasing plant growth and fruit production. Fruit yield was 133 to 258% higher in the solarized as compared with the nonsolarized treatment. Based on these results, we suggest that soil solarization, which precludes chemical contamination and is suitable for organic farming, is an appropriate technology where the risk of branched broomrape infestation is high.

Pyrithiobac and imazaquin are persistent herbicides used in the midsouthern region of the United States. Certain rotational crops are extremely sensitive to each herbicide. Field studies were established near Brooksville, MS, and St. Joseph, LA, to examine crop injury from the previous year's application of pyrithiobac and imazaquin. In Mississippi, 690 g ai ha−1 of each herbicide was applied as a preplant-incorporated carryover treatment. The subsequent year, pyrithiobac-treated areas were planted with soybean, corn, and grain sorghum. Imazaquin-treated areas were planted with cotton, corn, and grain sorghum. In plots adjacent to the carryover treatment, pyrithiobac or imazaquin was applied at rates ranging from 0 to 173 g ha−1 as in-field bioassay plots. In Louisiana, pyrithiobac was applied at 70 and 240 g ha−1 as a postdirected broadcast layby treatment in cotton. The subsequent spring, pyrithiobac-treated areas were planted to corn. Assay rates of pyrithiobac ranging from 0 to 140 g ha−1 were established in areas adjacent to the carryover treatment. Grain sorghum was most sensitive to pyrithiobac soil residues, followed by corn and then soybean. Cotton was the most sensitive crop to imazaquin soil residues, followed by corn and then grain sorghum. Residual amounts of pyrithiobac and imazaquin were approximately 15 and 24 g ha−1, respectively. The approximated half-life of pyrithiobac and imazaquin was 61 and 71 d, respectively. Pyrithiobac applied at 280 g ha−1 as a layby treatment the preceding year injured corn and reduced yields in 1 of 2 yr. Pyrithiobac at 70 g ha−1 did not affect corn in either year. The difference in persistence between the 2 yr was attributed to reduced precipitation and lower soil temperatures in the year with observed carryover. Injury and yield reduction observed in the second year of the study correspond to an approximate pyrithiobac half-life of 60 d.

Remote sensing and associated spatial technologies provide tremendous opportunity to enhance weed management and improve–protect the environment through judicious use of the most efficacious control methods for a given site. They can also be invaluable assets for detection of invasions, assessment of infestation levels, monitoring rate of spread, and determining the efficacy of mitigation efforts for weed management. In combination with other technologies such as global positioning systems and geographic information systems, sampling strategies can be devised to efficiently determine the location of weed populations in agricultural and wildland situations. Maps created from remote sensing or sampling (or both) allow site-specific weed management of only the areas requiring corrective action. Potential benefits to the land managers and the ecosystem as a whole will come from reductions in inputs, reduced environmental liability from the detrimental effects of applying control measures to entire areas, crop yield increases through better management decisions, and early detection and effective management of invading species. Improvements in spatial and spectral resolution, temporal frequency, image turnaround time, and cost of image acquisition, combined with the realization of the value of the data, are enhancing the acceptance and usage of remote sensing technologies. However, remote sensing will be best used by providing accurate, site-specific data that can be converted into information used by decision support systems (DSSs). Advances in these DSSs, and their ability to incorporate remote sensing data, have been rapid and widespread in the past 10 yr. As a result, federal management and research agencies, academic institutions, and private entities have collectively developed efforts to use this information in monitoring and management efforts for invasive species in western rangelands, aquatic ecosystems and forestry, and site-specific weed management in agronomics.

Field experiments on suppression of three species (scentless chamomile, field poppy, and canola) by winter wheat sown in two different spatial patterns (normal 12.8-cm rows and a uniform, grid-like pattern) and three densities (204, 449, and 721 plants m−2) in two growing seasons were performed. The effects of crop-sowing density and pattern when weeds were controlled by herbicide were also investigated in one season. Weed and crop biomass were measured when weed biomass was at its maximum (late June/early July), and grain was harvested in August. Weed biomass comprised on average 30% of the total (crop + weed) biomass in the first year and only 5% in the second year. Weed biomass decreased and grain yield increased with increasing sowing density. Weed biomass was on average 23% lower and grain yield 14% higher in the uniform pattern than in rows. Weed biomass decreased 27% and 38% in the row pattern and 36% and 50% in the uniform pattern by increasing sowing density from low to medium and from low to high density, respectively. When weeds were controlled with herbicide, increasing sowing density had no influence on grain yield, but grain yield was 7% higher in the uniform pattern. Field poppy was the weed with the largest biomass and the largest impact on yield, whereas canola had the lowest biomass and the least impact on yield.

A greenhouse experiment was conducted to evaluate the herbicidal activity of five aliphatic (ethyl, propyl, butyl, allyl, and 3-methylthiopropyl) and three aromatic (phenyl, benzyl, and 2-phenylethyl) isothiocyanates (ITCs) on Palmer amaranth, pitted morningglory, and yellow nutsedge. All ITCs were applied to soil at 0, 10, 100, 1,000, and 10,000 nmol g−1 of soil and incorporated. All ITCs had a deleterious effect on Palmer amaranth and pitted morningglory emergence. LC50 values for Palmer amaranth emergence inhibition from aliphatic and aromatic ITCs ranged from a low of 32 nmol g−1 of soil for phenyl ITC to a high of 941 nmol g−1 of soil for propyl ITC. Pitted morningglory was slightly more tolerant than Palmer amaranth to each of the ITCs, with LC50 values for emergence ranging from 347 to 2,855 nmol g−1 of soil for 3-methylthiopropyl and butyl ITC, respectively. Yellow nutsedge was the most tolerant of the three species, with LC50 values for ethyl, butyl, benzyl, and 2-phenylethyl being greater than the highest evaluated concentration of 10,000 nmol g−1 of soil. Phenyl and 3-methylthiopropyl at 10,000 nmol g−1 of soil were the most effective ITCs against yellow nutsedge, reducing emergence by 92%. Effectiveness of the ITCs varied across structure and species, but 3-methylthiopropyl and phenyl ITC were generally the most efficacious for the three species evaluated.

Nonindigenous invasive weed species can have substantial negative impacts on the quantity and quality of outdoor recreational activities such as fishing, hunting, hiking, wildlife viewing, and water-based recreation. Despite the significance of impacts on recreation, very little research has been performed to estimate the corresponding economic losses at spatial scales such as regions, states, and watersheds. This is true primarily because in most jurisdictions the data necessary to estimate recreational impacts are scarce and incomplete. Because of the challenges involved in measuring recreational losses precisely, we illustrate a method that can provide indications of the ranges in which the true economic losses likely lie. To reflect underlying uncertainty in parameters such as the number of acres infested in a jurisdiction and the rate at which wildlife-related recreation decreases as a function of increasing weed infestation, we developed a range of estimates using lower, medium, and higher scenario combinations of parameter and variable values. Our case study jurisdiction is a western state (Nevada) in which nonindigenous weed infestations on public lands have expanded rapidly in recent years. Under conservative assumptions, the negative economic impacts stemming from the adverse influence of nonindigenous weeds on wildlife-related recreation in Nevada likely range from $6 million to $12 million per year. Using the most conservative findings for annual recreation losses, the predicted discounted stream of negative economic impacts over a future time horizon of 5 yr ranges from about $30 million to $40 million in Nevada, depending on actual future expansion rates of weeds.

The Brassicaceae contain glucosinolates, which hydrolyze to form compounds toxic to plants, fungi, nematodes, and certain insects. Lower weed density and biomass in crops grown following incorporation of brassica cover crops suggest that they may contribute to weed management in agricultural systems. Field experiments were conducted to determine whether incorporated brassica cover crops, including canola, rapeseed, and yellow mustard, reduce subsequent weed and crop establishment; a companion paper describes separate but related field experiments that examined the influence of brassica cover crops on plant growth. Emergence rate and total emergence of sixteen weed and crop bioassay species were measured following brassica cover crops, fallow, or incorporated residues of other short-season cover crops including oat, crimson clover, and buckwheat. The bioassay species, representing a range of seed sizes, were chosen to determine whether larger seed size confers protection from residue-mediated effects on emergence. Averaged over bioassay species, brassica cover crops reduced emergence by 23 to 34% compared with fallow; emergence following brassicas was delayed by approximately 2 d. The effects of the incorporated brassica residues were similar to those of the other short-season cover crops, which reduced emergence of the bioassay species by 19 to 39% and delayed emergence by 2 d. Seed size was a poor predictor of a species' establishment. These results suggest that brassica residues are capable of delaying seedling emergence and reducing establishment, although the magnitude of their effects were comparable to other widely available cover crops.

Weeds are an important plant resource for insects, although feeding by insects on weeds can have both positive and negative effects on crop productivity. Weeds also indirectly affect crops via their influence on beneficial insects, and by harboring plant and insect diseases. Weeds may affect the ability of dispersing insects to locate crop plants. The host relationship between insects and plants is highly variable, ranging from very specialized to generalized feeding behaviors. Despite the myriad interactions of weeds and insects, many aspects of the relationship are predictable. Most insects, including crop pests, are specialists, and preadapted to feed only on some plants, often within a single plant family. Even polyphagous insects often have a distinct preference hierarchy, feeding more widely only when preferred hosts are unavailable. Use of plants by insects is a dynamic interaction, with characteristics of the insect (e.g., mandible structure) and the plant (e.g., allelochemicals) affecting feeding behavior. Thus, weeds that are closely related to crops are particularly important in harboring insects that attack those crops. Crop production practices should seek to sever the taxonomic association between the crop and the weeds found within the crop, and nearby, by eliminating weeds related to the crop. This will make it less likely that insects will move easily from weed to crop plants, that damaging population densities of insects will develop in the field, and that insect vectors that harbor plant diseases will be harbored in the field. Particularly important integrated pest management practices include crop rotation, reduced use of chemical herbicides, and management of weeds in noncultivated areas.

Field studies have shown that weed density and biomass were lower in crops following incorporation of brassica cover crops compared with fallow but have not determined whether weed-suppressive effects are solely a consequence of reduced establishment, as evidenced in our companion paper, reduced growth of established plants, or both. In 2002 and 2003, canola and yellow mustard were seeded in early May, mowed in early July, and the residues incorporated. Green bean and redroot pigweed were then planted at fixed densities. Plant height and biomass were measured weekly; leaf area and biomass of component plant parts were measured at three harvests. Based on analysis of variance (ANOVA) at discreet sampling points, growth of redroot pigweed and green bean in monoculture or mixture were similar following fallow and incorporated brassica cover crops. However, based on aboveground biomass fitted to a Richards function, redroot pigweed growth in monoculture was reduced by the yellow mustard cover crop compared with fallow in both years (P = 0.007), but the magnitude of this effect was small; the canola cover crop did not affect growth (P = 0.179). Brassica cover crops did not reduce redroot pigweed growth when it was grown in mixture with green bean (P ≥ 0.382). Redroot pigweed competition reduced green bean yield, but incorporated brassica cover crops did not affect green bean growth and yield, nor did they confer a competitive advantage to the crop. Thus, brassica cover crops may suppress the growth of established weed and crop plants, but the magnitude of suppression was less than previously documented for effects on weed establishment.

A study was conducted near Garden City, KS with irrigated corn to determine how the integration of a terminated winter wheat cover crop with various atrazine rates would affect Palmer amaranth control and corn water use efficiency (WUE). Without atrazine, the presence of a winter wheat cover crop, killed in the boot stage, resulted in a threefold weed biomass reduction in irrigated corn. The highest rate of atrazine completely masked the weed control effect of the cover crop, producing a greater than 15-fold reduction regardless of the presence or absence of the cover crop. A terminated winter wheat cover crop without atrazine elevated corn yield in only two of nine location-yr, and in one instance, depressed yield. However, a terminated wheat cover crop elevated corn yield in six of nine location-yr combinations when used in conjunction with 1.6 kg ha−1 atrazine. Although increases in WUE associated with reductions in soil water evaporation produced by the cover crop seemed to be responsible for some of the increase in corn grain yield and stored soil water at the end of the summer growing season, end of season Palmer amaranth biomass had a more profound impact.

Annual crop fields typically are simple habitats dominated by a few plant species where pesticides play a major role in managing weed and insect infestations. Recently, there has been significant interest in the potential to reduce reliance on pesticides by manipulating plant species and communities to benefit natural enemies of insects and weeds. Such efforts aim to enhance natural enemy impact by providing appropriate food, shelter, and hosts, and efforts typically are accomplished by manipulation of plant species, populations, or communities. Habitat management is generally viewed as an important factor in maintaining stable insect and natural enemy populations in agricultural systems and may have a similar function in increasing weed seed predation. Crop and noncrop habitats provide resources to natural enemies either directly through floral nectar and pollen, indirectly by increased host or prey availability, or through emergent properties of the habitat such as by moderating the microclimate. These critical resources for natural enemies can be provided in agricultural ecosystems at several scales: within fields, at field margins, or as a component of the larger landscape. Because individual natural enemy species may require quite specific resources at different times and spatial scales, not all attempts to manipulate habitat diversity are equally effective. We review the role of plant resources, including weeds, in supporting natural enemy communities and provide case studies of how varying plant diversity at different spatial scales can influence the effectiveness of biological control in agricultural landscapes.

Soil and surface residues from cotton field studies in Stoneville, MS (1994 through 1996) and Florence, SC (1995 through 1996) were sampled to evaluate effects of cover crop and tillage on herbicide dissipation. Mississippi treatments included tillage (conventional [CT]; none [NT]) and cover crop (ryegrass; none [NC]). South Carolina treatments included tillage (CT; reduced tillage [RT]) and cover crop (rye; NC). Fluometuron was applied preemergence (PRE) in both Mississippi and South Carolina, and norflurazon was applied PRE in Mississippi. Soils were sampled various times during the growing season (depths: 0 to 2 cm, 2 to 10 cm). Cover crop residues were sampled from RT or NT cover crop areas. Soil and cover crop sample extracts were analyzed for herbicides. Soil organic carbon tended to increase with tillage reduction and presence of cover crop and was positively correlated with herbicide sorption, especially in the surface. Across locations, herbicide half-lives ranged from 7 to 15 d in the soil surface. Tillage had mixed effects on herbicide persistence in surface soil, with higher herbicide concentrations in CT at early samplings, but differences were insignificant later on. The most consistent effects were observed in RT/NT with cover crops, where cover crop residues intercepted applied herbicide, impeding subsequent movement into soil. Herbicide dissipation in cover crop residues was often more rapid than in soil, with half-lives from 3 to 11 d. Herbicide retention in cover crop residues and rapid dissipation were attributed to strong herbicide affinity to cover crop residues (e.g., fluometuron Kd = 7.1 [in rye]; Kd = 1.65 [in Mississippi Dundee soil CT, NC]) and herbicide co-metabolism as cover crop residues decomposed. A fluometuron metabolite, desmethyl-fluometuron, was observed in most soil and cover crop samples after 1 wk. Only minimal herbicide or metabolite moved into the subsurface, and little treatment effect could be ascribed to herbicide or metabolite movement below 2 cm.

Interactions between weeds and organisms in other pest categories are inevitable. Weeds are plants and therefore ecologically are producers. All other pest organisms are consumers; they are herbivores or pathogens and can thus use weeds directly as a food source. Beneficial organisms are primary carnivores that feed on herbivores; weeds can support beneficials indirectly when they feed on herbivores living on weeds. Weeds can also serve to mask crop plants from herbivore pests; the mechanisms by which this occurs are still debated. Presence of a weed canopy modifies ecosystem microclimate and provides shelter for pests and beneficials that would otherwise not survive. Tactics used to control pests can have impacts on nontarget organisms in other pest categories. Changes in tillage for weed control can impact population development of other pests. Pesticides can affect nontarget organisms resulting in unanticipated changes in crop tolerance and pest control. Development of true integrated pest management programs requires a multidisciplinary approach that incorporates interactions between organisms in different pest categories.

Herbicides targeting grass plastidic acetyl coenzyme A carboxylase (ACC) are effective selective graminicides. Their intensive use worldwide has selected for resistance genes in a number of grass weed species. Biochemistry and molecular biology have been the means of determining the herbicidal activity and selectivity toward crop plants of ACC-inhibiting herbicides. In recent years, elucidation of the tridimensional structure of ACC and identification of five amino acid residues within the ACC carboxyl transferase domain that are critical determinants for herbicide sensitivity shed light on the basis of ACC-based resistance to herbicides. However, metabolism-based resistance to ACC-inhibiting herbicides is much less well known, although this type of resistance seems to be widespread. A number of genes thus endow resistance to ACC-inhibiting herbicides, with the possibility for various resistance genes that confer dominant resistance at the herbicide field rate to accumulate within a single weed population or plant. This, together with a poor knowledge of the genetic parameters driving resistance, renders the evolution of resistance to ACC-inhibiting herbicides unpredictable. Future research should consider developing tactics to slow the spread of resistance. For this purpose, it is crucial that our understanding of metabolism-based resistance improves rapidly because this mechanism is complex and can confer resistance to herbicides with different target sites.

Weeds and native plants should be considered when endeavoring to manage and control plant pathogens of cultivated plants. Whether as a pest itself, vector of a pathogen, or reservoir of a pathogen or its vector, weeds can significantly influence disease incidence. The relationship between these factors plays a critical role in determining disease incidence and impact. Weeds can interact with pathogen management in several ways, including provision of weed biological control, parasitic weeds can directly serve as vectors of plant pathogens, weeds can serve as reservoir alternative hosts for pathogens and vectors, weeds may be obligate alternate hosts for some pathogens, and herbicides used for weed control can interact with plant pathogens. A recent concern is the advent and deployment of plants genetically engineered for pathogen resistance, raising the question of “super weeds” resulting from genetic drift of genes from crops into surrounding weed populations, the impact of which has yet to be determined.