Preliminary field experiments with 3,4-dichloropropionanilide (propanil) indicated that methods of application had little effect on the activity of this herbicide. Growth of tomato (Lycopersicum esculentum Mill.) plants was reduced for about 7 days. Susceptible plants were killed in 3 to 7 days. Propanil inhibited the growth of tomato radicles and the auxin-induced growth of Avena coleoptiles. Propanil destroyed the permeability of red beet (Beta vulgaris L.) membranes and also destroyed chloroplast membranes in vitro. Oxygen uptake by isolated mitochondria and treated leaf tissues, and phosphate esterification in mitochondria were severely inhibited by propanil. Photosynthesis was completely inhibited within 20 min after treatment but began to return to normal in resistant plants (tomato) after 6 hr.

Field studies were conducted to determine the influence of phosphorus (P) fertility and method of application (banded versus broadcast) on the competitive interaction of lettuce and spiny amaranth. Weed density significantly affected lettuce head weight and spiny amaranth shoot biomass after 5 wk of interference. Weed density and duration of interference had little or no effect on P content of lettuce tissue. Duration of interference did affect P concentration of spiny amaranth; however, weed density did not Spiny amaranth competition reduced lettuce yield, but P was not the limiting factor. Duration of interference and method of P application interactively affected lettuce head weight; however, only duration of interference affected spiny amaranth biomass. Seven wk of interference caused a decrease in lettuce head weight of 20, 8, and 24% when P was broadcast, banded, or not applied, respectively. Banding of P reduced the negative impact of spiny amaranth on lettuce. Although method of P application influenced the interaction between lettuce and spiny amaranth, interspecific competition between the two species probably was not due to competition for P but some other factor.

Field experiments were conducted to evaluate the interactions of tillage systems with imazaquin and imazethapyr on weed control and soybean injury and yield. Control of jimsonweed, common cocklebur, ivyleaf morningglory, velvetleaf, and giant foxtail from imazaquin and imazethapyr in conventional tillage was generally equal to or greater than control in no-tillage. However, under limited rainfall, weed control in no-tillage was generally equal to or greater than control in conventional tillage. Reductions in soybean heights due to herbicide treatment were evident in both tillage systems in 1985 and 1986 but not in. Soybean yields were reduced in 1985 from imazaquin at 140, 210, and 250 g/ha and imazethapyr at 105 and 140 g/ha. Yields were not reduced in 1986 and. Imazaquin and imazethapyr appear to provide adequate control of jimsonweed, common cocklebur, ivyleaf morningglory, velvetleaf, and giant foxtail in conventional and no-till systems.

Downy brome in dryland winter wheat presents a major constraint to the adoption of reduced tillage cropping systems in the Pacific Northwest of the United States. Effective suppression of downy brome during fallow periods depletes seed in the soil and reduces infestations in subsequent winter wheat crops. Delayed tillage operations or delayed herbicide applications in the spring increase the risk for production of viable downy brome seed during fallow periods. In a series of studies, downy brome panicles were sequentially sampled at Pendleton, OR, and Pullman, WA, in 1996 and 1997, and at nine locations around the winter wheat growing region of the western United States in 1999 and 2001. Cumulative growing degree days (GDD) were calculated using local, daily maximum, and minimum air temperature data. A simple GDD model based on the formula GDD = (daily maximum temperature [C] + daily minimum temperature [C])/2, with a base temperature of 0 C and a starting point of January 1, was used to calculate cumulative GDD values for panicle sampling dates. Number of seed germinating per collected panicle was recorded in greenhouse germination tests. Estimations of degree days required for production of viable downy brome seed were made using nonlinear regression of germination on GDD. The GDD value at which viable seed can be found on plants (i.e., when seed germination > 0) was of interest. Estimates of the GDD values at which viable seed could be found in the three studies ranged from 582 GDD at Bozeman, MT, to 1,287 GDD at Stillwater, OK, with a group of GDD values for Pendleton and Pullman around 1,000. Variation in seed-set GDD among locations may be attributed to differing climatic conditions that control vernalization at the various locations or to differences in vernalization requirements among downy brome biotypes (or both).

Leafy spurge can be detected during flowering with either aerial photography or hyperspectral remote sensing because of the distinctive yellow-green color of the flower bracts. The spectral characteristics of flower bracts and leaves were compared with pigment concentrations to determine the physiological basis of the remote sensing signature. Compared with leaves of leafy spurge, flower bracts had lower reflectance at blue wavelengths (400 to 500 nm), greater reflectance at green, yellow, and orange wavelengths (525 to 650 nm), and approximately equal reflectances at 680 nm (red) and at near-infrared wavelengths (725 to 850 nm). Pigments from leaves and flower bracts were extracted in dimethyl sulfoxide, and the pigment concentrations were determined spectrophotometrically. Carotenoid pigments were identified using high-performance liquid chromatography. Flower bracts had 84% less chlorophyll a, 82% less chlorophyll b, and 44% less total carotenoids than leaves, thus absorptance by the flower bracts should be less and the reflectance should be greater at blue and red wavelengths. The carotenoid to chlorophyll ratio of the flower bracts was approximately 1:1, explaining the hue of the flower bracts but not the value of reflectance. The primary carotenoids were lutein, β-carotene, and β-cryptoxanthin in a 3.7:1.5:1 ratio for flower bracts and in a 4.8:1.3:1 ratio for leaves, respectively. There was 10.2 μg g−1 fresh weight of colorless phytofluene present in the flower bracts and none in the leaves. The fluorescence spectrum indicated high blue, red, and far-red emission for leaves compared with flower bracts. Fluorescent emissions from leaves may contribute to the higher apparent leaf reflectance in the blue and red wavelength regions. The spectral characteristics of leafy spurge are important for constructing a well-documented spectral library that could be used with hyperspectral remote sensing.

When peanuts (Arachis hypogaea L.) were not kept weed-free, a single cultivation 4 weeks after emergence increased yields substantially over those of non-cultivated peanuts. Cultivation had no effect, however, when peanuts were maintained weed-free for 4 or 8 weeks. The length of the weed-free periods, cultivation, and the presence or absence of peanuts all strongly influenced the density of the sicklepod (Cassia obtusifolia L.) plants which grew above the canopy of peanut foliage. Peanut foliage, which was released from weed-free maintenance at 8 weeks but then competed with sicklepod until harvest, reduced the green weight of sicklepod 95 to 98% thus illustrating the competitive capacity of the peanut plant. Moderate stands of Florida beggarweed [Desmodium tortuosum (Sw.) DC] did not markedly affect yield of peanuts. Application of a growth regulator, succinic acid, 2,2-dimethylhydrazide (SADH), did not influence weed competition or yield of in-shell peanuts consistently at any of the three locations. Quality analyses showed that treatment variables did not modify the taste of peanuts; however, in some experiments, components of the market grade of in-shell peanuts were changed by SADH, cultivation, length of the weed-free period, and cultivation X length of the weed-free period.

Amaranthus retroflexus L. is a common annual weed worldwide. It can be found in a wide range of habitats and causes substantial yield reduction in many crops mainly through competition. Alternaria spp. are airborne molds that are considered to have potential for the biological control of weeds. The aim of this study was to assess the effect of spore concentration, host-plant growth stage, dew period, and temperature on the pathogenicity of three Alternaria alternata isolates against A. retroflexus. The pathogenicity of A. alternata increased with increasing spore concentration and length of dew period. A spore concentration of 107 spores ml−1 in a rapeseed oil emulsion and given a 24 h dew period caused 100% mortality of A. retroflexus plants at the four-leaf stage. Infection and mortality in older plants (>four-leaf stage) was lower. The highest levels of plant mortality were obtained at post-inoculum temperatures between 20 and 30 C. These experiments confirm the potential of A. alternata as a mycoherbicide under specific environmental conditions.

Invasive plants can respond to injury from natural enemies by altering the quantity and distribution of biomass among woody materials, foliage, fruits, and seeds. Melaleuca, an Australian tree that has naturalized in south Florida, has been reunited with two natural enemies: a weevil introduced during 1997 and a psyllid introduced during 2002. We hypothesized that herbivory from these and other adventive organisms (lobate-lac scale and a leaf-rust fungus) would alter the distribution and allocation of biomass on melaleuca trees. This hypothesis was tested by temporally assessing changes in aboveground biomass components in conjunction with the presence of natural enemies and their damage to melaleuca trees. Melaleuca trees of different diameters representing the range (1 to 33 cm diam at 1.3 m height) within study sites were harvested during 1996, prior to the introduction of herbivorous insects, and again during 2003 after extensive tree damage had become apparent. Aboveground biomass, partitioned into several components (woody structures, foliage, fruits, and seeds), was quantified both times in Broward, Miami–Dade, and Palm Beach county sites located in south Florida. The two harvests within each site were performed in closely-matched melaleuca stands, and changes in biomass components were compared between years. Total biomass and woody portions decreased in Broward, whereas they increased in Miami–Dade and Palm Beach sites. Reductions in foliage (on all trees) and seed biomass (among seed-bearing trees) were greatest at Broward and least at Miami–Dade County site. Hence, overall seed and foliage production was severely reduced at the Broward site where both the natural enemy incidence and damage were more abundant compared to other sites. We therefore attribute the reduced foliar biomass and reproductive capability of melaleuca trees to infestations of natural enemies. These findings highlight the role that natural enemies can play in the long-term management of invasive tree species.

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

Black nightshade plants were controlled by hoeing in the same cotton plots each year (1982 to 1986) for 3 to 15 weeks after crop emergence to evaluate the influence of several black nightshade-free periods on cotton yield, reproduction of black nightshade, and longevity of weed seeds in soil. Similar information, although limited, was also collected for Palmer amaranth that escaped the initial herbicidal treatment each year. Except for 1982, black nightshade competing with cotton for the duration of the growing season in nonhoed plots severely reduced yields (60 to 100%), with greatest yield reductions (82 to 100%) occurring in 1983 and 1984 when 0.5 to 0.7 cm of rain fell within 10 days after cotton planting. When combined with cultivation, a 3-week nightshade-free period at cotton planting was of sufficient duration to protect cotton yields. Weed seed production for all hoed treatments was less than 1% of the nonhoed treatment, and after five consecutive cotton crops (1982 to 1986), the amount of both black nightshade and Palmer amaranth seeds in soil was similar for all hoed treatments. These populations were 60 to 80% and 95 to 97% less than beginning populations of black nightshade and Palmer amaranth in 1982, respectively. After 5 yr of continuous treatments, cotton was grown in 1987, with standard cultivation as the only method of weed control, to evaluate how the weed-free periods in 1982 to 1986 influenced weed seed populations in the soil and the resultant effect on weed competition and cotton yields. Reduction of cotton yields in 1987, in the absence of weed-free periods, indicated that black nightshade seed survival in soil appears to be sufficiently long for ample establishment of this weed to compete with cotton. Thus, fields will have to be kept weed free for greater than 5 yr to reduce black nightshade populations to a level that will not reduce cotton yields.

Weed-free sugarbeets [Beta vulgaris (L.) Beauv.] growing at a spacing of 60 cm (1/2 stand) yielded the same weight of roots as those spaced 30 cm (full stand), and those spaced 90 cm (1/3 stand) yielded about 90% as much. Gross sucrose yield was reduced somewhat more, because the larger roots growing in partial stands contained a lower percentage of sucrose than the roots in full stands. Annual weeds that emerged after July 1 were suppressed and. killed by competition from sugarbeets of normal vigor in a full stand, but became competitive when the sugarbeets were spaced at 1/2 or 1/3 of a full stand. Competition from barnyardgrass [Echinochloa crus-galli (L.) Beauv.] and pigweed (mixture of Amaranthus retroflexus L. and A. powellii S. Wats.), which reached a height of 150 cm, reduced root yields 5 to 39% in 1/2 stands and 19 to 49% in 1/3 stands, as compared with weed-free sugarbeets in the same stands. Hairy nightshade (Solarium sarachoides Sendt.) was abundant but never became taller than the sugarbeets. Its competition had no measurable effect on the sugarbeets.

Witchweed (Striga asiatica), a parasitic chlorophyll-bearing plant (Scrophuladiaceae) which attacks corn, sorghum, sugarcane, and more than 60 other important crop plants and weeds belonging to the grass family, was found in North Carolina and South Carolina in 1956. It presents a potential hazard to the production of grain crops with an annual value of more than 5 billion dollars in the United States. Continued extensive surveys indicate that witchweed is now present in 20 counties in North Carolina and 10 in South Carolina on a total of 9,566 farms containing 210,671 acres. Federal interstate and state intrastate quarantines (the first for a weed in the U. S.) were established to reduce its spread. Witchweed produces up to 500,000 microscopic longlived seeds per plant. These do not usually germinate until stimulated by a chemical stimulant secreted by the roots of certain plants. A stimulant has been isolated from corn but its exact chemical structure has not be determined. More than 300 chemicals have been evaluated in the laboratory in attempts to find a synthetic stimulant. Of these, 18 stimulated witchweed seed germination in the laboratory but they have not been evaluated for their effectiveness in controlling witchweed under field conditions. After 4 years of catch or trap crops, excellent control of witchweed has been obtained and highly profitable crops of corn have been produced on heavily infested land. Corn grown continuously with witchweed seed production prevented by 2,4-dichlorophenoxyacetic acid [2,4-D] has resulted in excellent witchweed control and high corn yields. Corn grown continuously receiving 2,3,6-trichlorophenylacetic acid [fenac] as a pre-planting soil-incorporated treatment and 2,4-D as a post-emergence spray as needed have proved effective full-season control of witchweed. Tobacco and certain other crops cannot be grown in rotation with corn for a period of 2 years after this treatment because of fenac residues in the soil. Other herbicides as pre-planting soil-incorporated or premergence treatments to control broadleaved and grass-type weeds plus 2,4-D to prevent witchweed seed production have been highly effective combinations for witchweed control. Quick eradication of witchweed will not be easy but effective control now and future eradication look favorable.

The concentration of three sulfonylurea herbicides in soil was determined by a modified petri dish bioassay procedure. The method involved planting pregerminated seed of selected species in petri dishes containing 65 to 100 g of treated soil and measuring the radicle lengths after 24 h. Chlorimuron was detected in two soils at 0.002 μg g–1 using either corn, sorghum, or sicklepod as the assay species. Chlorsulfuron and CGA-131036 were detected in soils at 0.001 μg g–1 using corn, and chlorsulfuron was detected at the same level using sicklepod. In contrast to other methods, the bioassay procedure described could be completed in 48 h, including pregermination of the seed, growth of the plants on treated soil, and plant measurements.

Field studies were conducted in 1996 and 1997 to determine the response of Cyperus rotundus and Cyperus esculentus, Kyllinga brevifolia, and Kyllinga gracillima to mowing regimens common to recreational turfgrass. Treatments were selected to simulate Cynodon dactylon golf course management and included mowing at 1.3 and 3.8 cm with mowing frequencies of three times per week and once a week, respectively. A nonmowed check was included for comparison. Reductions in C. rotundus shoot number were observed beginning 6 wk after initial treatment (WAIT) in 1996 and 9 WAIT in 1997 for the 1.3-cm mowing regime. The 3.8-cm mowing regime did not reduce C. rotundus shoot number until the final evaluation of each year. Reductions in C. rotundus rhizome length, tuber number, and tuber size were observed for both mowing regimes in both years. Cyperus esculentus shoot number was reduced by the 1.3-cm treatment at each evaluation date in 1996 and 1997. Cyperus esculentus shoot number reductions in the 3.8-cm regime were first observed 4 and 6 WAIT in the 2 yr and continued until termination. The 1.3-cm regime reduced C. esculentus spread beginning 6 WAIT in 1996 and 3 WAIT in 1997. Cyperus esculentus spread was also reduced by the 3.8-cm treatment, but reduction began at later evaluations (8 and 9 WAIT). Tuber production by C. esculentus was completely inhibited by the two mowing regimes in both years. The only treatment effect observed in K. brevifolia and K. gracillima in 1996 was a reduction in internode length of K. gracillima by the 1.3-cm mowing regime. In 1997, the 1.3-cm regime reduced K. brevifolia shoot number at 15 and 18 WAIT and plant spread beginning 6 WAIT and continuing until termination. The 3.8-cm treatment did not affect K. brevifolia shoot number and reductions in spread were only observed at the final evaluation. Kyllinga gracillima shoot number and plant spread were reduced by the 1.3-cm mowing regime at each 1997 evaluation. Reductions in K. gracillima shoot number occurred at the final evaluation, and reductions in spread began 12 WAIT when subjected to the 3.8-cm treatment. Both mowing regimes reduced K. brevifolia and K. gracillima internode length. Kyllinga brevifolia total rhizome length and total node number were reduced by the 1.3-cm regime only. Kyllinga gracillima rhizome length, internode length, and node number were reduced by both regimes in 1997.

Early patent records show that a rotary-hoe type of implement has been in existence since 1839 (2). The rotary hoe was used on soybeans as early as 1915, and important considerations for weed control were discussed in “Soybeans in Indiana, 1928,” (1) but its use did not become widespread until about 1929.

Weed Science—a new scientific discipline, has evolved and caused a revolution in agricultural technology. Weeds and their control are basically ecological and economic problems. The losses caused by weeds and the cost of their control, now estimated to exceed $4½ billion annually, are motivating forces in the revolution.

The technological revolution in Weed Science is characterized by the rapid and extensive acceptance and use of chemical weed control methods, expanded scope and diversity of weed research, advances in Weed Science technology, needs and challenges for advances in future technology, impact of chemical weed control on farm management practices, and its effect on the objectives and direction of agricultural research and educational programs.

Chemical methods of weed control greatly reduce energy requirements on farms. Herbicides reduce the manpower, machine hours, and machine horsepower requirements in crop production. Thus, herbicides provide a new dimension for improving farming efficiency and lowering production costs.

In 1962, chemical methods of weed control were used on more than 70 million acres of agricultural land at a cost of more than $272 million, as compared to approximately 53 million acres treated in 1959 at a cost of $128 million, a 33 percent increase in the acreage treated in 1962. The acreage treated in 1962 was equivalent to about 25 percent of the national crop acreage. These use data do not include chemical weed control on industrial sites and other non-agricultural lands.

Chemical weed control is having a far-reaching impact on all phases of crop production. New chemical methods of weed control will affect choice of crop and variety; seedbed preparation; method of seeding; seeding rates; row spacing; plant spacing in the row; plant populations; fertilizer practices including type, time of application, and placement; cultivation; irrigation practices; harvesting; seed-cleaning operations; erosion control; fallow practices for weed control; disease and insect control practices; pasture renovation; pasture and range management; clearing new lands for crops or pasture; forest management; utilization of farm water resources for irrigation and recreation; and maintenance of drainage ditches, ditchbanks, irrigation canals, and farm roadsides.

Agriculture is on the horizon of a second-generation breakthrough in production efficiency because of the revolution in Weed Science. Already the accomplishments made possible by this revolution have established the necessity for a new research cycle in crop production that can result in new plateaus of crop yields, crop quality, and production and harvesting efficiency.

All herbicide treatments controlled the initial stand of Canada thistle [Cirsium arvense (L.) Scop.] and musk thistle (Carduus nutans L.) on a subirrigated range site. Annual retreatment controlled seedlings and the few plants that emerged from surviving root remnants after the first application. Average perennial grass production on unfertilized, herbicide-treated plots increased 110, 314, and 212%/yr over unfertilized check plots during the 3-yr period of treatment, i.e., 960, 3450, and 4300 kg/ha, respectively. Grasses did not fully reoccupy the site at the end of 3 consecutive yr of excellent thistle and forb control. The increase in grass production varied considerably among several herbicide treatments that controlled thistles equally. Grass production in the third year was greater under the 3,6-dichloropicolinic acid and 2,4-D [(2,4-dichlorophenoxy)acetic acid] treatment series than all other treatments. Ammonium nitrate enhanced the recovery of grass damaged by certain herbicides, but favored the competitive ability of the thistle more than that of the grasses.

Rapid detection methods were developed for discriminating between resistant (R) and susceptible (S) biotypes of Echinochloa colona to either propanil or fenoxaprop-P at all growth stages. In the pregerminated seed assay for fenoxaprop-P, seeds were placed on 1.0% agar containing a range of concentrations of herbicides and kept under humid conditions. For propanil, pregerminated seeds were placed on moist filter paper in the lid of a petri dish and, when one leaf had developed, the lid was inverted for 1 min into propanil solutions at a range of concentrations. For the fenoxaprop-P and propanil test, seedling length and fresh weight were measured after 1 wk. For juvenile plants with four-leaf to one-tiller, shoots and roots were trimmed and placed in 20-ml glass tubes containing 0.2% (wt/v) agar and a range of concentrations of herbicides. Shoot extension and weight were recorded after 7 d. Larger plants with several small tillers were also assayed by this method. Tillers were removed from larger plants and were evaluated as described for juvenile plants. At later growth stages from ear emergence to flowering, excised stem node segments (8 cm) were soaked in water to stimulate rooting. Rooted nodes were placed in 30-ml glass bottles containing 0.2% agar with a range of concentrations of herbicides, and the test was conducted as described for juvenile plants but with the final assessment of new shoot extension and weight being recorded after 10 d. Discrimination between R and S biotypes was possible on the basis of GR50 values for shoot length and fresh weight in all testing methods. With few exceptions, GR50 values for the length of new shoot were very similar to those for the new shoot fresh weight. We concluded that all testing methods in our study provided reliable and quick discrimination between biotypes for both propanil and fenoxaprop-P susceptibility, covering various growth stages from seed to flowering stage. Trimming plants before herbicide treatment gives a rapid method of discrimination by measuring not only newly grown shoot fresh weight but also shoot length.

The effects of 22 herbicides on oxidative phosphorylation in mitochondria from cabbage (Brassica oleracea var. capitata L.) have been studied. Carbamates, phenoxyacids, 3,5-dimethyltetrahydro-l,3,5,2H-thiadiazine-2-thione (DMTT), N-1-napthylphthalamic acid (NPA), 2,3,6-trichlorobenzoic acid (2,3,6-TBA), 3-(p-chlorophenyl)-1,1-dimethylurea (monuron), and 2-chloro-2-N,N-diallylacetamide (CDAA) inhibited phosphorylation to a greater extent than oxygen uptake in cabbage mitochondria. The compounds 1,2-dihydropyridazine-3,6-dione (MH), 2,2-dichloropropionic acid (dalapon), 2,2,3-trichloropropionic acid (hereinafter referred to as 2,2,3-TPA), sodium chlorate, and 3-amino-l,2,4-triazole (amitrole) at concentrations as high as 1 × 10–2 M did not have a large effect on oxidative phosphorylation in cabbage mitochondria. Respiration which was stimulated by 2,4-dinitrophenol (DNP) or adenosine diphosphate (ADP) was inhibited by 2,4-dichlorophenoxyacetic acid (2,4-D). Although 2,4-D did not affect Mg++-stimulated adenosine triphosphatase (ATPase), it inhibited an oleate-stimulated ATPase activity and the ATP-32Pi exchange reaction in cabbage mitochondria. The results suggest that 2,4-D may inhibit respiration in cabbage mitochondria by an effect on a reaction involved in coupling phosphorylation with electron transport.

Common cocklebur is a very competitive and difficult to control weed species in soybean production. Field research was conducted at Rosemount, MN, from 1989 to 1991 to evaluate reduced rates of bentazon applied broadcast or banded over the crop row in combination with interrow cultivation for common cocklebur control in soybean. Bentazon at 0.6 kg ai ha−1 applied in a 38-cm-wide band over the soybean row followed by two cultivations controlled almost 90% of the common cocklebur when environmental conditions were favorable, and the majority of the common cocklebur emerged prior to bentazon application. However, when precipitation was below normal prior to bentazon application and a high proportion of the common cocklebur emerged after the initial bentazon application, sequential bentazon treatments controlled more common cocklebur and resulted in greater soybean yields than combinations of bentazon plus cultivation. Differences in soybean yield were attributed to differences in common cocklebur control rather than injury from cultivation or bentazon.