Rozprawy doktorskie na temat „Weedy biomass”

After termination, cover crop residue can suppress weeds by reducing sunlight, decreasing soil temperature, and providing a physical barrier. Experiments were implemented to monitor horseweed suppression from different cover crops as well as two fall-applied residual herbicide treatments. Results suggest that cover crops, other than forage radish in monoculture, can suppress horseweed more consistently than flumioxazin + paraquat or metribuzin + chlorimuron-ethyl. Cover crop biomass is positively correlated to weed suppression. Subsequent experiments were designed to determine the amount of weed suppression from different cover crop treatments and if carbon to nitrogen (C:N) ratios or lignin content are also correlated to weed suppression or cover crop residue thickness. Results indicate that cereal rye alone and mixtures containing cereal rye produced the most biomass and suppressed weeds more than hairy vetch, crimson clover, and forage radish alone. Analyses indicate that lignin, as well as biomass, is an important indicator of weed suppression. While cover crops provide many benefits, integrating cover crops into production can be difficult. Hairy vetch, a legume cover crop, can become a weed in subsequent seasons. Multiple experiments were implemented to determine germination phenology and viability of two hairy vetch cultivars, Groff and Purple Bounty, and to determine when viable seed are produced. Almost all germination occurred in the initial cover crop growing season for both cultivars. Both cultivars had <1% of viable seed at the termination of the experiment. These results indicate that seed dormancy is not the primary cause of weediness.
Master of Science in Life Sciences

After termination, cover crop residue can suppress weeds by reducing sunlight, decreasing soil temperature, and providing a physical barrier. Experiments were implemented to monitor horseweed suppression from different cover crops as well as two fall-applied residual herbicide treatments. Results suggest that cover crops, other than forage radish in monoculture, can suppress horseweed more consistently than flumioxazin + paraquat or metribuzin + chlorimuron-ethyl. Cover crop biomass is positively correlated to weed suppression. Subsequent experiments were designed to determine the amount of weed suppression from different cover crop treatments and if carbon to nitrogen (C:N) ratios or lignin content are also correlated to weed suppression or cover crop residue thickness. Results indicate that cereal rye alone and mixtures containing cereal rye produced the most biomass and suppressed weeds more than hairy vetch, crimson clover, and forage radish alone. Analyses indicate that lignin, as well as biomass, is an important indicator of weed suppression. While cover crops provide many benefits, integrating cover crops into production can be difficult. Hairy vetch, a legume cover crop, can become a weed in subsequent seasons. Multiple experiments were implemented to determine germination phenology and viability of two hairy vetch cultivars, Groff and Purple Bounty, and to determine when viable seed are produced. Almost all germination occurred in the initial cover crop growing season for both cultivars. Both cultivars had <1% of viable seed at the termination of the experiment. These results indicate that seed dormancy is not the primary cause of weediness.
Master of Science in Life Sciences

Camelina sativa (L.) Crantz (large-seeded false-flax) is a recently introduced oilseed crop in Montana and has potential for large scale production for the biofuel market. However, due to weedy characteristics, the invasive potential of this species is of concern. A need exists to create a pre-entry protocol to accurately assess risk to minimize uncertainty inherent in qualitative weed risk assessment approaches. We assessed the probability of C. sativa to invade rangeland ecosystems of southwest Montana to address this concern. The objectives of this study were to 1) quantitatively assess the invasion potential of C. sativa by collecting demographic data over two years and developing a population dynamics model, 2) compare experimental results and modeling outcomes to predictions suggested by a qualitative weed risk assessment system, and 3) assess the impact of growing conditions on the relative competitiveness of C. sativa and Brassica napus (L.) (canola). Objective 1 was carried out in two contrasting rangeland ecosystems to assess the effects of disturbance and seeding season on emergence, survival, and fecundity rates of C. sativa. Population growth (lambda) was forecasted by developing a population dynamics model. Resulting lambda values from simulations using observed data never exceeded 0.03 and the maximum time to extinction was six years. The low lambda values indicate that the threat of invasion by this species in the studied ecosystems is low. Objective 2 compared quantitative results to predictions from the Australian weed risk assessment (WRA) model. In contrast to experimental results, outcomes from the WRA suggested that this species should not be allowed entry into the region. These opposing results highlight the need for a more comprehensive approach to weed risk assessment. Objective 3 was conducted over three trials in two greenhouses. A replacement series design was used to estimate the effects of soil conditions and the presence of an invasive weed, Bromus tectorum (L.) (cheatgrass, downy brome), on the competitive outcomes between C. sativa and canola. Replacement series diagrams determined that competition occurred and that canola was the superior competitor in all treatments, thus providing further evidence that the invasion potential of C. sativa is low.

Field studies examined the strategies of mowing, herbicide, fertility, and all combinations on tall ironweed populations, weed biomass, pasture yield, grazing, economics, and forage quality at three Kentucky locations. Mowing was performed in July 2008 and 2009, herbicide applied in August 2008, and fertilizer applied in September 2008 and 2009 at all locations. Weed populations were measured in 2008, 2009, and 2010, and forage and weed biomass collected in May or June of 2009 and 2010. Herbicide treatments reduced weed biomass at all locations, and reduced tall ironweed stems by 64% or greater in 2009 at all locations. Weed biomass did not differ when comparing all treatments with and without mowing or treatments with or without fertilizer. Forage grass biomass produced was greatest with herbicide plus fertilizer and with the combination of mowing plus herbicide plus fertilizer at all locations in both years. Two years of grazing did not reduce weed populations. Grazing did reduce forage grass and clover biomass at one location, and weed biomass at two locations. Two locations had positive economic returns based on herbicide treatment for weed control and forge yield. Herbicide treatments reduced crude protein at one location and in-vitro true digestibility at two locations.

Low cover crop biomass production is a major obstacle to the success of conservation agriculture currently promoted as panacea to the inherent problem of soil erosion and loss of soil productivity in the Eastern Cape (EC). Therefore, this study evaluated cover crop management strategies for optimizing biomass production for better soil cover, soil nitrogen (N) and phosphorus (P) fertility, weed control and maize yields. The strategies tested are cover crop bicultures, selection of an adapted lupin cultivar and seeding rate, and the feasibility of rain fed winter cover cropping. The cover crop experiments were carried in rotation with summer maize between the winter of 2009 and summer of 2010/2011. Biculture trial was carried out by seeding oat (Avena sativa) and vetch (Vicia dasycarpa) at three mixture ratios and as sole crops under irrigation. On a separate irrigated trial, two lupin cultivars (Lupinus angustifolius var Mandelup & Qualinock) were seeded to a range of seeding rate, 40 to 220 kg ha-1. To study the feasibility of rain fed winter cover cropping, oat, vetch, rye (Lolium multiflorum), barley (Hordeum vulgare), radish (Raphanus sativa) and triticale (Triticale secale) were relayed into a maize crop in February, March and April of 2010. The irrigated trials were followed with SC701 maize cultivar, whilst the rain fed trial was followed with DKC61-25 maize cultivar. Bicultures gave higher cover crop biomass than sole vetch, increasing with an increase in the oat component of the mixture. Increased N and P uptake was observed with bicultures compared to sole oat, however, the levels were comparable to sole vetch. Sole vetch increased soil inorganic N and P at maize planting, whilst the slow decomposition by sole oat residue resulted in mineral lock up. Bicultured cover crop residues had intermediate decomposition rates and resulted in optimum levels of inorganic N and P for prolonged periods compared to sole crops. Weed suppression by the bicultures was comparable to sole cover crops. Biculturing technology significantly (P<0.05) increased maize grain yield compared to sole oat and the yields were comparable to those from sole vetch. For lupins, 206 kg ha-1 seeding rate gave the optimum biomass yield. Weed dry weights in both cover crop and maize crop decreased with an increase in lupin biomass. Comparable soil total N and inorganic P values at maize planting, were observed from plots planted to 120, 180 and 220 kg ha-1. Maize grain yield increased with an increase in lupin seeding rate. The study on rain fed winter cover cropping had most cover crop species’ biomass decreasing with each delay in planting except for radish, which increased. Vetch produced the highest amounts of biomass from February and March planting whilst radish had the highest biomass in April planting. The two species resulted in the greatest N improvement compared to the other species. Regardless of the grazing, the grass specie residues managed to persist to the next cropping season and the residue remaining were comparable to that of radish and vetch. Late-planted cover crops had the greatest residue remaining than early-planted, as a result, April planted cover crops provided better weed suppression than March and April planted. Vetch provided the highest maize grain yield (4005 kg ha-1) whilst all other species tested had comparable grain yields. The results suggested that bicultures could be grown to give sufficient biomass for both weed suppression and soil fertility improvement. Furthermore, increasing lupin plant densities improve its function as a cover crop with respect to weed suppression, soil fertility improvement and maize yields. The study also showed that for dry land systems, February and March planted vetch and April planted radish can provide the greatest biomass and maize yield improvement.

Loblolly pine (Pinus taeda) and coppiced sweetgum (Liquidambar styraciflua) seedlings were grown in competition with a native weed community using soil and seed bank collected near Appomattox, Virginia. Seedlings and weeds were exposed to CO2 (ambient and elevated) and water (water stressed and well watered) treatments for approximately one growing season in closed top chambers.

Weed growth had an effect on tree growth, but the amount of variation in tree biomass explained by weed biomass was very low. It appears that the tree seedlings benefited more from available resources than the herbaceous weeds. The influence of competition with loblolly pine and elevated CO2 did not have an influence on total weed biomass; however, it did favor C3 weed community development regardless of water availability. This suggests that weed community composition may shift toward C3 plants in a future elevated CO2 atmosphere.

Loblolly pine height, diameter, needle, shoot and total biomass were significantly greater in the well watered treatment than the water stressed treatment. Pine root, needle, shoot and total biomass were significantly greater in the elevated treatment than the ambient treatment. While not significant, root biomass of water stressed pine seedlings was 63% greater in the elevated CO2 treatment than the ambient treatment. There was a significant water and CO2 interaction for pine root:shoot ratio. Under elevated CO2, root:shoot ratio was significantly greater in the water stressed treatment than the well watered treatment. In contrast, root:shoot ratio in the ambient treatment was nearly identical under both water treatments. These results indicate that loblolly pine will respond favorably in an elevated CO2 atmosphere, even under dry conditions.

The coppiced sweetgum seedlings responded favorably to well watered conditions with significant increases in leaf area, specific leaf area, leaf, shoot and total biomass compared to water stressed conditions. Leaf, root, shoot+stump and total biomass of sweetgum significantly increased and specific leaf area decreased under elevated CO2 compared to ambient CO2, but differences were smaller than previous findings. This indicates that coppicing may dampen the growth response to elevated CO2, at least in the initial growth stage after coppicing.
Master of Science

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The determination of the critical period of weed interference is very important because it indicates the phase of the culture in which the management of weed shall be performed, furthermore, it limits the number of weeding and other management practices to the minimum, allowing the plant express its maximum yield potential at lower cost to the producer. The aim of this study was to identify the critical period of weed interference in the culture of orange ‘Pera’ tree by parameters valuated: falling immature fruits, productivity, juice yield, juice chemical characteristics, production of photosynthetic pigments and proline in leaves of orange tree, identify the weed species and their accumulation of biomass and nutrients. The experiment was installed in october 2012 and conducted until september 2014, in Rio Preto da Eva - AM. To define the treatments with and without interference of weeds in orange tree, was taken as reference the water balance in the region. The interference periods were defined considering the degree of water availability or absence in the soil: from october to january; february to may; june to september; october to may; october to January, june to september and february to september; period without weed interference (control treatment), and without producer management practices interference. The control of weed was obtained using the herbicide glyphosate (1.720 g ha-1 e.a.). The characteristics evaluated were: Falling immature fruits, productivity, juice yield, total soluble solids (SS), titratable acidity (TA), technological index (TI), chlorophyll production and carotenoids, proline content, biomass accumulation and nutrients by weeds. The treatment with weed interference in the period from october to may increase the fruit drop, reduced the number of fruits per plant and productivity, promoted an increase in soluble solids (SS), total acidity (TA) and reduced values of SS/TA ratio. The different periods of weed interference did not promote significant changes in the contents of chlorophyll a, b, total and of carotenoids in orange tree leaves, but viii promoted significant changes in the free proline content in the leaves. The weed species differ from each other in the accumulation of biomass and nutrients. The critical period of weed interference to the culture of orange ‘Pera’ was from october to may.
A determinação do período crítico de interferência de plantas infestantes é muito importante, pois seu conhecimento indica a fase da cultura em que o manejo das infestantes deve ser realizado, limita o número de capinas e de outras práticas de manejo ao mínimo necessário, possibilitando que a planta expresse o seu máximo potencial produtivo com menor custo para o produtor. O objetivo deste trabalho foi identificar o período crítico de interferência de plantas infestantes na cultura da laranja ‘Pera’ pela avaliação dos parâmetros: queda de frutos imaturos, produtividade, rendimento em suco e suas características químicas, produção de pigmentos fotossintéticos e de prolina nas folhas de laranjeira e identificar as espécies infestantes assim como a acumulação de biomassa e de nutrientes por elas. O experimento foi instalado em outubro de 2012 e conduzido até setembro de 2014, no município de Rio Preto da Eva – AM. Para definição dos tratamentos de interferência ou não de plantas infestantes em laranjeira, tomou-se como referência o balanço hídrico da região. Os períodos de interferência estabelecidos levaram em consideração o grau de disponibilidade ou não de água no solo: de outubro a janeiro; fevereiro a maio; junho a setembro; de outubro a maio; outubro a janeiro e de junho a setembro; fevereiro a setembro; sem interferência das plantas infestantes – tratamento controle; e sem interferência com práticas de manejo do produtor. O controle das plantas infestantes foi obtido com uso do herbicida glyphosate (1.720 g ha-1 e.a.). As características avaliadas foram: queda de frutos imaturos, produtividade, rendimento em suco, sólidos solúveis totais (SS), acidez titulável (AT), índice tecnológico (IT), produção de clorofila e de carotenoides, teor de prolina, acumulação de biomassa e nutrientes pelas infestantes. O tratamento com interferência das plantas infestantes no período de outubro a maio aumentou a queda de frutos prematuros, reduziu o número de frutos por planta e a produtividade, promoveu incremento dos sólidos solúveis (SS), da acidez total (AT) e reduziu vi os valores da relação SS/AT. Os diferentes períodos de interferência de plantas infestantes não promoveram alterações significativas nos teores de clorofila a, b, total e de carotenoides em folhas de laranjeira, mas promoveram mudanças significativas no conteúdo de prolina livre nas folhas. As espécies infestantes diferiram entre si na acumulação de biomassa e de nutrientes. O período crítico de interferência de plantas infestantes para a cultura da laranjeira foi de outubro a maio.

Achieving high biomass yields of cover crops has been a challenge to the success of Conservation Agriculture (CA) practices in the Eastern Cape (EC). A study was conducted to evaluate strategies for optimizing cover crop biomass production. Trials were carried out to screen summer and winter cover crops, as well as evaluate intercropping patterns and planting dates for biomass, weed suppression and subsequent maize yield under irrigation. Four summer legume cover crop species were evaluated under a Randomised Complete Block Design (RCBD) design. The cover crops were fertilized with 13.34 kg ha-1 of N, 20 kg ha-1 P and 26.66 kg ha-1 K. In the 2008/09 summer season a maize crop was superimposed on the 2007/08 screening trial under no-till. The crop was fertilized with 60 kg ha-1 of N. An intercropping trial was conducted over two seasons as a way of investigating the best way of incorporating cover crops into farmers cropping systems. This was done bearing in mind the limitation of resources such as land. The trial evaluated 3 factors laid as a 2 x 2 x 3 factorial arranged in a split-plot design. The main factor was cover crop planting date (planting at maize planting or 2 weeks after maize planting). The sub plot factor was intercropping pattern (strip intercropping and between row intercropping). A trial was also conducted to evaluate the effect of planting date (End of April and mid May) and four winter legume cover crop species on cover crop biomass, weed suppression and maize grain yield. The experiment was laid out as a Randomised Complete Block Design (RCBD) replicated 3 times. In the subsequent summer season a maize crop was superimposed on the winter trial to test the residual effects of the cover crop species. Another study was conducted to evaluate winter cereal cover crop species for biomass accumulation, weed suppression and subsequent maize grain yield. The cover crops as well as a weedy fallow control plot treatments were laid out as a Randomised Complete Block Design replicated 3 times. In the subsequent summer season a maize crop was superimposed on the site under no-till to evaluate the residual effect of the cover crops on maize. The results showed sunhemp, cowpea and lablab as the best cover crops with high biomass and weed suppression whilst mucuna was the least. Sunhemp consistently yielded higher cover biomass averaging 11200 kg ha-1 over the two seasons whilst mucuna had a consistently lowest average biomass yield of 4050 kg ha-1. These cover crops were above the critical 6 t ha-1 for effective weed suppression. There was a significant (p<0.01) relationship of cover crop dry weight and weed dry weight in both seasons. Subsequent maize grain yield was significantly higher in the sunhemp plots (64.2 %) than the weedy fallow plot. Mucuna, lablab and cowpea had maize grain yield increases of 16.6%, 33% and 43.2% respectively. Intercropping cover crops at maize planting yielded higher cover crop dry weights than a delay in intercropping cover crops. A delay in intercropping resulted in significantly higher average maize grain yield of 4700 kg ha-1 compared to intercropping at maize planting (3800 kg ha-1) and sole maize (4300 kg ha-1) over the two seasons. Strip intercropping also yielded higher (5000 kg ha- 1) average maize grain yield compared to row intercropping (3600 kg ha-1) and sole maize (4300 kg ha-1). There was a significant (p<0.05) relationship between cover crop dry weight in the 2007/08 season and maize grain yield in the 2008/09 season. Early planting grazing vetch gave the highest biomass yield of 8100 kg ha-1 whilst early planted red clover had the lowest biomass of 635 kg ha-1. Low weed dry weights were also obtained from the early planted grazing vetch as opposed to the other treatments. There was a significant (p<0.001) relationship of cover crop dry weight and weed dry weight. In the subsequent 2008/09 summer season early planted grazing vetch had the highest maize yield of 7500 kg ha-1 which was 56.3 % more than the weedy fallow plot had 4800 kg ha-1. The weedy fallow plot also had high weed infestation than the cover crop plots. There were significant (p<0.01) relationships between cover crop dry weight and maize grain yield, winter weed dry weight and maize grain yield and summer weed dry weight and maize grain yield. The results also showed triticale (13900 kg ha-1) as the best winter cover crop for biomass production. Italian ryegrass (6500 kg ha-1) produced the least amount of biomass. In The subsequent maize crop white oats gave highest maize grain yield (6369 kg ha-1) which was 33 % more than the weedy fallow plot (4784 kg ha- 1). There were also significant (p< 0.01) relationships of maize grain yield and winter weed dry weight, maize grain yield and summer growing weeds. The various studies demonstrated that there is opportunity for high biomass production under small scale farmers irrigated conditions using cover crops both in winter and summer. Best bet cover crops were sunhemp, cowpea and lablab for summer and triticale, white oats, barley, Italian ryegrass and grazing vetch for winter. Cover crops can also be incorporated into farmers cropping systems as sole crops or intercrops within the maize based cropping systems. Strip intercropping can be used by farmers as a way of introducing cover crops. Critical to achievement of high biomass is the time of planting cover crops with high biomass when planting is done early. A 2 week delay in strip intercropping cover crop into maize can be used as a way of incorporating cover crops into farmers cropping systems with minimal maize yield reduction.

The reported requirement for a higher weeding effort due to increased weed infestations under conservation agriculture (CA) relative to conventional mouldboard plough tillage is perceived by both smallholder farmers and extension workers as the main limiting factor to the widespread adoption of CA by smallholder farmers in southern Africa. However, proponents of CA argue that weeds are only a problem under CA in the initial two years and decline afterwards resulting in reduced labour requirements for weeding under CA. They further posit that weeds are only major problem where minimum tillage (MT) is adopted without crop residue mulching and diverse crop rotations. This thesis explores the effect of time under CA on weed population dynamics and crop growth under the recommended CA practices and actual smallholder farmer practice in semi-arid Zimbabwe. Assessment of weed and crop growth on a long-term CA experiment at Matopos Research Station revealed that the MT systems of planting basins and ripper tine were associated with higher early season weed density and biomass than conventional early summer mouldboard tillage (CONV) in both the fifth (cowpea phase) and sixth (sorghum phase) years of CA. This increased weed infestation within the first four weeks after planting in CA necessitated early weeding to provide a clean seedbed and avert significant crop yield loss. Maize mulching only suppressed early season weed growth in sorghum mostly at a mulch rate of 8 t ha-1 which is not a mulching rate that is attainable on most smallholder farms. However, the lower maize residue mulch rate of 4 t ha-1 was consistently associated with increased weed emergence and growth as from the middle of the cropping season in both crop species. The increased weed infestations under the mulch were probably due to the creation of ‘safe sites’ with moist conditions and moderate temperatures. The high weed growth under the mulch contributed to the low sorghum grain yield obtained under mulched plots. In addition, maize mulching was also associated with a less diverse weed community that was dominated by the competitive Setaria spp. and difficult to hoe weed Eleusine indica (L.) Gaertn. However, the weed community under CA was similar to that under CONV tillage with no evidence of a shift to the more difficult to control weed species. The increased early season weed growth and high weed pressure under CA meant that it was still necessary to hoe weed four times within the cropping season to reduce weed infestations and improve crop growth even after four years of recommended CA practices. Early and frequent weeding was effective in reducing weed growth of most species including Setaria spp. and E. indica demonstrating that on smallholder farms where labour is available hoe weeding can provide adequate weed control. The wider spacing recommended for use in CA contributed to the low cowpea and sorghum grain yields obtained under CA compared to CONV tillage. On smallholder farms in Masvingo District, the MT system of planting basin (PB) was the only conservation farming (CF) component adopted by farmers. There was no difference in the total seedling density of the soil weed seed bank and density of emerged weeds in the field in PB and conventional mouldboard ploughing done at first effective rains (CONV tillage). However, the first weeding in PB was done at least 15 days earlier (P < 0.05) than in CONV tillage suggesting high early season weed growth in PB relative to CONV tillage. As weed density did not decline with time in PB, weed management did not differ with increase in years under PB. Shortage of inputs such as seed and fertiliser was identified by smallholder farmers as the most limiting factor in PB crop production with the area under PB was equivalent to the seed and fertiliser provided by CARE International for most farmers. On this small area, weeds could be managed by available family labour. Double the maize grain yield was obtained in PB (mean: 2856 kg ha-1) due to improved weed management and soil fertility. However, the use of poorly stored composts was found to introduce weeds into some PB fields. The findings of this study demonstrated that weed pressure was still high and weed management were still a challenge under the practice recommended to smallholder farmers in Zimbabwe even in the sixth year of practice. There is, therefore, a need for research on the economic feasibility of using herbicides, intercropping and optimal crop density to ameliorate the high weed pressure under CA.
Thesis (PhD)--University of Pretoria, 2013.
Plant Production and Soil Science
unrestricted

Aphthona spp. flea beetles were released in the Little Missouri National Grasslands (LMNG) in western North Dakota in 1999 to control leafy spurge (Euphorbia esula L.). The changes in soil seed bank composition and leafy spurge density were evaluated on two ecological sites five (2004) and ten years (2009) after Aphthona spp. release to monitor the effectiveness of the insects on weed control and associated change in plant communities. In 2009, leafy spurge stem density averaged 2 and 9 stems m-2 in the loamy overflow and loamy sites, respectively, compared to 110 and 78 stems m-2, respectively, in 1999 and 7 and 10 stems m-2, respectively, in 2004. Leafy spurge constituted nearly 67% of the loamy overflow seed bank in 1999 compared to 17% in 2004 and 2% in 2009. In the loamy seedbank, the weed represented nearly 70% in 1999 compared to approximately 11% in 2004 and 15% in 2009. As leafy spurge was reduced, native species diversity and seed count increased ten years following Aphthona spp. release. High-seral species represented 17% of the loamy overflow seedbank in 2009, an increase from 5% in 1999. However, Kentucky bluegrass, a non-target weedy species, increased over 250% in the loamy overflow seedbank from 2004 to 2009. The reestablishment of native plant species has often been slow in areas where leafy spurge was controlled using Aphthona spp. A bioassay was completed to evaluate native grass establishment when grown in soil from Aphthona spp. release and non-release sites throughout North Dakota. Native grass production was not affected when grown in soil collected from established Aphthona spp. sites (1.5 g per pot) compared to soil without insects (1.6 g per pot). The cause of reduced native grass production in sites with Aphthono spp. previously observed is unknown but may have been due to a chemical inhibition caused by the insects within the soil that no longer exists. The native warm-season switchgrass (Ponicum virgotum L.) may be an alternative to corn for efficient biofuel production; however, control of cool-season grassy weeds has been a problem in switchgrass production. Various herbicides were evaluated for smooth bromegrass (Bromus inermis Leyss.) and quackgrass [Elymus repens (L.) Gould] control in an established switchgrass stand near Streeter, ND and a weed-infested field in Fargo, ND. Switchgrass yield was higher than the control 14 mo after treatment (MAT) when aminocyclopyrachlor or sulfometuron were applied early in the growing season, but no treatment provided satisfactory long-term grassy weed control. Herbicides were reevaluated at increased rates for smooth bromegrass or quackgrass control in Fargo. Sulfometuron provided 99% smooth bromegrass control when applied at 280 g ha-1 in the fall but injured other grass and forb species as well. Sulfometuron would likely be injurious to switchgrass and could not be used for biofuel production. Aminocyclopyrachlor did not injure other grass species but only reduced smooth bromegrass control by 76% when applied at 280 g ha-1 in the fall. No treatment provided satisfactory long-term quackgrass control.

Giant reed (Arundo donax L.) is a candidate to provide feedstock for the Portland General Electric power plant in Boardman, Oregon. Giant reed is a fast perennial grass, producing 23-27 metric tons ha⁻¹ of biomass and has the ability to adapt to diverse environments making it a good candidate for biomass production. This study tested postemergence and preemergence herbicides for controlling weeds in giant reed during the establishment year in which giant reed plants are more sensitive to weed competition. The greenhouse study demonstrated that among the tested herbicides, bromoxynil plus MCPA at 0.841 kg ai ha⁻¹, nicosulforun at 0.035 kg ha⁻¹, and dimethenamid-p at 0.735 kg ha⁻¹ did not injure giant reed. In a field study, preemergence application of dimethenamid-p at 0.735 kg ha⁻¹ followed by a postemergence application of 2,4-D amine at 0.560 kg ha⁻¹ and a postemergence application of bromoxynil plus MCPA at 0.841 kg ha⁻¹ did not injure giant reed. The presence of weeds in a field does not always mean that crop yield will be reduced and there are some periods during the growing season when weeds will not cause considerable yield loss. Therefore, predicting a critical period of weed control (CPWC) that includes the best time for weed control in giant reed could improve weed management in the field. The length of the CPWC could be different depending on the level of acceptable yield loss (AYL). Our results are reported for AYL of 5 and 10%. The CPWC started at 290 accumulated growing degree days (GDD) and ended at 820 for a 5% AYL, while for a 10% AYL, it started at 333 GDD and ended at 727 GDD. Based on the results, there are some herbicides which could be selected for further study for weed control in the giant reed and the estimated CPWC which could be used to inform weed management practices in giant reed production.
Graduation date: 2013

Thesis (M.S.)--University of Wisconsin--Madison, 1999.
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The investigations reported in this thesis examined the relationships between biomass reduction (burning and mowing), soil disturbance, soil nutrient levels (N, P and K) and weed invasion in native grassland remnants in Victoria, Australia. The study was carried out at three native grassland remnant sites, Derrimut grassland reserve, Laverton grassland reserve and Victoria University of Technology, St. Albans campus grassland reserve on the westem edge of Melboume, Australia. These grassland remnants belong to the Western Basalt Plains grassland community which is listed as threatened in Victoria, and which forms part of the lowland temperate grasslands which once extended across south eastern Australia. A major threat to the survival of these communities is invasion by a variety of weeds. Weed invasion is often markedly enhanced by disturbance of the soil due to ploughing and overgrazing. It is known that some type of biomass reduction (burning, mowing or grazing) is required to prevent the dominant native perennial tussock grasses from over shadowing the diversity of forbs that grow between the tussocks, but the effects of various biomass reduction methods on weediness, and the underlying causes of weediness, have not been studied in any depth. The outcomes of the investigation are currently being applied to the management and rehabilitation of lowland grassland remnants. Most previous studies of lowland grassland rehabilitation and management have assumed that the key determinant of competition between native and exotic plant species in lowland grasslands is above-ground cover. The results reported in this thesis clearly show that below-ground competition for nutrients is also important in determining the extent of exotic and native plant growth in intact and disturbed areas. The successful establishment of native species during revegetation, with low levels of weed growth, will depend on the extent to which below ground growth of natives is achieved to lower available nutrient levels quickly, and prevent large scale growth of exotics. The management of lowland grassland remnants with low levels of flatweeds and annual grasses will likewise depend on the maintenance of a healthy sward of native vegetation which can keep available nutrients in the soil at low levels, especially when above-ground cover is removed by biomass reduction actions such as burning, mowing and grazing.

Canola is a cool-season, oilseed crop grown throughout Europe, Canada, and the Northern Great Plains region of the United States. The expansion of canola production into new growing regions, such as the Southern Plains region, has resulted in new production challenges. The Southern Plains region cultivates canola as a winter annual compared to a spring annual for the Northern Great Plains and Canada. Given the difference in climate and weed spectrum, region-specific weed management systems need to be developed. Agronomic practices can affect seed oil content, protein content, and fatty acid composition, however the effect of herbicides on these and other characteristic of canola are unknown. Therefore, experiments were conducted in 2010 and 2011 to evaluate a broad spectrum of herbicides for potential use in South Texas canola production with respect to crop injury, effects on canola seed oil content, fatty acid composition, weed control, biomass yield, and forage quality. Visual crop injury at 42 DAE was unacceptable for saflufenacil at both 0.12 and 0.06 kg ai ha-1 and ethalfluralin at 1.05 kg ai ha-1. Trifluralin at 1.12 and 0.56 kg ai ha-1, S-metolachlor at 2.14 and 1.07 kg ai ha-1, pyroxasulfone at 0.24 and 0.12 kg ai ha-1, and pendimethalin at 0.8 kg ai ha-1 had lowest visual injury of all treatments. Fluroxypyr applied EPOST caused severe injury at both 0.21 and 0.11 kg ae ha-1. All other EPOST treatments did not cause any visible injury. Seed oil content was not affected by the herbicides evaluated. Fatty acid composition, specifically stearic acid, oleic acid, linolenic acid, and oleic to linolenic acid ratio, was affected by herbicide treatments. This research found that protoporphyrinogen oxidase (PPG oxidase) inhibitor herbicides, such as carfentrazone-ethyl and saflufenacil, negatively affect canola oil quality. Biomass yield was improved for all herbicide treatments except pendimethalin PRE when compared to the untreated plots. Crude protein content of canola forage was not affected by herbicide treatment. Digestible dry matter appeared to be reduced by treatments that included an EPOST application of sethoxydim. The research shows that pendimethalin and S-metolachlor may be suitable for canola production in South Texas based on low crop injury and effective weed control. Neither pendimethalin nor S-metolachlor is currently labeled for use in canola. The herbicides trifluralin, ethalfluralin, quizalofop P-ethyl, ethametsulfuron-methyl, sethoxydim, glyphosate, clethodim, and clopyralid are currently labeled for use in canola and were confirmed suitable for canola production in South Texas. Carfentrazone-ethyl is currently labeled for use in canola but the effects on oil quality should be considered.