Nicosulfuron provides POST weed control in corn, pastures, and grassy roadsides, and has potential for use in fine turfgrass. The objective of this research was to evaluate tolerance, absorption, translocation, and metabolism of nicosulfuron in annual bluegrass and four turfgrass species. In greenhouse experiments, relative tolerance of grasses to nicosulfuron (35, 70, or 140 g ai ha⁻¹) from high to low was bermudagrass  =  zoysiagrass > tall fescue > creeping bentgrass > annual bluegrass. In laboratory experiments, grasses had similar foliar and root absorption of ¹⁴C-nicosulfuon. Annual bluegrass and creeping bentgrass averaged 80% greater radioactivity per unit dry mass in shoots than bermudagrass following root uptake of ¹⁴C-nicosulfuron, but other species were similar to these grasses. At 72 h after treatment (HAT), annual bluegrass metabolized 36% of absorbed ¹⁴C-nicosulfuron, which was less than bermudagrass, tall fescue, and zoysiagrass that metabolized 47 to 58%. Creeping bentgrass metabolism of nicosulfuron was similar to annual bluegrass. Tall fescue had similar levels of metabolism to bermudagrass and zoysiagrass, averaging 67%, at 168 HAT but produced fewer metabolites. Overall, turfgrass tolerance to nicosulfuron is associated with relative herbicide concentrations in shoots and differential species metabolism.

Nomenclature: Annual bluegrass (Poa annua L.); bermudagrass (Cynodon dactylon × transvaalensis Burtt-Davy) ‘Princess 77’; creeping bentgrass (Agrostis stolonifera L.) ‘Penn A-4’; tall fescue [Lolium arundinaceum (Schreb.) S.J. Darbyshire] ‘Titan’; zoysiagrass (Zoysia japonica Steud.) ‘Zenith’.

Field, greenhouse, and laboratory experiments were conducted to investigate resistance to glyphosate in tropical sprangletop biotypes (Lv8 and Lv9) collected in Persian lime from Veracruz, Mexico. Assays to determine the dose required to reduce seedling fresh weight by 50% indicated a resistance factor (RF) of 4.9 and 3.2 for biotypes Lv8 and Lv9, respectively; whereas the LD₅₀ showed a RF of 4.4 and 3.3 for biotypes Lv8 and Lv9, respectively. On the other hand, the RFs using whole plant dose–response assays were lower (RF of 3 for Lv8 and 2.3 for Lv9). The susceptible biotype (LvS) accumulated 5.5 and 11.8 times more shikimate than biotypes Lv8 and Lv9, respectively, at 96 h after treatment (HAT). In field experiments, alternatives to glyphosate-resistant tropical sprangletop management were identified. Indaziflam glufosinate and paraquat diuron provided over 80% control of in-field populations of tropical sprangletop at 60 d after treatment (DAT). These results confirmed the first reported case of glyphosate-resistant tropical sprangletop.

Nomenclature: Diuron; indaziflam; paraquat; glufosinate; glyphosate; tropical sprangletop, Leptochloa virgata (L.) P. Beauv.; Persian lime, Citrus latifolia Tan.

Horseweed is a surface-germinating ruderal facultative winter annual. The ruderal nature is a key adaptive characteristic that implicates emergence timing as an important recruitment factor. Experiments were established at three sites in southern Ontario, Canada, from 2009 to 2012 to determine the possible effect of emergence timing of horseweed on plant number, fecundity, and flowering timing. Emerged seedlings were tagged in 0.25-m² plots in five 2-wk cohorts in the fall and spring of each experimental season. Each plot was followed though until the plants contained within each plot completed their life cycle. Generally, spring-emerging plants were found to flower earlier than fall-emerging plants, but with fall emergence there were higher plant densities in August each season compared with spring emergence. Overall, there was no difference in fecundity between spring- or fall-emerging cohorts, but when cohorts were parsed beyond just spring or fall emergence, we found that plants emerging in early fall and early spring were more fecund and flowered earlier than plants emerging in late fall and late spring. Disturbance (tilled versus not-tilled) significantly affected emergence levels but not emergence timing. The differences in performance among emergence cohorts are likely due to spatial or temporal density-dependent growth advantages. These results show that spring-emerging cohorts of horseweed, especially early spring–emerging cohorts, should not be discounted when considering the weediness of this species, and this may hold true for other facultative winter annual weeds as well.

Nomenclature: Horseweed, Conyza canadensis (L.) Cronq. var. canadensis.

Chinese sprangletop, a problematic weed in aerobic rice fields, has developed resistance to various groups of herbicides. In search of natural herbicides, the phytotoxic effects of napiergrass extracts on Chinese sprangletop were investigated. Phytotoxicity-directed extraction and fractionation of the culm plus leaves of napiergrass led to the isolation and identification of three major compounds: 2,4-di-tert-butylphenol (2,4-DTBP), cis-9-octadecenoic methyl ester (methyl oleate), and phthalic acid, mono-(2-ethylhexyl) ester (MEHP). These compounds showed different degrees of inhibition against the tested bioassay species. 2,4-DTBP was the most potent of the three compounds and completely inhibited the germination of Chinese sprangletop at the concentration of 0.5 g L⁻¹, followed by MEHP, where 62% inhibition of germination was obtained. In contrast, methyl oleate showed only slight inhibition (< 10%) of germination. A soil bioassay further demonstrated that 2,4-DTBP is a strong inhibitor of root growth and completely prevented root growth of Chinese sprangletop at an application rate as low as 0.60 kg ai ha⁻¹. Under aerobic conditions, 2,4-DTBP at 2.4 kg ai ha⁻¹ reduced the emergence and shoot fresh weight of Chinese sprangletop by more than 60%, with negligible effect on root and shoot growth of aerobic rice seedlings, suggesting that 2.4 kg ai ha⁻¹ 2,4-DTBP is the most suitable rate to control Chinese sprangletop without injuring rice seedlings. Reduction in shoot height of rice plant was evident at 0 d after sowing across 2,4-DTBP rates. However, the rice plants became less susceptible with increasing growth stages. The present findings imply that 2,4-DTBP may potentially be developed as a PRE soil-applied natural herbicide for control of Chinese sprangletop and perhaps other weeds in aerobic rice system.

Nomenclature: Napiergrass, Pennisetum purpureum Schumach.; Chinese sprangletop, Leptochloa chinensis (L.) Nees; aerobic rice, AERON, Oryza sativa L.

Acetyl-coenzyme A carboxylase (ACCase)–resistant Italian ryegrass is one of the most difficult-to-control weeds in United States wheat-production systems. Seed was collected from a suspected ACCase-resistant Italian ryegrass population in a winter wheat field with a history of ACCase-inhibitor herbicide use. This study investigated cross-resistance patterns in this Italian ryegrass population. Resistance was identified to the commercial dose of the ACCase herbicides pinoxaden, clethodim, sethoxydim, and clodinafop. Partial chloroplastic ACCase sequences revealed aspartate-to-glycine or isoleucine-to-asparagine substitutions at positions 2078 or 2041 in individuals of the resistant population. This is the first report, to our knowledge, of Asp-2078-Gly and Ile-2041-Asn substitutions in ACCase-resistant Italian ryegrass in the United States. Associating the occurrence of resistance alleles with resistance to specific active ingredients provides a better understanding of ACCase cross-resistance in Italian ryegrass and possibly options for its control.

Nomenclature: Clethodim; clodinafop; pinoxaden; sethoxydim; Italian ryegrass, Lolium perenne L. ssp. multiflorum (Lam.) Husnot LOLMU; winter wheat, Triticum aestivum L.

At germination, seedlings may be encouraged by the surrounding vegetation. This facilitation was studied in dandelions in a manipulative experiment to determine whether a nurse plant, either maple trees or alfalfa, assisted in the germination of dandelion seeds and whether there was a difference in germination when additional moisture (irrigation) was provided on top of rainfall. Seeds were sown into 33-cm² “arenas” and surrounded by plastic collars, which prevented the seed from being eaten by predators. Germination was recorded at 1- to 2-d intervals, and the percentage of the seeds that germinated and their time to 50% germination (T₅₀) was calculated. The nurse plant was consistently associated with seed germination. Compared with bare ground, the germination percentage was lower under tree cover, whereas T₅₀ was shorter under the cover of herbaceous plants (alfalfa). The values for the germination percentage and T₅₀ were similar under tree and herb nurse plants when the soil was irrigated. Nurse plants facilitated germination via their effects on understory humidity. Facilitating the germination of dandelions through the presence of nurse plants contributes to their propagation in crops.

Nomenclature: Dandelion, Taraxacum officinale G.H. Weber ex Wiggers TAROF; alfalfa Medicago sativa L.; Norway maple, Acer platanoides L.

Cowcockle, an introduced summer annual weed of the Northern Great Plains, is being considered for domestication because of its high quality starch, cyclopeptides, and saponins. Loss of seed dormancy is one of the key desirable traits for domestication. To determine the potential for domestication of this species, an understanding of the seed dormancy and germination patterns is required. The objectives of this study were to evaluate seed dormancy in cowcockle ecotypes and determine how temperature and light affect seed dormancy. We evaluated 15 populations of cowcockle for primary dormancy by exposing them to five temperatures (5, 7.5, 10, 15, and 20 C) under two temperature regimes (constant and alternating) in both dark and light conditions. Freshly matured seeds of all the populations showed high levels of primary dormancy except ‘Mongolia’. Lower levels of dormancy at medium temperatures (10 and 15 C) and greater dormancy at low and high temperatures suggest conditional dormancy, a state at which seeds germinate over a narrower range of conditions compared to nondormant seeds. The effects of temperature regime, light, and their interaction was significant only at suboptimal (5 and 7.5 C) and supraoptimal (20 C) temperatures. Under these conditions, alternating temperatures were more effective in breaking the conditional dormancy, followed by light. The variation in optimum temperature, light, and their interactions among the cowcockle populations may be due to the plants evolving to adapt to their local environments. From a domestication perspective, the conditional dormancy in cowcockle can be observed as an evolutionary mechanism that prevents untimely germination following maturity and may not be a major obstacle for its domestication.

Nomenclature: Cowcockle, Vaccaria hispanica (P. Mill.) Rauschert, VAPPY.

The synthetic auxin herbicides, aminocyclopyrachlor and clopyralid, control dicotyledonous weeds in turf. Clippings of turfgrass treated with synthetic auxin herbicides have injured off-target plants exposed to herbicide-laden clippings. Labels of aminocyclopyrachlor and clopyralid recommend that clippings of treated turfgrass remain on the turf following a mowing event. Alternative uses for synthetic auxin-treated turfgrass clippings are needed because large quantities of clippings on the turf surface interfere with the functionality and aesthetics of golf courses, athletic fields, and residential turf. A white clover bioassay was conducted to determine the persistence and bioavailability of aminocyclopyrachlor and clopyralid in turfgrass clippings. Aminocyclopyrachlor and clopyralid were each applied at 79 g ae ha⁻¹ to mature tall fescue at 56, 28, 14, 7, 3.5, and 1.75 d before clipping collection (DBCC). Clippings were collected, and the treated clippings were recycled onto adjacent white clover plots to determine herbicidal persistence and potential for additional weed control. Clippings of tall fescue treated with aminocyclopyrachlor produced a nonlinear regression pattern of response on white clover. Calculated values for 50% response (GR₅₀) for visual control, for normalized difference vegetative index (NDVI), and for reduction in harvested biomass were 20.5, 17.3, and 18.7 DBCC, respectively, 8 wk after clippings were applied. Clippings of tall fescue treated with clopyralid did not demonstrate a significant pattern for white clover control, presumably because clopyralid was applied at a less-than-label rate. The persistence and bioavailability of synthetic auxin herbicides in clippings harvested from previously treated turfgrass creates the opportunity to recycle clippings for additional weed control.

Nomenclature: Aminocyclopyrachlor; clopyralid; tall fescue, Lolium arundinaceum (Schreb.) S.J. Darbyshire; white clover, Trifolium repens L. ‘Dutch’.

A four site-year study was conducted in North Carolina to evaluate the effects of soybean planting timing and row spacing on soil moisture, weed density, soybean lodging, and yield in a cover crop-based no-till organic soybean production system. Soybean planting timing included roll-kill/planting and roll-kill/delayed planting where soybean planting occurred either on the same day or approximately 2 wk later, respectively. Soybean row spacing included 19, 38, and 76 cm, and all treatments included a weedy check and weed-free treatment. Rye biomass production averaged above 10,000 kg ha⁻¹ dry matter, which resulted in good weed control across all sites. Despite having good weed control throughout all treatments, weed coverage was highest in the 76-cm row-space treatment when compared to both the 19-cm and 38-cm row spacing in two of the four site-years. Soybean lodging is a potential consequence of no-till planting of soybeans in high residue mulches, and of the three row spacings, the 19-cm spacing exhibited the greatest incidence of lodging. Row spacing also influenced soybean yield; the 19- and 38-cm row spacing out yielded the 76-cm spacing by 10%. Soil volumetric water content (VWC) was higher in the cereal rye mulch treatments compared to the no rye checks. Furthermore, delaying soybean planting lowered soil water evaporation. However, the increased soil VWC in the rolled-rye treatment did not translate into increased soybean yield. The rolled-rye treatment exhibited significant (P < 0.01) increases in soil VWC when compared to the no-rye treatment at three of the four site-years. These results highlight planting date flexibility and potential risk to lodging that producers face when no-till planting organic soybeans.

Nomenclature: Glyphosate; s-metolachlor; imazethapyr; redroot pigweed, Amaranthus retroflexus L.; cereal rye, Secale cereal L.; DM, dry matter.

The efficacy, absorption, translocation, and metabolism of thiencarbazone-methyl (TCM) in hedge bindweed, field bindweed, ivyleaf morningglory, tall morningglory, and wild buckwheat were evaluated in greenhouse experiments and field trials. Forty-eight hours after foliar microapplication, ¹⁴C-TCM absorption was highest in ivyleaf morningglory (60%), followed by field bindweed (50%), wild buckwheat (35%), tall morningglory (17%), and hedge bindweed (9%). In all species, ¹⁴C-TCM was translocated systemically. By 24 h after application, ¹⁴C-TCM was detected in all parts of the plants. The translocation pattern is species-specific, with more translocation in tall morningglory and wild buckwheat. In all vining weeds ¹⁴C-TCM was almost not metabolized whereas corn metabolized almost all ¹⁴C-TCM 48 h after application. The efficacy of TCM was analyzed using dose–response curves. Wild buckwheat had the lowest value for the dose at which 50% of the activity occurs (2.1 g ai ha⁻¹ TCM), followed by hedge bindweed and ivyleaf morningglory. Field studies confirmed the high control of wild buckwheat with TCM. Even at the lowest tested concentration of TCM, wild buckwheat was controlled by over 90%. In contrast, efficacy of TCM in field trials against perennial vining weeds was very low, 25% for field bindweed and 65% against hedge bindweed. Control efficacy could be well explained by the translocation pattern of TCM in vining weeds.

Nomenclature: Thiencarbazone-methyl; field bindweed, Convolvulus arvensis L. CONAR; hedge bindweed, Calystegia sepium (L.) R.Br. CAGSE; ivyleaf morningglory, Ipomoea hederacea Jacq. IPOHE; tall morningglory, Ipomoea purpurea (L.) Roth PHBPU; wild buckwheat, Polygonum convolvulus L. POLCO; corn, Zea mays L. ZEAMA.

Organic agricultural systems increase the complexity of weed management, leading organic farmers to cite weeds as one of the greatest barriers to organic production. Integrated Weed Management (IWM) systems have been developed to address the ecological implications of weeds and weed management in cropping systems, but adoption is minimal. Organic agriculture offers a favorable context for application of IWM, as both approaches are motivated by concern for environmental quality and agricultural sustainability. However, adoption of IWM on organic farms is poorly understood due to limited data on weed management practices used, absence of an IWM adoption metric, and insufficient consideration given to the unique farming contexts within which weed management decisions are made. Therefore, this study aimed to (1) characterize organic weed management systems; (2) identify motivations for, and barriers to, selection of weed management practices; and (3) generate guiding principles for effective targeting of weed management outreach. We surveyed Midwestern organic growers to determine how specified psychosocial, demographic, and farm structure factors influence selection of weed management practices. Cluster analysis of the data detected three disparate, yet scaled, approaches to organic weed management. Clusters were distinguished by perspective regarding weeds and the number of weed management practices used. Categorization of individual farms within the identified approaches was influenced by primary farm products as well as farmer education, years farming, and information-seeking behavior. The proposed conceptual model allows weed management educators to target outreach for enhanced compatibility of farming contexts and weed management technologies.

Biannual applications of hexazinone have been applied in many lowbush blueberry fields in Nova Scotia for more than 30 years. Persistent reliance on a single herbicide chemistry may have selected for hexazinone-resistant red sorrel. The recommended rate of hexazinone (1.92 kg ai ha⁻¹) no longer controls red sorrel in many growing regions. Six levels of hexazinone (0, 0.48, 0.96, 1.92, 3.84, and 7.68 kg ai ha⁻¹) were applied to red sorrel plants grown in a greenhouse from seeds collected from three commercial fields and a no blueberry area to determine if they were hexazinone resistant. Red sorrel from two sites where hexazinone had not been applied regularly died at the 0.96 kg ai ha⁻¹ rate of hexazinone whereas red sorrel from two commercial fields survived at 7.68 kg ai ha⁻¹. It is concluded that red sorrel is hexazinone-resistant in some wild blueberry fields. A portion of the psbA gene was sequenced and it was determined that resistant plants had a Phe to Val substitution at position 255 in the D1 protein. This is the first recorded instance of hexazinone resistance in a perennial broadleaf weed in blueberry fields.

Nomenclature: Hexazinone, Velpar; red sorrel, Rumex acetosella L.; lowbush blueberry, Vaccinium angustifolium Ait.

Research studies evaluated effects of the auxin transport inhibitor, diflufenzopyr, on the biokinetics and efficacy of aminocyclopyrachlor-methyl ester (AMCP-ME) applications to black nightshade and large crabgrass. Absorption, translocation, and metabolism of ¹⁴C-AMCP-ME was quantified with and without diflufenzopyr (35 g ai ha⁻¹). Diflufenzopyr had minimal effects on translocation of radioactivity in either species. Accumulation of radioactivity in aboveground plant sections of black nightshade was greater than or equal to that in large crabgrass by 72 h after treatment (HAT). In both species, metabolism of ¹⁴C-AMCP-ME was rapid, as 60 to 78% of the extracted radioactivity was the free acid metabolite 8 HAT. In the greenhouse, black nightshade and large crabgrass were treated with AMCP-ME (9, 18, and 35 g ai ha⁻¹) alone and in combination with diflufenzopyr (35 g ha⁻¹). Mixtures of AMCP-ME plus diflufenzopyr did not increase large crabgrass control compared with AMCP-ME alone at any time. Diflufenzopyr (35 g ha⁻¹) increased black nightshade control with AMCP-ME (18 and 35 g ha⁻¹) 7 d after treatment (DAT). However, this increase in control was not observed 14 or 28 DAT. All treatments containing AMCP-ME controlled large crabgrass 70 to 79% 28 DAT compared with > 93% for black nightshade at the same time point.

Nomenclature: Aminocyclopyrachlor; aminocyclopyrachlor-methyl ester; diflufenzopyr; black nightshade, Solanum nigrum L.; large crabgrass, Digitaria sanguinalis (L.) Scop.

Seedbank persistence in canola seeds is related to their potential to develop secondary dormancy. This can result in volunteer weed problems many years after canola production. The potential to be induced into secondary dormancy is controlled by both the canola genetics and the environment of the mother plant. However, the effect of time of harvesting on secondary dormancy potential is not known. The objective of this study was to determine the effect of harvest timing on potential to develop seed dormancy in canola. Six harvest samples were collected weekly from two canola genotypes (5440 and 5020) starting from 10 to 20% seed color change on the main stem until they were fully ripened. Freshly harvested seeds of 5440 and 5020 showed 13 and 16% primary dormancy at 32 and 33 d after flowering (DAF), respectively, but dormancy decreased with harvest timings and no dormancy was observed when seeds were fully mature (78 DAF). After dormancy induction, 10% of 5440 seeds were dormant at 32 DAF, but 94% of seeds were dormant at 78 DAF. Similarly, 70% of 5020 seeds were dormant at 33 DAF, but 90% of seeds were dormant at 68 DAF. Thus, seeds had lower potential to secondary dormancy at early development but have a high potential to secondary dormancy induction at full maturity. This study suggests that windrowing these canola genotypes at the recommended time (60% seed color change on the main stem) may reduce ability of the seed to develop secondary dormancy and thus reduce the persistence of seeds in the soil seedbank.

Nomenclature: Brassica napus L., canola.