Weed scientists face complex and difficult challenges. Within our discipline, we must increase the sustainability of current weed management approaches and help respond to invasive plants as a component of global change. There also are major challenges that we share with other agricultural disciplines, such as mounting comprehensive efforts to address the problems of current agriculture. We believe that any effective response to these challenges will require public work, i.e., projects in which a diverse group of people work together—across lines of difference (professional, cultural, etc.)—to produce broad-based, systemic innovations that meet complex challenges. We propose that weed scientists should join relevant public-work projects by practicing “public scholarship.” We define public scholarship as original, creative, peer-evaluated intellectual work that is fully integrated in a public-work project. By full integration we mean that the scholar's work serves to fuel the social (i.e., collective) learning of the public-work group. This condition requires that the scholar be a full participant in the group rather than just being in a consultative or advisory role. We present several case studies of weed scientists practicing public scholarship. These scientists found this mode of scholarship to be a highly effective means by which to address their professional priorities. Barriers to the practice of public scholarship include the lack of relevant guidelines and norms within academic culture, e.g., with regard to quality-assurance standards. But public scholarship offers weed scientists a new way of responding to increasingly urgent demands to show that our work effectively produces public value in return for public investment. We believe that graduate programs in weed science should begin to offer students opportunities to learn skills that are relevant to public scholarship.

Green algal cell suspensions are used in agrochemical research for screening herbicides. This paper reports on the inhibitory effects of 39 herbicides from 19 different chemical classes with nine different modes of action on green algae cultured in liquid media. The results show that herbicide screening using Chlorella as a model organism is simple, quick, and sensitive. In vitro systems can be a second screening net to ensure that potential new structures are not missed. A set of biotests using cell suspension cultures of Chlorella are suggested as a herbicide prescreen to eliminate inactive compounds before testing on higher plants in the greenhouse. The method has been shown to be suitable for the requirements of microscreen and high-throughput screening (HTS) in vitro. A large-scale microscreen has been devised, which cultures algal cells using self-made Erlenmeyer flasks containing 5 ml of liquid medium. They are particularly useful for HTS compounds with dissimilar modes of action.

Nomenclature: Green algae; Chlorella pyrenoidosa chick.

Glufosinate efficacy was examined in two major grass weed species, rigid ryegrass and sterile oat. Dose–response pot experiments under controlled environmental conditions showed that sterile oat was more successfully controlled by glufosinate than was rigid ryegrass. Glutamine synthetase was extracted from both species and assayed in vitro. Glufosinate readily inhibited glutamine synthetase activity in both species, indicating no differential sensitivity to the target enzyme. Thin-layer chromatography analysis of glufosinate showed no significant metabolism of glufosinate in either species. Absorption and translocation studies with ¹⁴C-glufosinate showed that the radiolabel was rapidly absorbed into the leaves of both species. However, translocation of radiolabeled glufosinate from the treated leaf to the meristematic regions was significantly greater in sterile oat, whereas translocation to the tip of the leaf was significantly greater in rigid ryegrass. This indicates that there is a difference in glufosinate distribution between the two species. It is likely that this difference in the distribution of glufosinate results in sterile oat being more easily controlled by glufosinate than is rigid ryegrass.

Nomenclature: Glufosinate; phosphinothricin; rigid ryegrass, Lolium rigidum Gaud. LOLRI; sterile oat, Avena sterilis L. AVEST.

Several seed production fields of the Italian ryegrass cultivar ‘Tetrone’ were destroyed in 1988 by 280 to 350 g ai ha⁻¹ racemic fenoxaprop applied for wild oat control. Because similar rates of fenoxaprop had possessed adequate safety when applied to ‘Oregon common’ Italian ryegrass, suspicion arose that the cultivars differed in tolerance. Seedlings of 21 commonly grown cultivars were screened in the greenhouse at the three-leaf growth stage to determine their fresh weight GR₅₀ for fenoxaprop. The GR₅₀ values for the two most tolerant cultivars, ‘Marshall’ and ‘Torero’, were more than threefold greater than the two most sensitive cultivars, ‘Futaharu’ and ‘Ace’. Cultivars could be separated into sensitive, intermediate, and tolerant groups, but the distribution of the GR₅₀ values appeared to be continuous rather than discrete. Tolerance increased with growth stage, and the average GR₅₀ for tillered plants was 80% higher than that for the two-leaf stage and 41% higher than that for the four-leaf stage seedlings. Cultivars differed slightly in the specific activity of acetyl–coenzyme A carboxylase (ACCase) (EC 6.4.1.2) and in the I₅₀ values for the inhibition by fenoxaprop, but the only clear relationship between these biochemical factors and whole-plant tolerance was a threefold increase in ACCase activity at the tillered stage over that present in the younger seedlings.

Nomenclature: Fenoxaprop; wild oat, Avena fatua L. AVEFA; Italian ryegrass, Lolium multiflorum Lam. LOLMU.

Site of uptake, absorption, translocation, and metabolism of ethofumesate were studied to determine the differential responses of annual bluegrass, creeping bentgrass, and perennial ryegrass. Using barriers to isolate foliage or soil, the primary site of ethofumesate absorption was found to be the foliage. Symptoms resulting from foliar and over-the-top spray applications of ethofumesate to annual bluegrass included stunting of growth, epinasty, swelling of the crown, darker green color, and fusing of expanding leaf blades. Averaged over the course of the experiment, annual bluegrass, creeping bentgrass, and perennial ryegrass absorbed 33, 11, and 13% of ethofumesate applied. No ethofumesate was detected distally from the treated leaf, but overall translocation of ¹⁴C 14 d after treatment (DAT) was 2.2, 5.8, and 2.2% of the absorbed ¹⁴C for annual bluegrass, creeping bentgrass, and perennial ryegrass, respectively. Ethofumesate was rapidly metabolized in each species with half-lives ranging from 4.5 to 7 d. The three grasses had metabolized, on average, 67% of the absorbed ethofumesate 14 DAT.

Nomenclature: Ethofumesate; annual bluegrass, Poa annua L. POANN; creeping bentgrass, Agrostis stolonifera L. AGSST ‘Penncross’; perennial ryegrass, Lolium perenne L. LOLPE ‘Manhattan II’.

Experiments were conducted in a growth chamber to study alachlor and metolachlor metabolism in soil and corn and to determine if alachlor and metolachlor and their metabolites are exuded from corn roots to the growth medium. Alachlor was more readily absorbed by corn than was metolachlor. The absorption of alachlor and metolachlor was 72 and 55%, respectively, 10 d after seedling emergence (DAE). Alachlor and metolachlor were rapidly metabolized in corn, although metabolism rates were higher with metolachlor than with alachlor. Ten similar alachlor metabolites were detected in roots and shoots. In addition, two metabolites were detected only in the shoots, and one metabolite was detected only in the roots. Metolachlor metabolism in corn produced fewer metabolites than did alachlor metabolism. At 5 DAE, 10 and 9 metabolites were detected in shoots and roots, respectively. The metabolism of alachlor and metolachlor in soil showed patterns similar to the metabolism in corn but produced fewer metabolites. One unique alachlor metabolite appeared in soil but not in corn. Roots of corn seedlings treated with ¹⁴C-alachlor or ¹⁴C-metolachlor released significant amounts of radioactivity to the surrounding growth medium 5 d after treatment. Plants treated with alachlor released more radioactivity than did plants treated with metolachlor.

Nomenclature: Alachlor; metolachlor; corn, Zea mays ‘Patriot 6168’.

Wild oat, a troublesome weed in small grain cereals, infests about 11 million ha of cropland in the United States. Diversifying cereal production with alternative crops, such as yellow mustard and canola, provides flexible cropping systems, decreases production risks, and may allow more effective weed suppression. A greenhouse study was conducted to assess the competitive ability of yellow mustard and canola with wild oat in 1999 and 2000, using replacement series interference experiments to relate the results to plant development stages. Yellow mustard, regardless of its proportion in mixture, reduced aboveground biomass of wild oat 33 to 66%, leaf and tiller number 34 to 36%, and panicle production 58% compared with wild oat in monoculture. Canola did not affect wild oat biomass in mixtures. Yellow mustard per plant biomass in 2000 and inflorescence production in 1999 decreased 30 and 20% with increased density of yellow mustard in mixtures. Yellow mustard biomass was not affected by the addition of wild oat to the mixture, indicating the greater importance of intraspecific competition between yellow mustard relative to interspecific competition with wild oat. Canola per plant biomass was affected more by interspecific competition with wild oat than by intraspecific competition. A second greenhouse experiment was conducted to compare plant height and biomass accumulation by the three species over 7 wk. Yellow mustard had the greatest biomass accumulation and plant elongation rate, followed by canola and wild oat. The greater competitive ability of yellow mustard with wild oat, compared with canola, is likely associated with the rapid growth and canopy elevation of yellow mustard.

Nomenclature: Wild oat, Avena fatua L. AVEFA; canola, Brassica napus L. ‘Sunrise’; yellow mustard, Sinapis alba L. ‘Idagold’.

Weed management could be more efficient and require less herbicide if growers could afford to estimate the composition, density, and distribution of weed seed banks. Spatial distribution of a weed seed bank will affect the accuracy of both mean estimates and interpolated maps of density. Consequently, information about the general characteristics of spatial distributions of seeds in a seed bank is needed to identify the most efficient strategies for sampling. Seed banks were sampled on 8.4-m square grids in eight irrigated corn fields to identify the common features of distributions of seed banks of annual weeds. Spatial dependence was described with correlograms for four to eight species in each field. Spatial dependence was detected for 36 of 45 distributions, and seed counts were correlated to an average distance of 25 to 150 m for a distribution. Seed banks of different species and fields had common features of spatial correlation: spatial pattern accounted for less than half of the total variability of seed counts, spatial correlation decreased rapidly over short distances, and ranges of spatial dependence varied with direction. For half of the distributions, the maximum range of spatial dependence was at least twice as long as the minimum range. Seed counts were correlated for the longest distances in the direction of the crop row for 16 distributions, and the distance was longer in the direction of the crop row than across rows for 26 of the 36 samples. Researchers should be able to design more efficient sampling plans for growers if the common features of spatial dependence are considered. For seed banks like these, the accuracy of maps and estimates of seed bank density may be improved by collecting multiple cores around each sampling location to mitigate the effect of short-scale spatial variability. In addition, sampling may be more efficient with grids and interpolation methods that account for ranges that are 1.5 to 2 times longer in the direction of the crop row than perpendicular to the row. With a 55- by 30-m sampling grid, adjacent observations would be correlated, and maps could be made for 80% of these seed banks. More closely spaced observations would be needed to describe the rapid decline in spatial correlation with distance for a more accurate or finer-scale map. Whether sampling seed banks for making management decisions will be cost-effective is not clear. However, potential methods to sample and map seed bank distributions more efficiently have not been exhausted.

Nomenclature: Corn, Zea mays L.

Common waterhemp has become a problem weed species in Midwest soybean production. Determining the critical interference period after soybean and common waterhemp emergence is necessary for the implementation of weed control practices before soybean seed yield loss occurs. Field experiments were conducted during 1996, 1997, and 1998 to determine the influence of duration of common waterhemp interference on soybean seed yield. Removal of common waterhemp 2 wk after soybean unifoliolate leaf expansion resulted in soybean seed yield equivalent to a season-long weed-free control. Delaying common waterhemp removal until 4 wk after soybean unifoliolate leaf expansion resulted in decreased soybean seed yield. Allowing common waterhemp interference to persist 10 wk after soybean unifoliolate leaf expansion reduced soybean seed yield by an average of 43% over 3 yr. These results suggest that soybean producers should implement common waterhemp management strategies earlier than 4 wk after soybean unifoliolate leaf expansion in order to reduce the potential loss of soybean seed yield.

Nomenclature: Acifluorfen; sethoxydim; common waterhemp, Amaranthus rudis Sauer. AMATA; soybean, Glycine max (L.) Merr. ‘Pioneer 9362’, ‘Pioneer 9363’.

Ultraviolet-B (UVB, 280 to 320 nm) radiation levels reaching the earth's surface are increasing because of depletion of the stratospheric ozone layer. Adverse biological effects of this harmful radiation are a serious concern. The effects of UVB radiation on spotted chickweed, dandelion, downy brome, green foxtail, redstem filaree, and common catsear seed germination, shoot and radicle elongation, and seedling growth and development in a UVB-free environment after exposure to UVB were investigated in a greenhouse study. UVB radiation (4, 7, or 11 kJ m⁻² d⁻¹) did not influence seed germination of any species except downy brome. Postgermination UVB exposure (8 h d⁻¹, for 10 d), however, inhibited radicle elongation in all species, except in spotted catsear. Shoot elongation was also inhibited in some species but to a lesser extent than radicle elongation. A significant inhibition by UVB treatment (11 kJ m⁻² d⁻¹) of root and shoot biomass, leaf area, and leaf number was observed in some species after transfer of UVB-treated seedlings to a UVB-free environment for 3 wk. The inhibition of postgermination shoot and radicle elongation by UVB radiation, the continuation of growth inhibition after transfer of the treated seedlings to a UVB-free environment, and the differential responses of weedy species could have significant implications for their ability to compete with each other and with associated crops in a UVB-enriched environment that is likely as the stratospheric ozone layer gets depleted.

Nomenclature: Common chickweed, Stellaria media (L.) Vill. STEME; dandelion, Taraxacum officinale Weber in Wiggers TAROF; downy brome, Bromus tectorum L. BROTE; green foxtail, Setaria viridis (L.) Beauv. SETVI; redstem filaree (stork's-bill in Canada), Erodium cicutarium (L.) L'Her. ex Ait. EROCI; spotted catsear, Hypochoeris radicata L. HRYRA.

Wild oat is the most important weed in Argentine barley and one of the worst weeds in wheat and barley worldwide. During 1997 and 1998, field experiments were performed to determine the response of wild oat to N fertilizer at planting and at early tillering at 55 kg ha⁻¹ with and without the use of diclofop-methyl. Seedling emergence and survival rate, individual fecundity, and preharvest seed dispersal rate of wild oat were assessed. The effect on the individual growth of wild oat was also studied. N fertilizer increased both wild oat seedling survival rate and fecundity, especially when it was applied at early tillering. However, the effect of N fertilization was negligible with diclofop-methyl treatment. In addition, the application of N fertilizer reduced the seed dispersal rate immediately before crop harvest. Quantification of the effects of each agronomic practice, such as N fertilization, is useful to develop simulation models that predict the population dynamics. When N was applied at sowing, the annual growth rate of the wild oat population was almost 25% lower than when applied at early tillering.

Nomenclature: Diclofop; wild oat, Avena fatua L. AVEFA; barley, Hordeum vulgare L.; wheat, Triticum aestivum L.

Gene flow via pollen and inheritance of acetyl-coenzyme A carboxylase (ACCase) inhibitor resistance in giant foxtail have not been previously characterized. Therefore, experiments were conducted to quantify outcrossing rates and flowering periods of giant foxtail accessions resistant or susceptible to ACCase inhibitors and to determine the inheritance pattern of gene(s) that confer resistance. In greenhouse experiments, outcrossing rates between resistant and susceptible parents ranged from 0.24 to 0.73%, as gauged by the response of F₁ plants to fluazifop-P. Resistant plants and susceptible plants flowered at similar times, with peak flowering occurring 63 to 67 d after planting. During this period, flower initiation was nearly twice as great for resistant plants as for susceptible plants. In inheritance experiments F₂ plants segregated in a 1:2:1 ratio of resistant–intermediate–susceptible phenotypes after exposure to fluazifop-P, indicating that resistance was associated with a single nuclear, incompletely dominant allele. The very low rates of outcrossing among giant foxtail plants in the greenhouse indicate that gene flow of resistance traits in the field is likely to occur primarily by seed immigration rather than by pollen movement. However, gene flow of herbicide resistance traits via pollen provides a mechanism in which multiple resistance may develop among giant foxtail populations that are resistant to other classes of herbicides.

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

Common lambsquarters is an annual weed of many important crops. Ascochyta caulina is a plant pathogenic fungus that causes necrotic lesions on the leaves and stems of common lambsquarters. The objective of the present study was to estimate the effect of plant N supply on the biocontrol activity of A. caulina isolates against common lambsquarters. In greenhouse experiments replicated groups of common lambsquarters plants raised with different N supplies were sprayed with various isolates and concentrations of A. caulina 3 wk after planting. Height, number of leaves, total leaf area, fresh and dry weight, and tissue N concentration of common lambsquarters 4 wk after emergence increased significantly with increasing N supply. Disease development was positively related to increasing plant tissue N and also to increasing spore concentration. Fungal spore concentration also had a positive effect on the plant tissue N percentage. Ascochyta caulina isolate W90-1 caused a greater dry weight reduction in common lambsquarters than isolates I-001 and NW-6 did.

Nomenclature: Ascochyta caulina (P. Karst) v.d. Aa & v Kest.; common lambsquarters, Chenopodium album L. CHEAL.

Managing fertilizer in cropping systems may be an important component of integrated weed management programs. A field study was conducted to determine the effect of various application methods of the ¹⁵N-enriched nitrogen (N) fertilizer on N uptake in green foxtail, wild mustard, and spring wheat and on crop–weed competition. N application methods consisted of ammonium nitrate in solution applied broadcast on the soil surface, applied in pools on the soil surface at 20-cm intervals between every second wheat row, and point injected 10 cm deep at intervals similar to those of the surface pools. An unfertilized control treatment was also included. N uptake by green foxtail throughout the growing season was often greater from surface broadcast than from surface pools or point-injected N and was sometimes greater from surface pools than from point-injected N. In contrast, N uptake by wild mustard was rarely affected by the fertilizer placement method. In the presence of weeds, the ranking of N uptake by wheat usually was point injected > surface pools > surface broadcast. Weed biomass was often greater with surface broadcast than with either surface pools or point-injected N. In the absence of weeds, wheat yields were similar across the three N application methods. However, in the presence of green foxtail, wheat yields were greater with point-injected N than with surface broadcast N in two of the 3 yr and with surface pools of N in one of the 3 yr. In the presence of wild mustard, wheat yields were greater with surface pools and point-injected N compared with the unfertilized control in two of the 3 yr, whereas yields with broadcast N were never greater than the unfertilized control. Study findings suggest that point-injected N results in suppressed weed growth, not by reduced N uptake by weeds but instead by greater N uptake by wheat that increases its competitiveness with weeds. Information gained in this study will be utilized to develop a more integrated program for weed management in spring wheat.

Nomenclature: Green foxtail, Setaria viridis (L.) Beauv. SETVI; wild mustard, Brassica kaber (DC.) L. C. Wheeler SINAR; spring wheat, Triticum aestivum L. ‘Katepwa’.

An experiment conducted at five locations in North Carolina during 1998 and 1999 evaluated weed management systems in cotton with CGA-362622 and pyrithiobac. Weed management systems evaluated different combinations with or without fluometuron preemergence (PRE) followed by (fb) CGA-362622 early postemergence (EPOST), postemergence (POST), or EPOST POST; or pyrithiobac EPOST fb prometryn plus MSMA late postemergence directed (LAYBY) or no LAYBY treatment. The weed species evaluated include common ragweed, entireleaf morningglory, pitted morningglory, prickly sida, sicklepod, tall morningglory, and yellow nutsedge. Fluometuron PRE improved the control of all weed species by at least 17 percentage points and increased cotton lint yield compared with the systems that did not use fluometuron PRE. Prometryn plus MSMA LAYBY improved the control of all weed species and increased lint yield compared with the systems that did not use prometryn plus MSMA LAYBY when PRE or POST herbicides were used. Control with CGA-362622 at all application timings was greater than 70% for all weed species evaluated (common ragweed, entireleaf morningglory, pitted morningglory, sicklepod, tall morningglory, and yellow nutsedge), except prickly sida. Control of all three morningglory species and prickly sida was at least 70% with pyrithiobac, whereas control of common ragweed, sicklepod, and yellow nutsedge was lower. The only cotton that yielded over 800 kg ha⁻¹ was treated with fluometuron PRE fb CGA-362622 EPOST, POST, or EPOST POST fb prometryn plus MSMA LAYBY. Cotton treated with pyrithiobac EPOST gave yields that were similar to those given by cotton treated with CGA-362622 EPOST in systems with fluometuron PRE and less than those given by cotton treated with CGA-362622 EPOST in systems without fluometuron PRE. Early-season injury with CGA-362622 was greater than 60% at Clayton and Rocky Mount in 1998, whereas 12% or less injury was observed at the other locations. Pyrithiobac resulted in 25 to 45% injury at these two locations. No injury was observed 45 d after treatment.

Nomenclature: CGA-362622; fluometuron; MSMA; prometryn; pyrithiobac; common ragweed, Ambrosia artemisiifolia L. AMBEL; yellow nutsedge, Cyperus esculentus L. CYPES; entireleaf morningglory, Ipomoea hederacea var. integriuscula Gray IPOHG; pitted morningglory, Ipomoea lacunosa L. IPOLA; tall morningglory, Ipomoea purpurea (L.) Roth PHBPU; sicklepod, Senna obtusifolia (L.) Irwin and Barnaby CASOB; prickly sida, Sida spinosa L. SIDSP; cotton, Gossypium hirsutum L. ‘Stoneville 474’.

An open grassland at Sugarloaf Ridge State Park, Sonoma County, CA, was burned during three consecutive summers (1993–1995) to control yellow starthistle. By 1996, the yellow starthistle seedbank, seedling density, and mature vegetative cover were reduced by 99, 99, and 91%, respectively, and the plant community had greater diversity and species richness, particularly of native forbs. After the cessation of the prescribed burning after 1995, the community was monitored for 4 yr to determine if the reduced yellow starthistle population represented a stable state or if the population would quickly recover. The yellow starthistle seedbank rose dramatically over 4 yr. Seedling counts and summer vegetative cover also rose, though less rapidly. Total forb cover, particularly native species, total plant cover, and plant diversity decreased significantly after cessation of the burning. Grass cover did not show any strong trends, and year-to-year variation in the grass cover appeared to be more important than the treatment effects. In the absence of some overall changes in management, e.g., periodic prescribed burning, herbicide treatments, or revegetation, it may not be possible to establish and maintain a stable state with a low population of yellow starthistle in annual grasslands in California.

Nomenclature: Yellow starthistle, Centaurea solstitialis L. CENSO.

Metabolites of the fungus Fusarium solani (Sud 96) inhibited Striga hermonthica germination induced by the germination stimulant GR24. The active principles were identified as trichothecenes acuminatin, neosolaniol, 8-acetylneosolaniol, and tetraacetoxy T-2 tetraol (neosolaniol diacetate) on the basis of their chromatographic behavior and nuclear magnetic resonance and mass spectra. Inhibitory activity of the four trichothecenes against Striga germination increased with acetylation of the hydroxyl moieties. The most abundant inhibitor produced by the fungus, 8-acetylneosolaniol, completely inhibited Striga germination at 24 μM. The fungal toxin did not affect the germination of sorghum, a host crop, but retarded root and shoot elongation of the seedlings by 60 and 30%, respectively, at the same concentration.

Nomenclature: Fusarium solani (Mart.) Sacc.; sorghum, Sorghum bicolor (L.) Moench; Striga hermonthica (Del.) Benth.

Landscape-induced differences in soil variability and other parameters have potential effects on herbicide sorption, persistence, degradation, and, ultimately, efficacy. This study examined the spatial variability of herbicide efficacy across an eastern South Dakota field in continuous corn. Atrazine and alachlor had been applied for the previous 10 yr. The spatial variability observed in weed control was compared with herbicide sorption (Kd), mineralization rate, and first-order half-life (t_1/2), and field herbicide dissipation rates (DT₅₀). Spatial structure was present in atrazine mineralization, weed biomass, and corn biomass data. The amount of atrazine and alachlor sorbed to soil collected from the summit position of the field was 10 and 20% less, respectively, than the amounts sorbed to backslope or toeslope soils. Generally, both herbicides had faster mineralization rates and shorter t_1/2 in summit than in backslope or toeslope soils. Weed biomass was correlated positively with elevation and total amount of atrazine mineralized, whereas corn biomass was correlated negatively with these parameters. These findings suggest that weed control can be improved by accounting for the landscape positional effects on differential herbicide mineralization and dissipation in fields.

Nomenclature: Alachlor; atrazine; corn, Zea mays L.

In the San Joaquin Valley of California the presence of preemergence herbicides in runoff water from citrus orchards during winter rainfall can cause surface water and groundwater contamination. In 1999 and 2000 this study was conducted in a citrus orchard to evaluate the effect of irrigation incorporation with rotor sprinklers on simazine losses in runoff water. Four treatments included the application of simazine at 2.0 kg ai ha⁻¹ without irrigation incorporation and the application of simazine at 2.0 kg ai ha⁻¹ with three levels of irrigation incorporation (0.5, 1.25, and 1.75 cm of water). Simulated rainfall with 3.5 cm of water was then applied on the plots to generate surface runoff. Simazine concentrations in runoff water from the first simulated rainfall event averaged 0.85, 0.77, 0.45, and 0.30 mg L⁻¹ for treatments without irrigation incorporation and for those receiving 0.5, 1.25, and 1.75 cm of water incorporation, respectively; simazine concentration in runoff water from the second event applied 1 wk later averaged 0.42, 0.38, 0.21, and 0.16 mg L⁻¹, respectively. Total simazine mass recovered in runoff water from both simulated rainfall events was estimated to be 6.5, 6.0, 3.6, and 2.5% of the application for treatments without irrigation incorporation and for those receiving 0.5, 1.25, and 1.75 cm of water incorporation, respectively. Applying sufficient water for herbicide incorporation into the soil matrix is a practical approach to mitigate off-site movement from the soil.

Nomenclature: Simazine.

The genetic variation of quackgrass as a species and the array of environments in which it is found indicate that selection in these different environments could lead to differentiation among quackgrass populations. Yet, a highly diverse environment might not promote the genetic divergence of quackgrass if it poses contradictory selection pressures. To assess the extent of divergence among quackgrass populations, this study compared the morphology of populations of quackgrass for 1 yr in Rosemount, MN, in a “common garden” study. The quackgrass was initially collected from three different farming systems in southeast Minnesota: corn–soybean (CS), oats–hay–corn (OHC), and permanent pasture (PP). The systems represent pasture or arable land and differ in cropping rotations and levels of disturbance. Although no differences among farming systems were detected in multivariate or univariate comparisons, a significant farming system effect was detected between CS and PP systems when the most diversified system, OHC, was excluded from the analysis. Consistent with this result, a principal components analysis suggested that plants from two of the three farming systems exemplified contrasting modes of perennial plant growth. Relative to each other, the CS plants showed more features of the “guerrilla” growth mode (longer intra-ramet distances, sparse, large patches), whereas PP plants showed more “phalanx” mode features (short intra-ramet distances, dense, smaller patches). Plants from the most diversified system, OHC, did not fit into either growth form, and for this farming system, the variation among populations was the highest. The results suggest that the CS and PP systems selected for distinct growth forms, whereas the diversified OHC system did not. This is consistent with the hypothesis that diversification of a farming system and weed management decreases the risk of evolution of a weed population highly adapted to control measures used in that farming system.

Nomenclature: Quackgrass, Elytrigia repens (L.) Nevski AGRRE; corn, Zea mays L.; oats, Avena sativa L.; soybean, Glycine max (L.) Merrill.