Field experiments determined the potential interaction of insecticides and propanil [N-(3,4-dichlorophenyl) propanamide] in drill-seeded rice (Oryza sativa L. ‘Bond’). The carbamate insecticides carbaryl (1-naphthylmethylcarbamate) and methomyl {S-methyl N-[(methylcarbamoyl) oxy] thioacetimidate} interacted more severely with propanil than the organophosphate methyl parathion [O,O-dimethyl O-(p-nitrophenyl) phosphorothioate] in emulsifiable or encapsulated formulations. Carbaryl and methomyl injured rice vegetatively when applied from 7 days before to 4 days after propanil. Carbaryl was the only insecticide that interacted with propanil to reduce rice grain yield. Insecticides interacted adversely with propanil on rice grain yield when they were applied 2 days before propanil treatment. Insecticide interactions with propanil from most to least phytotoxic to rice plants for all times of application were: carbaryl > methomyl > emulsifiable methyl parathion > encapsulated methyl parathion.

Dormant barnyardgrass seeds imbibed in darkness for 8 days at 38 C displayed a very low fluence response to phytochrome, and very little germination occurred in darkness. However, if seeds were imbibed at temperatures >42 C or exposed to 46 C for only 0.5 h, considerable germination was evoked in the absence of a light stimulus. It is proposed that the high temperatures caused a transition in dormancy from a light-requiring to a light-independent state. The finding may have relevance to field behavior of barnyardgrass seeds where burial could prevent reception of the required light stimulus.

Fourteen tomato genotypes, eastern black nightshade, and 35 Lycopersicon accessions were screened for tolerance to acifluorfen. Tolerant and susceptible genotypes occurred in most species. Fifteen genotypes were chosen for further study depending on their fresh weight after acifluorfen treatment Over all 15 genotypes, there was no correlation between trichome density and acifluorfen tolerance; however, in L. esculentum, cultivars with the most trichomes were the most tolerant There was an inverse relationship between stomata density and tolerance. Amount and composition of epicuticular wax and cuticle thickness did not correlate to acifluorfen tolerance.

Atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triaazine] diffused at a rate of 15.2 × 10−8 sq cm/sec at 25 C which was faster than the diffusion rate of 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) and 2-chloro-4,6-bis(isopropylamino)-s-triazine (propazine) in the same soils. Diffusion rates were highly correlated with total surface area of the soil. Increasing soil moisture content, soil pH, and soil temperature resulted in greater diffusion rates. A relative indication of herbicide mobility may be obtained from some of the physical and chemical properties of the soils and herbicides involved; however, rate of diffusion appears to be governed by a combination of soil and herbicide properties.

Leafy spurge is a serious problem because of the speed with which it spreads and the difficulty of controlling it with available means. A rangeland economics model was developed to estimate the economic impacts of leafy spurge infestation on both ranchers and regional economies. A leafy spurge-induced carrying capacity reduction of about 580 000 animal unit months (AUMs), or enough for 77 000 cows, reduced ranchers' annual net income nearly $9 million. Ranchers did not spend another $14 million on input costs, which reduced regional business activity. The regional impacts are about $75 million in reduced business activity for all sectors. These impacts on rancher incomes and regional economies suggest the potential economic returns of leafy spurge control could be substantial.

Greenhouse and field experiments were conducted to investigate degradation rates and efficacy of EPTC (S-ethyl dipropyl carbamothioate), butylate [S-ethyl bis(2-methylpropyl)carbamothioate], vernolate (S-propyl dipropylcarbamothioate), and cycloate (S-ethyl cyclohexylethylcarbamothioate) in soils with and without prior history of thiocarbamate use. Herbicide persistence was measured by an oat (Avena sativa L.) bioassay, and giant foxtail (Setaria faberi Herrm. # SETFA) control was visually estimated in field studies. Both greenhouse and field studies indicated that enhanced thiocarbamate degradation occurred when the same herbicide (EPTC, butylate, or vernolate) was applied over successive years. Addition of dietholate (O,O-diethyl O-phenyl phosphorothioate) or fonofos (O-ethyl-S-phenylethylphosphonodithioate) increased thiocarbamate persistence and giant foxtail control in soils with prior thiocarbamate exposure. Use of dietholate or fonofos two or three consecutive years with EPTC or butylate again resulted in enhanced degradation. EPTC persistence and giant foxtail control were not affected by prior treatments or tank mixes with carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate). Rotation away from successive applications of the same thiocarbamate herbicide (EPTC, butylate, or vernolate) improved herbicide efficacy.

Spurred anoda [Anoda cristata (L.) Schlect.] seed germination averaged 0.5 to 3.0% for seeds buried in the field for up to 20 months under natural conditions. These same seeds averaged over 76% germination after scarification with concentrated sulfuric acid, indicating hard seed coat involvement in seed dormancy. Germination was unaffected by light variables but increased as temperature increased. Germination of scarified seed increased from 50% to 85% as temperature was increased from 18 C to 30 C. Spurred anoda developed more slowly than did cotton (Gossypium hirsutum L. ‘Coker 310′) for the first 45 days. After that spurred anoda grew more rapidly until by 60 days after emergence, the weed was larger than cotton as measured by shoot height and total leaf surface area. This difference in development between cotton and spurred anoda indicates postemergence directed sprays of herbicides might be useful as a control tool.

Tebuthiuron {N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N′-dimethylurea} was evaluated on three Wyoming soil types for control of big sagebrush (Artemisia tridentata Nutt.), silver sagebrush (Artemisia cana Pursh.), and phytotoxic effects on associated vegetation. Big sagebrush was controlled with tebuthiuron at application rates of 0.6 to 1.1 kg ai/ha, whereas silver sagebrush was not completely controlled. Prairie junegrass [Koeleria pyramidata (Cam.) Beauv.], western wheatgrass (Agropyron smithii Rydb. ♯3 AGRSM), needleandthread (Stipa comata Trin. ♯ STDCO), and green needlegrass (Stipa viritdula Trin. ♯ STOVI) were not significantly reduced with tebuthiuron up to 1.1 kg ai/ha. Blue grama (Bouteloua gracilis H.B.K. ♯ BOBGR), downy brome (Bromus tectorum L. ♯ BROTE), plains pricklypear (Opuntia polyacantha Haw. ♯ OPUPO), and broom snakeweed [Gutierrezia sarothrae (Pursh.) Britt. and Rugsby, ♯ GUESA] were tolerant to tebuthiuron at rates up to 1.1 kg ai/ha.

Control of annual broadleaf winter weeds in dormant bermudagrass [Cynodon dactylon (L.) Pers. ‘Common’] by a variety of herbicides was examined. Nearly all treatments allowed good to excellent control of spur weed (Soliva sessilis P. and K.). Metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H)one] was the only tested herbicide that controlled all weed species consistently with a single treatment. Bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-(4) 3H-one 2,2-dioxide] applied alone as single or repeated treatments controlled spur weed, parsley-piert (Alchemilla microcorpa Boissier Reuter), and chickweed [Stellaria media (L.) Cyrillo], but a combination of bentazon and silvex [2-(2,4,5-trichlorophenoxy)propionic acid] were needed for acceptable control of henbit (Lamium amplexicaule L.) and corn speedwell (Veronica arvensis L.). None of the herbicide treatments affected green-up of bermudagrass the following spring.

The herbicides glyphosate [N-(phosphonomethyl)glycine], hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione], and pronamide [3,5-dichloro(N-1,1-dimethyl-2-propynl)benzamide] were evaluated for effectiveness in quackgrass (Agropyron repens L. Beauv.) control when applied in the fall, a time that is desirable in a short-growing-season area such as Atlantic Canada. Glyphosate at 1.12 kg/ha applied to quackgrass in grain stubble gave good control. Glyphosate at 1.12 kg/ha in late October gave better control than the same rate applied in late September. At low rates of 0.56 and 1.12 kg/ha, glyphosate gave better quackgrass control when fall-applied than when spring-applied. No differences were noted between seasons at the 2.24-kg/ha rate of glyphosate. Pre-treatment with nitrogen at 35 kg/ha 30 or 15 days before application, or on the day of application, did not improve quackgrass control with glyphosate. Pronamide gave fair to good control. Hexazinone at 2.24, 4.48, or 6.72 kg/ha gave poor control when fall-applied, but excellent control when spring-applied.

Common purslane (Portulaca oleracea L.) seeds, produced by the same plants, had different degrees of dormancy. The dormancy variations were caused neither by low seed viability nor by location on the plant where seeds were produced. Seed water content and seed age at time of collection contributed to, but were not solely responsible for, the dormancy variations. Immature, brown seeds of high water content were less dormant than the more mature, black seeds of low water content. The immature seeds germinated better in the dark than did the more mature seeds. A puncture in the seed, over the radicle, broke purslane dormancy. Dormancy was not caused by blockage of water imbibition by seeds. Purslane dormancy developed during later stages of seed maturation on the mother plant.

Nalta jute and redweed are widespread and significant broadleaf weed species of the tropics. Experiments were conducted to determine the effects of various environmental factors on seed germination and seedling emergence of these species. Light was not required for germination in either species. Germination was stimulated by seed scarification, suggesting that inhibition of germination in these species is mainly due to the hard seed coat. Scarified seeds of both species germinated over a wide range of alternating temperatures (25/15, 30/20, and 35/25 C). Both species were moderately tolerant of salt and osmotic stress, but nalta jute tolerated more stresses than redweed. Seedling emergence of nalta jute and redweed was greater than 87 and 93%, respectively, at soil depths of 0 to 2 cm but decreased as depth increased, with no emergence at 8 cm. Seedling emergence of both species was reduced by the addition of rice straw, though a high amount (4 to 6 t ha−1) of straw was required to suppress emergence significantly. The information gained from this study could facilitate the development of effective weed control programs.

Concern over the consequences of increased weed seed inputs to the soil seed bank during the transition period from conventional to organic production is one obstacle to grower adoption of reduced input and nonchemical weed management strategies. An 11-yr study was established in southwest Montana to investigate the effect of a single pulse of wild oat seeds on subsequent seed bank dynamics. In 1993, wild oat seeds were sown at five densities (0, 20, 80, 320, and 800 seeds m−2) in eight wheat–small grain cropping systems that differed in the number of crops in rotation and fallow periods. Wild oat seed banks were measured each spring from 1994 to 2004 in half of the cropping systems and from 2001 to 2004 in all eight systems. In 1994, seed bank densities in response to the pulse were as much as 11 times higher than controls that received no seeds in 1993. By 1996, after mechanical fallowing of all cropping systems, wild oat seed bank densities were not significantly different from densities in control plots regardless of the size of the initial seed pulse and remained so through 2004. These data suggest that increases in wild oat seed inputs during the organic transition period will have relatively few long-term agronomic effects on the dynamics of wild oat seed banks in these systems. In addition, wild oat seed banks may be constrained by factors other than cropping sequence when herbicides are not used, such as possible density-dependent regulation as a result of increased soil pathogen attack and seed predation.

Yield loss was related to weed species composition and density in permanent plots, recorded several weeks after sowing of spring cereals in southern Sweden. A range of agronomic situations was included by experimentally varying fertilizer application and sowing density in 33 field trials in different locations during 3 yr. Direct gradient analysis, using yield loss as the sole predictor, arranged weed community composition in the spring along a gradient of small to large yield losses. Yield loss could be explained, to some extent, by the species composition in the spring. Species associated with situations with large losses were hempnettle and wild radish, whereas several benign species were identified based on their association with lack of yield loss. The results suggest that possible predictive tools using spring species composition would be improved if they also considered soil type and seed rate. Some agronomically important weed species were not identified as associated with yield loss when assessed in terms of their abundance in the spring, which may limit the possibilities of basing management decisions on weed plant density in the spring.

Widely varying weed densities that had been desiccated by freezing did not reduce combine efficiency (gathering plus threshing losses) in harvesting corn (Zea mays L., var. Nebraska 508), sorghum [Sorghum bicolor (L.) Moench, var. RS 610], or soybeans (Glycine max L., var. Ford). Average combine gathering losses with corn, sorghum, and soybeans were 2, 7, and 16%, respectively. Average threshing losses with the combine were less than 1% with corn and sorghum and less than 3% with soybeans. The high gathering losses in this study were due partly to delayed combining to allow the weeds to be desiccated by freezing temperatures.

Johnsongrass is a common weed of corn in Chile, which is most often controlled by nicosulfuron, an acetohydroxyacid synthase (AHAS)-inhibiting herbicide. Recurrent nicosulfuron use has resulted in selection for resistant johnsongrass biotypes. We conducted studies to determine nicosulfuron resistance levels in two johnsongrass biotypes from Chile and to investigate if this resistance was target-site mediated. Whole-plant resistance to nicosulfuron was 33 and 46 times higher in resistant (R) than in susceptible (S) plants grown from seed and rhizomes, respectively. The nicosulfuron concentrations for 50% inhibition of AHAS enzyme activity in vitro were more than 11 times higher in R than in S plants. Sequencing analysis of the AHAS coding sequence revealed a Trp-574-Leu substitution in both R biotypes. This study shows that resistance to nicosulfuron in the two R biotypes is conferred by an altered target site. We also report the first consensus sequence of the johnsongrass AHAS gene corresponding to the known mutation sites conferring resistance to AHAS-inhibiting herbicides.

The triethylamine salt of triclopyr [(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid and the potassium salt of picloram (4-amino-3,5,6-trichloropicolinic acid) were applied as soil, foliar, and soil-plus-foliar treatments to greenhouse-grown huisache [Acacia farnesiana (L.) Willd.]. At 1.12 kg/ha, picloram was more effective (97 to 100% defoliation) than triclopyr (51 to 83%) in defoliating huisache. Triclopyr was more effective at 2.24 kg/ha than at 1.12 kg/ha in defoliating huisache, except when applied to the soil. Soil or soil-plus-foliar treatments of triclopyr at 2.24 kg/ha were not significantly different from those of picloram at 1.12 kg/ha 15 weeks after treatment. Triclopyr and picloram content was determined in and on leaves, stems, and roots at 0, 3, 10, and 30 days after treatment. More picloram than triclopyr was found in the leaves 30 days after treatment with foliar sprays at equal rates. Soil, foliar, and soil-plus-foliar treatments resulted in greater accumulation of picloram than triclopyr in stem tissue. Herbicide concentrations in roots were usually low. Reasons for the greater herbicidal activity of picloram than of triclopyr in huisache are not clear; however, the greater accumulation of picloram than of triclopyr in leaf and stem tissue may partially explain the difference in herbicide activity.

A greenhouse experiment used a replacement series design to compare the vegetative growth 6 wk after emergence in pure cultures and mixtures of winter wheat and Italian ryegrass, with phosphorus (P) levels recommended by soil testing. The planting proportions of wheat and Italian ryegrass were 100 and 0%, 75 and 25%, 50 and 50%, 25 and 75%, and 0 and 100%, respectively. There was no alleopathic interaction between the species. Both species in all pure and mixed cultures had substantially less growth in the low-P than in the recommended P treatment. However, the relative performance of the two species differed between P treatments. In the recommended P treatment in pure culture, Italian ryegrass had more tillers and greater root weight and length than wheat. Pure culture wheat in the low-P treatment exceeded pure culture Italian ryegrass in leaf area, weights of leaves, stems, and roots, and root length. Thus, the growth of wheat was inhibited less by P deficiency than the growth of Italian ryegrass in pure culture. In the 50:50 mixture of the recommended P treatment, wheat had greater leaf, stem, and root weights than Italian ryegrass. In the 50:50 mixture of the low-P treatment, the two species were very similar in growth, except that Italian ryegrass had about three times more tillers than did wheat. Whereas P deficiency limited the growth of wheat less than Italian ryegrass in pure culture, P deficiency did not affect the relative competitiveness of Italian ryegrass as much as wheat in mixed cultures. The ability of Italian ryegrass to compete with wheat when P was limiting may result from a difference in root growth. Italian ryegrass had a greater fresh root length to fresh root weight ratio than did wheat in the low-P treatment in pure culture and in the 50:50 mixture. The greater surface area of Italian ryegrass roots likely enhanced the competitiveness of Italian ryegrass relative to wheat under P-deficit conditions. Thus, the use of the recommended P nutrition from soil testing may be a key component to diminish Italian ryegrass competition in wheat fields.

Invasive watermilfoils, specifically Eurasian watermilfoil and the interspecific hybrid of Eurasian watermilfoil × northern watermilfoil, continue to be problematic for water resource managers. Herbicides are often used to control these nuisance weeds and have been historically successful in controlling Eurasian watermilfoil. A population of hybrid watermilfoil from Townline Lake in Michigan has shown increased tolerance to the herbicide fluridone. The objective of this work is to determine if cross- and multiple tolerance have also developed in this population. Eurasian watermilfoil plants collected from multiple sites and plants from Townline Lake were treated with 0, 5, 10, 20, 40, or 80 µg L−1 of fluridone, norflurazon, or topramezone. Fluridone and norflurazon inhibit phytoene desaturase, whereas topramezone is a 4-hydroxyphenylpyruvate dioxygenase-inhibiting herbicide. Chlorophyll fluorescence (Fv/Fm) and pigment content was measured at 10 d after treatment. Townline Lake plants responded differently from susceptible plants when treated with fluridone, norflurazon, and topramezone at 40 µg L−1. These results indicate that the Townline population of hybrid watermilfoil has inherent tolerance to multiple herbicide modes of action. These results are especially significant as topramezone has recently been labeled for aquatic use. Screening of additional herbicides to determine potential herbicide tolerance of the Townline Lake population is recommended.

Acetolactate synthase (ALS) inhibitors are widely used for POST control of sedges in turfgrass. A suspected resistant (R) biotype of annual sedge was collected from a bermudagrass turf in Georgia with a history of exclusive use of halosulfuron. Research was conducted to evaluate the resistance level of this biotype to halosulfuron, efficacy of ALS-inhibiting herbicides and other mechanisms of action for control, and the molecular and physiological basis for resistance. In greenhouse experiments, the halosulfuron rate required to reduce shoot biomass 50% in comparison with the nontreated at 8 wk after treatment (WAT) were 8 and > 1,120 g ai ha−1 for the S (susceptible) and R biotypes, respectively. Imazapic, sulfosulfuron, and trifloxysulfuron reduced biomass of the S biotype greater than 60% at 8 WAT, but biomass was reduced less than 20% for the R biotype. Glufosinate, glyphosate, MSMA, and sulfentrazone reduced shoot biomass of the R biotype by 93, 86, 97, and 45%, respectively. In laboratory experiments, the halosulfuron concentration required to inhibit ALS activity by 50% in excised leaf tissues was 5.8 and > 1,000 μM for the S and R biotypes, respectively. Gene sequencing of the R biotype revealed a Pro-197-Ser substitution that confers resistance to ALS inhibitors. This is the first report of ALS-inhibitor resistance in annual sedge and herbicide resistance in a sedge species from a turfgrass system.