The extensive and intensive use of herbicides has resulted in the spread of herbicide-resistant weeds in many crop production systems; therefore, it is imperative to devise new organic weed control methods. Recently, the application of spent coffee grounds (SCG) in agricultural fields has been found to inhibit plant growth and germination and is thus considered a potentially effective weed control measure. This study aimed to evaluate the effects of different amounts and methods of SCG application on weed growth through field experiments. The field experiments were conducted in an upland field converted from a paddy in western Japan. The results show that the plow-in application of over 10 kg m−2 of SCG and mulching application of 20 kg m−2 decreased the weed dry weight compared with the control. In addition, the growth of weed species of families other than Gramineae, such as wingleaf primrose-willow and horseweed, was not significantly affected by SCG application. Weed species of families other than Gramineae are dominant in some upland fields. Hence, the inhibitory effect of SCG on weeds may be lower in original upland fields than in the upland field converted from paddy field that was investigated in the present study. Overall, this study demonstrated that the plow-in application of 10 kg m−2 of SCG every 4 mo was effective for weed control in an upland field converted from a paddy field. Because SCG worked against grass weeds under the specific conditions in this study, it would be valuable to explore other potential applications of this novel means of weed control.

Hairy fleabane and horseweed are pervasive weed species in agriculture. Glyphosate-resistant (GR) and glyphosate/paraquat–resistant (GPR) biotypes challenge current management strategies. These GR and GPR biotypes have non–target site resistance, which can confer resistance to herbicides with different sites of action (SOAs). This study’s objective was to characterize the response of GR, GPR, and glyphosate/paraquat–susceptible (GPS) biotypes of both weed species to herbicides with a different SOA. Whole-plant dose–response bioassays indicated a similar response among tested biotypes of both weed species to rimsulfuron, dicamba, hexazinone, glufosinate, flumioxazin, saflufenacil, or mesotrione. The hairy fleabane GR and GPR biotypes were 2.7- and 2.9-fold resistant to 2,4-D relative to the GPS biotype (GR50 766.7 g ai ha–1), confirming 2,4-D resistance in hairy fleabane for the first time in California. The GR and GPR biotypes were not cross-resistant to dicamba. No differences in response to 2,4-D were observed among horseweed biotypes with a GR50 ranging from 150.2 to 277.4 g ai ha–1. The GPR biotypes of both species were cross-resistant to diquat, with a 44.0-fold resistance in hairy fleabane (GR50 863.7 g ai ha–1) and 15.6-fold resistance in horseweed (GR50 563.1 g ai ha–1). The confirmation of multiple resistances to glyphosate, paraquat, and 2,4-D in hairy fleabane curtails herbicide SOA alternatives and jeopardizes resistance management strategies based on herbicide rotation and tank mixtures, underscoring the critical need for nonchemical weed control alternatives.

In seasonal climates, germination timing is mainly controlled by temperature, especially in species with physiological seed dormancy. The germination response to temperature may, however, vary among populations across the distribution range of species. Understanding how populations along climate gradients vary in their sensitivity to temperature is important for determining their vulnerability to climate variability and change. Here, we investigated the germination response of two Erica species with physiological seed dormancy (E. australis and E. umbellata) to changes in temperature throughout the seasons (simulated autumn through to spring) and to the local climate in six localities across a latitudinal gradient in western Iberia. Effects were studied with and without exposing the seeds to a heat shock. The local climate of seed provenance emerged as a key factor in modifying the germination sensitivity to germination temperature and their variation through the seasons. Although each species showed idiosyncratic germination responses to temperature treatments and across the gradient, germination of both species was sensitive to warmer temperatures and to a heat shock. Both showed similar seasonal germination patterns: as we moved from south to north, populations tended to have a larger germination peak in spring, which was greater at colder temperatures. We conclude that rising temperatures associated with climate change will affect these species, particularly at their northern ranges, where many seeds will remain dormant during warmer winters. Arguably, models aiming at assessing climate change impacts in these species need to include such variability across latitude.

Conservation tillage systems for cotton production were monitored for their influence on horseweed populations. Without fall tillage, horseweed became established and was present the following spring. Disking in the fall or spring controlled horseweed for the subsequent cotton crop.

Populations of horseweed and Virginia pepperweed resistant to the herbicide paraquat were found in several fruit orchards in Essex County, Ontario, Canada, where paraquat had been applied four to five times per year for at least 10 yr. Dose–response experiments were conducted in the greenhouse for each species, using 2-to 4-mo-old plants from populations that had received repeated exposure to paraquat and those that had not. The paraquat dose that reduced shoot dry weight by 50% (ED 50) was estimated for each population. Resistant populations of horseweed had ED 50 values 25 to 35 times higher than susceptible populations. A resistant population of Virginia pepperweed had an ED 50 10 times higher than susceptible populations.

Research was conducted in 1987 and 1988 to identify effective horseweed control strategies in no-tillage soybean production. Paraquat at 560 g ai ha-1 plus metolachlor at 2200 g ai ha-1, linuron at 840 g ai ha-1, and nonionic surfactant at 0.25% (v/v) applied preemergence provided less than 61% horseweed control. Glyphosate at 840 g ai ha-1, 2,4-D ester at 560 g ai ha-1, HOE-39866 at 840 g ai ha-1, or BAS-514 at 70 g ai ha-1 applied early preplant controlled more than 96% when followed by the above preemergence herbicide combination. Substituting either glyphosate at 840 g ha-1 or HOE-39866 at 840 g ha-1 for paraquat in the preemergence application program significantly increased control. Adding BAS-514 at 70 g ha-1 to preemergence treatments containing paraquat significantly improved control. The substitution of metribuzin, metribuzin plus chlorimuron (10:1 ratio), or linuron plus chlorimuron (16:1 ratio) for linuron resulted in greater control and soybean yield. Sequential applications controlled horseweed equal to or better than a single preemergence application at the same total rate.

Field experiments were conducted from 1979 to 1989 to determine the influence of conventional, reduced, and no-tillage systems and different herbicide combinations on weed species and population, weed control, and soybean injury, population, and yield. In no-till (NT) non-treated plots, there was an abrupt shift from horseweed as the dominant early spring emerging weed to gray goldenrod in 1985. Following its initial observation, gray goldenrod became the dominant species within 2 yr, with giant foxtail as the only other species observed in these plots. Giant foxtail was the dominant weed species from 1980 to 1989 in conventional till (CT) and reduced-till (RT) plots. There also was a shift in the frequency of occurrence and in density of several broadleaf weed species during the 11-yr study. Most herbicides provided excellent control of all weeds in all tillage systems, especially those that included POST herbicides. There was little difference between glyphosate and paraquat in controlling weeds present at the time of planting in NT. PRE herbicides caused 2 to 9% soybean injury with slightly greater injury occurring in CT and RT than in NT. The POST broadleaf herbicides did not significantly increase soybean injury. There were no differences in soybean population or yield among the herbicide treatments regardless of tillage. There also was no difference in soybean population or yield in NT compared with CT when averaged over all herbicide treatments.

Sweet corn was grown with a living mulch of white clover, a dead mulch of rye, and without mulch, in both till and no-till conditions. Unplanted controls were also included in the experimental design. Corn yields were highest in clover treatments early in the experiment but lowest in later years. The declining yields in the clover living mulch were related to the strip application of glyphosate which allowed establishment of perennial and biennial weeds, notably dandelion and horseweed. These overwintering weeds apparently prevented effective control of summer annuals, especially redroot pigweed, common lambsquarters and large crabgrass, by atrazine and metolachlor. Presence of a rye mulch decreased weed biomass and had no detrimental effect on corn yield. In general, corn yield was not affected by tillage, although the number of marketable ears was reduced in the no-till treatments during the drought year of 1988. The much greater weed biomass in the unplanted control treatments showed the importance of crop competition for weed control in sweet corn cropping systems.

Studies were conducted to determine the effects of tillage level on weed control and yield of asparagus grown on a sandy loam soil under sprinkler irrigation. Without herbicides, horseweed and kochia populations increased in no-till asparagus production over a 3-yr period, compared to asparagus that was rototilled either once in the spring or once in the spring and again at layby. Without herbicides, hairy nightshade populations increased most when asparagus was rototilled both in the spring and at layby. Volunteer asparagus was controlled best when herbicides and tillage were combined in the spring and again at layby. Common groundsel was controlled without herbicides by rototilling both in the spring and at layby. Diuron at 1.4 kg ai/ha plus prodiamine at 1.4 kg/ha or metribuzin at 0.9 kg/ha plus norflurazon at 2 kg/ha controlled all weeds under all three tillage levels when split-applied in the spring and at layby. Tillage level had no effect on total asparagus spear number or weight if tillage was performed in mid March before asparagus emergence.

Field experiments were conducted to compare performance of glyphosate with three different boom arrangements in a winter wheat-fallow rotation near North Platte, NE, in 1994 and 1995. One boom was optically controlled, and the other boom was for broadcast herbicide applications. Spraying with both booms at the same time was called “dual boom.” The sprayers were tested during May, June, and July on two weed density levels established by applying glyphosate at 0.42 kg ae/ha with and without atrazine at 0.84 kg ai/ha in October following wheat harvest. The dual-boom and the broadcast herbicide applications were more efficient in controlling weeds than the optically controlled system. The dual boom reduced weed density 4.5-fold compared with the optically controlled sprayer used alone. Horseweed < 8 cm tall was more difficult to control with the optically controlled sprayer than redroot pigweed and kochia because of its cylindrical-shaped growth patterns. Barnyardgrass and green foxtail seedlings with an erect growth pattern were also difficult for the sensors to detect. Poorer control with the optically controlled sprayer was associated with failure to identify small weeds, chlorotic plants, inconsistency among sensors, and too wide a field of view (FOV), as sensors were spaced farther apart than presently recommended. The number of sensors on a boom needs to be increased to improve the performance of the optically controlled sprayer.

The objectives of this study were to determine how the timing of weed management treatments in winter wheat stubble affects weed control the following season and to determine if spring herbicide rates in corn can be reduced with appropriately timed stubble management practices. Field studies were conducted at two sites in Ohio between 1993 and 1995. Wheat stubble treatments consisted of glyphosate (0.84 kg ae/ha) plus 2,4-D (0.48 kg ae/ha) applied in July, August, or September, or at all three timings, and a nontreated control. In the following season, spring herbicide treatments consisted of a full rate of atrazine (1.7 kg ai/ha) plus alachlor (2.8 kg ai/ha) preemergence, a half rate of these herbicides, or no spring herbicide treatment. Across all locations, a postharvest treatment of glyphosate plus 2,4-D followed by alachlor plus atrazine at half or full rates in the spring controlled all broadleaf weeds, except giant ragweed, at least 88%. Giant foxtail control at three locations was at least 83% when a postharvest glyphosate plus 2,4-D treatment was followed by spring applications of alachlor plus atrazine at half or full rates. Weed control in treatments without alachlor plus atrazine was variable, although broadleaf control from July and August glyphosate plus 2,4-D applications was greater than from September applications. Where alachlor and atrazine were not applied, August was generally the best timing of herbicide applications to wheat stubble for reducing weed populations the following season.

Horseweed emergence and survival were evaluated in no-tillage soybean and corn at Rosemount, MN, and Ames, IA, from the fall of 1992 through the summer of 1995. Most of the horseweed at both locations emerged in the fall. Winter survival of fall-emerged seedlings ranged from 59 to 91%. Timing and extent of horseweed emergence in spring varied by year and location. Spring emergence ranged from 5 to 32% of total emergence, with greater spring emergence at Rosemount than at Ames. Emergence occurred as late as early June at Rosemount and late May at Ames. Results indicated that horseweed may emerge well into the growing season, and spring emergence should be considered in weed management systems for no-tillage crop production.

Greenhouse and laboratory studies were conducted to evaluate the effects of rye residues on germination and growth of three triazine-resistant weed species, barnyardgrass, willowherb, and horseweed. Soil containing rye roots was used to determine rye residue influence on seed germination. Willowherb and horseweed germination was inhibited up to 50% with the highest rate of rye, but there was no inhibition of barnyardgrass germination. Aqueous extracts of rye shoot tissue inhibited incubator germination of horseweed and willowherb, but did not significantly influence barnyardgrass germination. Although there were no germination differences between the susceptible and resistant biotypes, both were totally inhibited when a 5% w/v solution was used. Growth inhibition of the selected weeds was evaluated using rye root residues in soil. All combinations of biotypes, species, and rye seeding rates showed significant growth inhibition.

Insensitivity to atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] and certain other herbicides was found in plants of horseweed (Conyza canadensis (L.) Crong. ♯4 ERICA) collected from vineyards where atrazine had been applied extensively. There were no differences in the uptake, translocation, or accumulation of radiolabeled atrazine between plants of the susceptible and resistant biotypes. However, atrazine at 1 × 10-4 M caused no inhibition of photosynthetic electron transport in resistant leaves. Photosynthetic characteristics of leaves of susceptible and resistant plants showed different sensitivities to atrazine, but similar sensitivities to diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] and ioxynil (4-hydroxy-3,5-diiodobenzonitrile). Isolated chloroplast membranes from resistant plants exhibited a resistance factor of 1000 to atrazine but only 10 to 30 to methylthio-s-triazines. DNOC (4,6-dinitro-o-cresol) was found to be more effective in susceptible chloroplasts than in resistant ones. A marked reduction in the galactolipid content, especially digalactosyl diglyceride, was found in resistant leaves. It is suggested that these alterations in lipid content and composition may contribute to atrazine resistance in horseweed.

The combination of glyphosate and 2,4-D at various rates was evaluated for controlling existing weeds at planting in no-till corn and soybeans. Herbicide combinations in soybeans also included paraquat plus 2,4-D, linuron, or diuron. Standard treatments included glyphosate (0.6 and 1.1 kg ae/ha) and paraquat (0.3 and 0.6 kg ai/ha), and 2,4-D (0.6 kg ae/ha) alone. For corn, the addition of 2,4-D to glyphosate did not improve weed control, although the addition of 2,4-D to paraquat did improve horseweed control. Corn yield with the herbicide combinations was higher than that for the nonselective herbicides alone. Although initial weed control was good in soybeans, weed regrowth in all paraquat alone treatments was substantial, especially with horseweed. The addition of 2,4-D to paraquat improved control of horseweed and tall morningglory. The addition of linuron or diuron to paraquat improved horseweed and common ragweed control, whereas the addition of 2,4-D to glyphosate improved the control of tall morningglory but not the other weed species. Generally, after 4 weeks, all glyphosate treatments provided better horseweed control than all paraquat treatments. Paraquat plus either linuron or diuron and glyphosate alone or in combination with 2,4-D gave the highest soybean yields.

Field research was conducted from 1987 to 1991 to evaluate the influence of four weed management systems on weed population and species dynamics in conventional-tillage and no-tillage grain sorghum production. These weed management systems included zero, low, medium, and high input systems. The weed seedbank increased faster in zero and low input weed management systems than in the high input weed management systems because of differences in weed control. Tillage influenced weed seed densities as well. Common ragweed, common lambsquarters, horseweed, and sicklepod seed densities often were greater in no-tillage than conventional-tillage plots. Common cocklebur and large crabgrass seed densities were usually greater in conventional-tillage than no-tillage plots. Smooth pigweed seed densities were not affected by tillage. Increasing weed management inputs diminished differences between tillage systems. Smooth pigweed dominated the weed populations after 4 yr in both tillage systems even in high input systems.

Field research was conducted at Hancock, WI, from 1985 through 1987 to evaluate effects of conventional tillage, chisel plow, ridge tillage, and no-tillage systems on population dynamics and control of annual weed species in corn grown continuously on a loamy sand soil without irrigation. In all years of the study, green foxtail densities were greater in chisel plow and no-tillage than in the conventional tillage system, while ridge tillage had densities lower than all other tillage systems. Common lambsquarters density in the chisel plow system reached nearly 500 plants m−2 compared to less than 75 plants m−2 in the other tillage systems when averaged over years. Average redroot pigweed densities in the no-tillage and chisel plow systems were 307 and 245 plants m−2 compared to less than 25 plants m−2 in the conventional and ridge tillage systems. Horseweed was observed only in no-tillage and ridge tillage plots. Green foxtail and redroot pigweed were more difficult to control in chisel plow and no-tillage than in the conventional and ridge tillage systems with several herbicide treatments. Corn yields were not affected by tillage systems under weed-free conditions. Corn yield differences among tillage systems when the same herbicide treatment was applied appeared to be due to differences in weed control.

Field studies were conducted for 4 yr with HOE-39866 [ammonium-(3-amino-3-carboxypropyl)-methylphosphinate] and for 2 yr with SC-0224 (trimethylsulfonium carboxymethylaminomethylphosphonate) to compare foliar activities with those of paraquat (1,1’-dimethyl-4,4’-bipyridinium ion) and glyphosate [N-(phosphonomethyl)glycine] in no-till corn (Zea mays L. ‘Pioneer 3320’ in 1981 and ‘Pioneer 3184’ in 1982 and 1983). Rates evaluated were 0.6 to 1.7 kg ai/ha for all herbicides except paraquat which was included at 0.3 and 0.6 kg ai/ha. Control of rye (Secale cereale L.) was excellent at all rates with SC-0224, paraquat, and glyphosate, but HOE-39866 was effective only when combined with residual herbicides. All four herbicides provided good initial control of horseweed [Conyza canadensis (L.) Cronq., ♯ ERICA], but recovery following paraquat application was extensive. Combinations of HOE-39866, SC-0224, and glyphosate at 0.6 kg/ha with atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] at 1.1 to 1.7 kg ai/ha + metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] at 1.7 kg ai/ha produced antagonistic responses against rye and sometimes horseweed. In 1983, the combination of SC-0224 + residual herbicides provided significantly better control of rye and horseweed 8 days after treatment than the combination of glyphosate + residual herbicides.

A study was conducted to evaluate efficacy of 17 PRE herbicides on seven weed species that are important competitors in container-grown ornamentals. Treatments included napropamide, metolachlor, acifluorfen, lactofen, oxyfluorfen, oryzalin, pendimethalin, prodiamine, trifluralin, isoxaben, oxadiazon, simazine and combination products. Control was evaluated by counting weed seedlings 42, 202, and 249 d after treatment. Some treatments provided nearly total control of less than 1.0 seedling per container at each rating over two years. Those treatments were: common groundsel—diphenyl ethers, isoxaben, oxadiazon, oxyfluorfen + oryzalin, and oxyfluorfen + pendimethalin; prostrate spurge—pendimethalin, prodiamine, prodiamine-ABD, trifluralin, oxadiazon, isoxaben + oryzalin, and methazole + prodiamine; common chickweed—isoxaben and isoxaben + oryzalin; hairy bittercress—oxyfluorfen, pendimethalin, prodiamine, prodiamine-ABE, oxadiazon, oxyfluorfen + oryzalin, oxyfluorfen + pendimethalin, and methazole + prodiamine; common ragweed—oxyfluorfen, prodiamine-ABD, methazole + prodiamine; Virginia pepperweed—lactofen, oxyfluorfen, oryzalin, pendimethalin, prodiamine, prodiamine-ABD, isoxaben, oxadiazon, oxyfluorfen + oryzalin, oryzalin + pendimethalin, isoxaben + oryzalin, and methazole + prodiamine; horseweed—oxyfluorfen, dinitroanilines, oxadiazon, oxyfluorfen + oryzalin, oxyfluorfen + pendimethalin, isoxaben + oryzalin, and methazole + prodiamine.

Field studies were conducted to evaluate the influence of herbicide application timing on weed control in no-till soybean production. Row spacing generally had no effect on weed control. Herbicide treatments containing chlorimuron plus metribuzin applied as many as 45 days prior to planting in 1988 and 1989 controlled broadleaf weeds throughout the growing season. Imazaquin applied 45 and 30 days prior to planting provided poor control of common cocklebur in 1989. Giant foxtail control was inconsistent with all herbicide treatments. Soybean yields subsequent to early preplant herbicide applications were greater than or equal to those in which applications were made at planting when late-season weed control was adequate. Herbicides applied preemergence did not control high densities of common lambsquarters in 1989.