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Wild poinsettia (Euphorbia heterophylla L.) is a troublesome broadleaf weed in grain production areas in South America. Herbicide resistance to multiple sites of action has been documented in this species, including protoporphyrinogen oxidase (PPO) inhibitors. We investigated the physiological and molecular bases for PPO-inhibitor resistance in a E. heterophylla population (RPPO) from Southern Brazil. Whole-plant dose–response experiments revealed a cross-resistance profile to three different chemical groups of PPO inhibitors. Based on dose–response parameters, RPPO was resistant to lactofen (47.7-fold), saflufenacil (8.6-fold), and pyraflufen-ethyl (3.5-fold). Twenty-four hours after lactofen treatment (120 g ha−1) POST, RPPO accumulated 27 times less protoporphyrin than the susceptible population (SPPO). In addition, RPPO generated 5 and 4.5 times less hydrogen peroxide and superoxide than SPPO, respectively. The chloroplast PPO (PPO1) sequences were identical between the two populations, whereas 35 single-nucleotide polymorphisms were found for the mitochondrial PPO (PPO2). Based on protein homology modeling, the Arg-128-Leu (homologous to Arg-98-Leu in common ragweed [Ambrosia artemisiifolia L.] was the only one located near the catalytic site, also in a conserved region of PPO2. The cytochrome P450 monooxygenase inhibitor malathion did not reverse resistance to lactofen in RPPO, and both populations showed similar levels of PPO1 and PPO2 expression, suggesting that metabolic resistance and PPO overexpression are unlikely. This is the first report of an Arg-128-Leu mutation in PPO2 conferring cross-resistance to PPO inhibitors in E. heterophylla.

Pennsylvania smartweed [Persicaria pensylvanica(L.) M. Gómez] is a common weed of rice (Oryza sativaL.) in the midsouthern United States and has recently become a concern for farmers because of reduced tillage systems. Acetolactate synthase (ALS) inhibitors have been extensively used for controlling smartweeds in imidazolinone-resistant and conventional rice. In the present study, we confirmed resistance to commonly used ALS inhibitors in rice and characterized the underlying resistance mechanism in aP. pensylvanicabiotype from southeast Missouri. A dose–response experiment was conducted in the greenhouse using bensulfuron-methyl, imazethapyr, and bispyribac-sodium to determine the resistance index (resistance/susceptibility [R/S]) based on GR50estimates. The target-siteALSgene was amplified from R and S plants, and sequences were analyzed for mutations known to confer ALS-inhibitor resistance. TheP. pensylvanicabiotype in question was found to be resistant to bensulfuron-methyl (R/S=2,330), imazethapyr (R/S=12), and bispyribac-sodium (R/S=6). Sequencing of theALSgene from R plants revealed two previously known mutations (Pro-197-Ser, Ala-122-Ser) conferring resistance to sulfonylureas and imidazolinones. This is the first report of ALS-inhibitor resistance inP. pensylvanica.

Studies were conducted to evaluate 14C-aminocyclopyrachlor absorption and translocation in alligatorweed, waterhyacinth, and waterlettuce. Alligatorweed plants were treated at the seven-node stage, waterhyacinth was treated at the five-leaf stage, and waterlettuce was treated at the eight-leaf stage. All plants were pretreated with nonlabeled aminocyclopyrachlor at 0.14 kg ai ha−1 with 1% (v/v) methylated seed oil (MSO). 14C-aminocyclopyrachlor was then applied to a protected leaf, and plants were harvested at 1, 2, 4, 12, 24, and 96 h after treatment (HAT). Radioactivity was determined in the treated leaf, shoots above treated leaf, shoots below treated leaf, roots, and growing solution. Absorption was rapid in all species and reached a maximum of 73, 72, and 73% of applied radioactivity for alligatorweed, waterhyacinth, and waterlettuce, respectively. In alligatorweed at 96 HAT, 43% of absorbed carbon-14 (14C) was translocated to shoots above the treated leaf and 17% was translocated to lower shoot tissue. In waterhyacinth at 96 HAT, 56% of absorbed 14C remained in the treated leaf, whereas 14 and 13% were found in parts above and below the treated leaf, respectively. In waterlettuce at 96 HAT, 50 and 33% of absorbed radioactivity was located above the treated leaf and in the growing solution, respectively. The low recovery of aminocyclopyrachlor in alligatorweed roots and growing solution might explain regrowth potential after herbicide treatment. These results also indicate that the lack of waterlettuce control with aminocyclopyrachlor is not due to reduced absorption or translocation.

Winter annual weeds are becoming prolific in agricultural fields in the midwestern United States. The objectives of this research were to understand the roles of soil temperature (daily average and fluctuation) and moisture on the emergence of nine winter annual weed species and dandelion and to develop predictive models for weed emergence based on the accumulation of modified thermal/hydrothermal time (mHTT). Experiments were established at Lincoln, NE; Mead, NE; and at two sites (irrigated and rainfed) near Clay Center, NE, in 2010 and 2011. In July of each year, 1,000 seeds of each species were planted in 15 by 20 by 6-cm mesh baskets installed between soybean rows. Soil temperature and water content were recorded at the 2-cm depth. Emerged seedlings were counted and removed from the baskets on a weekly basis until no additional emergence was observed in the fall, resumed in late winter, and continued until emergence ceased in late spring. Weather data were used to accumulatemHTT beginning on August 1. A Weibull function was selected to fit cumulative emergence (%) on cumulativemHTT (seven base temperature [Tbase] by six base water potential [Ψbase] by three base temperature fluctuation [Fbase] candidate threshold values = 126 models); it was also fit to days after August 1 (DAA1), for a total of 127 candidate models per species. The search for optimal base thresholds was based on the theoretic-model comparison approach (Akaike information criterion [AIC]). All three components (Tbase, Ψbase, andFbase) were only important for Virginia pepperweed. For downy brome and purslane speedwell, includingTbaseand Ψbaseresulted in the best fit, whereas for dandelion includingTbaseandFbaseresulted in the best fit. A model including onlyTbaseresulted in the best fit for most species included in this study (Carolina foxtail, shepherd's-purse, pinnate tansymustard, henbit, and field pansy). For field pennycress, the model based on DAA1 resulted in the best fit. Threshold values were species specific. Soil temperature was the major environmental factor influencing winter annual weed emergence. Even though soil moisture and often temperature fluctuation are essential for seed germination, ΨbaseandFbasewere not as critical in the predictive models as initially expected. Most seedlings (> 90%) of downy brome, pinnate tansymustard, Carolina foxtail, henbit, and field pansy emerged during the fall. Virginia pepperweed, purslane speedwell, dandelion, shepherd's-purse, and field pennycress seedlings emerged during both fall and spring. The results of this research provide robust information on the prediction of the time of winter annual weed emergence, which can help growers make better management decisions.

The Parasitic Plant Genome Project has sequenced transcripts from three parasitic species and a nonparasitic relative in the Orobanchaceae with the goal of understanding genetic changes associated with parasitism. The species studied span the trophic spectrum from free-living nonparasite to obligate holoparasite. Parasitic species used were Triphysaria versicolor, a photosynthetically competent species that opportunistically parasitizes roots of neighboring plants; Striga hermonthica, a hemiparasite that has an obligate need for a host; and Orobanche aegyptiaca, a holoparasite with absolute nutritional dependence on a host. Lindenbergia philippensis represents the closest nonparasite sister group to the parasitic Orobanchaceae and was included for comparative purposes. Tissues for transcriptome sequencing from each plant were gathered to identify expressed genes for key life stages from seed conditioning through anthesis. Two of the species studied, S. hermonthica and O. aegyptiaca, are economically important weeds and the data generated by this project are expected to aid in research and control of these species and their relatives. The sequences generated through this project will provide an abundant resource of molecular markers for understanding population dynamics, as well as provide insight into the biology of parasitism and advance progress toward understanding parasite virulence and host resistance mechanisms. In addition, the sequences provide important information on target sites for herbicide action or other novel control strategies such as trans-specific gene silencing. © 2012 Weed Science Society of America., This work is supported by National Science Foundation (NSF) Plant Genome award DBI-0701748 to J.H.W., C.W.D., M.P.T., and J.I.Y. Additional support is acknowledged to J.H.W. from the U.S. Department of Agriculture (Hatch project no. 135798), to M.P.T. from the NSF (IBN-0322420) and Kirkhouse Charitable Trust, and to J.I.Y. from NSF (IBN-0236545).

Changes in the weed flora of cropping systems reflect the impacts of factors that create safe sites for weed establishment and facilitate the influx and losses to and from the soil seedbank. This analysis of the annual surveys of the Southern Weed Science Society documents changes in the weed flora of the 14 contiguous southern states since the advent of transgenic, herbicide-resistant crops. In 1994 and 2009, the top five weeds in corn were morningglories, Texas millet, broadleaf signalgrass, johnsongrass, and sicklepod; in this same period Palmer amaranth, smartweeds, and goosegrass had the greatest increases in importance in corn. In cotton, morningglories and nutsedges were among the top five most troublesome weeds in 1995 and 2009. Palmer amaranth, pigweeds, and Florida pusley were also among the five most troublesome species in 2009; the weeds with the largest increases in importance in cotton were common ragweed and two species with tolerance to glyphosate, Benghal dayflower and Florida pusley. In soybean, morningglories, nutsedges, and sicklepod were among the top five weed species in 1995 and 2009. Two species with glyphosate resistance, Palmer amaranth and horseweed, were the second and fourth most troublesome weeds of soybean in 2009. In wheat, the top four weeds in 2008 were the same as those in 1994 and included Italian ryegrass, wild garlic, wild radish, and henbit. Crop production in the southern region is a mosaic of various crop rotations, soil types, and types of tillage. During the interval between the surveys, the predominant change in weed management practices in the region and the nation was the onset and rapid dominance of the use of glyphosate in herbicide-resistant cultivars of corn, cotton, and soybean. Because of the correspondence between the effects of glyphosate on the respective weed species and the observed changes in the weed flora of the crops, it is likely the very broad use of glyphosate was a key component shaping the changes in weed flora. Only eight of the top 15 most troublesome weeds of cotton and soybean, the crops with the greatest use of glyphosate, were the same in 1995 and 2009. In contrast, in corn and wheat where adoption of glyphosate-resistant cultivars lags or is absent, 12 of the 15 most troublesome weeds were the same in 1994 and 2008. These findings show on a regional scale that weeds adapt to recurrent selection from herbicides, currently the predominant weed management tool. Future research should seek methods to hinder the rapid spread of herbicide-tolerant and evolution of herbicide-resistant weed species. As new tools are developed, research should focus on ways to preserve the efficacy of those tools through improved stewardship.

While witchweed is nearing eradication in the United States, it continues to thrive in other parts of the world, especially in Africa, together with other witchweed species. The continuing problems from witchweeds and other parasitic weeds, the broomrapes, dodders and mistletoes, are outlined, including their extent, the degrees of damage caused, and the difficulties in their control. While a small minority are being successfully controlled by the use of immune varieties, most are currently controlled by existing techniques only partially, or on a local basis, and they may even be spreading or intensifying. The challenges they present are emphasised. Nomenclature: witchweed, Striga asiatica (L.) Kuntze; dodders, Cuscuta spp.; broomrapes, Orobanche spp.; mistletoes, Viscaceae, Loranthaceae.

Perennial weeds constitute a serious problem in Greek cotton-growing areas, as they strongly competing against the crop and downgrade the final product. Monitoring weeds at a regional scale and relating their occurrence with abiotic factors will assist in the control of these species. Purple nutsedge, field bindweed, bermudagrass, and johnsongrass were studied in cotton crops for three consecutive growing seasons (2007 through 2009) in a large area of central Greece. Weed densities and uniformities per sampling site were assessed in relation to soil and climatic data. Abundance index (AI), which is highly dependent on abiotic factors, was also estimated, and revealed purple nutsedge to the most persistent and damaging species among the recorded weeds. Field bindweed showed the highest correlation with soil properties and especially with clay content. Furthermore, correlation analysis was used over the sampling years in order to assess the stability of weed occurrence in the sampling sites. Purple nutsedge, field bindweed, and bermudagrass proved to be stable in location and intensity. The weed density spatial distribution was evaluated by using local indicators of spatial autocorrelation (LISA) statistics, and was mapped by ordinary kriging and co-kriging interpolation methods. Only 1 to 3 spatial outliers were identified in each 1 of the 3 yr. Between the two interpolation methods co-kriging delivered better results for field bindweed and purple nutsedge, indicating that soil data could improve the estimation of weed occurrence. These co-kriging interpolated weed maps would be a very useful tool for decision makers in taking appropriate weed control measures.

Field and pot studies were conducted in Central New York to determine the potential weed-management benefits of a buckwheat cover crop grown before winter wheat. Specific objectives were to determine buckwheat residue effects on (1) emergence and growth of winter annual weeds; (2) wheat establishment and yield; and (3) emergence of summer annual weeds in the spring following overwinter seed burial. In a field study, buckwheat was sown at two timings (July or August), mowed, and either incorporated or left on the soil surface. Winter wheat was drilled into buckwheat residue in September and weed and crop growth were monitored. In a complementary pot study, four winter annual weeds were sown in soil removed from buckwheat and bare-soil plots at 0 or 15 d after incorporation and monitored for emergence and early growth. To assess buckwheat residue effects on spring emergence from overwintering seeds, seeds of three weed species were buried in buckwheat residue and bare-soil plots in the fall, exhumed in April, and evaluated for emergence. To investigate the mechanism for possible effects of buckwheat residue on overwintering seeds, two levels each of seed treatment (none or fungicide) and fertilization (none or 170 kg ha−1) were applied before burial. Buckwheat residue had no negative effect on wheat yields but suppressed emergence (22 to 72%) and growth (0 to 95%) of winter annual weeds, although effects were often small and inconsistent. Buckwheat residue had no effect on the emergence of buried weed seeds in spring. However, fungicide treatment enhanced the emergence of Powell amaranth seeds by 12.5 to 25.5% and of barnyardgrass seeds by 0 to 12%. Our results suggest that buckwheat residue can contribute to weed management in wheat cropping systems, but that further studies investigating the mechanistic basis for the inconsistent selective effects of buckwheat residue on weeds are needed before buckwheat use can be optimized.

The objective of this study was to define further the environmental requirements for safe sites for germination of diffuse knapweed achenes (seeds). Germination temperature profiles were developed for diffuse knapweed seeds collected from sites in the Great Basin and Colorado. Each profile consisted of seeds germinated at 55 constant or alternating temperatures from 0 through 40 C. The resulting germination was used to develop quadratic response surfaces with regression analysis. Some germination occurred from 71 to 96% of the temperature regimes, depending on the accession being tested. Maximum observed germination ranged from 85 to 98%. Optimum germination, defined as the maximum observed minus one half the confidence interval at the 0.01 level of probability, occurred at a wide range of temperatures from cold periods of 0 through 20 C, alternating with warm periods of 10 through 35 C. The temperature regimes that most frequently supported optimum germination were 5/25 C (5 C for 16 h and 25 C for 8 h in each 24-h period) and 10/25 C. Germination of diffuse knapweed seeds was generally higher at alternating than constant temperatures.

Invasive plants impose threats to both natural and managed ecosystems. Downy brome is among the most aggressive invasive weeds that has infested the shrub-steppe ecoregion of eastern Washington. Hyperspectral remote sensing has potential for early detection and for monitoring the spread of downy brome—information that is essential for developing effective management strategies. Two airborne hyperspectral Advanced Visible Infrared Imaging Spectrometer (AVIRIS) images (electromagnetic spectrum ranging from 400 to 2,500 nm) were acquired at a nominal 4-m ground resolution over a study area in south-central Washington on July 27, 2000 and May 5, 2003. We used a mixture-tuned matched filtering (MTMF) algorithm to classify downy brome and predict its percent cover in each dataset plus a merged multiseasonal dataset using the transformed bands from a minimum noise fraction (MNF) output. The correlation coefficient was 0.79, calculated for the multidate MTMF predicted downy brome abundance, compared to 0...

Fluridone has been widely used for control of the submersed aquatic weed hydrilla in Florida for over 25 years. Recently, some hydrilla biotypes were suspected of having developed resistance to fluridone. Laboratory studies were conducted to monitor changes in phytoene and β-carotene contents as a function of suspected susceptible and resistant hydrilla biotypes to fluridone treatment. Hydrilla shoot tips from each biotype were exposed to 5, 10, 15, 20, 30, and 50 μg L−1 fluridone. Higher β-carotene and lower phytoene content was observed in all resistant hydrilla biotypes compared with the susceptible. The susceptible biotype showed an increase in phytoene or a decrease in β-carotene content when treated with as little as 5 μg L−1 of fluridone, whereas much higher doses were needed for the resistant biotypes. EC50 β-carotene values of 9 and 63 μg L−1 fluridone were found in the susceptible and the most resistant biotype, respectively. Consistent levels of hydrilla injury occurred at phytoene/β c...

Weed seedbanks have been studied intensively at local scales, but to date, there have been no regional-scale studies of weed seedbank persistence. Empirical and modeling studies indicate that reducing weed seedbank persistence can play an important role in integrated weed management. Annual seedbank persistence of 13 summer annual weed species was studied from 2001 through 2003 at eight locations in the north central United States and one location in the northwestern United States. Effects of seed depth placement, tillage, and abiotic environmental factors on seedbank persistence were examined through regression and multivariate ordinations. All species examined showed a negative relationship between hydrothermal time and seedbank persistence. Seedbank persistence was very similar between the two years of the study for common lambsquarters, giant foxtail, and velvetleaf when data were pooled over location, depth, and tillage. Seedbank persistence of common lambsquarters, giant foxtail, and velvet...

Since commercialization of Collego™ and Devine™ in the early 1980s, there has been a small but consistent research effort in the area of bioherbicides. The bioherbicide approach has promised effective weed management in cropping systems where the classical approach (using exotic natural enemies) is largely unsuitable. The overriding principle of the bioherbicide approach has been that a host-specific, coevolved natural enemy can be used as a bioherbicide when applied in simple formulations at inundative levels; however, two decades of research has effectively disproven this principle. Although research has revealed weaknesses in the bioherbicide approach, it has also revealed potential in a number of areas. A number of niche situations will remain in which host-specific plant pathogens can be developed as bioherbicides, such as for parasitic weeds and narcotic plants, but more research should be conducted with virulent, broad host range organisms, and more effort should be devoted to developing t...

Absorption and translocation of three commercial formulations of glyphosate, the isopropylamine salt formulated as Roundup Ultra™ (IPA1) and Roundup UltraMax™ (IPA2) and the diammonium salt formulated as Touchdown™ IQ (DA), were compared in three- to five-leaf velvetleaf, common waterhemp, and pitted morningglory. Absorption of 14C-glyphosate in velvetleaf was not significantly different among the three formulations up to 50 h after treatment (HAT). More absorption of 14C-glyphosate occurred in the IPA1 (26.0%) vs. the IPA2 (17.7%) formulation over 74 h. Of the total 14C-glyphosate absorbed, 20 to 35% was translocated from the treated leaf to the rest of the plant. Initial absorption of 14C-glyphosate was rapid in common waterhemp with the IPA1 (42.7%) and IPA2 (30.7%) formulations; both were higher compared with absorption of the DA formulation (11.5%) by 2 HAT. These differences continued up to 26 HAT, but no differences were evident by 74 HAT. Up to 65% of the 14C-glyphosate absorbed was trans...

The study objective was to use demographic information to adjust forage production practices to control the invasive weeds golden chervil and yellow-rattle without herbicides by defining the population dynamics traits that are directly involved in weed responses to farming practices. The principal population traits are capacity for dominance, sensitivity and accessibility of targeted developmental stages, and variation in weed population reactions from year to year. On the basis of demographic surveys of these two weed species when subjected experimentally to various cutting regimes (by date and number), we used matrix simulation models to describe each weed in terms of these traits and to construct species-specific management strategies. Management strategies for golden chervil need to prevent new recruitment by focusing on limiting or eliminating seed production and seedling survival because adult mortality is insensitive to cutting. Grazing to a low residual height is proposed in spring, when ...

Lanceleaved waterplantain is an exotic weed of rice that was introduced into Australia in the 1930s. Since its introduction, it has spread throughout much of the rice-growing region in southeastern Australia. The variability of lanceleaved waterplantain in these regions was studied using polymerase chain reaction-based, inter simple sequence repeat (ISSR) analysis. Deoxyribonucleic acid fingerprints from samples of the weed from southeastern Australia were compared between locations and with two samples of common waterplantain, a closely related species. The analysis indicated that there were two distinct groups of lanceleaved waterplantain that correlated with location. From the results of multidimensional scaling analysis, it is hypothesized that the Griffith group did not arise from hybridization between lanceleaved waterplantain and common waterplantain, and that it is more likely that the group arose from a separate introduction into the area. It is also suggested that there is seed movement...

The efficacies of bentazon and fomesafen in controlling annual weeds in dry and edible pod beans in New York State were investigated in greenhouse and field experiments. Dose responses to bentazon and fomesafen were studied for four weed species (ragweed, velvetleaf, eastern black nightshade, and hairy nightshade) under greenhouse conditions. Herbicides were applied at cotyledon to two-, two- to four-, and four- to six–true leaf stages, both with and without a crop oil concentrate (bentazon) or a nonionic surfactant (fomesafen). Field studies were conducted for 2 yr for all weed species except eastern black nightshade, for which no adequate field populations were found. Field studies confirmed greenhouse results, indicating that weed control could be improved by the use of an adjuvant, but there were exceptions. In general, adjuvant usage improved the efficacy of fomesafen more than it did with bentazon. The minimum rates of herbicide required for effective and consistent control was dependent on the particular combination of weed species, herbicide and its rate of application, growth stage at which the application was made, and adjuvant usage.

Ecological management of invasive weeds will require substantial increases in the application of ecological knowledge and its integration with other forms of knowledge. To enable these increases, we call for purposeful development of knowledge networks in which new knowledge about a complex situation is created by the interaction of different forms of knowledge. We believe that invasive-plant management must be based on a fine-tuning of managed ecosystems, in which operations (e.g., farm activities) are comprehensively adjusted to confront invasives with a wide array of control measures. Land managers must have the primary role in this tuning process because of their holistic knowledge of the ecosystems they manage. Additionally, such ecological management of invasives will require support from new or improved practices in many relevant sectors, e.g., involving extension workers, farm advisors, and researchers of many sorts. Knowledge networks facilitate the creation, application, and integration of knowledge that will be needed to support ecological invasive-plant management. Worldwide, knowledge networks are under very active development as promising solutions to ecological-management challenges. To develop, networks require proactive organization and facilitation. We have developed an experimental knowledge network to facilitate ecological management of field-crop weeds on the basis of collaborative learning groups that help farmers and other professionals develop necessary knowledge. These groups have been favorably evaluated by most participants, and this article describes the results of our project, including our insights into development of such networks.

A study was undertaken to determine whether the relative leaf stages of common annual weeds and crops could serve as a reliable indicator of the time of weed emergence. Ten annual broadleaved and grass weeds were sown at successive intervals in field corn and soybean at Harrow, ON, Canada, in 1997, 1998, and 1999. All weeds emerging at a particular crop leaf stage were assigned to a cohort. Leaf numbers of the crop and different weed cohorts were recorded at 2- to 3-d intervals up to the eight-leaf stage of corn and the fourth trifoliate of soybean. For each weed species, categorical data analysis revealed a high degree of association between the leaf stage of a crop and the leaf number expected for an individual weed of a given cohort. For example, by the third trifoliate of soybean, most of the weeds emerging with the crop (VE cohort) had 8 to 10 leaves, whereas weeds in the V1, V2, and V3 cohorts averaged about seven, four, and two leaves, respectively. Year to year variation in the correspond...

Striga hermonthica reduces cereal yields in large areas of sub-Saharan Africa. Recent research has shown that transplanting maize seedlings that are more than 15 d old reduced Striga emergence and improved crop yields. Field experiments were conducted in 1998 and 1999 to determine whether maize varieties with different maturity periods and susceptibility to Striga parasitism respond similarly to transplanting. There was a considerable difference in Striga emergence between varieties in direct-seeded maize, but transplanting clearly reduced Striga emergence for all varieties. Transplanting of maize in plastic tubes gave the best Striga control until 8 wk after transplanting, whereas transplants from nurseries provided better season-long control. Transplanting improved grain yields 50 to 100% compared with direct seeding for three of the four varieties tested. Only the early-maturing variety ‘Morogoro’ had lower yields with transplanting than with direct seeding, indicating that transplanting cause...

Greenhouse studies determined dose responses of winter annual weeds and winter wheat to preemergence (PRE) and postemergence (POST) treatments of MKH 6561 and the residual effects on kochia. MKH 6561 at 11 to 45 g ha−1 did not affect wheat. MKH 6561 at 11, 34, and 45 g ha−1 reduced winter annual weed densities compared to an untreated control. Weed growth was inhibited as the rate of MKH 6561 was increased, but downy brome, cheat, and Japanese brome were three to six times more susceptible than jointed goatgrass. The GR70 values for cheat, downy brome, Japanese brome, and jointed goatgrass were 7, 11, 4, and >45 g ha−1, respectively. Dry weights of kochia seeded after removal of the winter annual grasses decreased as MKH 6561 rate increased, with average control being 77% compared to the nontreated control. Regression analysis indicated that kochia was controlled 80% with MKH 6561 at 28 g ha−1. Nomenclature: MKH 6561, methyl 2-[[[(4-methyl-5-oxo-3-propoxy-4,5-dihydro-1H-1,2,4-triazol-1-yl)carbony...

Striga hermonthica is a major threat to cereal production in western Kenya and other parts of the world. In laboratory experiments, maize and sorghum showed some resistance to late Striga attachment and parasitism. Field experiments were conducted in 1996 and 1997 in western Kenya to assess the effect of transplanting maize and sorghum, under rain-fed field conditions, on grain yield and Striga parasitism. In transplanted sorghum, Striga emergence was not reduced and sorghum failed to produce grain yield in three out of four seasons. Transplanting maize in all experiments significantly increased grain yield compared to direct seeding. The improved productivity was largely associated with less Striga attachment. Striga densities were considerably lower if maize seedlings were more than 17 d old at transplanting, with decreasing levels of Striga with increasing age of the maize seedlings. Seedlings transplanted before they were 15 d old did not reduce Striga attachment. Transplanting maize under ra...

Subterranean turions (tubers) of hydrilla lose viability when desiccated. Experimental data showed that freshly collected tubers had a moisture content between 50 and 60% and more than 90% viability. When desiccated, there was an approximate 2% increase in tuber mortality with each percent decline in moisture content. However under field conditions, the tuber bank within the exposed sediments of a northern Texas reservoir showed no decline in number or tuber viability throughout a 12-mo continuous drawdown. Apparently, the buried tubers were never subject to sufficient dessication to damage them. Finally, an experimental pond with an extensive hydrilla tuber bank was manipulated through six flood/drawdown cycles to determine the effects of short-term drawdowns on tuber survival and quiescence. Initially, the pond had a tuber bank of about 676 and 305 tubers m−2 in the shallow and deep zones, respectively. Although the tuber number was reduced to fewer than 15 to 30 tubers m−2 by these repetitive ...

There is little information published on patch expansion of perennial weeds and none for Apocynum cannabinum. Studies were conducted to measure the between-season and in-season expansion patterns of natural A. cannabinum patches over three growing seasons. Regression analysis indicated strong relations between patch area in consecutive years 1996 to 1997 (r2 = 0.81) and 1997 to 1998 (r2 = 0.76). Patches less than 20 m2 in 1996 increased in area by more than 100% in 1997 during a fallow season. However, patches decreased in size 6 to 51% between 1997 and 1998 when Glycine max was grown. Evidence suggested that a late-season mowing of the A. cannabinum patches in 1997 contributed more to the decline in patch area than competition from G. max during the 1998 season. The relations between patch area and growing degree units (r2 = 0.97) indicated that greater than 89% of the terminal patch expansion occurred prior to the accumulation of 435 growing degree units (GDU) (June 19, 1997; May 31, 1998; June...

Parasitic plants, including the root holoparasites Orobanche spp., cause devastating damage to crops worldwide. Arabidopsis thaliana is widely used as an amenable model plant system to study host–pathogen interactions. Understanding the molecular basis involved in host–parasite interactions will provide practical tools for the detection of genes responsible for incompatibility and resistance responses. In preliminary petri dish experiments, A. thaliana induced seed germination of O. aegyptiaca, O. minor, and O. ramosa at the rate of 87, 72, and 67% of maximum seed germination, respectively. Arabidopsis thaliana induction of O. crenata and O. cumana seeds was negligible (less than 2% of maximum germination). In additional polyethylene bag studies, A. thaliana was parasitized by O. aegyptiaca, O. ramosa, and O. minor resulting in 12, 5, and 2 parasites per host plant, respectively. The results facilitate the use of A. thaliana in host-parasitic plant interaction research. Nomenclature: Orobanche ae...

Data on the spread of invasive weeds into arid western lands are used to evaluate the environmental and economic importance of controlling invasive weed infestations early. Variable rate and constant rate infestation expansion paths are estimated. The implications of variable vs. constant infestation growth rates for projecting both biophysical and economic effects are illustrated. The projections derived from both constant and variable growth rate expansion paths support the contention that it is expedient to control new infestations early.

Downy brome is one of the most troublesome winter annual weeds in winter wheat-fallow rotations in the central Great Plains. A 3-yr seed burial study was initiated to determine how long downy brome seed remained germinable when placed on the soil surface or 2.5 cm deep at four different times in four environments. Only 1 to 7% of the downy brome seed survived after 1 yr on the soil surface in chemical fallow and stubble mulch when deposited in August, but survival varied in September, October, and November. In 1970, a year with low fall and winter precipitation, 36 to 46% of the seed placed on the soil surface of chemical fallow in September, October, and November survived, compared with 1 to 8% for stubble mulch tillage. Early spring tillage covered more seed with soil, and downy brome seed survival decreased. When fall and winter precipitation was normal, stubble mulch and chemical fallow had 1 to 20% germinable seed remaining. Induced dormancy existed in some years. More downy brome seed survived when placed on the soil surface of crested wheatgrass sod (14 to 50%) than on smooth brome sod (0 to 36%). No differences existed among environments when downy brome seed was buried 2.5 cm deep. Only 0.4% of downy brome seed buried 2.5 cm survived after 1 yr when averaged across all environments.

Experiments were conducted to establish low temperature thresholds for initiation of bud sprouting in dormant vegetative propagules of johnsongrass, purple nutsedge, and yellow nutsedge, and seed germination in cotton. Weed propagule sprouting and cotton seed germination responses to temperature were determined in a series of experiments conducted on a temperature gradient bar. Four calculated indices were used to quantify germination: mean percent germination per day, reciprocal median response time, and two versions of germination rate index. Data were analyzed as a series of regressions of germination indices against temperature. Maximum and minimum temperatures for germination were derived directly from the regressions and compared among species. Yellow nutsedge had the lowest temperature threshold (6 C), while the other species had low temperature thresholds of 11 to 12 C. Upper temperature thresholds were similar among species and ranged from 42 to 44 C. The lower low temperature threshold of yellow nutsedge sprouting compared to those for cotton, johnsongrass, and purple nutsedge suggests that early establishment by yellow nutsedge is an important factor in competitiveness in mixtures of these species. The results presented here suggest that application of principles derived from studies in seed biology might advance our understanding and ability to manage perennial weeds.

A 3-yr field study was conducted to compare an in-row cultivator versus a standard row-crop cultivator to decisions made with WEEDCAM, a weed/corn management computer decision aid, for controlling annual weeds within the row in irrigated corn. In the absence of herbicides, weeds were always controlled better with the in-row cultivator than with the standard row-crop cultivator. However, grain yield and gross margin were affected only in 1991 when weeds emerged simultaneously with corn, and rain delayed the first cultivation 10 d. The in-row cultivator plots not only averaged 34% more grain ha-1than the standard row-crop cultivator plots, but gross margin was $143 ha-1more. Weed densities each year were about 95% less in plots managed in accordance with the computer model WEEDCAM simulations than in the non-herbicide treated post-planting tillage plots. Grain yields and gross margins were not affected by weed seedbank density, pre-cultivation tillage, or type of cultivator when weed management decisions were based on WEEDCAM simulation ranking. In the absence of herbicides, weeds can be controlled successfully in corn with an in-row cultivator, but success will depend on such factors as weed seedbank density, cultivation timeliness, and relative time of weed and corn emergence.

Tests with 18 growth regulators and chemicals revealed that the cytokinins [zeatin, kinetin, and 6-benzylaminopurine (BAP)], ethylene, and sodium hypochlorite stimulate germination of cowpea witchweed seeds at various concentrations. Witchweed seeds preconditioned in different concentrations of sodium hypochlorite for 3 wk had germination rates of 25 to 89% after incubation in distilled water, 10−5M solution of kinetin, 50 mg L−1of 2-chloroethylphosphonic acid, and exudates from cowpea roots. Witchweed seeds preconditioned in distilled water and subsequently incubated in distilled water did not germinate.

The relative effectiveness of eight acetanilide herbicides for controlling annual weeds in processing peas(Pisum sativumL.) was evaluated in field studies and the susceptibility of peas to injury from these herbicides was studied in both the field and greenhouse. Alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] was the most phytotoxic to peas grown in washed silica sand in the greenhouse when the herbicides were applied at concentrations of 2 and 8 mg/L. Field trials conducted over a 2-yr period and simulated rainfall experiments in the greenhouse demonstrated that the phytotoxicity to peas by acetanilide herbicides applied preemergence at 2.2 and 4.5 kg/ha was greatly influenced by subsequent rainfall. When 2.5 cm of simulated rainfall were applied immediately after herbicide application, pea injury increased. Injury to peas was avoided in the field by delaying alachlor application at 2.2 kg/ha until peas began to emerge. This treatment also gave excellent control of annual grass weeds. Greenhouse studies demonstrated that injury to peas by all eight acetanilide herbicides at 2.2 and 4.5 kg/ha could be avoided by delaying application until pea emergence. Alachlor, propachlor (2-chloro-N-isopropylacetanilide), and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] gave slightly superior weed control than butachlor [N-(butoxymethyl)diethylacetanilide] and H 22234 [N-chloroacetyl-N-(2,6-diethylphenyl)-glycine ethyl ester].

Winter applications of paraquat (1,1′-dimethyl-4,4′-bipyridinium ion) or combinations of paraquat with diquat [6,7-dihydrodipydrido (1,2-α: 2′,1′-c) pyrazinediium ion], propham (isopropyl carbanilate), or pronamide [3,5-dichloro-N-(1,1-dimethyl-2-propynyl)benzamide], usually controlled both winter annual grass and broadleaf weeds in established white clover (Trifolium repensL. ‘New Zealand,’ ‘Louisiana S-1,’ and ‘Tillman’). Diquat controlled broadleaf weeds, but was less effective than paraquat or combinations of paraquat with diquat, propham, or pronamide for grass control. Dichlobenil (2,6-dichlorobenzonitrile) controlled weeds in some experiments. Pronamide and propham usually controlled grass weeds but not broadleaf weeds. In experiments in which clover seed production differed significantly, paraquat or combinations of paraquat with propham or pronamide usually resulted in higher yields than the check. Yields increased more often where treatments included paraquat.

Twelve sequential herbicide treatments were compared to cycloate (S-ethylN-ethylthiocyclohexanecarbamate), a standard treatment, for control of annual weeds in sugarbeets (Beta vulgarisL.) in three field experiments conducted from 1971 through 1977. At harvest, seven sequential treatments had less than 10 annual broadleaf weeds per 30 m of row, whereas there were 40 broadleaf weeds per 30 m of row for the cycloate treatment. Four of these sequential treatments had significantly higher root yields and net returns than the cycloate treatment. Dependent on the sequential treatment and year, tonnage was increased 7.3 to 20.3 t/ha, and net returns $150 to $515/ha above those with cycloate. The most effective sequential treatment for control of weeds was a preplanting mixture of 2.2 kg/ha of ethofumesate [(±)-2-ethoxy-2,3-dihydro-3,3-dimethyl-5-benzofuranyl methanesulfonate] plus 1.7 kg/ha of diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid} followed by a postemergence mixture of 0,6 kg/ha each of desmedipham [ethylm-hydroxycarbanilate carbanilate (ester)] plus phenmedipham (methylm-hydroxycarbanilatem-methylcarbanilate). This sequential herbicide treatment increased root yields by an average of 20.3 t/ha and net returns by $515/ha above those with cycloate.

Greenhouse experiments were conducted to determine effects of watering frequency, drought stress, and a combination of watering frequency and glyphosate on growth of field bindweed. Watering daily, twice weekly, or weekly to field capacity affected field bindweed grown from seed differently than plants grown from root runner segments. Shoot and root dry matter of plants grown from seed increased when plants were watered frequently but dry matter allocated to root runners did not change when plants were watered less frequently. Plants grown from root runner segments increased their shoot and root runner dry matter when watered frequently, but root dry matter was not affected by watering frequency. Plants grown from seed had highest regrowth when watered weekly and plants grown from root runner segments had higher regrowth when watered daily. Drought reduced survival rate and dry matter of surviving plants. Frequency of watering and whether plants began from seeds or root runner segments influenced glyphosate action on field bindweed. No regrowth was observed 6 months after 1.5 kg ai/ha of glyphosate was applied to plants that had been grown for 8 weeks under three different watering frequencies.

In the greenhouse, postemergence treatments of 1.12 kg/ha of HOE 22870 [4-(4′-chlorophenoxy)-phenoxy-α-propionic-isobutylester] or HOE 23408 methyl 2-[4-(2,4-dichlorophenoxy)phenoxy] propanoate were applied to three perennial weeds (two grasses and one sedge) and to 27 species or varieties of annual grasses (crops and weeds). The perennials were resistant to both compounds, but control of the seedling grasses ranged from 0 to 100%, indicating the potential of controlling certain grass weeds in certain grass crops, as well as in broadleaf crops. Generally, the response of a given species was similar for both compounds. However, on some grasses one compound was considerably more effective than the other. Seven species or varieties of foxtail (Setariaspp.) were treated with various rates of either compound. Differential responses were observed, but it appeared that all the foxtail species or varieties studied should probably be controllable with HOE 23408. All but yellow foxtail [Setaria lutescens(Weigel) Hubb.], should probably be controllable with HOE 22870.

Mixtures of annual weeds emerging 1 week after and competing all season with potatoes (Solanum tuberosum L.) reduced tuber yields by an average of 54% compared to 16% in comparable treatments in which weeds emerged 3 weeks after potatoes. Weeds reduced tuber yields 19% 8 weeks after potatoes emerged and 25 to 45% 10 weeks after emer- gence. After full-season competition, for each additional 10% of the total dry biomass represented by weeds, there was a decrease of 12% in fresh tuber yield. This relationship was not valid if potatoes were harvested early, which reduced the deleterious effect of weeds on potato yields. Weed com - petition reduced both size and number of tubers, but had little effect on specific gravity of tubers. In some instances, the reduction in tuber size by weeds reduced the percentage of tubers that were marketable. Additional index words. Specific gravity, tuber size, tuber number, biomass.

Experiments were conducted to determine the influence of water depth, fall flooding, and burial depth on the development of common waterplantain (Alisma trivialePursh. # ALSPA) in wild rice (Zizania palustrisL.). The optimum water depths for common waterplantain from corms were 20 cm when grown outside in submerged pots at St. Paul and 2 and 15 cm when grown in a sloping field at Grand Rapids. Growth of common waterplantain from seeds was maximum at the lowest water depths (2 to 7 cm) at both locations. Wild rice dry weight and seed yield in the same experiments were maximum at the 20- and 30-cm water depths at St. Paul and the 15- and 28-cm depths at Grand Rapids. The increased wild rice seed yield and dry weight at these depths was related to increased tiller production. Wild rice yield was similar at all water depths at Grand Rapids when grown with common waterplantain from corms at a density of 11 plants/m2. Water depths that decreased the effect of common waterplantain interference with wild rice, reduced wild rice yield. Establishment of common waterplantain from corms was maximum at the 5- and 15-cm depths and was severely reduced at the 0- and 30-cm soil depths in soils not flooded in the fall but flooded in the spring. Fall flooding killed all corms regardless of burial depth. Corm mortality under fall-flooded conditions may be due to the effects of ice-encasement.

The efficacy of glyphosate as affected by rate, time of application, and addition of surfactant3was evaluated on blackberry, Japanese honeysuckle, poison ivy, sericea lespedeza, and trumpetcreeper. The addition of surfactant (0.5% v/v) to glyphosate had no effect on the control of the weeds studied. Glyphosate applied in mid-June to September at 1.1 or 2.2 kg/ha controlled blackberry. Mid-August glyphosate applications of 2.2 kg/ha controlled 83% of actively growing Japanese honeysuckle; there was less than 75% at one site due to moisture stress. Use of 2.2 kg/ha of glyphosate from mid-June through mid-August controlled 87% of poison ivy. Consistent commercially acceptable control of sericea lespedeza was obtained when glyphosate was applied at 1.1 or 2.2 kg/ha at the time of flowering. Applying glyphosate at 1.1 or 2.2 kg/ha from late July through early October controlled 50% or more of the trumpetcreeper.

The freezing resistance of overwintering buds of four perennial weed species was evaluated in artificial freezing experiments conducted in the winters of 1986 and 1987. Survival, dry weight of surviving shoots, and number of shoots produced/surviving section were determined at −4, −8, −12, −16, and −20 C for Canada thistle roots, leafy spurge roots, leafy spurge crowns, perennial sowthistle roots, and quackgrass rhizomes. The temperature required to reduce survival (LT50) and total dry weight (GR50) by 50% was determined. The response to freezing temperatures varied among species and in some cases between years. The LT50was −7, −13, colder than −20, −17, and colder than −20, and the GR50was −5, −11, −14, −15, and −13 for Canada thistle roots, leafy spurge roots, leafy spurge crowns, perennial sowthistle roots, and quackgrass rhizomes, respectively. The relative susceptibility to freezing among species appears to be directly related to the depth at which the overwintering buds generally are located.

Annual bluegrass (Poa annuaL.) growing in bermudagrass completes its life cycle in May and dies leaving open spaces in the turfgrass. This occurs at a time when summer annual weeds are germinating and these weeds fill the space formerly occupied by the annual bluegrass. As the summer annual weeds die in the fall, annual bluegrass germinates and fills the space formerly occupied by the summer annual weeds. To control annual bluegrass, it is important to control the summer annual grasses and manage the bermudagrass [Cynodon dactylon(L.) Pers.] to maintain a competitive groundcover especially during the peak germination period for the weeds. Herbicide programs over a three-year period were designed to control annual grasses with treatments in April for large crabgrass [Digitaria sanguinalis(L.) Scop.], in May or June for goosegrass [Eleusine indica(L.) Gaertn.] and in late August for annual bluegrass. Oxadiazon [2-tert-butyl-4-(2,4-dichloro-5-isopropoxyphenyl)-δ2-1,3,4-ozadiazolin-5-one] applied in August gave complete control of annual bluegrass. Although oxadiazon has a long residual life in the soil, annual bluegrass was poorly controlled with treatments made in June. Fall (August or September) applications of benefin [N-butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine], prosulfalinN-[[4-(dipropylamino)-3,5-dinitrophenyl] sulfonyl]-S,S-dimethylsulfilimine and butralin [4-(1,1-dimethylethyl-N-(1-methylpropyl)-2,6-dinitrobenzenamine] provided adequate control of annual bluegrass with only a few exceptions during the three-year period. Bensulide [O,O-diisopropyl phosphorodithioateS-ester withN-(2-mercaptoethyl)benzenesulfonamide] gave variable control of annual bluegrass; however, this was improved in programs with oxadiazon which provide goosegrass control during summer. In these studies, DCPA [dimethyl tetrachloroterephthalate], even with three applications a year, gave very little control of annual bluegrass. When oxadiazon was used in rotation with DCPA, adequate control was obtained.

Herbicide treatments of tank mixtures or sequential applications were applied at different growth stages and compared with standard treatments for control of barnyardgrass [Echinochloa crus-galli(L.) Beauv. ♯ ECHCM], bearded sprangletop [Leptochloa fascicularis(Lam.) Gray ♯ LEFFA], and aquatic weeds in rice (Oryza sativaL. ‘Starbonnet’). Some herbicides injured rice more than the propanil (3’,4’-dichloropropionanilide) or molinate (S-ethyl hexahydro-1H-azepine-1-carbothioate) standards, but injury for all treatments was inconsequential. Postemergence tank mixture treatments that controlled barnyardgrass (two- to four-leaf stages), bearded sprangletop (two-leaf to tillering stages), and aquatic weeds better than standard treatments were propanil plus butachlor [N-(butoxymethyl)-2-chloro-2’,6,-diethylacetanilide], oxadiazon [2-tert-butyl-4(2,4-dichloro-5-isopropoxyphenyl)-δ2-1,3,4-oxadiazolin-5-one] thiobencarb {S-[(4-chlorophenyl)methyl]diethylcarbamothioate}, or pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine].

Witchweed (Striga asiaticaLour. ♯4STRLU) seed remained viable for 6 yr under open-shelf laboratory conditions and for 14 yr when deep buried under field conditions. Seed kept in normal atmosphere of the laboratory for < 1 through 6 yr had the following germination percentages: 2, 95, 95, 94, 83, 38, and 7. Newly harvested seed will not germinate because of postmaturity ripening requirements. There were no viable seed after 7 yr of shelf storage in the laboratory. Seed buried in the field gave 60 to 90% germination the first season after production. Annual recovery and testing showed a gradual reduction in viability to 10% at year 14 for the deepest buried seed; shallower buried seed expired sooner. No germination occurred thereafter.

In a 3 year herbicide-crop rotation involving corn (Zea maysL. ‘Coker 811A’), cotton (Gossypium hirsutumL. ‘Carolina Queen’), and peanuts (Arachis hypogaeaL. ‘Argentine’), uncontrolled yellow nutsedge (Cyperus esculentusL.) and annual weeds drastically reduced all crop yields. By the end of the rotation sequences, intensive cultivation throughout the rotation killed 97 to 99% of the nutsedge tubers, but this level of control depended on hand-weeding the cotton. Moderate application of herbicides killed from 78 to over 99% of the yellow nutsedge tubers, while intensive treatment with herbicides produced at least 99% kill. In addition, all annual weeds present when the experiment was initiated were controlled satisfactorily by intensive herbicide applications except Florida beggarweed [Desmodium tortuosum(Sw.) DC.] in peanuts. The major change in composition of the weed population was a reduction in yellow nutsedge and in the total number of weeds; however, certain other population shifts also seemed affected by the weed control treatments. Compared to intensive cultivation, the investment in intensive herbicide treatment produced maximum yields and highest gross profit per acre. None of the treatments affected the market grade or organoleptic characteristics of peanuts. The fatty acid content of corn and cotton changed significantly only in 1967.

Several herbicides were sprayed on emerged London rocket (Sisymbrium irioL.), common sunflower (Helianthus annuusL.), ragweed parthenium (Parthenium hysterophorusL.), annual sowthistle (Sonchus oleraceusL.), and onion (Allium cepaL. ‘yellow granex PRR’), to determine selectivity. Onion yields were reduced with common sunflower and London rocket interference within 7 weeks after emergence. Oxadiazon [2-tert-butyl-4-(2,4-dichloro-5-isopropoxyphenyl)-δ2-1,3,4-oxadiazolin-5-one] at 2.2 kg/ha controlled all but common sunflower under most field conditions and reduced the yield of onion in one experiment out of six. Methazole [2-(3,4-dichlorophenyl)-4-methyl-1,2,4-oxadiazolidine-3,5-dione] and bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) reduced the yields of onion in two of four experiments and three of six experiments, respectively. Both herbicides controlled most weed species, but only bromoxynil controlled annual sunflower adequately. Onion was injured when low temperature, low light intensity, and high relative humidity occurred within 3 days of the application of herbicides. Bioassays showed that oxadiazon persisted 5 months in soil in three of five experiments. Seeded London rocket populations reduced the growth of onion 20% within the first 4 weeks and 65% within 12 weeks after planting.

Hydrilla (Hydrilla verticillata (L. f.) Royle #3 HYLLI) was cultured in flowing water in outside aquaria on rooting media differing in fertility and texture. Nutrient content was determined for rooting media at the beginning of the experiment and for plant tissues at harvest. Growth, as measured by rates of stem elongation, areal coverage of aquaria by plants, and biomass accrual, was highly related to N-, P-, and K-fertility of rooting media. Rooting media were the most important source of phosphorous since water supplies were not adequate to support plant growth. There was evidence of P-deficiencies in plants grown on infertile rooting media. Both N and K were adequately supplied from either water, rooting media, or both. Production of plant biomass was most closely related to P-levels in tissues which were closely related to supplies in rooting media. Pretest soil N (comparable to preestablishment conditions) was best related to biomass production, indicating that growth and establishment of hydrilla may be predictable through assessment of hydrosoil fertility. The need for sampling and analytical procedures to assess fertility is emphasized. Additional index words. Phosphorous, nitrogen, potassium, aquatic weeds, biomass.

The competitiveness of annual weeds in irrigated sweet Spanish onions (Allium cepaL.) was studied at North Platte, Nebraska, during 1969 and 1970. Weeds allowed to grow in the row for 2, 4, 6, and 8 weeks after onion emergence reduced onion yields 20, 20, 40, and 65%, respectively. When plots were kept weed-free until onion emergence and 2, 4, 6, 8, 10, and 12 weeks after emergence, onion yields were reduced 100, 99, 87, 75, 46, 25, and 5%, respectively. Redroot pigweed (Amaranthus retroflexusL.), kochia (Kochia scopariaL.), and grass weeds accounted for 54, 21, and 21%, respectively, of the total weed yield.

The use of herbivorous fish for the biological control of aquatic weeds has great potential. The susceptibility of most herbivorous fish to low temperature is the principal factor limiting their use in the United States. The white amur (Ctenopharyngodon idella Val.) can tolerate low temperature and other water quality extremes. This fish has a voracious appetite for many aquatic plants, and after attaining a length of 30 mm, it is almost exclusively phytophagous. Factors affecting the feeding habits of the white amur include the species of plants and water temperature. The white amur has been introduced for aquatic weed control in various countries. This fish might ameliorate some of the aquatic weed problems in the United States, and also provide a new source of protein.

A 3-year ecological study of physical, chemical, and biological factors affecting the growth of submersed aquatic weeds was conducted in four irrigation and drainage canals in south Florida. Growth of aquatic vegetation was influenced by light intensity which decreased in a geometric ratio to water depth. The light penetration into the water was influenced by dissolved coloring matter and turbidity. A high concentration of nitrogen was found in three of the canals. Nitrogen entered the canals as sewage effluent discharged into the water and as fertilizer runoff from surrounding farmland. Density of aquatic vegetation harvested at intervals of 6 months from clipped plots varied from zero in a newly dug canal to 22,000 lb green weight/A in an old established canal. During the 3-year study, there was a complete shift of the vegetation from southern naiad (Najas guadalupensis(Spreng.) Magnus) to Florida elodea (Hydrilla verticillataCasp.) in two of the canals, but there was no change in plant species in two canals. No other chemical and physical factors evaluated in the study limited submersed aquatic weed growth in these canals.

Cotton (Gossypium hirsutumL.) grown at two locations in Alabama from 1966 to 1968 produced maximum yields when kept free of annual weeds for approximately 8 weeks after emergence. Weed control for longer periods failed to increase yields. When initial weed control measures were delayed until 6 to 7 weeks after emergence at one location and 4 to 5 weeks at the other location, cotton was not injured. Prolonged weed competition past the critical periods usually resulted in drastic cotton yield reductions. Plant height and stem diameter of cotton were reduced by some of the weed competition treatments.