Thesis (Ph. D.)--Oregon State University, 2007. / Printout. Includes bibliographical references (leaves 204-236). Also available on the World Wide Web.

Thesis (MSc (Logistics))--University of Stellenbosch, 2008. / The primary question considered in this study is whether a small population of a biological species introduced into a resource-heterogeneous environment, where it competes for these resources with an already established native species, will be able to invade successfully. A two-component autonomous system of reaction-diffusion equations with spatially inhomogeneous Lotka-Volterra competitive reaction terms and diffusion coefficients is derived as the governing equations of the competitive scenario. The model parameters for which the introduced species is able to invade describe the realized niche of that species. A linear stability analysis is performed for the model in the case where the resource heterogeneity is represented by, and the diffusion coefficients are, two-toned functions. In the case where the native species is not directly affected by the resource heterogeneity, necessary and sufficient conditions for successful invasion are derived. In the case where the native species is directly affected by the resource heterogeneity only sufficient conditions for successful invasion are derived. The reaction-diffusion equations employed in the model are deterministic. However, in reality biological species are subject to stochastic population perturbations. It is argued that the ability of the invading species to recover from a population perturbation is correlated with the persistence of the species in the niche that it occupies. Hence, invasion time is used as a relative measure to quantify the rate at which a species’ population distribution recovers from perturbation. Moreover, finite difference and spectral difference methods are employed to solve the model scenarios numerically and to corroborate the results of the linear stability analysis. Finally, a case study is performed. The model is instantiated with parameters that represent two different cultivars of barley in a hypothetical environment characterized by spatially varying water availability and the sufficient conditions for successful invasion are verified for this hypothetical scenario.

No description available.

Invasive exotic plants can persist and successfully spread within ecosystems and negatively affect the recruitment of native species. The exotic invasive Ailanthus altissima and the native Robinia pseudoacacia are frequently found in disturbed sites and exhibit similar growth and reproductive characteristics, yet each has distinct functional roles such as allelopathy and nitrogen fixation, respectively. 1) A four-month full additive series in the greenhouse and 2) spatial point pattern analysis of trees in a silvicultural experiment were used to analyze the intraspecific and interspecific interference between these two species. In the greenhouse experiment, total biomass responses per plant for both species were significantly affected by interspecific but not by intraspecific interference (p <0.05). Competition indices such as Relative Yield Total and Relative Crowding Coefficient suggested that A. altissima was the better competitor in mixed plantings. Ailanthus altissima consistently produced a larger above ground and below ground relative yield while R. pseudoacacia generated a larger aboveground relative yield in high density mixed species pots. However, R. pseudoacacia exhibited more variation for multiple biomass traits, occasionally giving it an above ground advantage in some mixed species pots. Analysis of spatial point patterns in the field with Ripley's K indicated that the two species were positively associated with each other along highly disturbed skid trails in the majority of the field sites. Locally, increased disturbances could lead to more opportunities for A. altissima to invade, negatively interact with R. pseudoacacia (as was evident in the greenhouse study), and become established in place of native species. / Master of Science

Accounting for competition effects is an essential step in building any stand growth simulator. However, accurate modelling of competition effects depends upon a clear understanding of quantitative relationships of various aspects of stand dynamics, including distributional parameters and spatial statistics. This study addressed four aspects of competition effects: 1) competition effects on distributional parameter dynamics of tree size variables; 2) inter-specific (loblolly pine vs. hardwood) and intra-specific competition effects on basal area growth, 3) dynamics of spatial statistical characteristics of DBH and total height, and their relevance to intertree competition, 4) and spatial properties of competition measures by available stand simulators for loblolly pine (<i>Pinus taeda</i> L.) plantations. Competition was found to affect the distribution parameters such as coefficient of variation, skewness, and the general shapes of distributions for diameter measurements, total height, crown width and crown height. Competition expedites size differentiation and thereby increases distribution variability for all variables except crown height. Intertree competition also drives skewness of these variables negative, although the distributions of crown heights tends to be more symmetric. Normality assumption generally holds for diameter measurement, but distributions of total height and crown width deviate from, and those of crown height approach, normality with intensified competition. A set of competition driven equations was developed for the distribution parameters and was validated. The differences of distribution parameters among the variables studied could be attributed to their biological meanings. / Ph. D.

A predição de perdas de rendimento de uma cultura devido à interferência de plantas daninhas é fundamental para o planejamento e seleção de estratégias de manejo das plantas daninhas na agricultura. Diversos modelos empíricos foram descritos na literatura porém, há necessidade de validá-los e escolher variáveis que possam ser avaliadas de modo prático, sem reduzir a precisão da predição de perda de rendimento da cultura. Os objetivos foram os de avaliar a competitividade da cultura de feijão (Phaseolus vulgaris L.) em relação à planta daninha [Brachiaria plantaginea (Link) Hitchc.]; comparar a eficiência de modelos empíricos em predizer perdas de rendimento da cultura em que a variável independente descreve a população de plantas daninhas em termos de densidade, índice de área foliar relativa e cobertura relativa do solo e propor uma metodologia para estimar os coeficientes dos modelos avaliados. Os experimentos foram conduzidos na Universidade de São Paulo (USP), na Escola Superior de Agricultura 'Luiz de Queiroz' (ESALQ), no Departamento de Produção Vegetal, em Piracicaba/SP, Brasil. Em casa de vegetação, avaliou-se a resposta de cada espécie à sua própria densidade, determinando-se a densidade mínima de plantas a partir da qual a produção de massa vegetal/unidade de área torna-se independente da densidade e a resposta de cada uma em relação à presença da outra em diferentes densidades e proporções. No campo, avaliou-se a perda de rendimento da cultura devido à densidade e à época de emergência da planta daninha, simulada por diferentes datas de semeadura do capim-marmelada. Em casa de vegetação, o feijão, como espécie competidora, é superior ao capim-marmelada; considerando o efeito da competição sobre a massa seca produzida pela cultura, uma planta de feijão competiu com outra da mesma espécie com a intensidade de 21,3 plantas de capim-marmelada e, o efeito da competição sobre a massa seca produzida pela planta daninha, uma planta de capim-marmelada competiu com outra da mesma espécie com a intensidade de 0,68 plantas de feijão; há diferenciação parcial de nicho ecológico entre as espécies, isto é, ambas utilizam os mesmos recursos do meio mas o feijão evita a presença do capim-marmelada. No campo, o capim-marmelada, como espécie competidora, é superior ao feijão; a perda de rendimento diminui com o atraso na semeadura da planta daninha; o modelo que tem como variável a densidade da planta daninha não descreve a perda de rendimento da cultura causada pela época de semeadura da planta daninha mas aqueles cuja variável é o índice relativo da área foliar da planta daninha ou a cobertura relativa do solo pela planta daninha descrevem; a cobertura relativa do solo pela planta daninha, avaliada visualmente, é uma variável potencial para substituir o índice de área foliar relativa da planta daninha; com o método completo, proposto para estimar os coeficientes dos modelos, obtém-se melhor predição de perdas de rendimento da cultura do que com o método simplificado. / The prediction of crop yield losses due to weed infestation is fundamental for the selection of weed management strategies. Several empirical models have been presented in the literature. However, it is necessary to validate them and choose variables, which can be evaluated in more practical way without reducing the precision of the crop yield loss prediction. The goals of this study were to assess the competitive ability of bean (Phaseolus vulgaris L.) relative to the weed alexandergrass [Brachiaria plantaginea (Link) Hitchc.]; to compare the performance of empirical models on predicting crop yield loss based on weed density, weed relative leaf area index and weed relative ground cover and, to suggest a methodology to estimate the coefficients of the tested empirical models. The experiments were carried out at University of São Paulo (USP), Escola Superior de Agricultura 'Luiz de Queiroz' (ESALQ), in the Department of Crop Production, in Piracicaba, São Paulo State, Brazil. With the glasshouse experiments the response of the species to its own density and, the response of the species to the presence of each other at different densities and proportion between them was assessed. With the field experiment, the crop yield loss, due to weed density and weed emergence relative to the crop emergence, simulated by sowing the weed at different time, was assessed. In the glasshouse experiments, as a competitor species, bean is superior to alexandergrass; the competition as measured by effects on bean biomass, one plant of bean is, to another plant of the same species, competitively equivalent to 21,3 alexandergrass plants and, as measured by effects on alexandergrass biomass, one alexandergrass plant is, to another plant of the same species, equivalent to 0,68 plant of bean; there is partial niche differentiation; the species compete for the same resources however, bean plants avoid the alexandergrass presence. In the field experiment, alexandergrass as a competitor species is superior to bean; crop yield losses decreased with the weed sowing delay; the model which variable is the weed density does not predict de crop yield loss due the relative time of weed sowing however, those models which variable is the weed relative leaf area index or weed relative ground cover, visually estimated, do; the visually estimated weed ground cover is a potential variable to substitute for the weed relative leaf area index; the suggested advanced method for estimating the models coefficients predicts the crop yield loss better than the simple method.

Plants are the major source of food or energy required to sustain life on the planet, but humans are grappling with the deteriorating conditions of natural ecosystems such as compositional change, desertification, invasive plants and soil erosion. In the face of global climate change and growing demands for agricultural productivity, future pressures on grassland ecosystems will intensify, therefore sustainable utilization of all plant resources is of vital importance to enhance food security within the limits of good conservation. The semi-arid grasslands of southern Africa represent major grassland resources for grazing. Herbage production in these areas is determined not only by water and nutrient availability, but also by controlled and uncontrolled fires. Since fire is regarded as a natural factor in savannas, it is essential to develop a deeper understanding of the role of fire in community structure and function for the development of appropriate burning regimes. A study was conducted in the Eastern Cape of South Africa where the rural communities are faced with the challenges of rangeland degradation in the form of encroachment by unacceptable bush, karroid, macchia and less desirable grass species, as well as soil erosion. The main objective of this thesis was to investigate the roles of competition and disturbance regimes (fire and simulated non-selective grazing) on species composition, habitat productivity and the performances of selected species from this semi-arid savanna. Long-term effects of burning frequency on herbaceous species composition, Leaf Area Index (LAI), Photosynthetically Active Radiation (PAR) within the herbaceous canopy, biomass production and soil chemical properties were investigated. These studies were conducted on a fire trial set up in 1980 at the University of Fort Hare research farm in the Eastern Cape, South Africa. The treatments comprise an annual, biennial, triennial, quadrennial, sexennial and no burn control, all replicated twice in a Complete Randomized Design. The data from the trial collected between 1980 and 2008 were used to determine compositional variation for herbaceous species using the Non-metric Multidimensional Scaling and Bray-Curtis Dissimilarity tests. The PAR ceptometer was used to determine LAI and intercepted PAR, while random samples were harvested from 1m² quadrats from each plot. Soil samples were taken at four depths (0-2 cm; 2-4 cm; 4-6 cm and 6-8 cm) from each plot and analyzed for pH, Ca, K, P, total C and total N. The Resin-Bag technique was used to determine nitrogen mineralization. Burning frequency caused significant variation in herbaceous species composition over time. The species were distributed along gradients of increasing burning frequency, and these responses were in three categories: Those that increased with burning frequency such as Themeda triandra; those that decreased with burning frequency such as Melica decumbens, and those that showed little response such as Panicum maximum. The three-year burn resulted in the highest compositional variation, light interception, Leaf Area Index, aboveground biomass production, while the annual, biennial and no burn treatments resulted in the lowest. The fact that infrequent burning resulted in higher species variation, improved habitat productivity due to increased leaf area for light interception shows that appropriate use of fire can maintain a more diverse and productive savanna system. Burning frequency had significant effects on the soil properties, while soil depth did not show any significance. Frequent burning increased soil pH, K, Ca, and Na, but reduced C, N, P and N mineralization. There was a negative correlation between burning frequency and N mineralization, but no correlation existed between N mineralization and total N, total C or the C:N ratio. These results imply that frequent burning can cause nutrient losses and a greater nutrient limitation to plants in the long-term, especially soil C and N loss from combustion of organic material in the soil top layer. The ability of shade-tolerant plants to persist under shade and regular defoliation such as in burnt and grazed systems may be of greater importance for long-term productivity and sustainability of forage crops. It is therefore imperative to explore the mechanisms by which some species were favoured by frequent burning which created low shade conditions, while others were favoured by high shade conditions where burning is infrequent or absent. A pot experiment was conducted to investigate the shade tolerances of seven grass species that were abundant in the long-term fire trial. The test species were Cymbopogon plurinodis, Digitaria eriantha, Eragrostis curvula, Melica decumbens, Panicum maximum, Sporobolus fimbriatus and Themeda triandra. Individual grass tillers of each species were collected from the natural vegetation, propagated in separate seedling trays and transplanted into individual pots, and were grown under five shading treatments: full sun (0 % shading), 55 %; 70 %; 85 % and 93 % shading respectively. Shading significantly reduced the dry matter production of all the species. Biomass production of all the species decreased linearly to varying degrees with an increase in shade intensity. Digitaria eriantha and Eragrostis curvula were most adversely affected by shading, hence are classified as shade intolerant, while Melica decumbens was the least affected by shading, and is hence classified as shade tolerant. Cymbopogon plurinodis, Panicum maximum, Sporobolus fimbriatus and Themeda triandra are classified as moderately shade-tolerant. From the results it was apparent that some species could perform optimally in partial shade than in full sunlight, and these results lead to a conclusion that for satisfactory natural regeneration and seedling growth of this savanna vegetation would require a gap large enough to provide at least 30 % of ambient light. Investigating patterns in competitive effects and responses of species in these communities may not only explain the abundance of each species, but may also provide insight into the nature of forces that affect the structure and function of that community. Since fire, herbivory and soil nutrients are natural drivers of savanna community structure and function, their influence on competitive interactions of selected species were investigated. Two experiments were conducted to investigate the competitive effects and responses of eight selected common species in the area. The test species (phytometers) included one woody shrub, Acacia karroo and seven grass species namely: Cymbopogon plurinodis, Digitaria eriantha, Eragrostis curvula, Melica decumbens, Panicum maximum, Sporobolus fimbriatus and Themeda triandra. In an outdoor plot experiment the responses of the phytometers to competition from neighbours (0; 2 and eight neighbours respectively), fertility (fertilized, unfertilized) and clipping (clipping, no clipping) were investigated. The second comprised a pot experiment where the competitive effects of the species were investigated. Each species was grown under 3 levels of fertility (0 %; 50 % and 100 % Hoagland‘s solution) and clipping (clipping, no clipping) in pots filled with fine river sand and 4 neighbours. Competition intensity, soil fertility and clipping had significant effects on the biomass production of the phytometer species. Acacia karroo and Melica decumbens, exhibited the weakest competitive effects and responses, and incurred the highest mortalities after clipping and with 8 neighbours. Digitaria eriantha and Panicum maximum exhibited the strongest competitive effects and responses, especially in high fertility, and experienced the lowest mortalities. T.triandra exhibited stronger competitive effect after clipping in low fertility, while A. karroo and C. plurinodis exhibited stronger competitive effects in moderate (50 %) fertility. Cymbopogon plurinodis, Eragrostis curvula and Sporobolus fimbriatus ranked between these two extreme groups in terms of competitive effects and responses. Relative Competitive Interaction increased with soil fertility and number of neighbours in the absence of clipping. These results indicate that in general, taller or broad-leaved grass species outgrow the shorter ones, and this gives them a competitive advantage over light and soil resources. One of the range management objectives in the False Thornveld of the Eastern Cape is to promote the abundance of Themeda triandra, which is of high forage value and an indicator of rangeland that is in good condition. The general situation under livestock farming conditions in this area is that if the grass sward is optimally grazed and rested then there is a great potential for Themeda triandra to dominate.The results of the competition experiments indicated that the species exhibits strong competitive interaction, and also exhibited stronger competitive effect after clipping in low fertility. These results imply that it has a low response and a high effect, an attribute that would enhance its performance if it is moderately grazed or the area is burnt. The species is also moderately shade tolerant, and this may explain why it thrives in burnt and moderately grazed areas. These studies have demonstrated the important role that competition and disturbance in the form of fire and herbivory play in the maintenance of this savanna grassland. Through natural selection species are able to occupy different niches in the same area and coexist in a heterogeneous environment and minimize their chances of extinction. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2012.

Made available in DSpace on 2014-06-11T19:28:31Z (GMT). No. of bitstreams: 0 Previous issue date: 2008-02-26Bitstream added on 2014-06-13T20:58:20Z : No. of bitstreams: 1 demetrio_cs_me_jabo.pdf: 252291 bytes, checksum: 1265f694678a98b45a4678fbf67cb7f0 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O trabalho teve como objetivo avaliar a influência de espaçamentos entre as linhas e densidades populacionais, em dois híbridos de milho, no desempenho agronômico da cultura com ênfase nos componentes de produção, produtividade e composição glicídica dos grãos. O experimento foi conduzido na safra 2006/2007, em condições de campo, em Latossolo Vermelho eutrófico típico, textura argilosa, A moderado, a 21°15’22”de latitude sul e longitude 48°18’58” WGr, em Jaboticabal – SP. O delineamento experimental foi o de blocos ao acaso, compostos por 24 tratamentos, constituídos pela combinação de três espaçamentos entre as linhas (40, 60 e 80 cm) nas parcelas, dois híbridos (30K73 e 30F80) nas subparcelas, e quatro densidades populacionais (30, 50, 70 e 90 mil plantas ha-1) nas sub-subparcelas, com três repetições. As variáveis avaliadas foram: estatura de plantas, altura da 1ª espiga, n.º espigas por planta, diâmetro de colmo, acamamento e quebramento de plantas, n.º de grãos na 1ª espiga, massa de grãos na 1ª espiga, massa de mil grãos, produtividade de grãos e índice de colheita. Foram feitas análises químicas do grão para determinação do teor de sacarose, açucares redutores e amido. Verificou-se que a produtividade do milho aumentou com a redução do espaçamento entre as linhas de 80 para 40 cm, que o melhor arranjo de plantas, para os híbridos 30K73 e 30F80, nas condições de edafoclimáticas de Jaboticabal, é composto pela combinação de 40 cm de espaçamento entre as linhas e 75 e 80 mil plantas ha-1 de densidade populacional, respectivamente. Modificando o arranjo espacial de plantas de milho, a composição final de glicídios no grão não é alterada. / The present work had the objective of evaluating the influence of row spacing and population densities, in two corn hybrids, on the agronomic performance of the culture with enface on production components and productivity. The experiment was carried out in the agricultural year of 2006/2007, under field conditions, in a Typical Haplustox , clay texture, at 21°15’22” South latitude and longitude 48°18’58” W, at Jaboticabal – SP, Brazil. The experimental design was a complete randomized block, in split split-plot scheme with 24 treatments, where the plots were composed by three row spacing (40, 60 e 80 cm), the split-plots by two contrastive hybrids (30K73 ND 30F80) and the split split-plots by four population densities (30, 50, 70, e 90 thousand plants per hectare), with three replications. The following variables were evaluated: plant height, first ear height, number of ear per plant, stem diameter, plant lodging and breakage, number of grains per ear, mass of grains per ear, mass of a thousand grains, grain yield and yield index. Furthermore, grain chemical analyses were done to determine the content of saccharose, redacting sugars and starch. It was verified that corn productivity rises with the reduction of row spacing from 80 to 40 cm, that the best plant arrangement, for the hybrids 30K73 and P0F80, under Jaboticabal environment conditions, is the combination of 40 cm row spacing and 75 and 80 thousand plants per hectare, respectively. Final grain glicidic composition is not altered by the spacial arrangements of corn plants.

Resumo: O trabalho teve como objetivo avaliar a influência de espaçamentos entre as linhas e densidades populacionais, em dois híbridos de milho, no desempenho agronômico da cultura com ênfase nos componentes de produção, produtividade e composição glicídica dos grãos. O experimento foi conduzido na safra 2006/2007, em condições de campo, em Latossolo Vermelho eutrófico típico, textura argilosa, A moderado, a 21°15'22"de latitude sul e longitude 48°18'58" WGr, em Jaboticabal - SP. O delineamento experimental foi o de blocos ao acaso, compostos por 24 tratamentos, constituídos pela combinação de três espaçamentos entre as linhas (40, 60 e 80 cm) nas parcelas, dois híbridos (30K73 e 30F80) nas subparcelas, e quatro densidades populacionais (30, 50, 70 e 90 mil plantas ha-1) nas sub-subparcelas, com três repetições. As variáveis avaliadas foram: estatura de plantas, altura da 1ª espiga, n.º espigas por planta, diâmetro de colmo, acamamento e quebramento de plantas, n.º de grãos na 1ª espiga, massa de grãos na 1ª espiga, massa de mil grãos, produtividade de grãos e índice de colheita. Foram feitas análises químicas do grão para determinação do teor de sacarose, açucares redutores e amido. Verificou-se que a produtividade do milho aumentou com a redução do espaçamento entre as linhas de 80 para 40 cm, que o melhor arranjo de plantas, para os híbridos 30K73 e 30F80, nas condições de edafoclimáticas de Jaboticabal, é composto pela combinação de 40 cm de espaçamento entre as linhas e 75 e 80 mil plantas ha-1 de densidade populacional, respectivamente. Modificando o arranjo espacial de plantas de milho, a composição final de glicídios no grão não é alterada. / Abstract: The present work had the objective of evaluating the influence of row spacing and population densities, in two corn hybrids, on the agronomic performance of the culture with enface on production components and productivity. The experiment was carried out in the agricultural year of 2006/2007, under field conditions, in a Typical Haplustox , clay texture, at 21°15'22" South latitude and longitude 48°18'58" W, at Jaboticabal - SP, Brazil. The experimental design was a complete randomized block, in split split-plot scheme with 24 treatments, where the plots were composed by three row spacing (40, 60 e 80 cm), the split-plots by two contrastive hybrids (30K73 ND 30F80) and the split split-plots by four population densities (30, 50, 70, e 90 thousand plants per hectare), with three replications. The following variables were evaluated: plant height, first ear height, number of ear per plant, stem diameter, plant lodging and breakage, number of grains per ear, mass of grains per ear, mass of a thousand grains, grain yield and yield index. Furthermore, grain chemical analyses were done to determine the content of saccharose, redacting sugars and starch. It was verified that corn productivity rises with the reduction of row spacing from 80 to 40 cm, that the best plant arrangement, for the hybrids 30K73 and P0F80, under Jaboticabal environment conditions, is the combination of 40 cm row spacing and 75 and 80 thousand plants per hectare, respectively. Final grain glicidic composition is not altered by the spacial arrangements of corn plants. / Orientador: Domingos Fornasieri Filho / Coorientador: Jairo Osvaldo Cazetta / Banca: Aildson Pereira Duarte / Banca: Orivaldo Arf / Mestre

A predição de perdas de rendimento de uma cultura devido à interferência de plantas daninhas é fundamental para o planejamento e seleção de estratégias de manejo das plantas daninhas na agricultura. Diversos modelos empíricos foram descritos na literatura porém, há necessidade de validá-los e escolher variáveis que possam ser avaliadas de modo prático, sem reduzir a precisão da predição de perda de rendimento da cultura. Os objetivos foram os de avaliar a competitividade da cultura de feijão (Phaseolus vulgaris L.) em relação à planta daninha [Brachiaria plantaginea (Link) Hitchc.]; comparar a eficiência de modelos empíricos em predizer perdas de rendimento da cultura em que a variável independente descreve a população de plantas daninhas em termos de densidade, índice de área foliar relativa e cobertura relativa do solo e propor uma metodologia para estimar os coeficientes dos modelos avaliados. Os experimentos foram conduzidos na Universidade de São Paulo (USP), na Escola Superior de Agricultura 'Luiz de Queiroz' (ESALQ), no Departamento de Produção Vegetal, em Piracicaba/SP, Brasil. Em casa de vegetação, avaliou-se a resposta de cada espécie à sua própria densidade, determinando-se a densidade mínima de plantas a partir da qual a produção de massa vegetal/unidade de área torna-se independente da densidade e a resposta de cada uma em relação à presença da outra em diferentes densidades e proporções. No campo, avaliou-se a perda de rendimento da cultura devido à densidade e à época de emergência da planta daninha, simulada por diferentes datas de semeadura do capim-marmelada. Em casa de vegetação, o feijão, como espécie competidora, é superior ao capim-marmelada; considerando o efeito da competição sobre a massa seca produzida pela cultura, uma planta de feijão competiu com outra da mesma espécie com a intensidade de 21,3 plantas de capim-marmelada e, o efeito da competição sobre a massa seca produzida pela planta daninha, uma planta de capim-marmelada competiu com outra da mesma espécie com a intensidade de 0,68 plantas de feijão; há diferenciação parcial de nicho ecológico entre as espécies, isto é, ambas utilizam os mesmos recursos do meio mas o feijão evita a presença do capim-marmelada. No campo, o capim-marmelada, como espécie competidora, é superior ao feijão; a perda de rendimento diminui com o atraso na semeadura da planta daninha; o modelo que tem como variável a densidade da planta daninha não descreve a perda de rendimento da cultura causada pela época de semeadura da planta daninha mas aqueles cuja variável é o índice relativo da área foliar da planta daninha ou a cobertura relativa do solo pela planta daninha descrevem; a cobertura relativa do solo pela planta daninha, avaliada visualmente, é uma variável potencial para substituir o índice de área foliar relativa da planta daninha; com o método completo, proposto para estimar os coeficientes dos modelos, obtém-se melhor predição de perdas de rendimento da cultura do que com o método simplificado. / The prediction of crop yield losses due to weed infestation is fundamental for the selection of weed management strategies. Several empirical models have been presented in the literature. However, it is necessary to validate them and choose variables, which can be evaluated in more practical way without reducing the precision of the crop yield loss prediction. The goals of this study were to assess the competitive ability of bean (Phaseolus vulgaris L.) relative to the weed alexandergrass [Brachiaria plantaginea (Link) Hitchc.]; to compare the performance of empirical models on predicting crop yield loss based on weed density, weed relative leaf area index and weed relative ground cover and, to suggest a methodology to estimate the coefficients of the tested empirical models. The experiments were carried out at University of São Paulo (USP), Escola Superior de Agricultura 'Luiz de Queiroz' (ESALQ), in the Department of Crop Production, in Piracicaba, São Paulo State, Brazil. With the glasshouse experiments the response of the species to its own density and, the response of the species to the presence of each other at different densities and proportion between them was assessed. With the field experiment, the crop yield loss, due to weed density and weed emergence relative to the crop emergence, simulated by sowing the weed at different time, was assessed. In the glasshouse experiments, as a competitor species, bean is superior to alexandergrass; the competition as measured by effects on bean biomass, one plant of bean is, to another plant of the same species, competitively equivalent to 21,3 alexandergrass plants and, as measured by effects on alexandergrass biomass, one alexandergrass plant is, to another plant of the same species, equivalent to 0,68 plant of bean; there is partial niche differentiation; the species compete for the same resources however, bean plants avoid the alexandergrass presence. In the field experiment, alexandergrass as a competitor species is superior to bean; crop yield losses decreased with the weed sowing delay; the model which variable is the weed density does not predict de crop yield loss due the relative time of weed sowing however, those models which variable is the weed relative leaf area index or weed relative ground cover, visually estimated, do; the visually estimated weed ground cover is a potential variable to substitute for the weed relative leaf area index; the suggested advanced method for estimating the models coefficients predicts the crop yield loss better than the simple method.