Seeds of largeseed dodder (Cuscuta indecora, Choisy), field dodder (Cuscuta campestris, Yuncker), and small-seeded dodder (Cuscuta approximata, Bab.) were studied on the basis of (1) germination at different temperatures, and (2) spring emergence of seedlings as related to soil temperature. Small-seeded dodder germinated at 35 to 45 F with maximum germination at 60 F. Largeseed dodder germinated very little and field dodder not at all below 50 F. They germinated best at 60 F and above. Dodder seeds were planted outside at different depths in a sandy loam soil during the autumns of two years. Daily counts on seedling emergence during the spring showed that small-seeded dodder was first to germinate, at a soil temperature of 39 F, largeseed dodder emerged from 7 to 10 days later at a soil temperature of 51 F, and field dodder emerged 2 to 4 days still later. Very few seedlings of the small-seeded dodder emerged after mid-May, but largeseed and field dodders continued to emerge throughout the summer.

Experiments were conducted in controlled environments and in the field on winter-hardy Triticum aestivum and three weed species commonly found in cereal fields in the United Kingdom to examine whether overwinter shoot growth of individual plants could be described by accumulated thermal time calculated using base temperatures derived from growth cabinets. Individuals of each species were grown in a controlled environment at constant temperatures ranging from 5 to 20 C and harvested sequentially. Base temperatures for the increase of dry weight and green area were estimated by fitting a simple linear model describing growth in response to the thermal sum. The estimated base temperatures for shoot dry matter accumulation of Alopecurus myosuroides, Stellaria media, Galium aparine, and T. aestivum were −0.8, −3.3, −1.4, and 0.2 C, respectively, and estimated base temperatures for increase in green area were 0.4, −1.7, 1.9, and 2.2 C, respectively. Each species also was grown in monoculture in the field over 2 yr at a range of sites to examine whether the base temperatures estimated from the controlled environment studies could be used to model weed and crop growth in response to thermal time in the field. The field data were described well when an expolinear function was fitted to accumulated thermal time calculated using the base temperatures derived from the controlled environment studies.

Knowledge of economic threshold level and growth habits of weeds in chickpea is essential to implement timely, effective, and economical weed control treatments. The effect of weed density on growth and yield performance in chickpea using dragon spurge as a test weed was investigated for 2 consecutive yr. Dragon spurge density levels of 5, 10, 15, 20, 25, 30, 35, and 40 plants m−2 were compared with weed-free plots. Each increment in densities of dragon spurge from 5 to 40 plants m−2 reduced plant height, number of fruits, seeds, and dry weight per plant. Chickpea grain yield losses varied between 1 to 63% with dragon spurge density ranging from 5 to 40 plants m−2. The seed protein content of chickpea was found to be significantly reduced above a weed density of 25 plants m−2. Maximum N, P and K uptake by dragon spurge was 1,520, 1,020, and 7,350 mg m−2, respectively. Thus, dragon spurge should be controlled at densities above 5 plants m−2 to achieve optimum chickpea yield.

When weeds were controlled in sorghum by hand-weeding or a herbicide there was no advantage or need for cultivation. Control of weeds in sorghum with preemergence herbicides was feasible, but the combination of cultivation and herbicides gave more dependable weed control than either method used alone. Preemergence applications of 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine) were the most effective for controlling weeds and increasing sorghum yields. In decreasing order of effectiveness were 2-chloro-4,6-bis(isopropylamino)-s-triazine (propazine), 2-chloro-N,N-diallylacetamide (CDAA) + trichlorobenzyl chloride (TCBC), and postemergence atrazine applications.

Foliar absorption of 2,4-D-C14 was rapid in both species. Cotton decarboxylated the side chain of 2,4-D several times faster than sorghum. Both species released C14O2 faster from the carboxyl than from the methylene position. Cotton slowly converted 2,4-D to a chromatographically different material in young leaves and bolls. Sorghum rapidly converted 2,4-D to a product different from 2,4-D and the 2,4-D-complex in cotton. After cotton recovered from 2,4-D-treatment, the regulator could not be detected in subsequent vegetative or reproductive growth.

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.

The effects of 16 herbicides from 14 different chemical classes on levels of soluble protein, hydroxyphenolics, anthocyanin, and chlorophyll were determined in light- and dark-grown soybean [Glycine max (L.) Merr. ‘Hill’] seedlings. Growth-reducing concentrations of the herbicides were supplied to 3-day-old dark-grown soybean seedlings in liquid culture. Soluble protein (per axis) was reduced by fluridone {1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridinone}, paraquat (1,1′-dimethyl-4,4′-bipyridinium ion), perfluidone {1,1,1-trifluoro-N-[2-methyl-4-(phenylsulfonyl)phenyl] methanesulfonamide}, and propanil (3′,4′-dichloropropionanilide) 24 or 48 h after treatment. In light-grown plants, soluble hydroxyphenolic compound levels were decreased on a per axis basis after 48 h by all chemical treatments except by atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], the methyl ester of diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy] propanoate}, DSMA (disodium methanearsonate), fluridone, MH (1,2-dihydro-3,6-pyridazinedione), nitralin [4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline], TCA (trichloroacetic acid), and 2,4-D [(2,4-dichlorophenoxy)acetic acid]. Total chlorophyll content in hypocotyls of these seedlings was decreased by fluridone, metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5 (4H)-one], norflurazon [4-chloro-5-(methylamino)-2-(α,α,α-trifluoro-m-tolyl)-3 (2H)-pyridazinone], paraquat, and 2,4-D, but others had no significant effect. Anthocyanin accumulation in hypocotyls of 48-h light-grown seedlings was decreased by atrazine, fenuron (1,1-dimethyl-3-phenylurea), metribuzin, norflurazon, paraquat, propanil, and propham (isopropyl carbanilate). Phenylalanine ammonia-lyase (PAL) activity (previously reported work) was positively correlated with anthocyanin levels in tissues after treatment with these 16 herbicides, but not with glyphosate [N-(phosphonomethyl)glycine]. There was, however, no correlation between extractable PAL activity and chlorophyll, soluble hydroxyphenolic compounds, or soluble protein content. These results indicated that extracted PAL activities usually reflect relative in vivo activities and that PAL activity is limiting to phenylpropanoid synthesis in vivo.

Inconsistent control of barnyardgrass with bispyribac may be alleviated through adjuvant technology. Experiments were conducted to determine the effect of adjuvant and urea ammonium nitrate (UAN) on absorption and translocation of bispyribac in barnyardgrass. Additional experiments were conducted to determine when maximum absorption and translocation occurred with the use of a methylated seed oil/organosilicone adjuvant (MSO/OSL) plus UAN (0.37 L ha−1 and 2% v/v). In the initial experiment, 14C-bispyribac–treated leaves, nontreated leaves, and roots were collected 6 and 24 h after application. Absorption was greatest with tank-mixed MSO/OSL (0.37 L ha−1) plus UAN (2% v/v) and the proprietary blend of MSO/OSL/UAN (2% v/v) at 80 and 74% of applied 14C-bispyribac, respectively. Translocation to nontreated leaves and roots was also highest with these treatments. Increased translocation appeared to be due to greater herbicide absorption, not an increase in translocation rate. The addition of 32% UAN to MSO/OSL and nonionic organosilicone (OSL/NIS) adjuvant systems resulted in a four to fivefold increase in absorption compared with treatments without UAN. Recovery of 14C-bispyribac in additional experiments generally decreased as time after application increased; however, recovery was 86% or greater for all time intervals. By 12 h after application, 68% of applied 14C-bispyribac was absorbed. At this time, 14C-bispyribac was partitioned within the plant in the following manner: 48% in the treated area, 10% in leaf tissue from treated area to tip of the treated leaf, 1.9% in leaf tissue from treated area to collar region of the treated leaf, 1.6% in remaining leaves from collar of treated leaf upward, 5.3% in remaining leaves from collar of treated leaf downward to soil line, and 0.7% in the roots. These data indicate that maximum absorption was achieved within 12 h with a tank mix of MSO/OSL and UAN or the MSO/OSL/UAN blend.

Bearded sprangletop [Diplachne fusca (L.) P. Beauv. ex Roem. & Schult. ssp. fascicularis (Lam.) P. M. Peterson & N. Snow] is a noxious annual grass weed of paddy fields, distributed in coastal regions of the Jiangsu and Hebei provinces in China. Cyhalofop-butyl has been widely used to control grass weeds since 2006 in China. Overreliance on cyhalofop-butyl has led to the evolution of resistant weeds. In this study, the resistance level and cyhalofop-butyl resistance mechanisms were investigated in the putative resistant (JSHH) population. The dose–response experiments showed that the JSHH D. fusca population had evolved 8.9-fold resistance to cyhalofop-butyl. Acetyl-CoA carboxylase (ACCase) sequencing revealed a point mutation (GGC to GCC) at amino acid position 2096, resulting in a Gly-2096-Ala substitution in the resistant population. To our knowledge, this is the first case of cyhalofop-butyl resistance in D. fusca and the first report of a target-site mutation conferring resistance to ACCase-inhibiting herbicides in D. fusca. In addition, the resistant D. fusca population (JSHH) with the Gly-2096-Ala mutation was cross-resistant to the aryloxyphenoxypropionate herbicide metamifop, the cyclohexanedione herbicide sethoxydim, and the phenylpyrazolin herbicide pinoxaden.

Weeds compete with crops for light, soil water, and nutrients. Nitrogen (N) is the primary limiting soil nutrient. Forecasting the effects of N on growth, development, and interplant competition requires accurate prediction of N uptake and distribution within plants. Field studies were conducted in 1999 and 2000 to determine the effects of variable N addition on monoculture corn and velvetleaf N uptake, the relationship between plant N concentration ([N]) and total biomass, the fraction of N partitioned to leaves, and predicted N uptake and leaf N content. Cumulative N uptake of both species was generally greater in 2000 than in 1999 and tended to increase with increasing N addition. Corn and velvetleaf [N] declined with increasing biomass in both years in a predictable manner. The fraction of N partitioned to corn and velvetleaf leaves varied with thermal time from emergence but was not influenced by year, N addition, or weed density. With the use of the [N]–biomass relationship to forecast N demand, cumulative corn N uptake was accurately predicted for three of four treatments in 1999 but was underpredicted in 2000. Velvetleaf N uptake was accurately predicted in all treatments in both years. Leaf N content (NL, g N m−2 leaf) was predicted by the fraction of N partitioned to leaves, predicted N uptake, and observed leaf area index for each species. Average deviations between predicted and observed corn NL were < 88 and 12% of the observed values in 1999 and 2000, respectively. Velvetleaf NL was less well predicted, with average deviations ranging from 39 to 248% of the observed values. Results of this research indicate that N uptake in corn and velvetleaf was driven primarily by biomass accumulation. Overall, the approaches outlined in this paper provide reasonable predictions of corn and velvetleaf N uptake and distribution in aboveground tissues.

Halauxifen-methyl is a new synthetic auxin herbicide for control of broadleaf weeds, including preplant applications for corn (Zea mays L.) or soybean [Glycine max (L.) Merr.]. The objective of this study was to investigate the efficacy of halauxifen-methyl in comparison to the current auxin standards, 2,4-D and dicamba, on glyphosate-resistant (GR) horseweed (Erigeron canadensis L.) at different plant heights. In field experiments, a foliar application of halauxifen-methyl at the recommended use rate of 5 g ae ha−1 resulted in 81% control. Dicamba applied at 280 g ae ha−1 provided a comparable level of efficacy of 80%, while 2,4-D at 560 g ae ha−1 resulted in 49% control. The addition of glyphosate improved GR E. canadensis control with 2,4-D more than with halauxifen-methyl or dicamba, possibly due to the higher level of control observed with halauxifen-methyl or dicamba alone. Even though applied at 50 to 100 times lower application rates, the efficacy of halauxifen-methyl on E. canadensis was similar to dicamba and greater than 2,4-D. Thus, halauxifen-methyl should be an effective tool for management of GR E. canadensis before planting both conventional and herbicide-resistant soybean varieties, and it precludes the extended preplant application interval required for dicamba in some soybean management systems.

An understanding of environmental factors governing patchy weed distribution in fields could prove to be a valuable tool in weed management. The objectives of this research were to investigate the relationships between weed distribution patterns and environmental properties in two Mississippi soybean fields and to construct models based on those relationships to predict weed distribution. Two months before planting, fields were soil sampled on a 60- by 60-m coordinate grid, and samples were analyzed for calcium, magnesium, potassium, sodium, phosphorus, zinc, cation exchange capacity, percent organic matter, and soil pH. The relative elevation of each sample location was also recorded. Approximately 8 wk after planting, weed populations were estimated on a 30- by 30-m grid over the soil sample grid. Punctual kriging was used to estimate environmental values at each weed sample location. Discriminant analysis techniques were used to evaluate the associations between environmental characteristics on weed population densities of sample areas within each field. Generally, as sicklepod and pitted morningglory infestations increased, the prediction accuracy of the discriminant functions also increased; however, horsenettle infestations were not closely correlated to the environmental properties. Discriminant functions reasonably predicted presence or absence of sicklepod and pitted morningglory within the field. However, validation of the functions across years within the same field and with data collected from the other field resulted in poor classification of all species infestations. Prediction of weed infestations with environmental properties was specific for each field, year, and species.

The absorption and translocation of 14C in 14C-ring-labelled 4-(2,4-dichlorophenoxy)butyric acid (2,4-DB) at 1 and 7 days after foliar treatment of soybean [Glycine max (L.) Merr.] and cocklebur (Xanthium sp.) were investigated at 3, 5, 7, and 11 weeks after planting by means of radioautography and standard radioassay methods. Partitioning of radioactivity varied with plant age and with treatment time. Although a species difference was not observed in the total uptake of 2,4-DB, the rate of penetration of the herbicide was greater in cocklebur than in soybean. The 2,4-DB was retained in the treated leaf and was not translocated to other plant parts. Translocated radioactivity occurred principally in actively growing parts of both species, but only trace quantities were detected in soybean. No radioactivity was detected in the lower leaves of either plant. Plant response to 2,4-DB may be partially explained on the basis of differential penetration and translocation.

Most farmers now rely on herbicides and, to a lesser extent, cultivation to control weeds in Glycine max in the Midwest. However, the general public is concerned that widely used herbicides will contaminate surface and groundwater. Alternative ways to control weeds in field crops are needed to reduce or prevent herbicide contamination of surface and groundwater. A new between-row-mowing weed management system that consists of band-applied herbicides over crop rows + two or more between-row mowings was tested in G. max over 6 yr in Missouri. Mowing weeds close to the soil surface two or more times between crop rows killed or suppressed annual grass and broadleaf weeds, chiefly Setaria faberi, Ambrosia artemisiifolia, and Amaranthus spp., if properly timed. Shading by crop canopy closure contributed to weed suppression in this weed management system. G. max yield also could not be distinguished from weed-free check plots and was greater than the weedy check plots. Herbicide use was reduced 50% by banding because only 50% of the field area was sprayed. The between-row-mowing weed management system may have use in environmentally sensitive areas to help reduce soil erosion or water contamination by herbicides.

The objective of this study was to test the potential for using a single-entry procedure to revegetate Russian knapweed– and green rabbitbrush–infested rangeland. I hypothesized that simultaneously applying an herbicide and seeding in the fall would produce the highest establishment and growth of desired species. For Russian knapweed, treatments included three seeding rates (zero, low, and high) and two herbicides (with and without clopyralid plus 2,4-D) applied in either the spring (2003) or fall (2004). The seed mixture included pubescent wheatgrass, Siberian wheatgrass, and alfalfa. Grasses were seeded on October 21, 2003 (fall dormant), and April 5, 2004 (spring), using a no-till rangeland drill. The seeding rates consisted of 3.4, 6.8, and 9.3 (low) or 5.0, 9.0, and 12.3 (high) kg ha−1 of alfalfa, Siberian wheatgrass, and pubescent wheatgrass, respectively. For green rabbitbrush, treatments included two seeding levels (13.5 kg ha−1 of Siberian wheatgrass and not seeded) and three herbicides (clopyralid, dicamba, and 2,4-D) and a control applied in July. Clopyralid plus 2,4-D (0.21 plus 1.12 kg ha−1) and dicamba at 2.1 kg ha−1 gave 61 and 66% control of Russian knapweed and green rabbitbrush, respectively. Herbicides interacted with seeding to provide the highest density of seeded species on the Russian knapweed site. These data support the hypothesis that simultaneously applying an herbicide and seeding in the fall would produce the highest establishment and growth of desired species. Conversely, only seeding affected Siberian wheatgrass establishment on the rabbitbrush sites. It may be reasonable to seed Siberian wheatgrass without controlling rabbitbrush if forage production is the primary objective.

Bioassays using cell cultures and callus tissues of leafy spurge were devised to evaluate the potential of rhizobacteria as biocontrol agents. Rhizobacteria isolated from roots of leafy spurge seedlings were screened in suspension-cultured leafy spurge cells. Cell viability was assessed using the Evan's blue bioassay 48 h after bacterial inoculation. Among the 30 isolates tested, LS102 and LS105 consistently caused intensive cell death determined by measuring the A630 of the inoculated cell cultures. Cell death was 2.5 to 3 times higher in cultures inoculated with LS105 and LS102, respectively, than in the control. Population levels of the two isolates within cell cultures and callus tissues of leafy spurge increased during the first 48 h. Leafy spurge callus tissues were inoculated with rhizobacteria either directly or by using the Host Pathogen Interaction System (HPIS). The latter exposes calli to bacteria without any physical contact. LS102 caused cellular leakage and eventually death of the callus tissue. Callus growth was reduced by about 30 to 70% when exposed to LS102 and LS105, respectively. Results suggest that these two isolates may affect leafy spurge at the cellular level by different mechanisms. A screening method based on cell cultures and callus tissues offers a good and rapid technique for detecting deleterious rhizobacteria with potential as biocontrol agents for leafy spurge.

The objectives of this research were to characterize the extent of intraspecifc variation in seed characteristics of Powell amaranth and to evaluate whether such variation was correlated with crop rotation history of the collection sites. We compared characteristics of seeds originating from dairy farms with a corn–alfalfa crop rotation history with seeds originating from farms with a history of intensive vegetable production. We hypothesized that (1) multiple years of perennial alfalfa would select for greater seed dormancy and longevity in seeds of the summer annual Powell amaranth, (2) earlier spring planting dates of corn and alfalfa compared with most vegetable crops would select for earlier emergence, and (3) greater competition and lower soil moisture in the nonirrigated corn–alfalfa rotation would select for greater seed size. Seeds from 10 to 20 plants from each of 10 farms from each habitat were collected in the fall of 2002 and 2003 in central New York. To control for maternal effects on seed dormancy, a second generation of seeds was produced from plants grown under common greenhouse conditions. Germination in petri dishes was greater for second-generation seeds from vegetable farms (46%) than for those from dairy farms (32%). Total emergence following overwinter burial in the field was greater for seeds originating from dairy farms (62%) compared with those from vegetable farms (52%). Neither seed weight nor the rate of emergence varied by habitat of origin. Our results suggest that perennial alfalfa in dairy rotations may have selected for greater dormancy and longevity of Powell amaranth seeds. The large intraspecific variation in seed characteristics observed, underscores the importance of considering multiple populations when making comparisons of germination characteristics across biotyes (e.g., resistant vs. susceptible) or species, or when developing and interpreting models of weed emergence or weed population dynamics.

Levels of MSMA (monosodium methanearsonate) that simulated drift (0.2, 0.39, and 0.78 kg/ha) were applied over-the-top to rice (Oryza sativa L. ‘Vista’, ‘Starbonnet’, or ‘Labelle’) after flooding at four developmental stages between early tillering and late jointing. The MSMA rates used were equivalent to, or lower than, the 0.78-kg/ha rate that would be applied as an over-the-top application to cotton (Gossypium hirsutum L.) for early season weed control. Over-the-top applications of MSMA did not affect heading and maturity dates of filled grains regardless of the rate or application timing. Reductions occurred, however, in plant height, number of panicles, and yield. In addition, the percentage of erect panicles showing typical “straighthead” symptoms increased. Rice sensitivity to MSMA depended on MSMA rate and stage of growth with injury being most severe as rice approached the reproductive stages of development. Because aerial applications of MSMA to cotton are normally made before rice reaches the reproductive stage, injury resulting from MSMA drift should be minimal.

Postemergence applications of atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine], tridiphane [2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl) oxirane] and atrazine, and tridiphane and cyanazine {2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl] amino]-2-methylpropanenitrile} resulted in 90% or better control of giant foxtail (Setaria faberi Herrm. # SETFA) less than 2.5 cm in height. Delaying the application of tridiphane plus atrazine or atrazine alone reduced the control of giant foxtail in 1984, control of giant foxtail over 2.5 cm in height was significantly greater with a mixture of tridiphane and atrazine (0.4 and 1.7 kg ai/ha) than with atrazine alone (2.2 kg/ha) or tridiphane plus cyanazine (0.6 and 1.8 kg/ha). Two successive applications of tridiphane and atrazine combinations controlled giant foxtail 8 to 16 cm tall (four to five leaves). Cultivation following applications of tridiphane plus atrazine improved giant foxtail control but was less effective than a second application of atrazine or tridiphane plus atrazine. Giant foxtail control in greenhouse experiments was similar to control obtained in the field. Tridiphane plus atrazine controlled 70% of giant foxtail in the four- to five-leaf stage, while atrazine applied alone resulted in only 23% control, indicating a synergistic effect.

Greenhouse and field experiments were conducted to evaluate the efficacy of chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} on Russian thistle (Salsola iberica Sennen and Pau # SASKR) in a spring wheat (Triticum aestivum L.) summer-fallow rotation. In the greenhouse, when chlorsulfuron was applied to Russian thistle at three stages of growth, shoot dry weight was reduced similarly with either 4.4 g ai/ha plus surfactant or 140 g ai/ha plus surfactant. In the field, postharvest control (1981) of Russian thistle with 17.5 to 140 g/ha of chlorsulfuron ranged from 35 to 60% without surfactant and from 76 to 93% with surfactant. During the summer-fallow year (1982), Russian thistle treated in the preflowering stage of growth was not effectively controlled with 17.5 or 35 g/ha. In contrast, residual control from postharvest treatments 1 yr after application ranged from 90 to 100%. Chlorsulfuron applied in the crop (1983) controlled Russian thistle throughout the growing season. All treatments of chlorsulfuron applied in 1981, 1982, and 1983 increased wheat yield at least 75%.