Sweet corn is seeded under a wide range of population densities; however, the extent to which variable population density influences weed suppression is unknown. Therefore, field studies were undertaken to quantify the influence of sweet corn seeding level on growth, seed production, and post-harvest seed germination of wild-proso millet, one of the most problematic weeds in the crop. As crop seeding level increased, path analysis results indicated the crop canopy became taller and thicker, resulting in less wild-proso millet biomass, seed production, and germinability. However, at the level of individual fields, reductions in wild-proso millet growth and seed production were modest, at best, between a crop population currently used by growers and a higher crop population known to optimize yield of certain hybrids. These results indicate near-future increases in sweet corn seeding levels may play a minor role in improving weed management in individual sweet corn fields. Nonetheless, a reduction in crop populations, via weather- or management-driven phenomenon, increases risk of greater wild-proso millet seed production.

Cultivated and weedy clones of yellow nutsedge were analyzed using random amplified polymorphic DNA (RAPD) markers to assess the polymorphism within the species and determine if this approach was suitable for identification of cultivar and wild populations. The RAPD markers unambiguously identified all studied clones. Nei-Li similarities were computed and used in an unweighted pair group method using arithmetic average (UPGMA) cluster analyses. Cultivated and weedy clones were clustered in two groups, but two cultivated clones were more closely related to weedy clones than to cultivated clones. The results showed a high level of genetic variability among the clones tested, particularly among the cultivated ones. Identification of yellow nutsedge cultivars and analysis of genetic diversity within and among weedy populations is possible by using only a small number of primers. In this study, seven selected primers discriminated among the 10 tested clones.

Studies were conducted to characterize qualitatively and quantitatively the water-soluble carbohydrates in johnsongrass [Sorghum halepense (L.) Pers.]. Glucose, fructose, and sucrose were the only sugars detected in other than trace amounts in leaves, culms, or rhizomes. Sucrose was the major storage carbohydrate, although glucose and fructose were present throughout most of the life cycle in both leaves and rhizomes. Levels of glucose, fructose, and sucrose in rhizomes were maximum when plants began flowering and minimum at 10 to 30 days after plant emergence. Glucose, fructose, and sucrose were at a maximum in leaves within 30 days after plant emergence. Glucose was present only in trace amounts in leaves for about 10 days during initial flowering; fructose was at a maximum at that time. Sucrose in leaves occurred in greatest quantities soon after plant emergence, immediately before flowering, and after flowering. Leaf blades and sheaths contained nearly equal concentrations of carbohydrates, but stems contained greater concentrations than leaves. Basal shoots and secondary culms from nodes on primary culms contained greater concentrations of carbohydrates than leaves on primary culms. Regrowth from clipped shoots had significantly higher concentrations of glucose and sucrose than did unclipped shoots. Rhizomes of clipped plants had significantly less sucrose but more glucose than rhizomes of unclipped plants.

Fluridone {1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4 (1H)-pyridinone} and R-4-0244 [1-(m-trifluoromethylphenyl)-3-chloro-4-chloromethyl-2-pyrrolidone], herbicides that block carotenoid synthesis, increase the sensitivity of corn (Zea mays L. ‘Merit’) to exogenously-applied gibberellic acid (GA). Increase in shoot length due to the application of GA to imbibing corn seeds was considerably enhanced by simultaneously applying either fluridone or R-40244. Fluridone was more active in this respect than R-40244.

A fiber system for controlled delivery of fluridone {1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridinone} was evaluated for its potential use in the management of the submersed aquatic weed hydrilla [Hydrilla verticillata (L.f.) Royle # HYLLI] in slow-moving water. The release of fluridone from the fibers was first order and could be extended over several days simply by using fibers of larger diameters. Fibers of 0.8- and 1.2-mm diam released fluridone over a period of 40 and 50 days, respectively. Both effectively controlled hydrilla at a rate of 2.2 kg ai/ha in a 4-month study in replicated pools with water flow adjusted to provide one volume change every 24 h. Under the same conditions, the conventional liquid formulation was ineffective and a pellet formulation provided only marginal control.

Formulated and technical grade HOE-39866 [ammonium-(3-amino-3-carboxypropyl) methylphosphinate] at concentrations of 10–1, 10–2, and 10–3M were applied to leaf blade tissues of nonreproductive adult redroot pigweed (Amaranthus retroflexus L. # AMARE) and fall panicum (Panicum dichotomiflorum Michx. # PANDI). Tissues were sampled at regular intervals after treatment and prepared for light microscopic examination. The major response of both species involved rupture and contortion of the interveinal mesophyll cells with concomitant disorganization of the bundle sheath cells. Rapid epidermal collapse occurred in redroot pigweed but not in fall panicum. The absence of adjuvants resulted in nonuniform symptom expression as herbicide droplets accumulated in depressions and along leaf margins. No other adjuvant-specific effect was observed. Herbicide concentration did not alter the final response but the time-to-expression increased as concentration decreased.

Ammonium sulfate increased the magnitude and rate of 14C-picloram (4-amino-3,5,6-trichloropicolinic acid) absorption in detached strawberry guava (Psidium cattleianum Sabine) leaves. Absorption of 14C-picloram with or without ammonium sulfate leveled off at 6 to 12 hr. The magnitude of the ammonium sulfate-induced increase in 14C-picloram absorption was not affected by picloram concentration in the range 250 to 2000 ppmw, or by leaf age. The absorption of 14C-picloram was increased by 0.1, 0.5, 1.0, and 10% ammonium sulfate; 0.01% had no effect. Increased 14C-picloram absorption resulted from adding the ammonium salts of sulfate, nitrate, chloride, or dibasic phosphate, but ammonium sulfate was the only one of six sulfate salts to increase picloram absorption. A pH adjustment of the external solution from 6 to 4 increased picloram absorption, whereas a pH change from 6 to 8 had no effect. Both ammonium dibasic phosphate at a solution pH of 7.7 and ammonium monobasic phosphate at pH 4.6 increased picloram absorption, whereas potassium dibasic phosphate at pH 9.5 had no effect, and potassium monobasic phosphate at pH 4.6 did increase absorption.

This investigation was initiated in 1954 at Beltsville, Maryland, to determine the initial and long-term response of vegetation to, and persistence of, massive quantities of herbicides. Vegetative responses were determined 1, 2, 3, and 15 years after treatment. Residual phytotoxicity and herbicide residues were determined 14 years after treatment with bioassay and chemical analysis. After 3 years, revegetation was nearly complete in plots treated with massive (up to 400 times recommended agricultural rates) quantities of fenuron (1,1-dimethyl-3-phenylurea), monuron [3-(p-chlorophenyl)-1,1-dimethylurea], dalapon (2,2-dichloropropionic acid), chlorpropham (isopropyl m-chlorocarbanilate), sodium chlorate, 2,4-D [(2,4-dichlorophenoxy)acetic acid], borax, sodium chlorate plus borax, and 2,4-D plus borax. Only diuron [3-(3,4-di chlorophenyl)-1,1-dimethylurea], DMU [3-(3,4-dichlorophenyl)-1-methylurea] sodium arsenite, and sodium arsenite plus sodium chlorate gave residual control of vegetation for more than 3 years. Revegetated plots were identical to untreated check plots whether the vegetation was initially killed by chemical or mechanical methods. Phytotoxic soil residues of DMU, diuron, and arsenate were present 14 years after application.

Hand-held hyperspectral reflectance data were collected in the summers of 2002, 2003, and 2004 to differentiate unique spectral characteristics of common turfgrass and weed species. Turfgrass species evaluated were: bermudagrass, ‘Tifway 419’; zoysiagrass, ‘Meyer’; St. Augustinegrass, ‘Raleigh’; common centipedegrass; and creeping bentgrass, ‘Crenshaw’. Weed species evaluated were: dallisgrass, southern crabgrass, eclipta, and Virginia buttonweed. Reflectance data were collected from greenhouse and field locations. An overall classification accuracy of 85% was achieved for all species in the field. A total of 21 spectral bands between 378 and 1,000 nm that were consistent over the three data collection periods were used for analysis. Only centipedegrass, zoysiagrass, and dallisgrass were correctly classified less than 80% of the time. An overall classification accuracy of 69% was achieved for the greenhouse species. Spectral bands used in this analysis ranged from 353 to 799 nm. Creeping bentgrass and Virginia buttonweed were classified correctly at 96 and 92%, respectively.

Windmillgrass is a major weed in agricultural systems in northern Australia, and it has now become more common in southern Australia. Because little information is available on the biology of this weed species in southern Australia, studies were conducted to investigate plant development and seed biology. Under irrigated field conditions in South Australia, windmillgrass required 748 to 786 growing degree days from emergence to mature seed production. Freshly harvested seed had low dormancy with 16% to 40% germination. Seeds required light exposure for germination and less than 2% germination was observed in complete darkness. Seed could germinate over a wide temperature range (10 to 40 C) with maximum germination at 20 to 25 C. At 25 to 30 C, 50% germination occurred within 27.3 to 45.5 h, and the predicted base temperature for germination of the two populations investigated ranged from 9.2 to 11.2 C. The sodium chloride concentration and osmotic potential required to inhibit germination by 50% were 51 to 73 mM and −0.27 MPa, respectively. Seedling emergence was completely inhibited by burial of seed, which is consistent with its absolute requirement for light exposure to begin germination. Under field conditions, there was no clear effect of burial depth on seed viability in the first 2 yr with average rainfall, and seeds were completely nonviable after 12 mo. However, in the third year, with low spring–summer rainfall, buried seeds (37% viability after 14 mo) persisted longer than those left on the soil surface (6% viability after 14 mo). This study provides important information on plant development and seed biology of windmillgrass that will contribute to the development of a management program for this weed species in southern Australia.

Discs of potato (Solanum tuberosum L.) tuber tissue were immersed in buffered (pH 4.0 to 8.0) solutions of 4-amino-3,5,6-trichloropicolinic acid (picloram) (5 × 10−4M to 5 × 10−3M) for 1 to 36 hr. Uptake of picloram during incubation, and leakage after return of the discs to untreated buffer, were determined by gas chromatographic analysis of extracts of the tissue and ambient buffer. Picloram absorption increased with concentration and with time up to 24 hr. Maximum uptake occurred at pH 4.0 and very little picloram was absorbed at pH 7.0 and 8.0. Both absorption and leakage were temperature dependent. The rate and extent of leakage was greatest at the highest concentration. Typically, more than 90% of the picloram absorbed from 5 × 10−3M was lost to fresh buffer within 12 hr.

We constructed a mechanistic model of purple nutsedge tuber population dynamics to provide a theoretical framework for the integrated management of this weed. The model relies on a transition matrix with 10 age classes to simulate fluctuations in the tuber population. Parameters of the transition matrix are given by functions of density, age, and cumulated incident photosynthetically active radiation (PAR) underneath crop canopies. Sensitivity ratios based on a 10% reduction in parameter values indicated that the parameters of the birthrate function were most sensitive. Simulations showed that in the absence of weed control, cumulated incident PAR was by far the strongest determinant of population size; intraspecific interference was the strongest determinant of the rate of population increase. When weed control was introduced, the simulation suggested that 95% control would be required to eliminate this weed. The analysis of simulation results led to the formulation of five research hypotheses of practical relevance to the management of purple nutsedge. New insights gained by testing these hypotheses should lead to practical recommendations as well as a better understanding of the relationships between management practices and fluctuations in purple nutsedge populations.

The basis for 2,4-D [(2,4-dichlorophenoxy)acetic acid] antagonism of diclofop-methyl {methyl 2-[4-(2,4-dichlorophenoxy) phenoxy] propanoate} toxicity to wild oat (Avena fatua L.) was investigated by studying changes in the metabolism of diclofop-methyl in vitro. An esterase from wild oat, which hydrolyzes diclofop-methyl to the acid diclofop, was extracted, partially purified, and the reaction characterized. The rate of hydrolysis of 14C-diclofop-methyl was 0.14 ηmoles/2 h at standard assay conditions of 0.25 mg lyophilized enzyme preparation (19.6% protein) in 0.1 ml phosphate buffer (0.1 M, pH 7.0), substrate 5 μM. The addition of 2,4-D to this reaction did not inhibit 14C-diclofop formation. Higher levels of 2,4-D stimulated enzymic hydrolysis. 14C-diclofop-methyl was rapidly metabolized to 14C-diclofop and polar 14C-conjugates when vacuum-infiltrated into wild oat leaf segments. The addition of 2,4-D caused small increases in the rates of both 14C-diclofop-methyl de-esterification and 14C-diclofop conjugation. It is concluded that 2,4-D does not inhibit the in vitro de-esterification of diclofop-methyl.

Low rates of diphenylether herbicides were previously shown to increase efficacy of glyphosate. Experiments were conducted to determine effect of lactofen on uptake of glyphosate and movement into tissues distal from the treatment area as measured by shikimate accumulation. As expected, glyphosate caused shikimate to accumulate, and sublethal rates of lactofen increased this effect. Cuticle abrasion had no effect on activity of glyphosate, alone or with lactofen, suggesting that the influence of lactofen was not in aiding penetration into the leaf.

Preplant-incorporated applications of commercial formulations of EPTC (S-ethyl dipropylthiocarbamate) and butylate (S-ethyl diisobutylthiocarbamate) plus the herbicide antidote R-25788 (N,N-diallyl-2,2-dichloroacetamide) were evaluated for their interactions with preemergence applications of tebuthiuron {N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N′-dimethylurea} in corn (Zea mays L., ‘Pioneer 3780’) under greenhouse conditions. The thiocarbamate herbicides plus the antidote were used at 1.1, 2.2, 3.4, and 4.5 kg/ha, and they were evaluated against 0.3, 0.6, and 1.1 kg/ha of tebuthiuron. Shoot dry weight and shoot height of corn seedlings were measured at 24 days after treatment. Growth responses of corn to combination treatments of these herbicides were significantly reduced. Statistical analysis of the data showed that tebuthiuron interacted synergistically with both EPTC and butylate in the presence of the herbicide-antidote R-25788.

Root growth of three zoysia (Zoysia spp. ‘Emerald’, ‘Meyer’, and ‘Matrella’) cultivars and four bermudagrasses [Cynodon dactylon (L.) Pers. ‘Tifway’, ‘Tifgreen’, ‘Tifdwarf’, and ‘Ormond’] was determined after several consecutive annual herbicide treatments. Napropamide [2-(α-naphthoxy)-N,N-diethylpropionamide] reduced the root growth of all zoysia and bermudagrass cultivars. Prosulfalin {N-[[4-(dipropylamino)-3,5-dinitrophenyl] sulfonyl]-S,S-dimethylsulfilimine} retarded root growth of Tifgreen and Tifdwarf bermudagrass when treated at any rate. Tifway and Ormond bermudagrass and zoysia cultivars were retarded at the 3X rate (10.2 kg/ha). Zoysia grass roots were not affected by oxadiazon [2-tert-butyl-4-(2,4-dichloro-5-isopropoxyphenyl)-δ2-1,3,4-oxadiazolin-5-one], bensulide [O,O-diisopropyl phosphorodithioate S-ester with N-(2-mercaptoethyl)benzenesulfonamide], or benefin (N-butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine) treatments. DCPA (dimethyl tetrachloroterephthalate) and oxadiazon did not reduce root growth of bermudagrasses, except when Tifgreen was treated with oxadiazon at 3X rate (13.4 kg/ha).

An antagonistic interaction between S-ethyl dipropylthiocarbamate (EPTC) and (2,4-dichlorophenoxy)acetic acid (2,4-D) was observed on sorghum (Sorghum vulgare Pers.) and corn (Zea mays L.) after 21 days growth in soil incorporated treatments. Increasing the 2,4-D concentration increased the antagonism to EPTC. The antagonistic interaction was also observed on sorghum grown in solution culture. The antagonistic interaction occurred when the site of uptake was restricted to either the root or shoot zone of sorghum. Uptake studies with 14C-EPTC indicated that 2,4-D does not interfere with uptake or subsequent movement of 14C radioactivity within sorghum plants. Respiration of sorghum treated with the combination EPTC-2,4-D appeared to be normal. Respiration of EPTC-treated plants was inhibited.

The degree of control of purple nutsedge (Cyperus rotundus L.) provided by Bactra verutana Zeller, a native moth, was evaluated for various stress conditions. Two, three, or four weekly infestations each with three larvae per live shoot, reduced purple nutsedge dry weight an average 77% compared with an average 55% for one infestation. Eight weekly infestations reduced plant dry weight 98%. Neither caging of the larvae on the plants nor osmotic stress with NaCl to −8 bars influenced larval efficacy, but nitrogen deficiency did reduce the effect of the larvae about 75%. Purple nutsedge had no effect on the dry weight of cucumber (Cucumis sativus L., ‘Straight Eight’) after 1 month, whether larvae were or were not present. Purple nutsedge did reduce the plant dry weight of okra (Abelmoschus esculentus (L.) Moench, ‘Clemson Spineless’) 64%, but repeated (eight) weekly infestations of larvae gave a plant weight of okra equal to that of okra grown without purple nutsedge. Purple nutsedge reduced the bulb and root dry weight of onion (Allium cepa L., ‘Big Valley’) 37%, but repeated weekly infestations of larvae gave onion growth equal to that of onion grown without purple nutsedge.

The changes in the nucleic acid metabolism were studied in plants susceptible and resistant to 4-amino-3,5,6-trichloropicolinic acid (picloram). The total RNA and DNA content of the tissue correlated inversely with the herbicide resistance; the resistant plants were low in nucleic acids, whereas sensitive plants were high. The increase in the nucleic acids of the sensitive species 24 hr after picloram treatment appeared to be associated with lower levels of ribonuclease and deoxyribonuclease. The inability of the resistant species to make more RNA may be associated with high levels of nucleases in the tissue.

The relative volatilities of various technical and formulated ester and amine salts of 2,4-D [(2,4-dichlorophenoxy)acetic acid] were determined in a closed air-flow system consisting of a volatilization chamber and an ethylene glycol trap. The system was operated at a flow rate of 28.8 L/hr and a temperature of 30 C. Trapped herbicide vapor was analyzed by gas chromatography. The relative volatilities of technical grade iso-propyl, butyl, propylene glycol butyl ether, and iso-octyl esters, were 56, 25 to 38, 4, and 1, respectively. The relative volatilities of various formulated esters and amines were of the magnitude of 440:33:1 for the high volatile, low volatile, and amine salts, respectively.