Controlled-environment experiments were conducted to predict the dispersal distance of horseweed seed. Seed were released from a fixed height and collected at three distances from the introduction point along a 6-m wind tunnel. Dispersal potential was assessed at wind speeds of 8 and 16 km hr−1 and release heights of 50.8 and 76.2 cm. In separate experiments, settlement velocity was determined to be 0.323 m sec−1 (SD = 0.0687). These data were used to parameterize a mechanistic model and compared to a quantile extrapolation (QE) of wind-tunnel results. The QE method predicted a greater mean dispersal distance than the mechanistic model, with large disparities between maximum dispersal distances. Quantile extrapolation predicted dispersal distances over 100 m, whereas the mechanistic model predicted a maximum distance of approximately 30 m. Air turbulence within the wind tunnel and complex dynamics of seed flight may have contributed to the discrepancy between models. Predicting the mean and numerical distribution of seed dispersal distance is crucial when estimating the spread of wind-dispersed seed and for the design of a field-sampling protocol. Although controlled-environment experiments lack the wind variability present in natural systems, predictions from wind-tunnel studies provide a better first approximation of dispersal distance than the mechanistic model. Field experiments designed on the basis of these outcomes are more likely to capture the true dispersal distribution. This should provide more accurate data to inform management decisions for wind-dispersed species.

Studies were conducted to determine the effect of ensiling and/or rumen digestion by cattle on the germination and viability of several common weed species. Seed survival of grass species subjected to ensiling and/or rumen digestion tended to be less than that of broadleaf species. Downy brome, foxtail barley, and barnyardgrass were nonviable after either ensiling for 8 weeks or rumen digestion for 24 h. Some green foxtail (17%) and wild oats (0 to 88%) seeds survived digestion in the rumen but were killed by the ensiling process. Varying percentages of seeds of kochia, redroot pigweed, common lambsquarters, wild buckwheat, round-leaved mallow, and field pennycress remained viable after ensiling (3 to 30%), rumen digestion (15 to 98%), and ensiling plus rumen digestion (2 to 19%). A time course study of rumen digestion indicated that loss of seed viability often was not a gradual process. With some species, there was an initial lag phase while degradation of the protective seed coat likely occurred, followed by a rapid decline in embryo viability. The diet fed to livestock appeared to affect viability losses caused by rumen digestion. Estimates of seed survival with varying rates of passage through the rumen due to differing ratios of grain to forage in the diet are presented.

The use of scouting and economic thresholds has not been accepted as readily for managing weeds as it has been for insects, but the economic threshold concept is the basis of most weed management decision models available to growers. A World Wide Web survey was conducted to investigate perceptions of weed science professionals regarding the value of these models. Over half of the 56 respondents were involved in model development or support, and 82% thought that decision models could be beneficial for managing weeds, although more as educational rather than as decision-making tools. Some respondents indicated that models are too simple because they do not include all factors that influence weed competition or all issues a grower considers when deciding how to manage weeds. Others stated that models are too complex because many users do not have time to obtain and enter the required information or are not necessary because growers use a zero threshold or because skilled decision makers can make better and quicker recommendations. Our view is that economic threshold–based models are, and will continue to be, valuable as a means of providing growers with the knowledge and experience of many experts for field-specific decisions. Weed management decision models must be evaluated from three perspectives: biological accuracy, quality of recommendations, and ease of use. Scientists developing and supporting decision models may have hindered wide-scale acceptance by overemphasizing the capacity to determine economic thresholds, and they need to explain more clearly to potential users the tasks for which models are and are not suitable. Future use depends on finding cost-effective methods to assess weed populations, demonstrating that models use results in better decision making, and finding stable, long-term funding for maintenance and support. New technologies, including herbicide-resistant crops, will likely increase rather than decrease the need for decision support.

Integrated weed management (IWM) became an accepted, and frequently used, term by weed scientists in the early 1970's. It has been argued by some that IWM is simply new terminology for a system that was previously called “weed control”. In any event, the acceptance of the “management concept” has assured the grower that chemicals for weed control are merely a component part of a total system for managing weeds in crops (9, 65).

An intensive field survey of an eastern Nebraska corn and soybean field was conducted to characterize the spatial structure and temporal stability of broadleaf weed seedling populations over two growing seasons. Anisotropy, the effect of direction on the relationship between observations, is present in the semivariogram for the velvetleaf and common sunflower populations in 1992 and 1993. The directional trends in aggregation are visible in kriged maps as elliptical shapes oriented east to west across the study area. In addition, there are two distinct zones of aggregation from north to south. These two distinct areas of aggregation are reflected as a ‘plateau’ in the north-south semivariogram. The distance over which this plateau extends indicates that the shape or size of the patch is contracting in the north-south direction (perpendicular to the crop row). The slope of the semivariogram in the east-west direction (aligned with the crop row) remains consistent from 1992 to 1993 suggesting that the shape of the patch is not changing. For sunflower populations, the slope of the north-south empirical semivariogram changes at 20 m, similar to the velvetleaf population semivariograms. This change, however, is reflected as a downward trend in the empirical semivariogram. The distance over which this trend occurs increases from 1992 to 1993 suggesting that seedling patch size was smaller in 1993 compared to 1992. Weed seedling establishment resulting from seed dispersal, differential seed and seedling mortality, or emergence may have resulted in the observed patch dynamics.

In Argentinean crop fields, weed control is mainly achieved by intense use of glyphosate as a nonselective and/or selective herbicide. Glyphosate use is very high as more than 95% of the 16 million ha soybean crop consists of glyphosate-resistant cultivars, always treated with this herbicide. From initial success, inconsistent glyphosate control of Johnsongrass, an invading C4 perennial grass of soybean crops, has become evident to producers from northern Argentina over the last 3 yr. Prior to this, glyphosate provided good control. This study evaluated the nature of these recurrent glyphosate failures in Johnsongrass. Experiments conducted with Johnsongrass plants obtained from seed and rhizome phytomers collected from fields with intense glyphosate use history showed that these populations showed differential survival and biomass productivity when glyphosate treated than Johnsongrass plants obtained from similar propagules collected from field sites with no history of glyphosate use. This empirical evidence establishes that the Johnsongrass survival in glyphosate-treated transgenic soybean fields from northern Argentina is due to evolved glyphosate resistance.

Inhibition of cotton (Gossypium hirsutum L.) and soybean (Glycine max (L.) Merr.) lateral roots by alpha, alpha, alpha-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) was directly related to depth and method of incorporation. Root inhibition of the crop plants was reduced when trifluralin was incorporated shallowly after planting with either a power tiller or a ground-driven tiller, to a depth no greater than 0.5 in above seed placement. A bioassay of the soil showed that disking trifluralin-treated plots before bedding caused the greatest retention of the material 3 weeks later, and that retention was less when the herbicide was applied and shallowly incorporated after planting. Further studies on the relationship of soil persistence of trifluralin to depth of incorporation 6 months after application, showed no herbicide present in plots not incorporated while only minimal residues were found where incorporation was no deeper than 1 in. Residues in plots incorporated below the 2-in depth were moderate at 0.5 to 1 lb/A rates and high at the 2 lb/A rate and above.

Dormancy is a form of developmental arrest and is an adaptive trait that promotes the survival of many organisms. In flowering plants, dormancy occurs in seeds and vegetative propagules (Lang 1996). Seed dormancy increases the distribution of germination over time, thus enhancing the survival of plants in an ever-changing environment. Seed dormancy is of intrinsic interest to weed scientists because it is one of 12 adaptive characteristics associated with weeds (Baker 1974). The sporadic emergence of seedlings derived from populations of dormant and nondormant weed seeds in the soil (Benech-Arnold et al. 2000; Forcella et al. 2000) is a key factor that dictates the need to apply weed control measures repeatedly within, between, and across growing seasons. My objective in writing this paper is to provide weed scientists, advanced students, and others with limited background information, some recent findings concerning the physiological genetics of dormancy, and steps toward identifying genes that directly regulate seed dormancy and germination. Molecular aspects of dormancy and germination will not be covered here because they have been reviewed recently (Bewley 1997; Li and Foley 1997). Readers can obtain additional and more extensive information on the biology and ecology of seed dormancy and germination from several recent books and reviews (Baskin and Baskin 1998; Benech-Arnold et al. 2000; Bewley and Black 1994; Casal and Sánchez 1998; Cohn 1996, 1998; Fennell 1999; Forcella et al. 2000; Hilhorst 1995, 1998; Hilhorst and Toorop 1997; Kelley et al. 1992; Kigel and Galili 1995; Simpson 1990; Vleeshouwers et al. 1995).

Grass and broadleaf weed densities and seed numbers, weed control practices, and grain yields were included in a bioeconomic model that evaluates alternative weed management strategies for continuous corn (Zea mays L.). Weed seed numbers in soil and herbicide carry-over provided intertemporal links. Four weed management strategies – two fixed, one mixed, and one flexible – were evaluated with annualized net returns as the performance indicator. The flexible strategy (weed control based on observed conditions) had the largest annualized net return for high and low initial weed seed numbers. The fixed weed management strategy (weed control predetermined) of an annual application of only a preemergence herbicide ranked second in terms of annualized net returns for high weed seed numbers. The mixed weed management strategy of alternative year applications of preemergence herbicide and “as needed” applications of postemergence herbicide ranked second for low initial weed seed numbers. The fixed weed management strategy of alternate year application of preemergence herbicide only generated the lowest annualized net return, regardless of initial weed seed numbers.

Greenhouse and laboratory studies were conducted to evaluate responses of ivyleaf morningglory, pitted morningglory, palmleaf morningglory, and smallflower morningglory to several herbicides in relation to leaf structure, epicuticular wax, and spray droplet behavior. Two- to four-leaf stage plants of each species were highly susceptible to acifluorfen, bentazon, bromoxynil, glufosinate, and glyphosate. However, at the five- to eight-leaf stage, these species were less susceptible, and control was herbicide specific. Spray droplets of these five herbicides had a higher contact angle on ivyleaf morningglory than the other three species with a few exceptions. Stomata and glands were present on both adaxial and abaxial leaf surfaces of all species, and palmleaf morningglory and smallflower morningglory had more of these than did the other two species. Trichomes were present on all species except palmleaf morningglory. Epicuticular wax mass was highest in ivyleaf morningglory (57 μg cm−2) and lowest in smallflower morningglory (14 μg cm−2). Wax consisted of homologous short-chain (< C18) or long-chain (> C20) hydrocarbons, alcohols, acids, and triterpenes. Smallflower morningglory waxes lacked short-chain length components. Triterpenes were absent in palmleaf morningglory and smallflower morningglory epicuticular waxes. Untriacontane (C31 hydrocarbon) and tridecanol (C30 alcohol) were common major long-chain components in waxes of all four species. Heptadecane (C17 hydrocarbon) and octanoic acid (C18) were common major short-chain length wax components in pitted, ivyleaf, and palmleaf morningglory. In spite of some differences in leaf surface structures, wax mass, and wax components among the four species, there was no clear relationship between these parameters and herbicide efficacy.

Microbial degradation of imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} was monitored by measuring 14CO2 evolution for 7 months under controlled laboratory conditions. Up to 10% of the 14C chain-labeled imazaquin that was applied to a Crowley silt loam was evolved as 14CO2 in 7 months. Less evolution of 14CO2 occurred on a Sharkey silty clay, a soil with higher clay and organic matter content, than on silt loam soils. The loss of 66 to 100% of the imazaquin applied to a Crowley silt loam incubated for 8 months at 18 C or 35 C, respectively, suggested that metabolic changes in addition to CO2 evolution were occurring. Rapid loss of imazaquin phytotoxicity occurred when soils were held at warm-moist (35 C and −33 kPa) conditions conducive to microbial growth. Imazaquin was more persistent in soils stored under cool, dry (18 C and −100 kPa) conditions. Imazaquin on a soil surface dissipated rapidly when exposed to ultraviolet light or sunlight. Photodecomposition could be a major mode of imazaquin dissipation if this herbicide is allowed to remain on the soil surface.

Two series of experiments were conducted from 1976 through 1978 on Marvyn loamy sand in central Alabama to determine the competitive relationship of yellow nutsedge (Cyperus esculentus L.) with cotton (Gossypium hirustum L. ‘Stoneville 213’). In the first series of experiments yellow nutsedge was left undisturbed or removed from plots to give periods of competition of 0, 2, 4, 6, 8, 10, and 25 weeks. Seed cotton yield was reduced 2 of 3 yr with full season (25 week) competition, but was unaffected by shorter periods of competition in all years. Main stem diameter of cotton was also reduced 2 of 3 yr when competing full season while main stem height was reduced in only 1 of 3 yr with full season competition. In the second series of experiments yellow nutsedge densities were monitored in 48 individual plots and regression analysis showed yields of seed cotton were consistently reduced with full-season competition. Yellow nutsedge shoot densities ranged from zero to 93 shoots/m2 (measured 13 cm on either side of the drill) and approximately 18 kg/ha of seed cotton were lost for each additional yellow nutsedge plant per m2.

The extent of 14C-imazaquin and 14C-imazethapyr abiotic vs. biotic degradation in soil was investigated. Degradation was measured in an in vitro system which allowed 90% recovery of applied herbicide. Triallate biodegradation is well documented and therefore used as a standard. Herbicide degradation was compared in two soils, a Cisne silt loam and a Drummer silty clay loam. Herbicide degradation in gamma-irradiated soil was compared to fresh soil. Biomass quantities were measured for the duration of the experiments. 14CO2 evolution, extractable parent, metabolites, and unextractable residue were measured. After 12 weeks of incubation, 95% of the radioactivity could be extracted as parent from sterilized soil. In unsterilized soil, imazaquin and imazethapyr degraded at a similar rate which was dependent upon soil type. All herbicides degraded slower in the Drummer soil and triallate degraded two to three times faster than the imidazolinones in either soil. 14C-imazaquin degradation products included 14CO2 and unextractable residues. The major product from 14C-imazethapyr degradation was 14CO2. Evolution of 14CO2 from an imazethapyr-treated Cisne soil, containing a serial dilution of activated charcoal, demonstrated that adsorption of herbicide was negatively correlated with degradation. Therefore imidazolinone microbial degradation is regulated by the amount of herbicide in soil solution as determined by soil characteristics.

One of the important areas of current herbicide research is the investigation of herbicide mixtures. It is a common field experience that use of a single material—even of a “broad-spectrum” herbicide—exerts a selective pressure on the field population because of differential effectiveness on the various weed species. There is some evidence that a part of this derives from purely mechanical considerations (e.g., the spraying system in use may not reach plants of different growth habit equally well), but in any case the weed control program often appears to decrease in effectiveness after several seasons. In addition to favoring the survival of particular species, the non-eradicatory application of even a highly efficient chemical apparently aids in the establishment of populations of individuals which are physiologically most tolerant of the herbicide used. It will probably be granted, then, that the use of mixtures of toxicants will provide more effective control of populations of mixed weed species, and may also reduce the numbers of individuals and species exceptionally tolerant of the herbicide program, especially when the toxicants kill by affecting different physiological processes.

Herbicide desorption isotherms may be affected by the amount of nondesorbable herbicide present in soil. Nondesorbable alachlor (as determined after methanol extraction) generally increased on a Waukegan silt loam (Typic Hapludolls) and a Ves clay loam (Udic Haplustolls) during five 0.01 M CaCl2 desorptions. Atrazine was totally extracted with methanol from the Waukegan soil after one desorption using 0.01 M CaCl2. However, after five desorptions with 0.01 M CaCl2 an average of 5.5 and 15.5% of the total recovered atrazine from two atrazine application rates was methanol nondesorbable from the Waukegan and Ves soils, respectively. Freundlich desorption isotherms adjusted for nondesorbable herbicide accounted for as much as 71 % of the difference between adsorption and desorption isotherms. Only a portion of the hysteresis observed can be attributable to nondesorbable herbicide.

The use of biological control agents for the control of noxious pests is not new conceptual research. There are examples that predate by centuries the modern use of both herbicides and insecticides. For example, the successful control of insects by biological control was shown in 1762 by the introduction of the Mynah bird (Gracula religiosa Linnaeus) from India to Mauritius (4). In 1888, the Vedalia beetle (Rodolia cardinalis Muslant) was introduced to control the cottony-cushion scale (Icerya purchasi Maskell) of citrus (3). An example of biological control of weeds was reported in 1863 when it was observed that an infestation of prickly pear cactus (Opuntia humifusa Raf. # OPUHU) was decimated by the imported cochineal insect (Dactylopius tomentosus Lam.) (5).

Imopea purpurea (L.) Roth and /. hederacea (L.) Jacq. significantly and equally reduced soybean yields and plant height, and increased lodging and the difficulty of harvesting. Damage to soybeans increased as the population of morningglories increased. Control of morningglories was needed for 6 to 8 weeks from the date of soybean planting. Morningglories germinated throughout the season and emerged from a depth of 6 inches below the soil surface.

Competitive cereal cultivars are less susceptible than others to weed interference. Their characterization may provide selection criteria that can be used as guidelines to develop new, even more competitive cultivars. Root exudates are a potential means by which competitive cultivars reduce weed growth. The objectives of this study were to evaluate the effect of cereal root exudates on Brassica kaber (DC.) L. C. Wheeler growth, to isolate and characterize the allelochemical compounds released by spring cereal cultivars, and to determine if a relation exists between these allelochemicals and cultivar competitiveness. Highly competitive (HC) and lesser competitive (LC) cultivars of four crop kinds (Triticum aestivum L. [wheat], Avena sativa L. [wild oat], two- and six-rowed Hordeum vulgare L. [barley]) were selected based on previous work. Exudates from undisturbed root systems of B. kaber and cereals were collected and used in a bioassay test with B. kaber. Root exudates were analyzed for 16 common phenolic compounds using high-performance liquid chromatography (HPLC). Bioassays indicated that cereal exudates had no negative effect on B. kaber germination, but all concentrations of cereal root exudates inhibited B. kaber root and hypocotyl growth. As cereal root exudate concentration increased, B. kaber growth decreased. For each crop kind, B. kaber growth inhibition was greater with HC cultivars than with LC cultivars. The root exudates of all crop kinds and cultivars contained benzoic, caffeic, ferulic, o-coumaric, and vanillic acids as well as scopoletin. Para-hydroxybenzoic acid was found in exudates from T. aestivum, A. sativa, and two-rowed H. vulgare cultivars. Para-coumaric acid was not identified in root exudates from LC H. vulgare cultivars. Gentisic acid was produced by A. sativa and H. vulgare. Vanillic and o-coumaric acids along with scopoletin may be responsible for the allelopathic effects of H. vulgare, T. aestivum, and A. sativa cultivars. These three compounds may be useful as possible indicators of allelopathic potential of genotypes under development and thus considered for use in breeding programs.

The influence of light, soil moisture, and temperature on establishment of selected species through hairy vetch residue on the soil surface was investigated under controlled conditions in the greenhouse. Hairy vetch residue at rates ranging from 0 to 616 g m−2 had no effect on corn, slightly reduced velvetleaf and green foxtail establishment, and severely inhibited common lambsquarters establishment under full sunlight conditions. The same rates of hairy vetch residue reduced velvetleaf, green foxtail, and common lambsquarters establishment more under a shade cloth with 9% light transmittance than under full sunlight. Day/night temperatures of 24/16 or 32/26 C had no effect and soil moistures of 50 or 133% field capacity had little effect on response of all species to residue rates. Weed establishment was similar under shade cloth without residue as under residue with an equivalent light transmittance, suggesting that light was more important than allelopathy or physical impedance for weed suppression by hairy vetch residue.

Mulching the soil with polyethylene sheets before sowing during the hot season, increased the soil temperatures, which resulted in the control of soil-borne pathogens and weeds. This method was tested in a field heavily infested with Egyptian broomrape (Orobanche aegyptiaca L.). Soil was irrigated and mulched for 36 days during August–September 1977, prior to sowing carrot (Daucus carota L. ‘Nantes Tip Top’) seeds. Mulching increased soil temperatures by 8 to 12 C, up to 56 C in the top 5 cm. In the non-mulched plots the carrot plants became stunted due to heavy parasitization with broomrape and they were completely destroyed by the end of the season. In contrast, broomrape and other weeds were controlled in the mulched plots and the carrot plants grew normally. This effect was less pronounced in the border rows of the mulched plots. Mulching also greatly reduced the infestation of other weeds. Egyptian broomrape was also controlled in two other field experiments with carrots and eggplants (Solanum melongena L. ‘Black oval’). As compared with fumigation, this new method of control is economical, simple, nonhazardous, and does not employ toxic materials.