The genus Orobanche includes chlorophyll-lacking root parasites that parasitize many dicotyledonous species and cause severe damage to vegetable and field crops worldwide. In the United States, small broomrape is a federally listed noxious weed that has been found in red clover seed production fields in Oregon. In this study, we classified the susceptibility of red clover, white clover, and crimson clover to small broomrape infection under controlled conditions. Red, white, and crimson clover seeds were sown in pots containing small broomrape–inoculated soil. Differences in susceptibility to infection were observed among clover species; red clover was severely infected, white clover was slightly infected, and crimson clover was not infected.

Widespread use of crop yield loss models based on weed density has been limited on account of spatial and temporal variability. Furthermore, research characterizing crop yield loss associated with two or more weed species is lacking for many cropping systems. Therefore, research was conducted to characterize giant foxtail and common lambsquarters leaf area, height, and shoot volume in soybean, to quantify the relative competitive ability of giant foxtail and common lambsquarters in a mixed–weed species environment, and to assess weed density, weed relative leaf area, and weed relative volume as predictors of soybean yield loss. Based on weed density, coefficient estimates of percent soybean yield loss as giant foxtail or common lambsquarters densities approached zero differed between years. In contrast, coefficient estimates of maximum soybean yield loss were similar between years. Based on weed relative leaf area, estimates of giant foxtail or common lambsquarters damage coefficients differed between years. Similarly, estimates of maximum soybean yield loss associated with common lambsquarters leaf area differed between years, whereas estimates of maximum soybean yield loss associated with giant foxtail leaf area did not change over time within a growing season or between years. Based on weed relative volume, estimates of giant foxtail or common lambsquarters damage coefficients differed between years. Similarly, estimates of maximum soybean yield loss associated with common lambsquarters volume differed between years, whereas estimates of maximum soybean yield loss associated with giant foxtail volume did not change over time within a growing season or between years. Based on weed density, weed relative leaf area, or weed relative volume, giant foxtail was more competitive than common lambsquarters in terms of soybean yield loss. Temporal variability of weed density, weed relative leaf area, and weed relative volume indicates that additional parameters may be required to accurately predict weed–crop interactions in a multiple–weed species community.

The diversity of resistance among wild oat collected before and after commercial introduction of imazamethabenz, difenzoquat, diclofop, fenoxaprop-P, sethoxydim, and tralkoxydim was evaluated. Wild oat sampled in 1964 and 2000 from the Red River Valley of Minnesota and North Dakota were screened for resistance. Nearly 43% of the 1964 collections were susceptible (S) to all six herbicides, whereas only 9% were S by 2000. The frequency of resistance in 2000 compared with 1964 increased for all six herbicides, and 27 phenotype response groups to the six herbicides occurred in 2000 vs. 14 phenotype response groups for the 1964 collection. The proportion of resistant (R) plants increased faster for the aryloxyphenoxypropionate (APP) herbicides, diclofop and fenoxaprop-P, than for the cyclohexanedione (CHD) herbicides, sethoxydim and tralkoxydim. High diversity of resistance responses was observed in wild oat to acetyl-coenzyme A carboxylase–inhibitor herbicides, suggesting that there may be multiple APP herbicide– or CHD herbicide–specific resistance mechanisms in addition to those that confer cross-resistance. The trend of resistance response generally indicates that increased exposure to herbicides in wild oat may confer resistance to newly introduced but unrelated herbicides.

Environmental legislation may impose limitations on the quantity of nitrogen (N) used in corn production on the basis of soil type and ground water flow. If N rates are reduced, this might influence the relative competitiveness of weed species. Therefore, the objectives of this research were to develop a surface response model to provide estimations of the effect of differing N rates on threshold values of green foxtail in corn and to use this model as a theoretical framework for hypothesis testing. Field experiments were conducted from 1999 to 2001 to examine the interaction of N rate and green foxtail density on corn grain yield. The experiment was designed as a two-factor factorial with N levels ranging from 0 to 200 kg N ha−1 and targeted green foxtail densities ranging from 0 to 300 green foxtail plants m−2. The addition of up to 200 kg N ha−1 increased corn grain yield in both weed-free and weedy treatments. Corn yield loss attributed to green foxtail ranged from 35 to 40% at 0 kg N ha−1 to 12 to 17% at 200 kg N ha−1. Ridge analysis of the response surfaces indicated that optimal corn grain yield could be achieved at derived values of 131 to 138 kg N ha−1 while maintaining a green foxtail density of 8 to 9 green foxtail plants m−2 on a sandy soil with less than 2% organic matter. The analyses of simulation results led to the generation of hypotheses of practical relevance to N management. On the basis of the generated hypotheses, a legislated reduction in N or an increase in the cost of N fertilizer would result in a lower threshold value for green foxtail in corn. If legislation were to ban the use of all herbicides in corn production, higher N rates or an increase in mechanical weed control measures would be required to offset yield losses caused by green foxtail. The human health and environmental consequences of such legislation would be significant.

The combination of Aphthona spp. flea beetles and herbicides can increase leafy spurge control when compared with either method used alone, but the effect of herbicides on the biocontrol agent and conversely the effect of Aphthona on herbicides within the plant are unknown. Understanding this interaction will allow for maximum weed control with little or no negative effect on the establishment and reproduction of the biological agent. Fall application of picloram plus 2,4-D had a minimal negative effect on the A. nigriscutis population and no effect on overwintering fitness. The number of A. nigriscutis adults collected from soil cores taken from the field was similar regardless of the herbicide application date, although emergence was variable. Leafy spurge soluble and insoluble carbohydrate and soluble protein concentrations, which are indirect measurements of the herbicide's effect on nutrient availability to the insect, were similar in treated and untreated plants. Aphthona nigriscutis larval feeding did not affect the absorption or translocation of 14C–2,4-D or 14C-picloram alone or in combination in leafy spurge. The observed increase in leafy spurge control due to the combined treatment was likely an additive effect of herbicide toxicity to root tissue and A. nigriscutis larval feeding on root buds.

Field studies were conducted to compare the barnyardgrass suppression by four U.S. (‘Starbonnet’, ‘Kaybonnet’, ‘Lemont’, and ‘Cypress’) and three highly competitive, high-yielding Asian cultivars (‘PI 312777′, ‘Guichao’, and ‘Teqing’). The economic consequence of applying less than the recommended propanil rates to these cultivars was also evaluated. Grain yields increased, and barnyardgrass biomass decreased with increasing propanil rates. With or without propanil, the Asian rice cultivars consistently suppressed barnyardgrass more and consequently produced higher grain yields than did U.S. cultivars. The economic benefit derived from propanil application was less for Asian than for U.S. cultivars. Asian cultivars produced higher rough rice yields, resulting in higher net returns (not adjusted for milling) than did the commercial cultivars, but this advantage was usually reduced when adjusting for their lower milling yields. These results suggest that growing weed-suppressive Asian rice cultivars in conjunction with reduced herbicide rates could be an effective and economical weed management strategy for rice in the southern United States. However, first, their plant type and grain quality characteristics must be improved.

A growing number of invasive animal populations—both vertebrate and invertebrate—have been completely eradicated. These projects usually have been on islands, but some have been on large continental areas, and many technologies have been used. Total eradication of plant populations has been reported less frequently, but there are nevertheless many successes. Though biological features may tend to make plant eradication more difficult than animal eradication, the difference in the number of success stories is probably more due to greater enthusiasm, persistence, and perhaps resources devoted to animal eradication than to biological differences. There is every reason to believe that many plant populations could be eradicated, particularly if eradication campaigns were coupled with a monitoring system that detects invasions early. Features conducive to successful eradication are (1) resources must be adequate, and there must be a commitment to see the project through to completion; (2) clear lines of authority must be established; (3) the biology of the species must be appropriate; (4) the target species must be detectable at low densities; and (5) subsequent intensive management of the system, such as for restoration, may be necessary. For success in some eradication campaigns, rapid reinvasion must be unlikely; in other instances the economics of the situation may make the attempt worthwhile even if reinvasion ensues. A failed eradication effort need not be a mistake, particularly if the eradication method used would have been utilized in traditional maintenance management. Further, the inspirational value of an eradication campaign and its enlistment of citizen support may help sensitize the public to the entire problem of invasive introduced species. Expanded eradication efforts can potentially effect enormous ecological and economic savings.

Weeds are a major constraint to crop production in smallholder farms in tropical Africa. The weed seedbank and annual recruitment are the main sources of weed infestation in crops. This study was carried out in Ibadan, Nigeria, to evaluate the effect on the seedbank of two types of planted fallow (alley cropping with leucaena and live mulch with tropical kudzu) and a natural bush fallow under four land-use intensities. Type of fallow was the main plot. Land-use intensities, consisting of continuous cropping of maize intercropped with cassava, 1 cropping yr of maize–cassava followed by 1, 2, and 3 yr of fallow, were the subplots. Averaged over a 3-yr period, the seedbank was 55% lower in the tropical kudzu plots and 43% lower in the leucaena plots compared with natural bush. The difference in seedbanks between plots cultivated after leucaena and natural fallow was 23%. Seed density of annual broadleaf weeds was high and dominated the seedbank of both planted and natural fallow. Overall, seeds of grasses occurred at low densities in all plots cultivated after 2 to 3 yr of fallow, whereas seeds of sedges occurred more in continuously cultivated plots and plots cultivated after 1 yr of fallow in all the fallow types. Seed density of perennial broadleaf weeds increased as land-use intensity decreased. Live mulch with tropical kudzu, especially when combined with 2 to 3 yr of fallow, lowered the seedbank more than the leucaena and traditional bush fallow systems.

The critical period for weed control (CPWC) is the period in the crop growth cycle during which weeds must be controlled to prevent unacceptable yield losses. Field studies were conducted in 1999 and 2000 in eastern Nebraska to evaluate the influence of nitrogen application on the CPWC in dryland corn in competition with a naturally occurring weed population. Nitrogen fertilizer was applied at rates equivalent to 0, 60, and 120 kg N ha−1. A quantitative series of treatments of both increasing duration of weed interference and length of weed-free period were imposed within each nitrogen main plot. The beginning and end of the CPWC based on an arbitrarily 5% acceptable yield loss level were determined by fitting the logistic and Gompertz equations to relative yield data representing increasing duration of weed interference and weed-free period, respectively. Despite an inconsistent response of corn grain yield to applied nitrogen, there was a noticeable influence on the CPWC. The addition of 120 kg N ha−1 delayed the beginning of the CPWC for all site–years when compared with the 0-kg N ha−1 rate and for three of the four site–years when compared with the 60-kg N ha−1 rate. The addition of 120 kg N ha−1 also hastened the end of the CPWC at three of the four site–years when compared with both reduced rates. The yield component most sensitive to both nitrogen and interference from weeds was seed number per ear. Practical implications of this study are that reductions in nitrogen use may create the need for more intensive weed management.

Greenhouse experiments were conducted to determine the effect of salinity, imazethapyr, and chlorimuron on weed growth. Five species, barnyardgrass, common cocklebur, ivyleaf morningglory, common purslane, and yellow nutsedge, were grown in potting soil and irrigated with nonsaline (EC ∼ 2 dS m−1) or sulfate-dominated saline (EC ∼ 7 dS m−1) nutrient solution. Plants were treated after emergence with imazethapyr (Pursuit formulation) at 70 g ae ha−1 or chlorimuron ethyl (Classic formulation) at 8.8 g ai ha−1. Results indicated that irrigation with saline water had no overall effect on the growth or survival of most tested weed species. Growth of yellow nutsedge (maximum height and stem diameter) was reduced for plants irrigated with saline water. Observed growth and survival trends in saline and nonsaline treatments were consistent with information on the herbicide label. Complete control of common purslane was not achieved by either chlorimuron or imazethapyr. Growth and survival of ivyleaf morningglory and yellow nutsedge were greater when plants were treated with imazethapyr than when treated with chlorimuron, whereas for barnyardgrass, growth and survival were significantly greater when plants were treated with chlorimuron. Both herbicides affected common cocklebur growth and survival in a similar way. For all tested species, most surviving plants were not vigorous and would not be highly competitive with crop plants. The results of these experiments suggest that weed control information mentioned on the labels for these herbicide formulations will not require modification for moderately saline soils.

Research on best management practices, including vegetative filter strips, is needed to evaluate the potential for reducing herbicides in surface runoff. Laboratory studies were conducted to determine the influence of different filter strip components on fluometuron adsorption. Samples were taken from a switchgrass filter strip (1-m wide) established on a Brooksville silty clay. Sampled components included switchgrass stems clipped to 4 cm, plant residue on the soil surface, and topsoil < 5-mm, 0.5- to 1-cm, 1- to 3-cm, and 3- to 5-cm deep. The filter strip topsoil samples contained 1.8, 2.2, 2.3, and 2.8% organic matter for the aforementioned depths, respectively, compared with 1.8% in soil collected from an adjacent cropped area. Fluometuron adsorption on each sample was compared at initial concentrations of 0.017, 0.5, 1.0, 2.0, and 4.0 mg L−1, the last of these representing four times the peak concentration in surface runoff. Fluometuron adsorption was greater in soil from filter strip areas than in soil from cropped areas. Averaged over concentration, the soil–water partition coefficient (K d) value for soil 1- to 3-cm and 3- to 5-cm deep was greater than the value for soil from the cropped area. Stems and residue had K d values 4.9- and 4.1-fold greater, respectively, than soil from the cropped area. Average adsorption coefficients normalized for organic carbon content (K oc) of soil < 5-mm and 3- to 5-cm deep from filter strip areas were greater than the values for soil from cropped areas.

Plant invasions that arise from human introductions of new species to a region, or from range expansion of some native plant species, can profoundly affect biodiversity and alter the structure and function of ecosystems. Although preventive strategies may be an effective way to limit plant invasions, they are difficult to achieve because adequate descriptions of biological and environmental characteristics are often lacking, and truly predictive models of invasive biology have been elusive. On the basis of a history of repeated plant introductions and the absence of a general predictive theory, it may be best to assume that plant invasions will continue. This paper explores ways to empirically study and to predict plant invasions through a study of the invasion process. Several approaches are explored, such as species demography, DNA analysis, and geographic information system reconstructions, to characterize source and satellite populations and factors that influence the spread of these populations.

A field study was conducted during 1997 to 2001 on a Dundee silt loam soil at Stoneville, MS, to examine the effects of rye and crimson clover residues on weeds, soil properties, soil microbial populations, and soybean yield in conventional tillage (CT) and no-tillage (NT) systems with preemergence (PRE)-only, postemergence (POST)-only, and PRE plus POST herbicide programs. Rye and crimson clover were planted in October, desiccated in April, and tilled (CT plots only) before planting soybean. Both cover-crop residues reduced density of barnyardgrass, broadleaf signalgrass, browntop millet, entireleaf morningglory, and hyssop spurge but did not affect yellow nutsedge at 7 wk after soybean planting (WAP) in the absence of herbicides. Densities of these weed species were generally lower with PRE-only, POST-only, and PRE plus POST applications than with no-herbicide treatment. Total weed dry biomass was lower when comparing CT (1,570 kg ha−1) with NT (1,970 kg ha−1), rye (1,520 kg ha−1) with crimson clover (2,050 kg ha−1), and PRE plus POST (640 kg ha−1) with PRE-only (1,870 kg ha−1) or POST-only (1,130 kg ha−1) treatments at 7 WAP. Soils with crimson clover had higher organic matter, NO3–N, SO4–S, and Mn, and lower pH compared with rye and no–cover crop soils. Total fungi and bacterial populations and fluorescein diacetate hydrolytic activity were higher in soil with crimson clover, followed by rye and no cover crop. Soybean yields were similar between CT (1,830 kg ha−1) and NT (1,960 kg ha−1), no cover crop (2,010 kg ha−1) and rye (1,900 kg ha−1), and rye and crimson clover (1,790 kg ha−1), but they were higher in PRE plus POST (2,260 kg ha−1) than in PRE-only (1,890 kg ha−1) or POST-only (1,970 kg ha−1) treatments.

Glufosinate-resistant transgenic creeping and velvet bentgrass plants expressing a bar gene under the control of the maize ubiquitin promoter were inoculated separately with the fungal pathogens, Rhizoctonia solani and Sclerotinia homoeocarpa, before or after treatment with 560 mg L−1 of glufosinate at a rate of 0.56 kg ha−1. Application of the herbicide 3 h before or 1 d after fungal inoculation significantly reduced infection of these transgenic grasses by R. solani and S. homoeocarpa. Assessment of the in vitro antifungal activity of the herbicide showed that 336 and 448 mg L−1 glufosinate completely inhibited the mycelial growth of S. homoeocarpa and R. solani, respectively. The results suggest that the nonselective herbicide glufosinate may also be used to suppress the activity of some fungal pathogens in turf composed of these transgenic glufosinate-resistant creeping and velvet bentgrasses.

Yield loss was related to weed species composition and density in permanent plots, recorded several weeks after sowing of spring cereals in southern Sweden. A range of agronomic situations was included by experimentally varying fertilizer application and sowing density in 33 field trials in different locations during 3 yr. Direct gradient analysis, using yield loss as the sole predictor, arranged weed community composition in the spring along a gradient of small to large yield losses. Yield loss could be explained, to some extent, by the species composition in the spring. Species associated with situations with large losses were hempnettle and wild radish, whereas several benign species were identified based on their association with lack of yield loss. The results suggest that possible predictive tools using spring species composition would be improved if they also considered soil type and seed rate. Some agronomically important weed species were not identified as associated with yield loss when assessed in terms of their abundance in the spring, which may limit the possibilities of basing management decisions on weed plant density in the spring.

Field research was conducted for 2 yr to determine the effect of reduced rates of glufosinate on growth and yield of non–glufosinate-resistant cotton. Rates of 3.4, 6.7, 13, 26.5, 52.5, and 105 g ha−1, representing 0.008, 0.016, 0.031, 0.063, 0.125, and 0.25 of an effective use rate (420 g ha−1), were applied to cotton at the two-, five-, or nine-node growth stage. Based on analysis of visual injury, cotton response decreased as application timing was delayed in one of the three experiments. Injury response was increased slightly with application at the five- compared with the two-node growth stage and was not significant for the latest application timing (nine-node stage) in two of three experiments. In two of the three experiments, plant height reduction response was lowest at the five-node stage and greatest at the nine-node stage. Regardless of application timing, plant dry weight was negatively affected only with the highest rate of glufosinate. Glufosinate application, based on node above white flower number and percent open boll, did not result in a delay in maturity. Final plant population was reduced in all experiments at the two-node application and in one of the three experiments at the five-node stage. Glufosinate application did not adversely affect final plant population when applied to nine-node cotton. Negative effects on cotton growth were not manifested in seedcotton yield reduction after glufosinate application.

GESTINF is a decision tool for postemergence weed control based on the equivalent density approach. Using observed weed densities just before treatment, the program estimates the economic return from the treatment, thus indicating whether to treat or not and, if a treatment is needed, the most economical weed control solution. Each treatment is also characterized by an environmental pollution index. GESTINF has been tested in wheat and soybeans on a farm in northeastern Italy with a total cropping area of 60 ha of wheat and 40 ha of soybean. For both crops, weed control followed the suggestions of GESTINF, whereas the remaining cropped areas were treated according to standard farm weed control practices. To compare the two weed control systems, weed control efficacy, average crop yield, and the extra time required for scouting and treatments were measured. In both crops, the treatments suggested by GESTINF showed good efficacy, and yields proved to be no different from those obtained in the fields treated with standard farm weed control practices. In most cases, GESTINF selected treatments with a lower environmental effect. The most critical point was the time required to scout the weed population that, in low-value crops or when very cheap treatments were available, reduced the weed control economic return. In wheat, GESTINF indicated that fewer fields needed to be treated than did the conventional system. However, extra costs due to both scouting and more expensive treatments balanced the savings obtained from nontreated areas. For soybean, the treatments adopted by the farm were based on a combination of pre- and postemergence practices. In this case, GESTINF identified cheaper but still efficacious treatments, significantly reducing the total cost of weed control.

The integration of cover crops with selected postemergence herbicides was evaluated on the basis of weed control and grain yields in no-till soybean and corn. Soybean was planted into wheat residue, whereas corn was planted into hairy vetch residue. Full, half, and quarter rates and sequential herbicide applications were made. The wheat cover crop did not increase weed suppression but increased soybean grain yields. Half rates of thifensulfuron plus quizalofop-P as single or split applications were as effective as full rates in reducing weed weight in soybean. Soybean grain yields were similar in the half- and full-rate treatments in 1994, but yield was highest in the full-rate treatment in 1995. The hairy vetch cover crop did not increase weed suppression but lowered corn stands and grain yields in 1995 and enhanced corn grain yields in 1996. Full, half, and quarter rates (1996 only) of nicosulfuron plus primisulfuron were equally effective in reducing weed weight. Corn grain yields were similar at all herbicide rates in 1995 but were inversely related to herbicide rate in 1996. Split herbicide applications did not improve weed suppression over single applications of the same herbicide rate in either crop. Results indicate that cover crops can improve crop productivity and reduced rates of environmentally benign herbicides can minimize the herbicide requirements in no-till corn and soybean.

It is becoming increasingly clear that prescriptions for rangeland weed control are not sustainable because they treat the symptoms of weeds rather than their cause. Future restoration of invasive plant–infested rangeland must be based on ecological principles and concepts that provide for predictable outcomes. A generalized objective for ecologically based weed management is to develop and maintain a healthy plant community that is largely invasion resistant. Successional management based on ecological principles involves modifying the processes controlling the three general causes of succession: disturbance, colonization, and species performance. The processes controlling plant community dynamics can be modified to allow predictable successional trajectories. Successional management can lead to biomass optimization models for grazing management, spread vector analysis, and using resource availability to direct weedy plant communities toward those that are desired. Our challenge is to develop ecological principles on which management can be based.