Application of nitrogen (N) fertilizer to sorghum at planting is a common practice that could confound competitive relationships of the crop with weeds. We studied the competitiveness of grain sorghum (Pioneer Brand 8333) relative to that of the annual weed shattercane and the perennial weed johnsongrass. The taxa are closely related, so survival requirements should be similar thus increasing the likelihood of finding differences associated with traits of the crop vs. weediness. Objectives of this research were to establish a competitive hierarchy for this crop–weed complex and to determine if relative competitiveness was affected by added N. A replacement design experiment was used in which plants were grown for 31 d in soil-filled pots placed outdoors. Taxa were planted in monocultures and 50:50 mixtures, representing all possible combinations of taxa, at a total density of 16 plants pot−1. Soil moisture was maintained at field capacity by daily additions of water or 30 μg ml−1 N in the form of an inorganic salt solution (KNO3). There was no response to the solution containing exogenous N likely because the amount of N in soil was greater than demand. Actual shoot and root dry weights in mixtures were compared with the expected dry weights, which were calculated as 50% of the root and shoot dry weights in monoculture. For grain sorghum, actual dry weights in mixture were often better than expected. Replacement series indices calculated from dry weight data described grain sorghum as competitively superior to its weedy relatives. These results indicate that further research on N management for cultivated sorghum, as a means of increasing crop competitiveness relative to that of weeds, may be unwarranted. However, a better understanding of other competition mechanisms inherent in grain sorghum might suggest management alternatives to enhance crop competitiveness with weeds.

A survey conducted in a seven-county region of eastern Oregon during the summers of 1998, 1999, and 2000 characterized the occurrence of jointed goatgrass × wheat hybrids and the features of weed infestations promoting hybridization. During the survey, 93 infested sites were visited with jointed goatgrass collected from 57 sites and hybrids collected from 45 sites. Thirteen collection sites were located in uncultivated areas. Observations of jointed goatgrass infestations in and around cropped fields suggested that jointed goatgrass successfully escapes control where weed populations persist in fencerows, access roads, scablands, draws, and roadsides. Most jointed goatgrass and hybrid populations were located in winter wheat fields but were also found in five spring grain fields. Of the 754 hybrid plants collected, 44% contained backcross seed. For all 3 yr, a 1% backcross hybrid seed–production rate was found. A parentage analysis of a subsample of the total hybrid collection showed that the majority were F1 hybrids and that jointed goatgrass was most often the female parent. This observational study has established that F1 hybrids are common in jointed goatgrass–infested wheat fields. Their capability for backcross seed production suggests the potential development of advanced backcross forms that resemble jointed goatgrass. The survey results offer valuable input for the risk assessment of gene flow potential between jointed goatgrass and herbicide-resistant wheat.

Experiments were carried out to investigate weed seed production in widely spaced spring wheat crops that received aggressive mechanical weed control (hoeing and harrowing) compared with that in narrowly spaced crops receiving less aggressive mechanical control (harrowing only). Three species (wild buckwheat [Polygonum convolvulus], ladysthumb [Polygonum persicaria], and common chickweed [Stellaria media]) were studied in three row-spacing treatments (10, 20, and 30 cm) and two sowing densities (140 and 180 kg ha−1). Average seed production per surviving plant was up to three times higher in the 30-cm treatments compared with the 10-cm treatments. Taking into account the 40–50% weed mortality resulting from control in the 30-cm treatments, seed production per seedling was still higher in the 30-cm treatments than in the 10-cm treatments. Differences in wheat yield were not found among treatments. From the perspective of long-term weed population management, using a narrow row spacing would be more effective in spring wheat on the basis of experiments with the weed species considered here.

Studies were conducted to determine the effect of interference between ladysthumb and cotton on plant growth and productivity. Ladysthumb remained shorter than cotton until at least 70 d after cotton planting. However, ladysthumb grew over twice as tall as cotton and, depending on plant density, produced between 179 and 681 g dry biomass per plant by cotton harvest. Ladysthumb biomass per plant was not affected by weed density when grown with cotton. When grown alone, ladysthumb produced over 2,000 g dry biomass per plant, which was over four times greater than biomass produced by plants grown with cotton. Cotton lint yield decreased between 0.7 and 0.9 kg ha−1 with each gram increase in weed dry biomass per meter of the row. The relationship between ladysthumb density and cotton percent yield loss was described by the rectangular hyperbola model with the asymptote (coefficient a) constrained to 100% maximum yield loss. The estimated coefficient i (yield loss per unit density as density approaches zero) was 35 ± 5 and 14 ± 2 in 1998 and 2000, respectively. Ladysthumb seed production was also described by the hyperbolic function. Estimated seed production at 1 plant m−1 of cotton row was 33,000 and 47,000 seed m−2 in 1998 and 2000, respectively.

The exotic plant, swallow-wort, a twining perennial of the Milkweed family, has become increasingly invasive in natural areas, successional old fields, and tree plantations on calcareous soils of the lower Great Lakes basin. Because mechanical control strategies are inadequate, swallow-wort's response in natural areas to foliar spray and cut-stem applications of glyphosate and triclopyr was evaluated. Foliar spray applications of glyphosate (3.1 and 7.8 kg ae ha–1) and triclopyr (1.9 kg ae ha–1) were more effective than cut-stem applications of the herbicides (3.1 kg ae ha–1 glyphosate, 1.4 kg ae ha–1 triclopyr). There were no statistical differences in effect among the foliar spray applications. Response to cut-stem application of glyphosate (3.1 and 6.2 kg ha–1) and triclopyr (1.4, 2.8, and 5.6 kg ha–1) in the following year indicated glyphosate to be more effective than triclopyr at all concentrations tested. To effect long-lasting control, all treatments require additional herbicide application.

Sprouting percentage was estimated for purple nutsedge tubers in the field from daily fluctuating soil temperatures. Tuber sprouting under alternating temperatures ranging from 20 to 45 C for 14 d responded quadratically to alternations of high and low temperature. A response surface regression of the cumulative sprouting percentage accounted for 88% of the variation. The cumulative sprouting percentage curves were sigmoidal, and the Richards function satisfactorily regressed the characteristics of the curves. A simulation model was developed for the cumulative sprouting percentage by estimating sprouting from daily high and low temperatures and accumulating daily increments of tuber sprouting. Five weeks of soil solarization with clear polyethylene film at Waimanalo, Hawaii raised the mean soil temperature at 15-cm depth by 5.8 C in spring and by 7.2 C in summer. Solarization also increased the mean daily temperature difference from 1.5 to 3.7 C in spring and from 2.3 to 3.8 C in summer. Solarization increased the final sprouting percentage in the field from 74 to 97% in spring and from 97 to 100% in summer. The simulation model estimated the final field sprouting of tubers within 95% confidence intervals of the observed means.

Interest in using crop competitiveness as an integrated weed management tool is increasing. Our objective was to describe traits that could be sources of the competitiveness we previously observed in grain sorghum grown in association with shattercane, which is a common annual weed and a close relative of the crop. Such information could aid in developing management practices for cultivated sorghum to improve its competitiveness with weeds. A bioassay was conducted to compare emergence of the crop and the weed in the greenhouse, and vegetative growth was monitored for 31 d in a within-row competition study. Results described a crop that competed well with the weed and other crop plants and agreed with studies showing that relative time of emergence influenced competitiveness. The mechanism by which grain sorghum emerged before the weed was a by-product of domestication that reduced glumes surrounding the wild-type seeds. This could be shown experimentally by hulling shattercane seeds, which then emerged almost as quickly as the grain sorghum. When planted in the grain sorghum row, shattercane plants from hulled seeds decreased the number of leaves and the root mass of the crop. Similarly, the time between emergence of the crop and emergence of shattercane was lessened by planting shattercane seeds early, and this increased the leaf number of the weed and shoot mass of the crop. It might be possible to increase weed suppression in grain sorghum by using management practices, such as more equidistant crop planting patterns that exploit the competitiveness already present, but which is being lost to interactions among crop plants.

Weed management could be more efficient and require less herbicide if growers could afford to estimate the composition, density, and distribution of weed seed banks. Spatial distribution of a weed seed bank will affect the accuracy of both mean estimates and interpolated maps of density. Consequently, information about the general characteristics of spatial distributions of seeds in a seed bank is needed to identify the most efficient strategies for sampling. Seed banks were sampled on 8.4-m square grids in eight irrigated corn fields to identify the common features of distributions of seed banks of annual weeds. Spatial dependence was described with correlograms for four to eight species in each field. Spatial dependence was detected for 36 of 45 distributions, and seed counts were correlated to an average distance of 25 to 150 m for a distribution. Seed banks of different species and fields had common features of spatial correlation: spatial pattern accounted for less than half of the total variability of seed counts, spatial correlation decreased rapidly over short distances, and ranges of spatial dependence varied with direction. For half of the distributions, the maximum range of spatial dependence was at least twice as long as the minimum range. Seed counts were correlated for the longest distances in the direction of the crop row for 16 distributions, and the distance was longer in the direction of the crop row than across rows for 26 of the 36 samples. Researchers should be able to design more efficient sampling plans for growers if the common features of spatial dependence are considered. For seed banks like these, the accuracy of maps and estimates of seed bank density may be improved by collecting multiple cores around each sampling location to mitigate the effect of short-scale spatial variability. In addition, sampling may be more efficient with grids and interpolation methods that account for ranges that are 1.5 to 2 times longer in the direction of the crop row than perpendicular to the row. With a 55- by 30-m sampling grid, adjacent observations would be correlated, and maps could be made for 80% of these seed banks. More closely spaced observations would be needed to describe the rapid decline in spatial correlation with distance for a more accurate or finer-scale map. Whether sampling seed banks for making management decisions will be cost-effective is not clear. However, potential methods to sample and map seed bank distributions more efficiently have not been exhausted.

No-tillage field studies were conducted in 1999 and 2000 at Columbia, MO, to determine the interaction of grass weed interference and side-dressed N fertilization on corn and weed growth, corn yield, and the N content of the soil and plant biomass at various intervals early in the growing season and at harvest. Ammonium nitrate (112 kg N ha−1) was applied before planting. A herbicide was applied to the entire experimental area before planting to control winter vegetation and to reduce broadleaf weed emergence. A mixture of large crabgrass, giant foxtail, and barnyardgrass was allowed to reinfest the experiment after corn planting and was sprayed with glyphosate when the weeds were 8, 15, 23, or 31 cm tall. Each removal date treatment was duplicated; one series received 45 kg N ha−1 side-dressed when the corn was approximately 60 cm tall, and the other series received no additional N. Corn dry weight and the N content of the corn biomass early in the growing season were similar between the weed-free control and treatments with grass interference up to 23-cm height. Side-dressed N applications before grass weeds were controlled did not increase the early-season N content of the corn biomass in 1999, but the N content of the grass weeds increased by 11 kg ha−1. In 2000, side-dressed N increased the N content of the corn and grass biomass by 13 and 18 kg ha−1, respectively. This suggests that grass weeds should be controlled before the side-dressed N applications to ensure that the N applied can be used by the corn rather than by the grass weeds. Grass weed interference beyond 15-cm height reduced corn yield by at least 1.13 Mg ha −1 and N content in the corn biomass at corn harvest by at least 30 kg ha−1.

Common waterhemp has become a problem weed species in Midwest soybean production. Determining the critical interference period after soybean and common waterhemp emergence is necessary for the implementation of weed control practices before soybean seed yield loss occurs. Field experiments were conducted during 1996, 1997, and 1998 to determine the influence of duration of common waterhemp interference on soybean seed yield. Removal of common waterhemp 2 wk after soybean unifoliolate leaf expansion resulted in soybean seed yield equivalent to a season-long weed-free control. Delaying common waterhemp removal until 4 wk after soybean unifoliolate leaf expansion resulted in decreased soybean seed yield. Allowing common waterhemp interference to persist 10 wk after soybean unifoliolate leaf expansion reduced soybean seed yield by an average of 43% over 3 yr. These results suggest that soybean producers should implement common waterhemp management strategies earlier than 4 wk after soybean unifoliolate leaf expansion in order to reduce the potential loss of soybean seed yield.

Today, the aim of weed management is to keep the weed community at an acceptable level rather than to keep the crop totally free of weeds. Satisfactory control of weeds may often be obtained when herbicides are used at lower doses than normally recommended. To facilitate the decision of what is an adequate dose in a specific field, the farmer needs support. In 1988 and 1989, a total of 10 field trials in spring cereals were initiated in Sweden with the objective of studying long-term effects of herbicide application according to recommendations from guidelines: the guidelines were developed at the Swedish University of Agricultural Sciences and consisted of printed cards designed for in-field use. Treatments also included a full and a half dose and an untreated control. As an average over the experimental time, i.e., until 1997, the dose used in the guideline treatment varied at different sites between 20 and 70% of a full dose. In 1998, i.e., 1 yr after the last herbicide application, the plant densities of annual weeds in the guideline treatment, the half and the full doses were 51, 57, and 67% lower, respectively, than in the untreated control when averaged over sites. At two and four sites, the half and full doses resulted in significantly lower weed densities than where guidelines had been used. Compared with the control, the full and half doses increased the proportion of difficult-to-control weed species significantly at five and four sites by 21 and 24%, respectively. In the guideline treatment the proportion of difficult-to-control weeds was increased at one site. In 1998, weed counts were higher where guidelines had been used than in the full dose for common lambsquarters and common chickweed at three sites each and for wallflower mustard, catchweed bedstraw, field violet, Galeopsis spp., and Lamium spp. at one site each. At three sites, no significant treatment effects on crop yields were found, whereas yields at the remaining seven sites were higher where guidelines had been used than in other treatments in several years. It is concluded that application of dose rates according to recommendations from guidelines can be a fruitful way to reduce herbicide use.

Understanding weed–crop interactions is critical in predicting crop yield loss, but it is also important to understand how these interactions affect weed productivity. Therefore, research was conducted to characterize the weed relative leaf area and weed relative volume of several giant foxtail cohorts in soybean, and to assess weed density and cohort emergence time, weed relative leaf area, and weed relative volume as predictors of giant foxtail shoot biomass and fecundity. Giant foxtail cohorts emerged at VE (emergence), VC (cotyledon), V1 (first node), and V3 (third node) soybean growth stages and were thinned to densities of 0, 4, 16, 36, and 64 plants m−2. Based on weed density and cohort emergence time, the maximum shoot biomass per square meter or the maximum fecundity per square meter differed between years. In contrast, shoot biomass or fecundity per plant, as weed density approached zero, and the rate at which shoot biomass or fecundity decreased exponentially, as time increased, were similar between years. Based on the weed relative leaf area, the cohort effect on giant foxtail shoot biomass differed between years, whereas the cohort effect on giant foxtail fecundity was similar between years. Maximum giant foxtail shoot biomass per square meter or fecundity per square meter differed between years when estimated from weed relative leaf area. Based on the weed relative volume, the cohort effect on giant foxtail shoot biomass per square meter or fecundity per square meter was similar between years, as was the maximum giant foxtail shoot biomass per square meter or fecundity per square meter. The temporal stability of weed relative volume, used to describe giant foxtail shoot biomass or fecundity, may aid in improving bioeconomic weed management models.

The effects of simulated trampling on shoot morphology and ethylene production of a trampling-tolerant perennial forb asiatic plantain were investigated. Trampling increased the number of leaves or inflorescences per plant, the petiole diameter, and the leaf blade length to width ratio but decreased the leaf blade width to petiole diameter ratio and the inflorescence length. Ramets subjected to trampling produced more ethylene than did nontrampled ramets originating from the same root crown. Moreover, an ethylene releaser ethephon decreased the leaf blade width to petiole diameter ratio and increased the leaf blade length to width ratio, in a manner similar to the changes induced by trampling. These results suggested that trampling-induced ethylene might be closely related to some of the adaptive morphological changes in asiatic plantain in response to trampling.

Red rice is a major weed in rice production in the southern United States. Red rice and rice intercross because they are the same species. Our objectives were to determine the genetic diversity represented by accessions of red rice and to identify DNA markers that might be useful in identifying hybrids between red and cultivated rice. Red rice accessions were collected from Arkansas and other rice-producing states. Seventy-nine red rice accessions, 10 known or putative hybrid derivatives of red rice and cultivated rice (RC hybrids), and seven rice cultivars were analyzed using microsatellite DNA markers developed for cultivated rice. Microsatellite markers differentiated awned and awnless red rice accessions, six of the seven rice cultivars, and all 10 RC hybrids tested. Thus, these markers were useful in identifying red rice types and RC hybrids.

Ultraviolet-B (UVB, 280 to 320 nm) radiation levels reaching the earth's surface are increasing because of depletion of the stratospheric ozone layer. Adverse biological effects of this harmful radiation are a serious concern. The effects of UVB radiation on spotted chickweed, dandelion, downy brome, green foxtail, redstem filaree, and common catsear seed germination, shoot and radicle elongation, and seedling growth and development in a UVB-free environment after exposure to UVB were investigated in a greenhouse study. UVB radiation (4, 7, or 11 kJ m−2 d−1) did not influence seed germination of any species except downy brome. Postgermination UVB exposure (8 h d−1, for 10 d), however, inhibited radicle elongation in all species, except in spotted catsear. Shoot elongation was also inhibited in some species but to a lesser extent than radicle elongation. A significant inhibition by UVB treatment (11 kJ m−2 d−1) of root and shoot biomass, leaf area, and leaf number was observed in some species after transfer of UVB-treated seedlings to a UVB-free environment for 3 wk. The inhibition of postgermination shoot and radicle elongation by UVB radiation, the continuation of growth inhibition after transfer of the treated seedlings to a UVB-free environment, and the differential responses of weedy species could have significant implications for their ability to compete with each other and with associated crops in a UVB-enriched environment that is likely as the stratospheric ozone layer gets depleted.

A new strain of the fungus Plectosporium tabacinum was isolated from naturally-infected false cleavers plants and evaluated as a bioherbicide for the control of both herbicide-resistant and herbicide-susceptible false cleavers in canola. Plectosporium tabacinum was nonpathogenic to both Argentine canola and Polish canola and to both herbicide-tolerant and conventional cultivars. Plectosporium tabacinum killed false cleavers seedlings when applied at a concentration of 1 × 107 conidia ml−1 and provided with 16 h dew. The efficacy of P. tabacinum on herbicide-resistant false cleavers was identical to that on herbicide-susceptible false cleavers. Catchweed bedstraw was also highly susceptible to P. tabacinum. Further host-range tests on 34 plant species in 26 genera and 12 families demonstrated that P. tabacinum is sufficiently host specific for false cleavers control in western Canada. This fungus may provide a novel approach for managing herbicide-resistant false cleavers.

Greenhouse and field studies were conducted in 1998 and 1999 to compare the growth and development of one imidazolinone-susceptible (S) and four -resistant (R1, R2, R3, and R4) smooth pigweed biotypes under noncompetitive and competitive conditions. Under noncompetitive conditions in the greenhouse, S plants accumulated biomass, grew faster during early seedling development, and accumulated leaf area sooner than plants from R2, R3, and R4 biotypes. At various times during the experiment, S plants grew faster and more efficiently used leaf area to accumulate more biomass than did R2, R3, and R4 plants. In addition, leaves emerged faster on S plants than on R2, R3, and R4 plants. R3 and R4 biotypes had significantly less chlorophyll per gram of plant tissue compared with S. In contrast, most growth parameters measured for S and R1 plants were similar. Biomass production in the field under intra- and interbiotypic competition was similar for S and all R biotypes. Findings from noncompetitive growth studies in the field were inconclusive, and further investigations are warranted. On the basis of these findings, S displayed an advantage in vegetative growth and development over three out of four imidazolinone-resistant biotypes during the early stages of development, but competitive differences were not confirmed in the field.

Studies quantifying weed seed production as a function of weed density are expensive and difficult, and lack of these data is a common limitation in modeling weed population dynamics over time. Observed empirical and theoretical relationships between crop yield loss curves and weed seed production curves led us to the hypothesis that there should be a strong relationship between the shapes of these two curves. Data from literature sources were evaluated to test this hypothesis for hyperbolic curves and to determine if the data describing the crop yield loss caused by weeds could provide estimates of the shape parameter of a hyperbolic equation for describing density dependence in weed reproduction. For each of 162 data sets, a shape parameter (N50) and a scale parameter (U) were estimated for an increasing hyperbolic model both for absolute crop yield loss as a function of weed density (N50YL, UYL) and for weed yield (either total biomass yield or seed yield) as a function of weed density (N50WY, UWY). N50YL was strongly correlated with N50WY across all data sets, with an apparent 1:1 relationship between the two. This relationship suggests that the shape parameter of the yield loss model may substitute for the shape parameter of a hyperbolic model describing the density-dependence of weed seed production. This substitution will be most useful in weed population modeling situations where data describing crop yield loss as a function of weed density are already available, but data describing weed seed production as a function of weed density are not available.

Different herbicide-application strategies may serve as part of an integrated weed management (IWM) system. In 1987 and 1988, ten field trials were initiated in the south-central part of Sweden, with the objective of studying the long-term effects of herbicide-application strategies on the development of weed populations and productivity of spring-sown cereals. Each year until 1997, herbicides were applied at 25, 50, 75, or 100% of the full recommended dose. Treatments included herbicide exclusion every second year and herbicide application at 25 or 50% of a full dose during two out of three years, with a full dose applied in the third year. An untreated control was included. In 1998, weed densities in 25, 50, 75, and 100% of a full dose were reduced 43, 58, 64, and 67% compared with the control. At seven sites, the exclusion of herbicides every second year resulted in 43 to 178% higher weed densities than in 50% of a full dose, although the same amount of herbicide was applied when summarized over 2 yr. At four sites, herbicide application each year at 50, 75, or 100% of a full dose increased the density of difficult-to-control species by 24% compared with the untreated control. Averaged over sites, the untreated control contained 30% more weed species than herbicide application at 75 or 100% of a full dose. At three sites in 1996, the full dose resulted in 520 kg ha–1 (pooled over sites) higher barley yields than in the control. Neither in 1996 nor in 1997, was there any difference in crop yield between herbicide application at 25 and 100% of a full dose. This study demonstrates the potential of reducing the input of agrochemicals for weed control by using herbicides at reduced rates in competitive crops.

A field experiment was conducted between 1988 and 1993 to determine the number of wild-proso millet seed that could be returned to the soil without increasing future soil seed bank populations. The first seed bank (1988) was 15,300 seed m−2 and ranged from 14,000 to 21,000 seed m−2, which represented a high seed population for the area. Seed rain treatments of 0, 3, 6, 12, 24, and 48% of the 1988 soil seed bank were returned each fall. By 1993, more than 90% of the original millet seed bank had been depleted for all treatments. It was estimated that 77% of the seed rain and 68% of the spring seed bank were lost each year. As a result, wild-proso millet must produce approximately three times the seed in the soil to maintain a constant seed bank population.