Cover crops (CCs) have shown great potential for suppressing annual weeds within agronomic cropping systems across the United States. However, the weed suppressive potential of CCs may be moderated by environmental and management factors that are specific to certain geographic areas and their associated characteristics. This may be particularly true within the U.S. Southeast, where higher mean annual temperature and precipitation generate favorable conditions for both CC and weed growth. To understand the effects of this regional context on CCs and weeds, a meta-analysis examining paired comparisons of weed biomass and/or weed density under CC and bare ground conditions from studies conducted within the Southeast was conducted. Data were identified and extracted from 28 journal articles in which weed biomass and/or weed density were measured along with cash crop yield data, if they were provided. Fourteen studies provided 142 comparisons for weed biomass; 23 studies provided 139 comparisons for weed density; and 22 studies, pooled over both weed response variables, provided 144 comparisons for cash crop yield. CCs had a negative effect on weed density (P = 0.0016) but no effect on either weed biomass (P = 0.16) or cash crop yield (P = 0.88). The mean relative reduction in weed density under CCs was 44%. Subsequent analyses indicated that CC biomass was the key factor associated with this reduction. Weed density suppression was linearly related to CC biomass; a 50% decrease in weed density was associated with 6,600 kg ha−1 of CC biomass. Edaphic, geographic, and other management factors had no bearing on this suppressive effect. This highlights the importance of generating adequate CC biomass if weed suppression is the primary objective of CC use and the potential for CCs to reduce weed density over diverse soil, climate, and farm management conditions.

Hairy galinsoga (Galinsoga quadriradiata Cav.) is a troublesome weed in lettuce (Lactuca sativa L.) and other low-growing crops. Many troublesome weed species are more responsive to fertilizer than major crops, and hence potentially more competitive under fertile conditions. This study examined the responses of G. quadriradiata and lettuce, in monoculture or competition with each other, to nitrogen and phosphorus. A greenhouse experiment assessed the biomass and flower production of G. quadriradiata grown in 16 nutrient treatments. The nutrient treatments were arranged in a phosphorus series, in which phosphorus varied, while nitrogen was either low or high, and a nitrogen series, in which nitrogen varied, while phosphorus was either low or high. A field experiment examined G. quadriradiata biomass, flower production, and competition with lettuce using the same nutrient treatments as the greenhouse experiment. Galinsoga quadriradiata dry biomass increased with phosphorus addition, especially when nitrogen was abundant. In the phosphorus series at high nitrogen, G. quadriradiata produced more biomass when competing with lettuce than in monoculture. Nutrient addition generally increased flower production per unit mass. Lettuce fresh biomass increased with phosphorus, but lettuce dry biomass was largely unaffected by fertility. Lettuce fresh and dry biomass were sometimes decreased by competition from G. quadriradiata, relative to intraspecific competition. We conclude that high phosphorus rates increase the productivity of G. quadriradiata and may increase its competitiveness. Therefore, high phosphorus rates should be avoided unless good weed control can be assured.

More than 80% of soybean [Glycine max (L.) Merr.] in Brazil is cultivated in no-till systems, and although cover crops benefit the soil, they may reduce the amount of residual herbicides reaching the soil, thereby decreasing herbicide efficacy. The objective of this study was to evaluate sulfentrazone applied alone, sequentially after glyphosate, and in a tank mixture with glyphosate before planting no-till soybean. Experiments were performed in two cover crop systems: (1) pearl millet [Pennisetum glaucum (L.) R. Br.] and (2) forage sorghum [Sorghum bicolor (L.) Moench ssp. bicolor]. The treatments tested were: glyphosate (720 g ae ha−1) at 20 d before sowing (DBS) followed by sulfentrazone (600 g ai ha−1) at 10 DBS; glyphosate + sulfentrazone (720 g ae ha−1 + 600 g ai ha−1) for cover crop desiccation at 10 DBS; and sulfentrazone alone at 10 DBS without a cover crop. The accumulation of straw was 31% greater using sorghum rather than pearl millet. In the sorghum system, the concentration of sulfentrazone at 0 to 10 cm was 57% less with sequential application and 92% less with the tank mixture compared with the treatment without cover crop straw at 1 d after application (DAA). The same occurred in the pearl millet system, where the reduction was 33% and 80% for the sequential application and tank mixture, respectively. The absence of a cover crop resulted in greater sulfentrazone concentrations in the top layer of the soil when compared with the sequential application or tank mixture. At 31 and 53 DAA, the concentration of sulfentrazone at 10 to 20 and 20 to 40 cm did not differ among treatments. Precipitation of 90 mm was enough to remove the herbicide from the cover crop straw at 31 DAA when using sequential application. An additional 90-mm precipitation was necessary to promote the same result when using the tank mixture.

Yellow nutsedge (Cyperus esculentus L.) is one of the most serious weeds in New York State. This weed has an extensive distribution in the state where it is a strong competitor with row crops especially corn, beans, and potatoes. Nutsedge is also an important weed problem in many other leading agricultural areas of the United States. The problem in all of these areas has increased recently, possibly due to a reduction of competition from other weeds which are being satisfactorily controlled by selective herbicides.

Rose clover (Trifolium hirtum All.), subterranean clover (T. subterraneum L.), and crimson clover (T. incarnatum L.), were sprayed at different growth stages with 2,2-dichloropropionic acid dalapon), and two formulations of 4-(2,4-dichlorophenoxy)-butyric acid [4-(2,4-DB)], in two growing seasons. Herbage yields, seed yields, and seed quality were for the most part unaffected by either formulation of 4-(2,4-DB) at the dates required for weed control. Only subterranean clover was tolerant of dalapon and then only at very low rates.

The atomization of jet streams from circular orifices, 279, 483, and 1016 microns in diameter, was measured under operating pressures less than 5 psi. The jet streams disintegrated into uniform large drops approximately twice the diameter of the jet orifice, and small drops making up less than 1% of the liquid volume from the small jet. About 80% of the spray from a jet discharging horizontally was deposited in a band 2 inches wide. The distribution pattern of horizontal jet streams positioned on a 2 inch spacing was improved by angling the jet streams 10–15° from the direction of travel.

Good plot sprayers are essential in weed control investigations where chemicals are to be applied uniformly and in specified quantities. Derscheid described such a machine in 1952 and this machine, or similar models, have been used quite widely. The purpose of this paper is to report on a sprayer which was built as an improvement on Derscheid's machine. A number of these modified sprayers have been built at Swift Current and used successfully by other weed workers in Canada.

Research was conducted to formulate a temperature-dependent population-level model to predict rhizome johnsongrass development to the four-leaf stage. A nonlinear poikilotherm rate equation was used to describe development rates as a function of temperature. Development rate was highest at 36 C and declined at higher temperatures. A temperature-independent Weibull function adequately distributed development times for the population. Coupling the poikilotherm rate equation and the Weibull distribution function yielded a model suitable for characterizing rhizome johnsongrass development to the four-leaf growth stage. The model was tested and validated against independent data sets. Model predictions of 80% of rhizome johnsongrass population at the four-leaf stage were used as the central point of a 4-d application window for using reduced rates of herbicides in johnsongrass management programs. This application window included an average interval of 85 to 99% of johnsongrass population at the desired growth stage in field validation experiments.

The transition from puddled-transplanted rice (Oryza sativa L.) (PTR) to direct-seeded rice (DSR) is gaining popularity in central China. In contrast, the PTR system is the most common practice in southwest China. Weeds are a major problem in the paddy fields of the DSR systems, and herbicides are widely used for weed control. However, the increased frequency and rate of herbicide use leads to the rapid evolution of resistance. Smallflower umbrella sedge (Cyperus difformis L.) is a troublesome weed species in rice fields of China and is usually controlled by the acetolactate synthase (ALS)-inhibiting herbicide bensulfuron-methyl. Here, we collected 32 C. difformis populations from DSR systems (Hunan Province) and PTR systems (Guangxi Province) and investigated their resistance to bensulfuron-methyl. Results revealed 80% (8 out of 10) populations from Hunan Province and 14% (3 out of 22) populations from Guangxi Province had evolved resistance to bensulfuron-methyl. Five populations from Hunan Province (HN-2, HN-3, HN-5, HN-9, HN-10) possessing the Trp-574-Leu mutation had high-level resistance (ranging from 169- to >1,309-fold) based on GR50 ratios. The resistant populations from Guangxi Province had a lower level of resistance to bensulfuron-methyl due to a Pro-197-Ser mutation. The Asp-376-Glu mutation was only identified in the HN-4 population. In addition, the GX-3 population from the PTR systems was resistant to bensulfuron-methyl without ALS gene mutations, indicating non–target site resistance (NTSR). Although some resistant populations of both regions exhibited cross-resistance to multiple ALS-inhibiting herbicides, including pyrazosulfuron-ethyl, bispyribac-sodium, penoxsulam, and imazapic, sensitivity was also detected to the auxin herbicide MCPA and the photosystem II–inhibiting herbicides bentazone and propanil. These results indicate that cultivation practices affect resistance evolution in C. difformis. DSR systems exert high selection pressure by selecting the Trp-574-Leu mutation, resulting in high-level resistance. In contrast, a mutation at Pro-197 plus NTSR likely plays a significant role in ALS resistance in the PTR systems.

Plant control methods have been developed to reduce weed species that are often problematic in agricultural systems. However, these methods can create new challenges, such as herbicide resistance. Determining which plant traits are associated with herbicide resistance can assist managers in identifying species with the potential to develop herbicide resistance and to better understand factors contributing to the evolution of herbicide resistance. We used random forest models to model herbicide resistance of noxious weeds as a function of 10 biological and ecological plant characteristics. Three noxious weed characteristics—plant life span, seedbank persistence, and occurrence in riparian or wetland microsites—predicted herbicide resistance with 87% accuracy. Species with persistent seedbanks and with short life spans (i.e., annuals) that occurred outside riparian or wetland areas were most likely to develop herbicide resistance. Short life spans indicate short generation times enabling faster evolution for herbicide resistance. Persistent seedbanks may increase the survival of resistant genotypes within a population or may be co-selected as an alternate form of escape from control methods. Species occurring in riparian or wetland microsites may be a case of “avoidance” rather than resistance, as managers typically avoid applying herbicide in these areas. Currently, 47 of the noxious weed species analyzed in this study are herbicide resistant, and our models identified an additional 63 species with traits that are highly associated with herbicide resistance, potentially indicating species that are at risk of developing resistance under conducive conditions. Further data-driven analyses with more plant traits and species from around the world could help refine current risk assessment of herbicide-resistance development.

This study was developed based on a goosegrass [Eleusine indica (L.) Gaertn.] population from Primavera do Leste, MT, Brazil, with resistance to multiple herbicide modes of action (5-enolpyruvylshikimate-3-phosphate synthase [EPSPS] inhibition: glyphosate; acetyl-coenzyme A carboxylase [ACCase] inhibition: aryloxyphenoxypropionate chemical group). The objective was to identify possible mechanisms of resistance associated or not with herbicide sites of action. Several experiments and analyses were carried out with the contribution of different laboratories and institutions. The results obtained allowed us to conclude that: (1) the Asp-2078-Gly mutation conferred resistance to ACCase inhibitors, without overexpression of ACCase or changes in herbicide absorption and translocation; (2) overexpression of EPSPS, Thr-102 and Pro-106 mutations, and changes in absorption and translocation are not involved in E. indica resistance to glyphosate; (3) the metabolism of glyphosate in resistant E. indica plants requires further studies to elucidate the final destination of this herbicide in these plants. The mechanism of resistance of E. indica biotypes to ACCase-inhibiting herbicides was elucidated: it involves a change in the action site. However, the mechanism of resistance to EPSPS inhibitors was not conclusive, indicating that some hypotheses, mainly those regarding the metabolism of glyphosate in resistant plants, require further testing.

Glyphosate [N-(phosphonomethyl)glycine] applied to cranberry (Vaccinium macrocarpon Ait.) as a spray at 4 kg/ha in March controlled aster (Aster subspicatus Nees) 77 to 87%. Glyphosate swab treatment with 6% solution in July or August, 1978 controlled spirea (Spiraea douglasii C.) 90%. A single application of terbacil (3-tert-butyl-5-chloro-6-methyluracil) in April, 1976 at 2 kg/ha controlled inflated rush (Carex vesicaria L.) 70% for 3 yr. Phytotoxicity was not observed on ‘McFarlin’ cranberry vines from any of these glyphosate or terbacil applications. Phytotoxicity did result from April, July, or August spray applications of glyphosate. Swab treatments did not cause any injury to cranberry.

Germination of fruit of common lambsquarters (Chenopodium album L.) from three out of four sources was enhanced by 9 to 41% by removal of the fruit coat, and by 21% by washing the fruit for 70 h. Brown-black seed germinated more rapidly than the black and brown seed after washing for 70 h. The washing treatment appeared to remove some unknown inhibitors. After washing, seed germination was found to be positively correlated with seed size.

When a range of weed densities is needed for competition experiments, one method of reducing existing populations is to remove all weeds in strips of specified length down the crop row. This technique also can be used to create different sizes of weed dusters. Two methods of sampling weed densities after such thinning were compared: unrestricted sampling (under which quadrats are placed randomly within the row) and restricted sampling (under which quadrats are randomly placed within a row under the restriction that they coincide with the beginning of an infested strip). The bias of estimators of weed density under each of these two approaches was derived and bias-corrected estimators of weed density from the two methods were compared on the basis of their variance. The variance under restricted sampling is less than or equal to the variance under unrestricted sampling so that, by a minimum variance criterion, restricted sampling using a bias correction is the better technique.

Pendimethalin is commonly applied for PRE weed control in container nursery production. Field and laboratory trials were conducted to determine herbicide effectiveness and leaching of two pendimethalin formulations in pine bark, the primary component used in the growing medium of container-grown plants in the South. The microencapsulated (ME) formulation of pendimethalin gave lower control of southern crabgrass and leached deeper than the emulsifiable concentrate (EC) formulation in pine bark. The EC formulation controlled southern crabgrass 93% compared with only 70% with the ME formulation at 4.48 kg ai ha−1. After applying 3.4 kg ai ha−1 pendimethalin and 17.8 cm of irrigation water, the ME formulation showed greater leaching into the 3- to 6- and 6- to 9-cm depths than the EC formulation on the basis of a southern crabgrass bioassay. Using a microwave extraction method, only 0.91 mg kg−1 pendimethalin was found in the 3- to 6-cm pine bark depth compared with 4.0 mg kg−1 for the ME formulation. Below the 6-cm depth, no pendimethalin was detected when the EC formulation was applied, but 0.5 ppm was found for the ME formulation. No pendimethalin was detected in effluent collected from irrigation water for the EC; however, 3.0 mg kg−1 was collected from leachate for the ME formulation. Pendimethalin ME is leaching much deeper than the EC formulation in the pine bark profile, which results in lower weed control. This extensive leaching of ME formulation may be due to capsule movement with the irrigation water, combined with a delayed release of pendimethalin, which then binds to the pine bark much lower in the container profile.

Field experiments were conducted in northern Greece during 2001 and repeated in 2002 and 2004 to evaluate the effects of pronamide on sugar beet. Total leaf area, leaf area index (LAI), leaf and root dry weights, photosynthetic yield (quantum yield of photochemical energy conversion in photosystem II), chlorotic index, and yield components of sugar beet were monitored after pronamide application. Three sugar beet cultivars, ‘Avantage’, ‘Dorothea’, and ‘Bianca’, requiring short, intermediate, and long vegetative periods, respectively, were subjected to treatment. Pronamide was applied on sugar beet either as a double application of 0.63 kg ai ha−1 at the two- to four-leaf and 0.63 kg ai ha−1 at the four- to six-leaf stage or as a single application of 1.26 kg ai ha−1 performed at the latter leaf stage. Both application procedures were combined with a split application of phenmedipham at 0.04 kg ai ha−1 plus desmedipham at 0.04 kg ai ha−1 plus metamitron at 0.70 kg ai ha−1 plus ethofumesate at 0.10 kg ai ha−1 plus mineral oil at 0.50 L ha−1 applied POST at the cotyledon–to–two-leaf as well as at the four-leaf growth stages. Pronamide (both single and double application) initially caused chlorosis and reduction of sugar beet growth. LAI and photosynthetic yield were also significantly affected for a 2-mo period following the final application, after which the negative effects caused by pronamide were ameliorated. At harvest, sugar beet root and sugar yield, sucrose, K+, Na+, and N-amino acid concentrations were not affected by the herbicide treatments compared with those produced in weed-free and herbicide-free plots, indicating that all cultivars managed to overcome the transient pronamide stress. Regarding sugar beet cultivars, root and sugar yield of Avantage and Dorothea at harvest were higher than that of Bianca, whereas sucrose concentration of Avantage was the lowest. There was not an apparent relationship between the order of sugar yield per cultivar (Dorothea > Avantage > Bianca) and the length of the vegetative period (Avantage < Dorothea < Bianca).

Cucumber [Cucumis sativus L. ‘Calypso′] susceptibility to the ethyl ester of quizalofop {2-[4-[(6-chloro-2-quinoxyalinyl)oxy] phenoxy] propanoic acid} was investigated under field and greenhouse conditions. Cucumber yield was significantly reduced under field conditions with a single or repeated application of quizalofop at 0.14 or 0.28 kg ai/ha. Under greenhouse conditions, quizalofop significantly suppressed cucumber plant fresh weight with or without the presence of an adjuvant. Enhancement of herbicide activity was related to the concentration of adjuvant. At a higher rate of quizalofop, addition of the lower concentration of an adjuvant increased the herbicidal effect (growth suppression and visually estimated phytotoxicity). At a lower concentration, increasing adjuvant concentration enhanced visually estimated phytotoxicity.

Ridge truncation methods affected weed populations, weed control, and corn yields in a ridge-tillage production system. Ridges were left undisturbed, partially truncated, deeply truncated, or destroyed by full-width tillage. The effect of truncation on weed densities and control (visual estimates of biomass reduction) varied by weed species, herbicide treatment, and time of observation. Without herbicide, deep truncation resulted in the lowest densities of all species except giant foxtail. Band application of herbicide controlled all weed species as well as broadcast application following deep truncation. When herbicides were broadcast, weed control was usually not affected by ridge truncation. Ridge truncation often increased corn yields compared with no truncation due to better weed control. While deep truncation often resulted in lower weed populations following herbicide treatment than shallow truncation or disking, differences were not great enough to affect corn yields. When ridges were truncated, broadcast herbicide did not increase corn yields compared with banded herbicide.

This study documented an undescribed aspect of medusahead germination behavior. Fibrous hairs were consistently observed growing from the coleorhizal surface of germinating medusahead seeds. Three medusahead accessions were investigated to determine temperature effects on coleorhizal hair growth and germination. Coleorhizal hair emergence at 8 C was first noted at 72 h. By 24 h at 18 C, 15 to 74% of seeds had hairs, and at 28 C, hairs were visible on 75 to 94% of seeds. Germination at 18 and 28 C was 80% at 96 h; germination at 8 C did not exceed 80% until 168 h. Less than 4% of germinated seeds were without coleorhizal hairs. Appearance of coleorhizal hairs confirmed that germinating medusahead embryos were active at least 72 h before germination was complete. These data affirm coleorhizal hairs are a normal part of medusahead germination biology. Coleorhizal hairs probably aid early phases of medusahead seedling establishment.