Southern and India crabgrass are important grass weeds of rice in many tropical countries. Environmental factors influenced seed germination and seedling emergence of these weeds. Seeds of both species germinated at a range of alternating temperatures (25/15, 30/20, and 35/25 C day/night), though the germination of southern crabgrass was reduced at the lowest alternating temperatures (25/15 C). Light stimulated germination of both species; however, a small proportion of southern crabgrass seeds germinated in the dark. Germination of India crabgrass was influenced to a greater degree by increasing salt and water stresses than was southern crabgrass. Seeds of both species germinated over a wide range of pH between 5 and 10. Seedling emergence of both species (98% for southern crabgrass and 94% for India crabgrass) was greatest for seeds placed on the soil surface. Seed burial depth of 2 cm completely inhibited emergence of India crabgrass, whereas for southern crabgrass, this depth was 8 cm. Knowledge gained from this study is expected to contribute to developing components of integrated weed management strategies for these species.

The effects of shade on dry-matter production, leaf area, and biomass partitioning in purple nutsedge (Cyperus rotundus L.) and yellow nutsedge (C. esculentus L.) were determined in a controlled-environment greenhouse with a day/night temperature regime of 32/26 C. For comparison with a full-light treatment, screening provided 40, 70, and 85% shade. Shading significantly reduced height of yellow nutsedge and dry-matter production, leaf-area production, and rhizome and tuber formation of both species. Shading decreased the partitioning of plant biomass into tubers and rhizomes and increased partitioning into leaves. Yellow nutsedge had a higher net-assimilation rate than purple nutsedge at all levels of shading, but the greater leaf-area duration or total amount of leaf area present in purple nutsedge resulted in greater dry-matter production by that species. The responses of the two species showed no differences in shade tolerance.

Glyphosate [N-(phosphonomethyl)glycine] applied at 1.1 kg/ha to wheat [Triticum aestivum (L.) ‘Arthur 71′] plants increased height and fresh weight of soybean [Glycine max (L.) Merr. ‘Ransom′] seedlings planted in the pot at time of application of the glyphosate as the number of wheat plants treated increased from 5 to 30/pot. Height and fresh weight of the soybean seedlings also increased as the rate of glyphosate applied to wheat plants (5/pot) increased from 1.1 to 6.7 kg/ha. Increasing the rate of glyphosate from 1.1 to 6.7 kg/ha, however, reduced the height and fresh weight of soybeans when 30 wheat plants/pot were treated. In addition, when 6.7 kg/ha of glyphosate were applied to wheat plants, soybean-seedling plant height and fresh weight decreased as the density of wheat plants per pot increased from 5 to 30. The 14C-glyphosate exuded into the soil from treated wheat plants was characterized by thin-layer chromatography. Trace amounts of the radio-label were present on thin-layer plates of leaf and stem extracts of corn (Zea mays L.) plants, which were growing in the same pots with the treated wheat plants. The zone of activity had the same Rf value as the glyphosate standard.

Field studies were conducted at Goiânia, GO, Brazil, on an Oxisol (clayey, kaolinitic, isothermic, Typic Haplustox) and at Jussara, GO, Brazil, on an Oxisol (loamy sand, kaolinitic, isothermic, Typic Haplustox) during 1995 and 1996 to determine the carryover effect of fomesafen, imazamox, and acifluorfen, applied to edible bean, on rotational crops (maize, sorghum, rice, and millet) and to estimate the level of soil residues under Brazilian Savanna conditions. Averaged across locale, year, and rate, fomesafen dissipation time (DT50) (37.5 d) was longer than acifluorfen (27.5 d) and imazamox (25.9 d). For both locations, soil herbicide persistence (average of herbicides) was longer in 1995 than in 1996. This was due to higher soil moisture content in 1996. The sensitivity of rotational crops to fomesafen and imazamox residues was, in decreasing order: sorghum, corn, millet, and rice, and for acifluorfen: sorghum, corn, rice, and millet. The period between herbicide application and rotational crop planting (PAP) varied in agreement with the sensitivity of rotational crops to herbicide residues in soil and the persistence of the herbicide. Considering both location and year, the PAP for fomesafen (250 g ai ha−1) ranged from 69 to 132 d for corn, 114 to 179 d for sorghum, 29 to 95 d for rice, and 52 to 111 d for millet; the PAP for imazamox (40 g ai ha−1) ranged from 68 to 111 d for corn, 78 to 139 d for sorghum, 25 to 75 d for rice, and 40 to 102 d for millet; and the PAP for acifluorfen (170 g ai ha−1) ranged from 56 to 89 d for corn, 96 to 139 d for sorghum, 61 to 95 d for rice, and 43 to 82 d for millet.

Many prime wetlands in North America have been degraded following encroachment by the exotic plant purple loosestrife. Conventional methods are unsuccessful in providing long-term control. Host specificity studies demonstrated the suitability of two leaf beetles, Galerucella calmariensis and G. pusilla, as biological weed control agents. Adults oviposited only on plants within the genus Lythrum. The only species other than purple loosestrife where adult feeding and oviposition occurred and that supported successful larval development was winged lythrum. Swamp loosestrife and winged lythrum may be vulnerable to limited attack by newly emerged teneral adults. Evaluation of the potential environmental impact of the two leaf beetles showed that benefits of an introduction outweigh potential risks to winged lythrum or swamp loosestrife. Their field release was approved in 1992.

Common cocklebur (Xanthium pensylvanicum Wallr. ♯ XANPE), velvetleaf (Abutilon theophrasti Medic. ♯ ABUTH), and jimsonweed (Datura stramonium L. ♯ DATST) are weeds that grow taller than soybeans [Glycine max (L.) Merr.] and compete for limited resources such as light. These weeds have an average area of influence of 0.5 m2, thus a density of 1 to 2 plants/m2 forms a full canopy of weed leaves above the soybeans that intercept 44 to 56% of the sunlight. Shade of 44 to 56% without weeds, placed above soybeans late in the season corresponding to times of weed shading, reduced soybean yield 19 to 26%. Since reported soybean yield reductions at weed densities of 0.7 to 2.5 plants/m2 were 18 to 54% for common cocklebur and 12 to 31% for jimsonweed and velvedeaf, it was concluded that most of the interference from jimsonweed and velvetleaf infestations in soybeans could be ascribed to competition for light, but only about one-half the interference was due to competition for light for common cocklebur. Soybean photosynthesis in the field was reduced about twice as much as soybean yield at equivalent levels of shading.

Sourgrass is a perennial weed infesting annual and perennial crops in Brazil. Three biotypes (R1, R2, and R3) of sourgrass suspected to be glyphosate-resistant (R) and another one (S) from a natural area without glyphosate application, in Brazil, were tested for resistance to glyphosate based on screening, dose-response, and shikimic acid assays. Both screening and dose-response assays confirmed glyphosate resistance in the three sourgrass biotypes. Dose-response assay indicated a resistance factor of 2.3 for biotype R1 and 3.9 for biotypes R2 and R3. The hypothesis of a glyphosate resistance was corroborated on the basis of shikimic acid accumulation, where the S biotype accumulated 3.3, 5.0, and 5.7 times more shikimic acid than biotypes R1, R2, and R3, respectively, 168 h after treatment with 157.50 g ae ha−1 of glyphosate. There were no differences in contact angle of spray droplets on leaves and spray retention, indicating that differential capture of herbicide by leaves was not responsible for resistance in these biotypes. The results confirmed resistance of sourgrass to glyphosate in Brazil.

The discovery of the first systemic or hormone herbicides, 2,4-D, 2,4,5-T, and MCPA, initiated an agricultural revolution and modern weed science. The finding of these herbicides was a striking case of multiple independent discovery by four groups of workers in two countries, the United Kingdom and the United States: William G. Templeman and associates at Imperial Chemical Industries; Philip S. Nutman and associates at the Rothamsted Agricultural Experiment Station; Franklin D. Jones at the American Chemical Paint Company; and Ezra Kraus, John Mitchell, and associates at the University of Chicago and the U.S. Department of Agriculture. Because of wartime and commercial secrecy, the usual procedures of scientific publication and patent disclosure were not followed; instead, the first scientific report on these herbicides occurred in a publication by workers who were not original discoverers. Considerable confusion consequently resulted concerning the discovery and the discoverers. This confusion has not been completely dispelled in subsequent years. The present report summarizes the complete story, clarifies the chronology of the discoverers and their publications, and makes the case that all four groups of workers deserve credit for this revolutionary advance. The scientific background of the discovery and events in its immediate aftermath, especially the ticklish patent situation, are also briefly chronicled.

Previous research has examined the extent to which red rice affects both yield and grain quality of cultivated rice. However, this research was conducted over 15 yr ago. Modern long-grain rice cultivars have the potential to produce yields above 10,000 kg ha−1; however, it is unknown whether modern rice cultivars sacrifice competitiveness to achieve higher yields, or if, in fact, they are more competitive. Field studies were conducted in 2002 and 2003 at the Southeast Research and Extension Center near Rohwer, AR, and at the University of Arkansas Pine Bluff Research Farm near Lonoke, AR, to investigate the effect of red rice density on interference between red rice and five rice cultivars (‘CL161’, ‘Cocodrie’, ‘LaGrue’, ‘Lemont’, and ‘XL8’). White rice yield reductions were between 100 and 755 kg ha−1 for every red rice plant m−2. The hybrid rice, XL8, had higher yields than the conventional cultivars. Red rice contamination in milling samples increased linearly as a function of red rice density at Lonoke and Rohwer in 2003. Dockage for each cultivar was calculated on the basis of the relationship between red rice density and red rice contamination. Semidwarf Lemont was the most contaminated and hybrid XL8 the least contaminated by the various densities of red rice.

Gene flow from imidazolinone (IMI)-resistant domestic sunflower to IMI-susceptible common sunflower and prairie sunflower was studied. Under greenhouse conditions, pollen from IMI-resistant domesticated sunflower was applied to flower heads of IMI-susceptible common and prairie sunflower. In addition, field studies were conducted in 2000 and 2001 near Manhattan, KS, to evaluate IMI-resistant gene flow from IMI-resistant domesticated sunflower to common and prairie sunflower under natural conditions. Common and prairie sunflower were planted in concentric circles at distances of 2.5, 5, 15, and 30 m around a densely planted IMI-resistant domesticated sunflower species. For both greenhouse and field studies, IMI-resistant gene flow was determined by treating the progeny of both wild species with 40 g ai ha−1 of imazamox. Greenhouse crosses made by hand showed that 94% of common sunflower and 79% of prairie sunflower were resistant or moderately resistant. The resistant plants were allowed to grow in the greenhouse and were backcrossed with the corresponding susceptible wild parents. Progeny of the backcross showed a 1:1 ratio of resistant to susceptible plants. In the field, gene flow was detected up to 30 m from the pollen source for both species, and it decreased as distance from the pollen source increased. In 2000, 11 to 22% of the progeny were resistant at 2.5 m from the pollen source and 0.3 to 5% were resistant at 30 m. In 2001, the number of resistant progeny did not exceed 7 and 2% at 2.5 and 30 m from the pollen source, respectively. The results of this study showed that IMI-resistant domesticated sunflower outcrosses with common and prairie sunflower over distances typically encountered near production fields. Also, backcrosses of resistant hybrids with wild parents are successful, further increasing the potential for the spread of IMI-resistant feral sunflowers.

Metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] has considerable potential for yellow nutsedge (Cyperus esculentus L.) control in crops commonly produced in the Texas High Plains. Little information is available, however, concerning adsorption characteristics of metolachlor in soils of this region. Adsorption and movement of metolachlor were determined in three commonly occurring soils of the Texas High Plains. Organic carbon contents of the soils by weight varied from 0.3 to 0.5%, and the clay fraction ranged from 16 to 33%. Freundlich adsorption isotherms exhibited two linear regions for each soil, suggesting the possibility of multilayer adsorption. K′oc values and coefficients of determination for organic carbon and clay content vs. Freundlich K values indicated that organic matter was the predominant adsorbent for metolachlor in the soils studied. Desorption, soil column leaching, and thin-layer plate studies demonstrated metolachlor to be sufficiently mobile in scils low in organic matter to cause possible crop injury or loss of efficacy.

Soil incorporation and site of uptake of preemergence herbicides was investigated under greenhouse conditions using green foxtail (Setaria viridis (L.) Beauv.) and the herbicides 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine), 2-chloro-N-isopropylacetanilide (hereinafter referred to as CP31393), 3-amino-2,5-dichlorobenzoic acid (amiben), 3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea (linuron), ethyl N,N-dipropylthiolcarbamate (EPTC), and α,α,α-trifluroro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin). As depth of incorporation of most herbicides was increased, control of green foxtail decreased, indicating a dilution effect. Subsurface application of herbicides at a 2-in depth generally was less than half as effective as surface application or shallow incorporation.

With three different moisture conditions, shallow incorporation of trifluralin to depth of 1 in was beneficial at all moisture levels. Shallow incorporation of atrazine and EPTC improved effectiveness under conditions analagous to furrow irrigation but not under conditions simulating moderate or high rainfall immediately after application. Shallow incorporation of the ammonium salt of amiben increased effectiveness significantly under high moisture but decreased effectiveness under low moisture. Effectiveness of linuron was reduced by incorporation under moderate and high moisture. Shallow incorporation did not significantly affect results with CP31393 under any moisture condition.

For the six herbicides tested, the expression of lethal action on tops of green foxtail was evident when herbicide was placed in the shoot zone but not when placed in the root zone during the 2-week period after seeding.

In two experiments beans were kept weed-free for various periods early in the season, and in a third, weeds were allowed to grow with beans for various periods before being removed. Weeds which emerged soon after planting caused extensive reductions in bean yields, but the actual yield-reducing competition occurred considerably later in the season. For 5 to 7 weeks after planting, the grower must control weeds by chemical or other means; for the remainder of the season, bean competition provides weed control.

Density and spatial arrangement (rectangularity) effects on the competitive relationships, yield performance, and dynamics in canopy dominance of winter wheat and Italian ryegrass were evaluated using two addition series experiments. In experiment 1, combinations of six densities of each species formed the treatment matrix of addition series. In experiment 2, each species was tested at four densities and three rectangularities (RE) of winter wheat. In monocultures, crop density (plants per square meter) explained 82 to 85% of the total variation in the per-plant biomass of winter wheat in experiment 1. In mixtures of crop and weed, initial wheat density (N1) and initial ryegrass density (N2) and interaction of N1 and N2 explained 74 to 80% of the total variation in the per-plant biomass of winter wheat and 68 to 79% of Italian ryegrass in experiment 1. Intraspecific competition was apparent between 15 and 90 days after emergence (DAE) in winter wheat and between 90 and 170 DAE in Italian ryegrass. In mixtures, RE influenced plant size of Italian ryegrass up to 50 DAE only. Maximum winter wheat intraspecific competition occurred at 170 DAE, but maximum interspecific competition occurred during reproductive stages in mixtures. High RE increased seed yield, seed size, and harvest index of winter wheat and reduced biomass of Italian ryegrass. Grain yield of winter wheat was reduced up to 92% by competition from ryegrass. Even nine ryegrass plants in 100 winter wheat plants m−2 reduced winter wheat grain yield by 33%. However, the extent of loss in winter wheat grain yield was less in RE 16 (wider spacing) than in RE 1 (square planting) or 4 (close row spacing). Winter wheat was the stronger competitor during vegetative stages, but Italian ryegrass became the stronger competitor during the reproductive stages of development. Winter wheat leaves dominated at the top canopy during the vegetative stage, but ryegrass dominated at the top canopy during the reproductive stages. In the top canopy of mixtures at 200 DAE, the leaf area indices (LAI) of ryegrass was 6.6 times greater than winter wheat at RE 1 compared to only 1.6 times at RE 16. Greater LAI of Italian ryegrass in the top canopy reduced photosynthetically active radiation available to winter wheat by 68% at booting stage.

Laboratory and field studies were conducted to compare the degradation rate of EPTC (S-ethyl dipropylthiocarbamate) in soils with a previous history of EPTC application vs. soils with no prior EPTC application. Laboratory experiments showed a rapid breakdown of 14C-carbonyl EPTC to 14CO2 in Kennebec silt loam (sil) and Tripp very fine sandy loam (vfsl) with prior exposure to EPTC. A single prior application of EPTC to Tripp vfsl was sufficient to increase the rate of 14CO2 evolution. The rate of EPTC degradation in Tripp vfsl with a history of EPTC exposure was dependent on soil moisture from below 3% and independent of moisture above 3%. Patterns of degradation at 5, 15, and 25 C in the Kennebec sil with and without prior history were described by exponential decay. EPTC was degraded more rapidly at 15 and 25 C in the Kennebec sil with a prior EPTC history. Under field conditions the breakdown of EPTC in a Kennebec sil with and without prior exposure to EPTC was similar to laboratory results.

Two major properties that determine weed seed germination are soil temperature and moisture content. Incident radiation is the primary variable controlling energy input to the soil system and thereby influences both moisture and temperature profiles. However, many agricultural field sites lack proper instrumentation to measure solar radiation directly. To overcome this shortcoming, an empirical model was developed to estimate total incident solar radiation (beam and diffuse) with hourly time steps. Input parameters for the model are latitude, longitude, and elevation of the field site, along with daily precipitation with daily minimum and maximum air temperatures. Field validation of this model was conducted at a total of 18 sites, where sufficient meteorological data were available for validation, allowing a total of 42 individual yearly comparisons. The model performed well, with an average Pearson correlation of 0.92, modeling index of 0.95, modeling efficiency of 0.80, root mean square error of 111 W m−2, and a mean absolute error of 56 W m−2. These results compare favorably to other developed empirical solar radiation models but with the advantage of predicting hourly solar radiation for the entire year based on limited climatic data and no site-specific calibration requirement. This solar radiation prediction tool can be integrated into dormancy, germination, and growth models to improve microclimate-based simulation of development of weeds and other plants.

Salicylic acid, p-hydroxybenzoic acid, caffeine, hydroquinone, and umbelliferone were evaluated in the greenhouse for their effects on shoot dry-weight accumulation of several crop and weed species. With the exception of caffeine, all the chemicals reduced shoot growth in oats (Avena sativa L. ‘Goodfield’). Chemicals applied preplant incorporated, preemergence, or postemergence were effective, depending upon the rate of chemical. When applied preplant incorporated at rates as high as 56.0 kg/ha, most of the chemicals reduced growth of corn (Zea mays L. ‘B73 × Mo17′), soybean [Glycine max (L.) Merr. ‘Corsoy’], velvetleaf (Abutilon theophrasti Medic.), redroot pigweed (Amaranthus retroflexus L.), and wild proso millet (Panicum miliaceum L.). Exceptions were caffeine on corn and soybean and hydroquinone on soybean. At 11.2 kg/ha the chemicals inhibited the weed species more than the crop species. Postemergence applications of caffeine and hydroquinone inhibited growth of the weed species more than the crop species. Hydroquinone at 1.1 kg/ha inhibited redroot pigweed, but rates as high as 11.2 kg/ha did not inhibit soybean. These experiments show that growth of agronomically important crops and weeds can be inhibited differentially by allelopathic chemicals.

Characterizing the response of weeds to canopy shade is important for improved understanding of crop–weed competition and weed population dynamics. In 2000 and 2001, field studies were conducted in central New York state to examine the influence of three neighbor types (none, broccoli, or broccoli plus winter rye) and two locations (between or within rows of broccoli) on the morphology, phenology, and seed germination characteristics of Powell amaranth. Reductions in light availability and in the ratio of red-to-far red light were associated with increases in (1) partitioning of dry weight to stem tissue, (2) stem elongation, and (3) specific leaf area. Canopy shade also resulted in fewer main leaves at flowering and a reduced rate of leaf appearance but had no effect on the number of days to flowering. The relationship between Powell amaranth fecundity and aboveground dry weight was allometric, with both parameters declining significantly under competition. The weight of seeds produced did not vary significantly according to the competitive environment experienced by the maternal parent. However, the germination percentage of viable seeds was 40 to 50% lower for seeds maturing on plants grown under competition than without competition. Reductions in the number of main leaves at flowering and increased seed dormancy may be adaptive responses to canopy shade. Both mechanistic crop–weed competition models and population dynamic models would benefit from incorporation of data on the phenotypic plasticity of morphology, phenology, and seed germination characteristics of weeds.

This paper describes the application of airborne video data with global positioning system and geographic information system technologies for detecting and mapping Chinese tamarisk infestations in the southwestern United States. Study areas were along the Colorado River in southwestern Arizona, the Rio Grande River in extreme west Texas, and the Pecos River in west-central Texas. Chinese tamarisk could be readily distinguished on conventional color video imagery in late November when its foliage turned a yellow-orange to orange-brown color prior to leaf drop. The integration of the global positioning system with the video imagery permitted latitude/longitude coordinates of Chinese tamarisk infestations to be recorded on each image. The global positioning system latitude/longitude coordinates were entered into a geographic information system to map Chinese tamarisk populations along the three river systems.

The influence of temperature and relative humidity on the activity of acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} on common cocklebur (Xanthium pensylvanicum Wallr.) and common ragweed (Ambrosia artemisiifolia L.) was studied in growth chambers. Treatment at high relative humidity (85 ± 5%) resulted in a significant increase in acifluorfen phytotoxicity and decrease in plant dry weight as compared to treatment at a lower (50 ± 5%) relative humidity (RH). Temperature (32 C day, 22 C night or 26 C day, 16 C night) was not as critical as relative humidity in influencing acifluorfen phytotoxicity. Addition of a non-ionic surfactant to the herbicide solution resulted in increased phytotoxicity. Penetration and translocation of acifluorfen in common cocklebur were increased when applications were made at high relative humidity.