Controlled-environment experiments were conducted to determine giant foxtail epicuticular wax (ECW) deposition and fluazifop-P absorption under different environmental conditions and with two adjuvants. Drought stress and low temperature increased leaf ECW content, whereas low light intensity decreased ECW content compared with medium light intensity. Drought stress conditions decreased the fatty acid and primary alcohol content of ECW and increased the hydrocarbon content compared with field capacity. Compositional changes would make the ECW more hydrophobic and reduce leaf wetting by herbicide spray. Increasing air temperature decreased the aldehyde content of ECW, whereas decreasing light intensity increased ECW fatty acid and aldehyde content while decreasing primary alcohols and esters. Compositional changes under low light intensity would make the ECW more hydrophilic and increase leaf wetting by a herbicide spray. Drought stress reduced fluazifop-P absorption regardless of the temperature but could not further reduce fluazifop-P absorption under low light intensity. Fluazifop-P absorption by plants under low light and drought stress conditions was similar to plants under low or medium light intensity and field capacity conditions. Similarly, the rate of fluazifop-P absorption was less under drought stress and low light conditions. Fluazifop-P absorption was greater when crop oil concentrate was added compared with 28% urea ammonium nitrate or no additive. Crop oil concentrate, added to the herbicide solution, overcame reduced fluazifop-P absorption under the low light conditions and in one of the two drought stress regimes but could not overcome reduced fluazifop-P absorption with the high temperature regime.

Nomenclature: Fluazifop-P; giant foxtail, Setaria faberi Herr. SETFA.

Landoltia was collected and cultured from a canal in Lake County, Florida, where diquat was used repeatedly during the past 20–30 yr for duckweed control. Recent applications of diquat failed to provide adequate control of duckweed, and a new commercial formulation of diquat was suspected. The new formulation was not the cause of reduced efficacy. Static exposures (48 h) to various concentrations of diquat were used to compare the susceptibility of the Lake County landoltia accession to one never exposed to diquat. These static tests indicated that landoltia, from a population with no prior history of herbicide treatment, was extremely susceptible to diquat. The accession from Lake County, FL had developed resistance to diquat, and was also cross resistant to paraquat. The resistance factor was 50 for diquat and 29 for paraquat. The Lake County accession also exhibited reduced ion leakage after diquat exposure under light and dark conditions. This suggests the resistance mechanism to the bipyridylium herbicides in landoltia is independent of photosynthetic electron transport. This research documents the first aquatic plant species that has developed resistance to the bipyridylium herbicides.

Nomenclature: Diquat, paraquat; duckweed, Lemna spp.; landoltia, Landoltia punctata (G. Meyer) D. H. Les and D.J. Crawford.

A glyphosate-resistant Palmer amaranth biotype was confirmed in central Georgia. In the field, glyphosate applied to 5- to 13-cm-tall Palmer amaranth at three times the normal use rate of 0.84 kg ae ha⁻¹ controlled this biotype only 17%. The biotype was controlled 82% by glyphosate at 12 times the normal use rate. In the greenhouse, I₅₀ values (rate necessary for 50% inhibition) for visual control and shoot fresh weight, expressed as percentage of the nontreated, were 8 and 6.2 times greater, respectively, with the resistant biotype compared with a known glyphosate-susceptible biotype. Glyphosate absorption and translocation and the number of chromosomes did not differ between biotypes. Shikimate was detected in leaf tissue of the susceptible biotype treated with glyphosate but not in the resistant biotype.

Nomenclature: Glyphosate; Palmer amaranth, Amaranthus palmeri S. Wats; AMAPA.

Laboratory studies were conducted to examine the leaf surface, epicuticular wax content, and spread area of primisulfuron spray droplet with and without surfactant on leaf surface of barnyardgrass and green foxtail. Adaxial and abaxial leaf surfaces were examined using scanning electron microscopy and leaf wax was extracted and quantified. The spread of 1-μl droplets of distilled water, primisulfuron solution (without surfactant), primisulfuron solution with a nonionic low foam wetter/spreader adjuvant (0.25% v/v), and with an organosilicone wetting agent (0.1% v/v) was determined on the adaxial leaf surfaces of each of the weed species. Stomata and trichomes were present on adaxial and abaxial leaf surfaces in both species. Green foxtail had more stomata per unit area on the adaxial as compared to the abaxial leaf surface. Barnyardgrass had more stomata on the abaxial than on the adaxial leaf surface. There was no significant variation in the number of trichomes per unit leaf area of green foxtail, and the number of prickles per unit area of leaf was significantly higher in adaxial than the abaxial leaf surface, in both young and old leaves. In barnyardgrass, there were more trichomes on abaxial than adaxial leaf surface. The mean value of the wax content per unit of leaf area in barnyardgrass and green foxtail was 35.9 μg cm⁻² and 19.1 μg cm⁻², respectively. On both species primisulfuron with a nonionic surfactant had more spread area than that without a surfactant, and the spread was even greater with organosilicone wetting agent. The spread area of primisulfuron droplet was higher on the leaf surface of barnyardgrass than on green foxtail when surfactant was added.

Nomenclature: Primisulfuron; barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; green foxtail, Setaria viridis (L.) Beauv.

The physiological basis of glufosinate resistance for two resistant (R) rice mutants, lines ‘R11-2’ and ‘R11-3’, was studied. Seven days after the application of 0.54 mM glufosinate, two susceptible (S) lines, i.e., variety (var.) ‘FSK’ and its inbred line ‘FSK-3’, and a reference var. Tainung 67 (TNG 67) suffered severe injury, whereas the two R lines exhibited resistance. Dose–response analysis and survival rate 14 d after treatment with 1.5 mM glufosinate also supported this observation. A ¹⁴C-glufosinate experiment showed that more labeled herbicide was absorbed by leaves of R11-2 than S lines 48 h after treatment (HAT), but the partitioning of absorbed glufosinate to each part of the shoot did not differ between R and S lines. Although a higher degradation of glufosinate in R line R11-2 was found as compared with the two S lines, i.e., 46% vs. 38 to 40%, the actual concentration of glufosinate in R line was still higher than that in S lines. Foliar application of glufosinate resulted in less inhibition of in vivo activity of glutamine synthetase (GS; EC 6.3.1.2) as well as a lower accumulation of ammonium 24 HAT in R line than in S lines. Further kinetic study of GS showed that cytosolic GS in line R11-2, with a higher enzyme-inhibition constant (Ki) value to glufosinate, was less sensitive to the toxic action of this herbicide. Therefore, a higher metabolism of, and more important, a lower susceptibility of, the target protein GS to this herbicide are suggested to contribute significantly to glufosinate resistance in these rice lines.

Nomenclature: Glufosinate; rice, Oryza sativa L.

Spurred anoda is a major competitor with cotton in the southern United States. Physiological and antioxidant responses of two species of cotton (Gossypium barbadense L. cv. ‘Pima S-7’ and Gossypium hirsutum L., Delta and Pine Land Company cv. ‘Delta Pine 5415’) and two accessions of spurred anoda [New Mexico (NM) and Mississippi (MS)] were investigated under nitrogen (N) -sufficient and -deficient conditions in the greenhouse. Pima S-7 had the highest leaf N content of all the plants regardless of treatment. Biomass decreased in all species when N was withheld, with Pima S-7 exhibiting the least reduction and MS the greatest. Plant height decreased in cotton but not spurred anoda under N stress. Height:node ratio increased 9% in MS, but decreased 8% in DP 5415 when they were deprived of N. Withholding N reduced photosynthesis 45% regardless of species. Comparable decreases were found in stomatal conductance and transpiration, suggesting strong stomatal regulation of gas exchange under N stress. The quantum efficiency of photosystem II (dark-adapted F_v/F_m) decreased 4% under N deficiency. Alpha-carotene decreased for all species when N was withheld, except for the NM accession, in which the levels increased. Total chlorophyll and lutein decreased under N stress regardless of species, but alpha-tocopherol and the xanthophyll cycle conversion state increased. Pima S-7 had the most chlorophyll and lutein, and both cotton species had more alpha-tocopherol, anthocyanins, and free-radical scavenging capacity than spurred anoda. These enhanced pigment and antioxidant profiles of cotton, particularly Pima S-7, may contribute to cotton's ability to compete for N with spurred anoda.

Nomenclature: Spurred anoda, Anoda cristata (L.) Schlecht. ANVCR; cotton, Gossypium barbadense L. ‘Pima S-7’; Gossypium hirsutum L. ‘DP 5415’.

A field experiment was conducted in 2000, 2001, and 2002 at Stoneville, MS, to determine the effect of spurred anoda interference on yield loss of two cotton cultivars, ‘Delta Pine 5415’ and ‘Pima S-6’, grown under wide (1 m) (WR) and ultra narrow (0.25 m) row (UNR) spacings. The relationship between spurred anoda density and dry weight per plot was linear each year. At a spurred anoda density of 8 m⁻², spurred anoda dry weight per plot was 507, 322, and 777 g m⁻² in 2000, 2001, and 2002, respectively. However, spurred anoda did not interfere with seed cotton yield in 2001, which was probably attributable to the low branch development in that year. Yield losses exceeded 55% at a spurred anoda density of 8 m⁻² compared with controls in both WR and UNR. The effect of spurred anoda density on boll numbers was nearly identical in 2000 and 2002, regardless of cotton cultivar and row spacing. Boll weights decreased in response to spurred anoda interference. Spurred anoda interference resulted in a decrease in cotton branch dry weight in WR but not in UNR. The yield decrease as a result of spurred anoda interference in WR was due to reduction in boll retention or fruiting sites (predicated on a decrease in branch weight). However, in UNR, the yield decrease was due to plant mortality; the plant density of both cotton cultivars decreased by one plant for each additional spurred anoda, but the yield per plant for surviving plants remained constant. Neither WR nor UNR cotton had significant advantage in response to spurred anoda interference. The decreased boll weight observed in UNR, and the failure to increase boll numbers m⁻² to compensate for decreased boll weight in UNR compared with WR, may limit its appeal to cotton producers.

Nomenclature: Spurred anoda, Anoda cristata (L.) Schlecht. ANVCR; cotton, Gossypium hirsutum L. ‘DP 5415’; Gossypium barbadense L. ‘Pima S-6’.

Differences in periodicity and depth of weed seedling recruitment due to agronomic management practices, such as reduced tillage, have implications for weed competitive ability and management strategies. Periodicity and depth of seedling recruitment of 10 different weed species was measured in the field in 2004 and 2005. The seedling recruitment of rigid ryegrass, threehorn bedstraw, and wild radish seedlings was higher under minimum tillage than under no-till scenarios. In contrast, the seedling recruitment of Oriental mustard, annual sowthistle, squirreltail fescue, little mallow, and turnipweed was higher under the no-till system. The seedling recruitment of wild oat and African mustard was not influenced by the tillage system. The mean seedling recruitment depth of wild oat, rigid ryegrass, threehorn bedstraw, wild radish, and turnipweed was greater under minimum tillage than under the no-till system. These weeds are able to emerge from deeper in the soil profile. In contrast, the seedling recruitment depth under minimum-tillage and no-till systems was similar for African mustard, Oriental mustard, annual sowthistle, little mallow, and squirreltail fescue. These are all small-seeded species, which failed to emerge from deeper depths under either tillage system. In addition, all of these species except African mustard showed higher total seedling recruitment under the no-till system. Results of this study will facilitate weed-control timing decisions and provide validation data for weed seedling recruitment models.

Nomenclature: Wild oat, Avena fatua L., AVEFA; African mustard, Brassica tournefortii Gouan, BRSTO; threehorn bedstraw, Galium tricornutum Dandy, GALTC; rigid ryegrass, Lolium rigidum Gaudin, LOLRI; little mallow, Malva parviflora L., MALPA; wild radish, Raphanus raphanistrum L., RAPRA; turnipweed, Rapistrum rugosum (L.) All., RASRU; Oriental mustard, Sisymbrium orientale L., SSYOR; annual sowthistle, Sonchus oleraceus L., SONOL; squirreltail fescue, Vulpia bromoides (L.) S.F. Gray, VLPBR.

Several studies were conducted to evaluate the effects of different tillage systems on the vertical seed distribution, seedling recruitment pattern, and persistence of the rigid ryegrass seed bank. Experiments were conducted in South Australia at two locations (Roseworthy Campus and Minlaton, a site on the Yorke Peninsula) in 2003 and 2005. The distribution of surface seeds through the soil profile was associated with the level of soil disturbance. The low–soil-disturbance tillage systems left more seed on the soil surface, whereas the high–soil-disturbance systems buried most of the seeds. The seedling recruitment of rigid ryegrass was lower under the low–soil-disturbance tillage systems than under the high–soil-disturbance tillage systems at both locations. The seedling recruitment was two- to fourfold greater under minimum tillage than under no-till. Not only was the seedling recruitment lower under the low–soil-disturbance tillage systems, biomass accumulation by rigid ryegrass seedlings was also lower under these systems. The carryover of residual viable seeds from one season to the next was similar between the tillage systems. However, seed decay under no-till (48 to 60%) was much greater than under minimum tillage (12 to 39%).

Nomenclature: Rigid ryegrass, Lolium rigidum Gaudin, LOLRI.

Specific knowledge about the dormancy, germination, and emergence patterns of weed species aids the development of integrated management strategies. The after-ripening period for jointed goatgrass seed was quantified, and the effects of germination conditions and spikelet structures on jointed goatgrass seed germination were measured. As the duration of after-ripening increased, jointed goatgrass seed germinated earlier, at faster rates, and to greater final percentages compared to non–after-ripened seed. Both primary and secondary positioned seed within jointed goatgrass spikelets were nondormant after 16 wk after-ripening at 22 ± 2 C. Germination of dormant seed depended on incubation temperature and dark/light conditions. Sixty-seven percent of spikelets produced a radicle when exposed to low temperatures in the dark, and light at warm temperatures increased germination by 7%. The relationship between light and incubation temperature was similar also for germination of the primary positioned seed in nondormant spikelets; however, the magnitude of the effect increased. Light increased germination of seed incubated at warm temperatures by 18%. Coleoptile emergence was dependent on planting depth for three jointed goatgrass populations, winter wheat, and spring wheat. Under optimum conditions in the greenhouse, no planting depth selectively allowed wheat germination and emergence while preventing jointed goatgrass germination and emergence. Glume removal increased jointed goatgrass secondary positioned seed final germination percentage to 96%, increased the germination rate, and decreased the number of days required to reach 50% germination to 6 d. Glume removal also promoted germination of the primary seed within jointed goatgrass spikelets. Glume removal resulted in 80% of the spikelets having two coleoptiles, but did not alleviate dormancy completely in jointed goatgrass seed. Tillage and herbicide applications for jointed goatgrass control will be most effective in the fall when primary dormancy is lost, but before secondary dormancy is imposed.

Nomenclature: Jointed goatgrass, Aegilops cylindrica Host, AEGCY; wheat, Triticum aestivum L., TRZAW.

Pollen movement between individuals can increase the rate of herbicide resistance evolution by spreading resistance alleles within or between populations and by facilitating the rapid accumulation of resistance alleles within individuals. This study investigated the level of pollen-mediated gene flow between paraquat-resistant and paraquat-susceptible populations of the self-pollinated weed species hare barley. The experiment was conducted in both directions, from resistant to susceptible and susceptible to resistant, across 2 yr. To maximize the potential for pollen flow, individual plants were grown in a single pot. The level of gene flow was similar across years and between genotypes. The level of pollen-mediated gene flow ranged from 0.06 to 0.15%. Gene flow from resistant to susceptible plants was confirmed by demonstrating segregation for resistance in the progeny of suspected crosses. This study suggests that pollen-mediated gene flow will occur in this species at frequencies less than 0.16% and could assist the accumulation of resistance alleles within a population. These low levels of gene flow through pollen movement suggest that cross-pollination over larger distances would be unlikely and pollen movement probably does not contribute to gene flow between populations.

Nomenclature: Paraquat; hare barley, Hordeum leporinum Link. HORLE

Jointed goatgrass is a troublesome weed in winter wheat in the Pacific Northwest of the United States. Wheat and jointed goatgrass (JGG) can cross and produce hybrids in the field that can serve as a potential bridge for gene migration between the two species. To determine the potential for gene movement it is important to be able to identify hybrids in the field. To study the effect of wheat genotype on hybrid phenotype, reciprocal crosses were made between JGG and two common wheat cultivars: ‘Brundage 96’, ‘Hubbard’, a common-type advanced breeding line: ‘87–52814A’, and a club wheat cultivar: ‘Rhode’. Hybrids and parents were measured for plant height, spike length, flag leaf length, flag leaf width, and number of spikelets. Reciprocal effects were nonsignificant for all characteristics measured, indicating that hybrid morphology was not affected by the direction of the cross. Hybrids were different from their wheat parents for spike length, plant height, and flag leaf width. Hybrids produced from each of the wheat parents were uniform in phenotypic characters. Spikes were intermediate in circumference (size) from crosses between JGG and common wheat lines; however, club wheat × JGG crosses resulted in spikes that were more similar to common wheat. Spike size and flag leaf width for all hybrids also were intermediate between their parents. Hybrids differed in spike size and awn characteristics because of unique characteristics of the wheat parent. Based on these results, it should be possible to identify hybrids in the field accurately, regardless of the wheat parent or direction of the cross unless the parent is a club wheat.

Nomenclature; Jointed goatgrass, Aegilops cylindrica Host., AEGCY; winter wheat, Triticum aestivum L.

Numerous studies have quantified the developmental responses of wheat to vernalization, but its response compared to a weedy relative, jointed goatgrass, remains relatively unknown. Six paired jointed goatgrass collections gathered from Washington and Oregon fields, and winter and spring wheat, were grown in field studies to quantify yield and germination in response to vernalization. Monthly planting dates initiated in October and concluded in March were used to vary the vernalization durations for plants sown at three Oregon locations (Corvallis, Moro, and Pendleton) over two growing seasons. Minimum vernalization requirements to produce reproductive spikes were similar among plants of six jointed goatgrass collections. Jointed goatgrass collections grown at Corvallis required a minimum of 89 and 78 vernalization days (January 17, 2003 and January 22, 2004 sowing, respectively) to produce reproductive spikes, and plants grown at Moro required 60 vernalization days (March 3 and February 23) in both years, and 48 and 44 vernalization days (March 3 and February 24) were required by plants to produce spikes at Pendleton. Jointed goatgrass spikelet and winter wheat seed yield were positively influenced by vernalization days, experiment location, and year. The strength of the interactions among these main effects differed among jointed goatgrass collections and winter wheat. The effects of vernalization on jointed goatgrass yields and seed quality were more pronounced at Pendleton, OR, a location where jointed goatgrass has adapted, compared to Corvallis, OR, where it has not adapted. The minimum vernalization days required to produce germinable seed differed among jointed goatgrass collections, winter and spring wheat. There was not a selection of spring-adapted jointed goatgrass populations in the populations tested. Yet if spring temperatures are cool, minimum conditions for vernalization may be satisfied, and the benefits of planting spring crops to control jointed goatgrass would be reduced.

Nomenclature: Jointed goatgrass, Aegilops cylindrica Host AEGCY; winter wheat, Triticum aestivum L. ‘Madsen’; spring wheat, Triticum aestivum L., ‘Penawawa.’

A trait-based community assembly approach to weed management may enhance our understanding of how weed communities respond to specific management practices and increase the utility of weed management based on ecological principles. Therefore, identifying management practices that operate as assembly filters and the species traits upon which they act is an important first step in developing a more predictive weed science. Here, I report results from a 3-yr investigation of the effects of timing of annual tillage (spring vs. fall) on the annual assembly of arable weed communities. The timing of tillage had consistent and dramatic effects on the composition of weed communities; spring tillage led to weed communities dominated by early emerging spring annual forbs and C4 grasses, and fall tillage led to communities dominated by later-emerging forbs and C3 grasses. Traits determining a species' susceptibility to tillage time likely include germination syndrome and life cycle, both of which influence how species respond to changes in soil resource levels and light availability driven by seasonal disturbance regime. Manipulating the timing of tillage and other major soil disturbances may therefore be an important tool for managers interested in influencing community composition or targeting species with similar germination and life-history traits.

There are many concerns about the effects of repeated use of glyphosate in glyphosate-resistant (GR) crops, including two that are seemingly contradictory. These are (1) weed escapes and (2) loss of weed diversity. Weeds that escape glyphosate treatment represent species that likely will become troublesome and difficult to control in the future, and identifying these future problems may allow more effective management. In contrast, complete weed control directly reduces the weed component of agroecosystem biodiversity and may lower other components indirectly (e.g., weed-dependent granivores). During 2001 and 2002 effects of glyphosate and conventional weed control treatments on weed community composition and GR soybean yields were studied. Field studies were conducted along a north–south transect of sites spanning a distance of 1600 km from Minnesota to Louisiana. Low-intensity use (single application yr⁻¹) of glyphosate allowed more escapes and maintained higher weed diversity than high-intensity use (two applications yr⁻¹) of glyphosate, and it was equivalent to or even higher than diversity in non-GR systems. Although the same weeds escaped from low- and high-intensity glyphosate treatments, frequency of escapes was higher with less intensive use. These results suggest that limited use of glyphosate would not have profound effects on weed diversity. In addition, crop yield did not differ between GR and non-GR treatments at high latitudes, but below 40° N latitude, with a longer cropping season, yields with low-intensity glyphosate use decreased by about 2% per degree latitude because of competition from escaped weeds.

Nomenclature: Soybean, Glycine max (L.) Merr.

The ability of an invasive species to acquire and use a limiting resource during critical life history stages governs its ability to establish and persist within an environment. Arid environments are generally considered more resistant to invasion and are defined by low and sporadic precipitation. Warm-season grasses are most susceptible to mortality during seedling emergence, but water requirements for emergence are rarely known. We examined the ability of the often invasive warm-season grass, buffelgrass, to emerge given a range of simulated precipitation delivered on 2, 3, and 4 consecutive days with the use of a line-source irrigation system in a glasshouse. The minimum amount of water required for buffelgrass emergence was observed to be 6.3 mm (3.14 mm on 2 consecutive days). With the use of probit analysis, the median emergence response (0.5 emergence probability) was predicted to require 17.4–19.9 mm of water. Emergence was concentrated within the first 5 days following initial simulated precipitation with the probability of new emergence highest on Days 3 and 4. Over the period from 1949–2001 in Tucson, Arizona within the Sonoran Desert, the total number of consecutive rainy-day sequences meeting the minimum per-day precipitation levels for a median and minimum emergence response was 27 and 92, respectively. Precipitation sufficient to result in emergence of 50% of viable buffelgrass caryopses has occurred in Tucson in about 1 of 2 years over this period. We compare the soil water requirements for emergence of buffelgrass to other perennial species in the Sonoran Desert and suggest that the invasion success of buffelgrass is due in part to its ability to emerge following relatively low precipitation levels.

Nomenclature: buffelgrass, Pennisetum ciliare (L.) Link PESCI

A 4-year study was initiated in 1997 to provide canola producers with information on the consequences of various rotational intervals with the use of new disease and herbicide-resistant canola varieties. The study was conducted at three locations in Alberta, Canada (Ellerslie, Strathmore, and Warburg). At each location, four canola rotations were established: continuous canola, and canola seeded in 1 of 2, 3, or 4 years. Canola varieties included the conventional varieties ‘AC Excel’ and ‘Quantum’, the glyphosate-resistant variety ‘Quest’, imidazolinone-resistant ‘45A71’, and a glufosinate-resistant hybrid, ‘Invigor 2153’. In the fourth year of the study, when canola was grown in all treatments, weed densities, weed species diversity, and evenness were determined preseeding and before and after in-crop herbicide application. Canola yield was greatest in the northern ecoregions of the Boreal Transition (Warburg) and Aspen Parkland (Ellerslie), and lowest in the Moist Mixed Grassland ecoregion (Strathmore). Weed populations increased and population diversity decreased and became less even where rotations were less diverse; in continuous canola and in the 1-in-4–year rotation. As expected, weed densities increased in poorly competitive, conventional canola varieties compared to herbicide-resistant varieties sprayed with their broader-spectrum herbicides. Where weed densities were high, variety/herbicide system became a critical factor affecting crop yield. Under these conditions, the herbicide-resistant varieties ‘Quest’ and ‘Invigor 2153’ typically outyielded the conventional varieties of ‘AC Excel’ and ‘Quantum’. Canola yield was highest when grown in a 1-in-3– or a 1-in-4–year rotation, although the 1-in-3–year rotation generally had lower weed densities, and allowed high-value canola to be grown more frequently in rotation.

Nomenclature: Canola; Brassica napus L.

An improved understanding of crop stress from multiple pests is needed for better implementation of integrated pest management (IPM) strategies. Field studies were conducted in 2003 and 2004 at two locations in eastern Nebraska to describe the effects of simulated early-season insect defoliation of soybean and duration of weed interference on soybean growth. Three levels of simulated defoliation (undefoliated, 30, and 60%) and seven durations of weed interference (weedy and weed free; weed removal at V2, V4, V6, R3, and R5) were evaluated in a split-plot design. Defoliation significantly reduced soybean leaf-area index (LAI), total dry matter (TDM), and crop height in season-long weedy treatments only. Biomass partitioning during vegetative and reproductive growth was affected by both defoliation and weed interference. Increase in soybean relative growth rate (RGR) and biomass production soon after defoliation occurred (e.g., V5 stage) indicated potential defense mechanism by which soybean is able to adjust its physiology in response to the loss of leaf area. Weed interference combined with defoliation caused the greatest yield losses up to 97%. Results from this study indicate the need for monitoring early-season insect density and weed growth to determine if simultaneous control of both pests may be needed.

Nomenclature: Soybean, Glycine max (L.) Merr.

Defatted seedmeals from 15 glucosinolate-containing plant species were analyzed for herbicidal activity by determining inhibition of seedling emergence when added to a sandy loam soil containing wheat and sicklepod seeds at concentrations of 0.1, 0.5, and 1% (w/w). In general, the seedmeals were more phytotoxic to wheat than sicklepod. For wheat, all of the seedmeals significantly inhibited seedling emergence at the 1.0% concentration. At the 0.1% concentration three of the seedmeals (Indian mustard, money plant, and field pennycress) completely inhibited wheat emergence. For sicklepod emergence, eight of the seedmeals were completely inhibitory at the 1% level (Indian mustard, field pennycress, garden rocket, Siberian wallflower, English wallflower, garden cress, sweet alyssum, and evening stock) and four were completely inhibitory at the 0.5% level (brown mustard, garden rocket, English wallflower, and sweet alyssum). Intact glucosinolates and their corresponding hydrolysis products varied among the seedmeals with the highest activity. Major hydrolysis products produced by the seedmeals with the most phytotoxicity, respectively, included 2-propenyl (allyl) isothiocyanate (AITC) by brown mustard seedmeal, allyl thiocyanate and AITC by field pennycress seedmeal, erucin (4-methylthiobutyl isothiocyanate) by arugula seedmeal, 3-butenyl isothiocyanate and lesquerellin (6-methylthiohexyl isothiocyanate) by sweet alyssum seedmeal, and isopropyl isothiocyanate by money plant seedmeal. From our data it appears that both the type and concentration of glucosinolates and their hydrolysis products present in the seedmeals affect seed-emergence inhibition.

Nomenclature: English wallflower, Erysimum cheiri (L.) Crantz; evening stock, Matthiola longipetala (Vent.) DC. MTLLB; Field pennycress, Thlaspi arvense L. THLAR; garden cress, Lepidium sativum L. ‘Cressida’ LEPSA; garden rocket, Eruca vesicaria (L.) Cav. subsp. sativa (Mill.) Thell. ‘Astro’ ERUVE; Indian mustard, Brassica juncea (L.) Czern. ‘Southern Giant Curled,’ BRSJU; money plant, Lunaria annua L.; Siberian wallflower, Erysimum × allionii; sicklepod, Senna obtusifolia (L.) H. S. Irwin & Barneby CASOB; sweet alyssum, Lobularia maritima (L.) Desv. LOUMA; wheat, Triticum aestivum L. ‘Cardinal’.

Wild radish is a major weed of field crops in southern Australia. The effects of various densities of wild radish and wheat on the growth and reproductive output of each other were investigated in field studies in 2003 and 2004. The experiments were established as a factorial combination of wheat (0, 100, 200, and 400 plants m⁻²) and wild radish (0, 15, 30, and 60 plants m⁻²) densities. The effect of wild radish density on wheat yield loss and wild radish seed production were described with a rectangular hyperbola model. The presence of wild radish in wheat reduced aboveground dry matter, leaf-area index (LAI), and grain yield of wheat, and the magnitude of this reduction was dependent on weed density. Increasing the density of wheat substantially reduced the adverse effects of wild radish on wheat. As crop density increased, wild radish dry matter, LAI, and seed production per unit area decreased. The maximum seed production of wild radish was achieved at its highest density (60 plants m⁻²), and was 43,300 and 61,200 seeds m⁻² for the first and second year, respectively. The results indicated that higher densities of wheat were able to suppress seed production of this weed species. From a practical viewpoint, this study shows that increased wheat density in the range of 200 to 400 wheat plants m⁻² can reduce wild radish seed production and also give some reduction in crop yield loss, and could be an important component of an integrated weed management program.

Nomenclature: Wild radish, Raphanus raphanistrum L., wheat, Triticum aestivum L.

Prescribed burning and the herbicide clopyralid are very effective tools for the management of yellow starthistle. However, repeated use of either can be impractical or can present other problems. The potential solution is the development of an effective integrated weed management strategy using a combination of the two approaches. In small plot studies (0.2 ha), we tested one of five possible treatments: (1) untreated control, (2) 2 consecutive yr of clopyralid (0.105 kg ha⁻¹), (3) 2 consecutive yr of prescribed summer burning, (4) first-year clopyralid followed by second-year prescribed burning, and (5) first-year prescribed burning followed by second-year clopyralid. Treatments were made in 1999 and 2000 at three study sites in California (San Benito, Yuba, and Siskiyou counties). In 2001, the year following the final treatment, 2 consecutive yr of clopyralid or first-year burning followed by second-year clopyralid consistently reduced yellow starthistle cover in the following year by 92 to 100%. However, at the Yuba site, clopyralid alone increased medusahead and ripgut brome cover. Although 2 consecutive yr of burning was effective in Yuba, very high levels of starthistle infestation in San Benito were not completely burned in the second year because of the lack of available consumable fuel. Clopyralid treatment the first year followed by prescribed burning in the second year stimulated yellow starthistle germination and did not reduce the infestation. In a large-scale study conducted at two sites (13 and 81 ha) in southern Monterey County, we used a first-year burn followed by either 2 yr of clopyralid (0.158 kg ha⁻¹) or a single year of clopyralid (0.210 kg ha⁻¹) and a subsequent burn. Results were in close agreement with those found in the small-scale studies. In the year following the final treatment, control of yellow starthistle was greater than 99% when the burn was followed by 2 yr of clopyralid. In contrast, when a prescribed burn was used in the last year of the program, the level of control was not as good, probably because of the increased germination of the remaining soil seedbank. These results indicate that a first-year prescribed burn followed by a second-year clopyralid treatment can provide consistently good control of yellow starthistle, as well as reduced levels of noxious annual grasses, including medusahead and ripgut brome.

Nomenclature: Medusahead, Taeniatherum caput-medusae (L.) Nevski ELYCM; ripgut brome, Bromus diandrus Roth BRODI; yellow starthistle, Centaurea solstitialis L. CENSO.

A 6-yr rotation study was conducted from 2000 to 2005 at Stoneville, MS to examine the effects of rotating glyphosate-resistant (GR) and non-GR (conventional) cultivars of cotton with corn under reduced tillage conditions on soil properties, weed control, crop yield, and net return. There were four rotation systems (continuous cotton, continuous corn, cotton–corn, and corn–cotton) for each non-GR and GR cultivar arranged in a randomized complete block design with four replications. Field preparation consisted of disking, subsoiling, disking, and bedding in the fall of 1999. After the fall of 2000, the experimental area received no tillage operations except rebedding after harvest each year to maintain reduced tillage conditions. A glyphosate-based program in GR cultivars and a nonglyphosate-based program in non-GR cultivars were used for weed management. Soil organic carbon in the top 5-cm depth progressively increased from the first year to the sixth year, regardless of rotation. In 2005, organic carbon was higher in corn grown continuously and in rotation compared to continuous cotton, partly due to higher plant residues from corn compared to cotton. Control of most grass and broadleaf weeds was sufficient to support cotton and corn production, regardless of rotation and herbicide program. Control of yellow nutsedge was reduced in continuous non-GR cotton; this apparent weed species shift toward yellow nutsedge was mitigated by breaking the cotton monocrop with corn. Plant populations of both GR and non-GR cotton rotated with corn were similar to that of continuous cotton suggesting cotton stand establishment was not affected by corn residues from the previous year. Cotton yield increased every year following rotation with corn by 10–32% in the non-GR cultivar, and by 14–19% in the GR cultivar compared to continuous cotton. Similarly, corn yield increased by 5–13% in non-GR cultivar and by 1–11% in the GR cultivar when rotated with cotton. As a result, net returns were higher from rotation management as compared with monoculture in both crops. This study demonstrated that alternating between cotton and corn is agronomically feasible and a sustainable option for farmers in the lower Mississippi River alluvial flood plain region who are looking for simple cultural practices that provide economic and environmental benefits.

Nomenclature: Glyphosate; corn, Zea mays L.; cotton, Gossypium hirsutum L.; yellow nutsedge, Cyperus esculentis L.

Growth habit is an important determinant of competitive ability in legume crops. Erect ‘Iron-Clay’ (IC), semi-erect ‘IT89KD-288’ (288), and prostrate ‘UCR 779’ (779) cowpea genotypes were planted with four densities of common sunflower or common purslane to determine which cowpea growth habit is more competitive to these weeds having markedly different statures. Both sunflower and purslane reduced cowpea relative growth rate (RGR) but at different phases of the growing season. Low-growing purslane had an effect in the earlier part of the season, and sunflower decreased cowpea RGR in the middle of growing season. RGR of genotype IC was the least affected and genotype 779 the most reduced by sunflower. Cowpea apparently compensated for early season purslane competition by increasing RGR after purslane flowering, with RGR for the IC genotype increasing the most and 779 increasing the least. All cowpea genotypes caused a similar reduction in sunflower biomass accumulation; but purslane biomass was most reduced by genotype IC and least affected by genotype 288. Erect genotype IC is more competitive due to its taller stature, greater height growth rate, and higher position of maximal leaf area density, despite a lower photosynthetic rate and light use efficiency than the other cowpea genotypes. Our results suggest that erect cowpea growth habit may be generally more competitive with weeds compared to semi-erect or prostrate growth habit.

Nomenclature: Common purslane, POROL; common sunflower, HELAN; cowpea, Vigna unguiculata (L.) Walp. VIGSI; ‘Iron-Clay’, ‘IT89KD-288’ ‘UCR 779’.

Using competitive crops and cultivars can be an important integrated weed management (IWM) tool, useful in both conventional and low-external-input (LEI) farming systems. Barley is considered a competitive crop, but cultivar competitiveness varies. There are two aspects of cultivar competitive ability; the ability to compete (AC) and the ability to withstand competition (AWC). However, the relationship between these aspects has not been addressed in barley. A study was conducted to explore aspects of barley cultivar competitive ability with oats, and to examine the feasibility of ranking cultivars based on either, or both, AWC and AC. Field trials were undertaken in 2001 and 2002 to determine cultivar competitive ability for 29 barley cultivars commonly grown on the Canadian prairies. Cultivars were selected from semidwarf and full height, hulled and hull-less, 2- and 6-row, and feed and malt classes. Yield loss ranged from 6 to 79% while weed seed return ranged from 10 to 83% of gross yield. As a class, semidwarf and hull-less cultivars were less competitive than full height and hulled cultivars, respectively. However, considerable variation existed within these classes, and an absolute relationship between class membership and competitive ability did not exist. Ability to withstand competition was significantly correlated with ability to compete, but correlation coefficients were not strong enough to attempt reliable co-selection within a breeding program. Ability to compete was a more consistent measure of competitive ability than AWC. Ranking barley cultivar competitive ability would make it a valuable IWM tool for farmers and extension personnel.

Nomenclature: Barley, Hordeum vulgare L.; oat, Avena sativa L. cv. AC Assiniboia.

Greenhouse and field studies were conducted with a population of common waterhemp resistant to POST protoporphyrinogen oxidase (protox)-inhibiting herbicides to compare its response to PRE and POST applications of selected herbicides. In the greenhouse, a dose–response study of PRE applications of acifluorfen, fomesafen, or lactofen was conducted on protox-susceptible and -resistant common waterhemp. The protox-resistant biotype was approximately 6.3, 2.5, and 2.6 times more resistant than the susceptible biotype to acifluorfen, fomesafen, and lactofen, respectively. In a separate study under field conditions, protox-resistant common waterhemp were treated with PRE and POST applications of acifluorfen, azafenidin, flumioxazin, fomesafen, lactofen, oxyfluorfen, or sulfentrazone. At 14 and 28 d after POST treatment (DAPT) in 2002 and 2004, all PRE applications of herbicides gave greater control than did POST applications. At 14 DAPT, oxyfluorfen had the greatest difference in PRE and POST control, with 85 and 10% control in 2002, respectively. An additional field study was conducted to determine the stage of growth at which resistance to protox-inhibiting herbicides becomes most prevalent. Protox-resistant common waterhemp were treated with herbicides at the 2-leaf, 4- to 6-leaf, and 8- to 10-leaf growth stage. Acifluorfen and fomesafen at 420 g ha⁻¹ gave greater than 90% control at the 2-leaf stage and 4- to 6-leaf stage, except in 2003 when control was 85% with acifluorfen. In 2003 and 2004, common waterhemp control at the 8- to 10-leaf stage ranged between 54 and 75% with acifluorfen or fomesafen. Results indicate that common waterhemp resistance to customary rates of POST protox-inhibiting herbicides becomes prevalent after the 4- to 6-leaf growth stage.

Nomenclature: Acifluorfen; azafenidin; bentazon; flumioxazin; fomesafen; lactofen; oxyfluorfen; sulfentrazone; common waterhemp, Amaranthus rudis Sauer AMATA.

The effects of time of precipitation and soil type on runoff losses of cyanazine and metolachlor were studied using a tilted-bed, microplot system. Two silt loam soils, Bosket and Dubbs, and a Sharkey silty clay were evaluated. Rainfall (22 mm h⁻¹) was simulated at 0, 2, and 14 days after treatment (DAT). Time of precipitation did not impact herbicide losses or any of the runoff parameters evaluated in this study. Water runoff occurred sooner and in greater quantities from the surfaces of Bosket and Dubbs silt loam soils than from the surface of Sharkey silty clay. Runoff losses of cyanazine did not vary by soil type. Soil drying produced large cracks in Sharkey silty clay, which greatly reduced runoff in this soil. Combined runoff and leachate losses were highest from Dubbs silt loam. Runoff losses of metolachlor were not affected by soil type. However, regression analyses indicated that time of precipitation and soil type interacted to affect initial metolachlor concentration. At 14 DAT, initial metolachlor concentration was highest in runoff from Sharkey soil. Time of precipitation ranked with respect to initial metolachlor concentration in runoff from Bosket and Dubbs silt loam soils were 0 > 2 > 14 DAT and 0 = 2 > 14 DAT, respectively.

Nomenclature: Cyanazine; metolachlor.

A two-year surface water reconnaissance of the Bogue Phalia and its tributaries was conducted in 1997 and 1998. Cyanazine and metolachlor in surface water samples were quantified using enzyme-linked immunosorbent assays (ELISA). Cyanazine and metolachlor were detected in 101 and 132 of 160 samples, respectively. Cyanazine concentrations ranged from 0.1 to 2.2 g L⁻¹ and exceeded the U.S. Environmental Protection Agency (EPA) lifetime health advisory level (HAL) of 1 g L⁻¹ in eight samples. However, concentrations never exceeded the HAL for shorter exposure times. Metolachlor concentrations never reached the lifetime HAL of 100 g L⁻¹. Metolachlor concentrations ranged from 0.1 to 20.6 g L⁻¹. Metolachlor was detected more frequently and found to be more persistent throughout the growing season than was cyanazine. Higher cyanazine and metolachlor concentrations were detected at sampling dates that coincided with herbicide applications. One of the Bogue Phalia's tributaries, Clear Creek, was found to be a point-source of cyanazine for the watershed.

Nomenclature: Cyanazine, metolachlor.