Paraquat and diquat were used from 1963 until 1986 to control capeweed and other annual weeds on an alfalfa field at Elmhurst, Victoria, Australia. From 1983, control of capeweed with these bipyridyl herbicides was no longer satisfactory. In experiments conducted on this field in 1986 and 1987, 800 g ai/ha of diquat was required to equal the control of capeweed obtained by 100 g/ha diquat in an adjacent pasture field with no history of herbicide use. There were substantial numbers of a diquat-resistant biotype within the population in the alfalfa field. It is concluded that the diquat-resistant biotype of capeweed became dominant due to the strong selection pressure provided by over 20 yr of use of the bipyridyl herbicides.

Ethylene production was stimulated by 2,4-dichlorophenoxyacetic acid (2,4-D) from light-grown corn (Zea mays L., var. XL-15) and soybeans (Glycine max Merr., var. Hawkeye). Ethylene had an inhibitory effect on the growth of corn and soybeans, but a reversal of the ethylene effect could not be clearly demonstrated using the competitive inhibitor, carbon dioxide. Ethylene did not mimic the ability of 2,4-D to cause growth curvatures. It was concluded that ethylene played a role in the activity of sublethal amounts of 2,4-D.

Activity, adsorption, mobility, and field persistence of one emulsifiable concentrate (EC) and three microencapsulated (ME) formulations of alachlor were studied with petri dish bioassay, based on root response of oats grown in sand or soil. Both bioassays indicated that activity of all formulations was increased with increasing herbicide concentration. EC-alachlor in sand showed the highest activity, while ME-alachlorL and ME-alachlorA, the lowest; the activity of ME-alachlorC was intermediate. In silty clay loam soil, EC-alachlor had the highest activity, while ME-alachlorL had the lowest; and ME-alachlorA showed intermediate activity and was similar to that of ME-alachlorC More alachlor was adsorbed on the soil or remained encapsulated (not biologically available) after ME-alachlorL and ME-alachorA application compared to EC-alachlor. An intermediate amount of alachlor was adsorbed or remained inside the capsules when the ME-alachlorC formulation was applied. Greater amounts of alachlor were leached through a silty clay loam after EC-alachlor application compared to the three ME-alachlor formulations. Biologically available alachlor was not detected below 15 or 10 cm after application of EC- and ME-alachlor formulations, respectively. All alachlor formulations applied alone or in mixture with atrazine showed similar field persistence. Biologically available alachlor was not detected in the 0- to 10-cm soil depth 30 d after their application. All alachlor formulations applied alone gave excellent control of redroot pigweed and black nightshade, but only partial control of jimsonweed. None of the herbicide treatments showed any detrimental effect on corn, and all of them increased corn yield to the level of weed-free control.

Several herbicides were applied in January, February, and March with a comparison of the intervals of treatment of 2 and 4 weeks after the initial treatment each month for postemergence control of winter annual weeds in bermudagrass [Cynodon dactylon (L.) Pers.] turf. Glyphosphate [N-(phosphonomethyl)glycine] treatments applied at 2-week intervals with the initial treatment made in January or February controlled a higher percentage of annual bluegrass (Poa annua L.) than when applied in March. Hop clover (Trifolium agrarium L.) control was also higher when glyphosate was initially applied in January or February than when applied in March regardless of time interval between first and second treatment. Combination treatments of (a) 2,4-D [(2,4-dichlorophenoxy)acetic acid] + dicamba (3,6-dichloro-o-anisic acid) and (b) 2,4-D + mecoprop {2-[(4-chloro-o-tolyl)oxy] propionic acid} + dicamba applied at 2-week intervals with the initial treatment made in January or February controlled more corn speedwell (Veronica arvensis L.) and hop clover than when applied in March. Highest henbit (Lamium amplexicaule L.) control was obtained from the combination 2,4-D treatments made at 4-week intervals when initial treatment was made in February and March. Weed control was not influenced by dates and interval of repeated treatments with either paraquat (1,1′-dimethyl-4,4′-bipyridinium ion) or atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] treatments. Germination and regrowth of weeds were greater in plots treated with glyphosate and paraquat initially in January or February than other herbicide-treated plots. Weeds were not reestablished in any of the atrazine-treated plots. Paraquat and combinations of 2,4-D + dicamba or 2,4-D + mecoprop + dicamba injured bermudagrass when applied initially in January and February even though they were applied to turf that appeared dormant. All herbicides injured bermudagrass more when applied to semi-dormant turf in March than to dormant turf in January or February. Atrazine affected bermudagrass less than any of the other herbicides tested when initial treatment was applied in March to semi-dormant turf.

A downy brome population in a winter wheat field at Córdoba, Spain, survived use rates of chlortoluron (2.5 to 3.5 kg ai ha−1) over 2 consecutive yr, where wheat monoculture and multiple annual chlortoluron applications had been carried out. The resistant (CR) biotype showed a higher ED50 value (7.4 kg ai ha−1; the concentration required for 50% reduction of fresh weight) than the susceptible (S) control (2.2 kg ai ha−1), with a 3.4-fold increase in chlortoluron tolerance. Chlortoluron resistance in the CR downy brome biotype was not caused by altered absorption, translocation, or modification of the herbicide target site but by enhanced detoxification. The inhibition of both the recovery of photosynthetic electron transport and chlortoluron metabolism in the CR biotype due to the presence of the Cyt P450 inhibitor 1-aminobenzotriazole (ABT) indicates that herbicide metabolism catalyzed by Cyt P450 monooxygenases is related to chlortoluron resistance in CR plants. Although both biotypes degraded chlortoluron by N-dealkylation and ring-methyl hydroxylation and seem to share the same ability to form polar conjugates, degradation in the resistant biotype is more efficacious as this biotype metabolizes the parent herbicide faster and to a greater extent than its susceptible counterpart. The ability of the susceptible biotype to ring-hydroxylate chlortoluron, albeit at much slower rate, probably explains its moderate tolerance to chlortoluron observed in the growth assays and its minor photosynthetic electron transport recovery observed in fluorescence measurements.

Absorption and translocation of the growth regulator 14C-chlorflurenol (methyl-2-chloro-9-hydroxyfluorene-9-carboxylate) and of the herbicide 14C-dicamba (3,6-dichloro-o-anisic acid) were studied in Canada thistle [Cirsium arvense (L.) Scop.]. Also investigated was how foliar treatments with chlorflurenol affected leaf uptake and distribution of dicamba by Canada thistle. Plants were grown in aerated nutrient solution. 14C-chlorflurenol showed strong acropetal and weak basipetal movement. Thirty-two percent of the foliar applied 14C-dicamba was absorbed and most of the radioactivity tended to accumulate in the young growing leaves. Only 0.6% of the applied radioactivity moved to the roots. Leaf absorption and basipetal translocation of 14C-dicamba were increased two- and ten-fold, respectively, by the simultaneous addition of chlorflurenol at 0.6 kg/ha. Root bud activity was increased when 4-week-old Canada thistle plants were foliarly treated with chlorflurenol at 0.6 kg/ha. Field work demonstrated that Canada thistle control could be enhanced by chlorflurenol addition to dicamba.

Johnsongrass [Sorghum halepense (L.) Pers.] was grown for 4, 8, and 12 weeks at 24, 32, and 40 C at 9, 13, and 19 klux illumination. Maximum growth and development after 12 weeks were obtained at 32 C and 19 klux. Maximum leaf growth and development were obtained at 32 C and 9 klux. Primary culm height and weight were greater at 24 C than at 40 C, regardless of the illumination, whereas secondary culm height and weight were greater at 40 C than at 24 C. Secondary culm production increased at 40 C and decreased at 24 C. Root and rhizome development increased with increasing light intensity. Growth of roots and rhizomes was maximum at 32 C and minimum at 40 C.

Studies were conducted to compare changes in germinability and/or viability of yellow starthistle achenes buried (in packets) in soil or stored dry in the laboratory, and to determine the rate of achene depletion from a soil seed bank. In one study, after 72 mo, 0 to 96% germinable achenes remained in packets buried 5 cm deep, and a mean above 99% after dry storage. In another study, total live (germinable plus viable) achenes declined from 100% at harvest to 77.6% after 24 mo of burial. Germinable pappus-bearing achenes remaining in packets increased with depth of burial. After 12 mo of burial, maximum survival of achenes occurred at depths of 5 cm or more. The density of yellow starthistle achenes and seedlings in a natural soil seed bank declined, when achene rain was prevented, to 3.9 and 1.1% of initial density, respectively, after 36 mo. Decline was attributed primarily to seedling emergence and achene death. Achene density in the upper 2.5 cm of soil appeared to be a good predictor of seedling emergence after autumn rains.

Cotton (Gossypium hirsutum L.) was grown in the presence of common cocklebur (Xanthium strumarium L. # XANST) on a Lucedale fine sandy loam. Cotton was maintained weed free or allowed to compete with common cocklebur for 0, 3, 5, 7, 9, or 11 weeks after planting. Mathematical predictions of seed cotton yield were optimum when maintained free from common cocklebur interference for 8 weeks or more after cotton emergence in 1981 and 1982, and 10 weeks or more in 1980. Common cocklebur adversely affected yield when allowed to compete longer than 4 weeks in 1981 and 1982, and 2 weeks in 1980.

Weed control in lentil is difficult because lentil is a poor competitor with weeds and few POST broadleaf herbicides are available. Imadazolinone-tolerant lentils have more herbicide options, but the optimum timing for herbicide application is not known. The critical period of weed control (CPWC) is the period in a crop's life cycle when weeds must be controlled in order to prevent yield loss. The objective of this research was to determine the CPWC for lentil. We made lentil remain weedy or weed-free from 0 to 11 aboveground nodes to investigate the durations of weed interference and weed-free period, respectively. It was found that lentil has a CPWC beginning at the five-node stage and continuing to the 10-node stage. There was an inverse relationship between weed biomass and lentil yield; that is, lentil yield was highest when weed biomass is minimal. We propose that the CPWC begins when weeds start to accumulate significant biomass and ends with crop canopy closure. Therefore, to maximize lentil yields, growers should consider using a POST residual herbicide that can control weeds during the CPWC.

The overall objective of this study was to identify common patterns in the spatial distribution of the major weed species present in the corn-growing region of central Spain, exploring the scale dependence of these patterns and the possible associations or dissociations between individual species. Weed density was assessed in 16 commercial fields using digital images acquired in a 9-m by 9-m sampling grid. A set of six species was found in all the fields: black nightshade, common cocklebur, fierce thornapple, johnsongrass, purple nutsedge, and velvetleaf. Spatial analysis by distance indices and inverse distance weighting interpolation methods were performed to create weed distribution maps. The results showed aggregated spatial distribution patterns for all individual species regardless their life cycle, annual or perennial. Some associations and dissociations among species were found in the analysis of interactions. Nevertheless, the spatial patterns of co-occurrence of weed species were field-specific and therefore cannot be considered general patterns of weed co-occurrence. In order to explore the scale dependence of these results, an additional study was conducted in an experimental field located in the same area using a 1.0-m by 0.75-m sampling grid. Although this resolution allowed for a better definition of the positions of the weed patches and weed-free gaps, the results obtained revealed similar patterns to those observed with a coarser sampling resolution.

Herbicide degradation in soil is a major research issue, as evidenced by the number of refereed articles on the subject (Figure 1). The approximate number of total citations per year has increased from about 70 in the mid-1990s to approximately 170 per year in the last few years (Figure 1a). From a weed science perspective, publications in the journals Weed Science and Weed Technology have tended to decline from an average of eight per year in the 1995 to 2005 interval to about three per year in the last 10 yr (Figure 1b). This discrepancy of total citations vs. Weed Science Society of America journals may be due to funding availability and other more immediate research needs in weed science. Other reasons might be a lack of reader interest (indicating low potential impact in this specific topic area) or the perception that Weed Science and Weed Technology are light venues for such papers and therefore not the first choice for publication. The authors believe this research topic to be important and relevant to the discipline of weed science, even more so as herbicide use patterns become more complicated because of glyphosate-resistant (GR) weeds.

Palmer amaranth control has become a major challenge for multiple cropping systems across the southeastern and midwestern United States. Despite extensive research on herbicide-resistance evolution, little research has been done exploring how Palmer amaranth might also be evolving other adaptive traits in response to different selection forces present in agricultural fields and the enrichment of soils with nutrients such as nitrogen. The objective of the present study was to determine whether Palmer amaranth populations have evolved different morphology and growth patterns in response to glyphosate use and fertilization history. Ten Palmer amaranth populations, including glyphosate-resistant (GR) and glyphosate-susceptible (GS) populations, were collected from different cropping systems with histories of high and low nitrogen fertilization in the states of Florida and Georgia. All populations were grown in pots filled with soil fertilized with either 0 or 40 kgNha−1, and their response to nitrogen was compared for morphological, growth, and nutrient-use traits. Populations differed in how they modified their morphology and growth in response to N, with major differences in traits such as foliar area, branch production, leaf shape, and canopy architecture. Populations with high nitrogen-fertilization histories had higher (>43%) nutrient-use efficiency (NUE) than populations with low nitrogen-fertilization histories. Similarly, GR populations have evolved higher NUE (>47%) and changed canopy architecture more than GS populations in response to nitrogen fertilization. The results of the present study highlight the importance of paying more attention to adaptations to cultural practices that might increase weediness and how genetic changes in traits involved in morphology and metabolism might favor compensatory mechanisms increasing the fitness of the population carrying herbicide-resistant traits.

Over 90% of Canadian kochia populations are resistant to acetolactate synthase (ALS)– inhibiting herbicides. We questioned whether the target site–based resistance could affect plant growth and competitiveness. Homozygous F2 herbicide-resistant (HR) kochia plants with an amino acid substitution at Trp574 (sources: Alberta [AB], Saskatchewan [SK], and Manitoba [MB]), or Pro197 (MB, AB with two populations) were grown in replacement series with homozygous F2 herbicide-susceptible (HS) plants from the corresponding heterogeneous population (total: six populations). In pure stands, growth of HR plants from AB and SK was similar to that of HS plants, regardless of mutation; conversely, MB2-HR plants (Trp574Leu) developed more slowly and were taller than MB2-HS plants. Final dry weight of HR plants in pure stands was similar across all six populations, whereas that for HS plants in pure stands and HR–HS plants in mixed stands (50–50%) varied with population. Results for AB and SK populations suggest little impact of either ALS mutation on kochia growth, whereas those for MB lines would suggest an unidentified factor (or factors) affecting the HS, HR, or both biotypes. The variable response within and between lines, and across HS biotypes highlights the importance of including populations of various origins and multiple susceptible controls in HR biotype studies.

An isolation and screening procedure was developed to identify soil actinomycetes that can produce germination and growth inhibitors on agar plates. Eight of 120 isolates tested demonstrated severe toxicity on cress (Lepidium sativum L. ‘Curly’) and barnyardgrass [Echinochloa crus-galli (L.) Beauv. ♯ ECHCG]. Several isolates were observed to stimulate growth. In replicated tests using the agar screening procedure, compounds produced by microorganisms significantly inhibited germination and growth of indicator plants. Organisms shown to produce toxins in agar media did not always produce toxins when grown in submerged culture.

The effects of several concentrations of glyphosate [N-(phosphonomethyl)glycine] and siduron [1-(2-methylcyclohexyl)-3-phenylurea] on germination and seedling growth of Kentucky bluegrass (Poa pratensis L. ‘Nugget,’ ‘Prato,’ ‘Fylking,’ ‘Park’), creeping bentgrass (Agrostis palustris Huds. Penncross,’ ‘Seaside’), and red fescue (Festuca rubra L. ‘Pennlawn,’ ‘Wintergreen’) were evaluated in petri dishes under controlled environmental conditions. A high concentration of glyphosate (10-4 M) reduced shoot growth in all but two cultivars. Siduron had no effect. Glyphosate or siduron applied to the soil and incorporated did not affect turfgrass germination or growth. When the glyphosate was sprayed over the soil and on the seed lying on the soil surface, red fescue germination was reduced and shoot growth of Kentucky bluegrass, creeping bentgrass, and red fescue was inhibited at the high rate of glyphosate application (17.9 kg/ha). Under field conditions, none of the glyphosate treatments reduced turfgrass germination or growth.

Of the grasses studied over a 3-year period, Alta tall fescue (Festuca arundinacea) was most competitive and Troy bluegrass (Poa pratensis L.) least competitive with Canada thistle (Cirsium arvense). Ladino white clover was more competitive than birdsfoot trefoil (Lotus corniculatus) when present in full stands. Winter-killing of Ladino clover (Trifolium repens L.) resulted in an increase in Canada thistle, particularly at low irrigation levels. Irrigation favored an increase in Canada thistle numbers in the year the grass stands were established. In the next two years thistle numbers declined more rapidly on irrigated than on non-irrigated plots. High rates of nitrogen increased the competitive ability of the grasses.

Experiments were initiated to determine if the tropical soda apple (TSA) biological control agent, Tobacco mild green mosaic tobamo virus (TMGMV), could be mixed with synthetic herbicides to provide effective broad-spectrum weed control. When TMGMV was mixed with 2,4-D ester or amine, metsulfuron, or hexazinone, TSA control ranged between 80 and 100%. On average, TMGMV increased TSA control by 81% as compared to these herbicides applied alone. Treatment applications were made by rubbing only three leaves, not as a broadcast application. Although this is not the optimum method for herbicide application, it does indicate the level of control the herbicide alone potentially provided relative to the herbicide/TMGMV mixture. Results indicate that the majority of TSA control was due to virus and that the herbicides mixed with TMCMV did not interfere with the virus's ability to infect TSA. Additions of organosilicone adjuvants or low rates of crop oil or nonionic adjuvants to TMGMV solutions resulted in greater infection of TSA. The finding that TMGMV remains infective when mixed with herbicides will allow greater flexibility for landowners attempting to control TSA and other troublesome weeds.

Three models that empirically predict crop yield from crop and weed density were evaluated for their fit to 30 data sets from multistate, multiyear winter wheat–jointed goatgrass interference experiments. The purpose of the evaluation was to identify which model would generally perform best for the prediction of yield (damage function) in a bioeconomic model and which model would best fulfill criteria for hypothesis testing with limited amounts of data. Seven criteria were used to assess the fit of the models to the data. Overall, Model 2 provided the best statistical description of the data. Model 2 regressions were most often statistically significant, as indicated by approximate F tests, explained the largest proportion of total variation about the mean, gave the smallest residual sum of squares, and returned residuals with random distribution more often than Models 1 and 3. Model 2 performed less well based on the remaining criteria. Model 3 outperformed Models 1 and 2 in the number of parameters estimated that were statistically significant. Model 1 outperformed Models 2 and 3 in the proportion of regressions that converged on a solution and more readily exhibited an asymptotic relationship between winter wheat yield and both winter wheat and jointed goatgrass density under the constraint of limited data. In contrast, Model 2 exhibited a relatively linear relationship between yield and crop density and little effect of increasing jointed goatgrass density on yield, thus overpredicting yield at high weed densities when data were scarce. Model 2 had statistical properties that made it superior for hypothesis testing; however, Model 1's properties were determined superior for the damage function in the winter wheat–jointed goatgrass bioeconomic model because it was less likely to cause bias in yield predictions based on data sets of minimum size.

Experiments were conducted in Lewiston, NC, in 1999 and 2000 and Rocky Mount, NC, in 1999 to evaluate weed management systems in strip- and conventional-tillage peanut. The peanut cultivars grown were ‘NC 10C’, ‘NC 12C’, and ‘NC 7’, respectively. Weed management systems consisted of different combinations of preemergence (PRE) herbicides including diclosulam and flumioxazin plus commercial postemergence (POST) herbicide systems. Dimethenamid plus diclosulam or flumioxazin PRE controlled common lambsquarters, eclipta, and prickly sida at least 91%. Diclosulam and flumioxazin provided variable control of three Ipomoea species (59 to 91%) and bentazon plus acifluorfen POST provided > 90% control. Only diclosulam systems controlled yellow nutsedge 90% late season. Annual grass control required clethodim late POST, regardless of tillage system. Dimethenamid plus diclosulam or flumioxazin PRE produced equivalent yields and net returns with no significant differences between the two PRE options. Both systems produced higher yields and net returns than dimethenamid regardless of the POST herbicide option. The tillage production system did not influence weed control of eight weeds, peanut yields, or net returns. The addition of diclosulam or flumioxazin to dimethenamid PRE improved weed control compared with dimethenamid PRE alone.