In the United Kingdom and Ireland, a major percentage of fertilized agricultural area is devoted to grasslands, which helps to support the associated milk and beef production industries. In temperate grasslands, perennial ryegrass (L. perenne) is the major forage grass and this species is particularly suitable as a forage grass due to its high yield and digestibility, when compared with other species. However, perennial ryegrass is not well adapted to abiotic stress conditions which are likely to occur in its natural environment. Some of the abiotic stress factors which have significant impacts on plant growth and development include water and nutrient availability. Therefore, this project set out to unravel some of the mechanisms involved in the adaptation of perennial ryegrass to limited water, phosphorous and nitrogen. In order to understand the metabolic mechanisms acting in response to these stresses, metabolite profiling was performed using GC-MS. Furthermore, for the water- and phosphorous-limitation studies this approach was complemented with transcript analysis.In order to study water-limitation a hydroponics system supplemented with polyethyleneglycol (PEG) was used to induce water-limitation for a period of one-week. A clear difference in the metabolic profiles of the leaves of plants grown under water stress was observed. Differences were principally due to a reduction in fatty acid levels in the more water stress-susceptible genotype Cashel and an increase in sugars and compatible solutes in the drought-tolerant PI 462336 genotype. Sugars exhibiting a significant increase included, raffinose, trehalose, glucose, fructose and maltose. Raffinose was identified as the metabolite exhibiting the largest accumulation under water-stress in the more tolerant genotype and may represent a target for engineering superior drought tolerance or form the basis of marker-assisted breeding in perennial ryegrass. The metabolomics approach was combined with a transcriptomics approach in the water stress tolerant genotype PI 462336 which identified genes in perennial ryegrass that were regulated by this stress.The characterization of the response to phosphorus-limitation was performed in a hydroponics system containing two solutions with different levels of phosphorus. Samples were collected from the roots and leaves of two genotypes 24 hours after being exposed to stress. Internal phosphate concentrations were reduced and significant alterations were detected in the metabolome and transcriptome of two perennial ryegrass genotypes. Results indicated a replacement of phospholipids with sulfolipids in response to P deficiency and that this occurs at the very early stages of P deficiency in perennial ryegrass. Additionally, the results suggested the role of glycolytic bypasses and the re-allocation of carbohydrates in response to P deficiency The characterization of the metabolic response of L. perenne leaves to different levels of nitrogen supply was performed for seven different genotypes with variability in the regrowth response rate to nitrogen supply in a hydroponics system. This facilitated the identification of common mechanisms of response between genotypes to nitrogen. The metabolic response observed included modifications of the lipid metabolism, as well as alterations of secondary aromatic metabolite precursors in plants exposed to nitrogendeficit. In contrast, plants grown in a nitrogen saturated media appeared to modify to some extent the metabolism of ascorbate. Additionally, it was found that amino acid levels increased with increasing concentrations of nitrogen supplied. This study suggested that the involvement of secondary metabolism, together with lipid and ascorbate metabolism, is of crucial importance in the early-adaptation of perennial ryegrass plants to different levels of nitrogen supply.

Typescript (photocopy) / Bibliography :leaves 310-329. / ix, 329 leaves : ill. (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Crop Protection, 1994

Thèse de doctorat : Génie des procédés et des produits : INPL : 2007. Thèse de doctorat : Génie des procédés et des produits : Universidade Federal de Pernambuco : 2007. / Thèse soutenue en co-tutelle. Titre provenant de l'écran-titre. Bibliogr.

Neotyphodium endophytes (Claviceptaceae) are asexual filamentous fungi found living between the cells of many cool season forage grasses including tall fescue, meadow fescue and perennial ryegrass. They produce a range of alkaloids, including ergovaline and lolitrem B, which have been shown to be directly associated with the livestock disorders fescue toxicosis and ryegrass staggers syndrome, while others, including peramine and the lolines, have been linked to increased insect and drought resistance of the grass host. In the past decade, the Neotyphodium strains AR1, MaxQ and MaxP were selected because they did not produce the alkaloids associated with livestock disorders. Subsequently, artificial associations were established between them and commercial forage grass cultivars. The slow growth rate of Neotyphodium endophytes in vitro and the low success rate of the present methods for establishing artificial associations between endophytes and grass hosts are limiting the rate at which new novel endophytes can be incorporated into plant breeding programs and eventually commercialised. In this thesis, the type and concentration of the growth medium was shown to affect radial growth rate, colony appearance and mycelial morphology of three strains of Neotyphodium endophytes. The floret inoculation of meadow fescue with the U2 strain of N. uncinatum using several techniques involving liquid culture was attempted but was unsuccessful in creating any artificial associations. Neotyphodium endophytes are unstable in stored seed. In New Zealand, it is critical that pastures are infected with protective Neotyphodium endophytes to ensure that they will not be destroyed by exotic pests. The present methods for determining the percentage of viable endophyte infection of a seed lot are too slow for efficient use in the commercial seed industry. In this thesis, primers specific to the â-tubulin gene of N. coenophialum, N. lolii and N. uncinatum were designed and successfully used to detect these species in planta. However, using these primers to develop a method to accurately determine the viable endophyte infection rate of a seed lot using RT-qPCR was unsuccessful.

Neotyphodium endophytes (Claviceptaceae) are asexual filamentous fungi found living between the cells of many cool season forage grasses including tall fescue, meadow fescue and perennial ryegrass. They produce a range of alkaloids, including ergovaline and lolitrem B, which have been shown to be directly associated with the livestock disorders fescue toxicosis and ryegrass staggers syndrome, while others, including peramine and the lolines, have been linked to increased insect and drought resistance of the grass host. In the past decade, the Neotyphodium strains AR1, MaxQ and MaxP were selected because they did not produce the alkaloids associated with livestock disorders. Subsequently, artificial associations were established between them and commercial forage grass cultivars. The slow growth rate of Neotyphodium endophytes in vitro and the low success rate of the present methods for establishing artificial associations between endophytes and grass hosts are limiting the rate at which new novel endophytes can be incorporated into plant breeding programs and eventually commercialised. In this thesis, the type and concentration of the growth medium was shown to affect radial growth rate, colony appearance and mycelial morphology of three strains of Neotyphodium endophytes. The floret inoculation of meadow fescue with the U2 strain of N. uncinatum using several techniques involving liquid culture was attempted but was unsuccessful in creating any artificial associations. Neotyphodium endophytes are unstable in stored seed. In New Zealand, it is critical that pastures are infected with protective Neotyphodium endophytes to ensure that they will not be destroyed by exotic pests. The present methods for determining the percentage of viable endophyte infection of a seed lot are too slow for efficient use in the commercial seed industry. In this thesis, primers specific to the â-tubulin gene of N. coenophialum, N. lolii and N. uncinatum were designed and successfully used to detect these species in planta. However, using these primers to develop a method to accurately determine the viable endophyte infection rate of a seed lot using RT-qPCR was unsuccessful.

At present, and more so in the future, irrigated agriculture will take place under water scarcity. Owing to the global expansion of irrigated areas and the limited availability of irrigation water, there is a need to optimize water production and use efficiency (WUE). In South Africa, annual ryegrass (Lolium multiflorum) is one of the most widely grown cool season pasture species under irrigation. It is mainly used in dairy farming enterprises. Shortages of water and nitrogen can, however, be limiting factors for the production of this pasture. By using appropriate irrigation and nitrogen management tools, water and nitrogen productivity of the pasture can be improved. The objective of this study was, therefore, to determine the effects of different water levels in combination with different N fertiliser applications on the growth rate and dry matter production, quality, water use and water use efficiency of annual ryegrass. For two seasons, the trial was conducted under a rain shelter on the Hatfield Experimental Farm of the University of Pretoria. Higher frequency of irrigation coupled with high nitrogen application significantly improved the dry matter yield. Canopy size influenced the LAI and FI which in turn affects the yield. The study showed that the treatments that were irrigated twice weekly and top-dressed with 60 kg N ha-1 after each cut consumed the most water, and this resulted in the production of higher yield, maintenance of the largest leaf area index and higher interception of the incoming solar radiation. The increase in these parameters may be due to the sufficient water and nitrogen fertiliser that induces rapid cell elongation as a result of higher water potential, higher turgor pressure and higher photosynthetic processes. As hypothesized, the decrease in the frequency of water application resulted in an increase in the DMC, digestibility, ME and CP values. Nitrogen application had an effect on the WU, as less water was used in the treatments that received no nitrogen. Highest KC value recorded was in the optimal range and this indicates that the treatments were not over-irrigated. As the irrigation interval increased, more water was depleted from the soil profile. Depletion rates increased as the season progressed but generally it was minimal in the frequently irrigated treatments. Increase in WUE was achieved by reducing the frequency of irrigation from twice a week to once a week without causing significant yield loss. A possible reason for the increase in the WUE by reducing the irrigation frequency could be ascribed in part to reduced evaporation from the soil resulting from the lower wetting frequency of the deficit irrigation treatments. Within the same irrigation frequency, higher WUE was achieved by alleviating a limiting factor, N fertiliser, in this case, through increases in dry matter production. The highest WUE was achieved by irrigating once every two weeks. However, in some treatments, the WUE was not improved with the reduction in the frequency of irrigation as the water saved was overshadowed by yield loss. In summary, it can be said that the hypotheses that pasture production will be positively associated with soil moisture content, water stress can improve the quality of the pasture, N fertiliser will increase the DM response to soil moisture content and WUE will increase by alleviating a limiting factor, N fertiliser in this case were accepted. A logical extension of this work would be to do the trial in an open field to analyze the effect of irrigation and nitrogen fertilization on the growth, yield and quality of the pasture and then extrapolate the results to other sites and soil types using models. / Dissertation (MSc(Agric))--University of Pretoria, 2011. / Plant Production and Soil Science / unrestricted

Glyphosate-resistant (GR) Italian ryegrass has been documented in many different countries around the world and has now become a major problem in row crop production areas of Mississippi. Field experiments were conducted from 2006 to 2008 in the Mississippi Delta to evaluate various herbicide and tillage treatment programs for its control. style='mso-spacerun:yes'> Highest level of control and reduction of GR Italian ryegrass biomass was observed with mechanically incorporated as well as surface applied residual herbicides in the fall of the year. style='mso-spacerun:yes'> Control of GR Italian ryegrass was 86-95% with surface applications of clomazone at 0.56, 0.84, and 1.12 kg ai/ha, s-metolachlor at 1.79 kg class=SpellE>ai/ha and KIH-485 at 0.16 kg ai/ha 171 days after emergence. Using a systems approach, preplant incorporated (PPI) class=SpellE>clomazone and/or s- class=SpellE>metolachlor followed by preemergence (PRE) application of paraquat + linuron+ non-ionic surfactant was also found to control and reduce biomass of GR Italian ryegrass.

Genótipos de azevém (Lolium multiflorum Lam.) diplóides e tetraplóides são cultivados como forrageira na produção animal, no entanto, quando sistemas de produção que envolve o plantio direto são estabelecidos após o cultivo da forrageira é comum a dessecação com o herbicida glyphosate. Portanto, é importante conhecer se existe suscetibilidade diferencial a este herbicida entre os genótipos. Desta forma, foi desenvolvida a presente pesquisa como o objetivo de avaliar o grau de tolerância dos genótipos diplóides e tetraplóides de azevém ao herbicida glyphosate em quatro estádios fenológicos de desenvolvimento. Para isso, foram instalados quatro experimentos, sendo um para cada estádio fenológico do azevém (duas folhas, quatro perfilhos, pré-florescimento e formação de grãos). Os tratamentos consistiram da combinação dos dois genótipos e seis doses do herbicida glyphosate (240; 480; 960; 1.920; 3.840 e 7.680 g e.a. ha-1), e uma testemunha sem aplicação de glyphosate, em delineamento experimental de blocos ao acaso, quatro repetições. Os parâmetros analisados foram porcentagem de controle e fitomassa seca das plantas. Os resultados foram submetidos à análise de variância e em seguida ajustados para modelo de curva de dose-resposta do tipo logística, sendo destes modelos calculados valores de controle correspondestes a 50, 80, 90 e 99%. As conclusões principais obtidas nesta pesquisa foram de que os genótipos de azevém diplóide e tetraplóide apresentam suscetibilidade diferencial ao herbicida glyphosate, sendo o genótipo tetraplóide mais tolerante ao herbicida. O grau diferencial de tolerância, medido pelo fator de tolerância (FT) diferencial entre os genótipos, expresso pelo valor médio dos quatro estádios fenológicos estudados, utilizando como base o controle de 50% das plantas pelo glyphosate foi de 1,6 vezes a dose de glyphosate no genótipo tetraplóide em relação ao genótipo diplóide. Os estádios fenológicos de desenvolvimento das plantas de ambos os genótipos estudados afetam o grau de tolerância ao glyphosate. De maneira geral, em estádios mais avançados de desenvolvimento fenológico dos dois genótipos a suscetibilidade do azevém é menor ao glyphosate, exceto para o estádio de préflorescimento, no qual a planta é mais suscetível que o estádio de quatro perfilhos, quando o parâmetro de análise é a dose necessária para controle de 50% das plantas. O parâmetro de análise de suscetibilidade fitomassa seca das plantas apresentou a mesma tendência diferencial entre os fenótipos diplóides e tetraplóides que o parâmetro porcentagem de controle visual. / Diploid and tetraploid genotypes of Italian ryegrass (Lolium multiflorum Lam.) are cultivated as forage crop for animal production, however, when cropping systems that involve no tillage is established after the forage cultivation it is common the dessecation with the herbicide glyphosate. However, it is important to know if there is differential susceptibility between the genotypes to the herbicide, in four phenological stages of development. Therefore, it was developed this research with the objective of evaluating the degree of tolerance of the diploid and tetraploid genotypes of Italian ryegrass to the herbicide glyphosate. For that, four experiments were installed being one for each of the Italian ryegrass phenological stages (two leaves, four tillers, pre-flowering, and grain formation). The treatments consisted of the combination of the two genotypes and six rates of glyphosate (240; 480; 960; 1.920; 3.840 and 7.680 g a.e. ha-1) and a check plot without glyphosate application, in randomized complete blocks design, four replications. The parameters that were analyzed were control percentage and dry biomass. Results were submitted to analysis of variance and subsequently adjusted to non linear model of logistic dose-response curves, and from these models control values were calculated at 50, 80, 90 and 99%. The main conclusions obtained in this research were that genotypes of Italian ryegrass presented differential susceptibility to the herbicide glyphosate. The differential degree of tolerance, measured by the tolerance factor (TF) between the biotypes, expressed by the mean value of the four development stages studied, using the 50% Italian ryegrass control, was 1.6 times more glyphosate rate for the tetraploid genotype compared to the diploid genotype. The phenological stages of development of both genotypes affected the tolerance degree to glyphosate. In general, the more is the advanced development stages of both biotypes, the lower is the susceptibility of Italian ryegrass to glyphosate, except for the stage of pre-flowering, in which the plant is less susceptible than the stage of four tillers, when the analyzed parameter is the rate necessary to control 50% of the plants. The parameter of analysis of susceptibility plant dry biomass presented the same tendency of differential control between the diploid and tetraploid genotypes than the visual control percentage.

O crescimento das atividades urbano-industriais tem provocado um significativo aumento nas emissões de poluentes aéreos para a atmosfera. A poluição do ar, um dos principais fatores determinantes da destruição do meio ambiente, causa estresse às plantas, já que apresenta compostos altamente reativos e danosos, como os metais pesados. O uso de plantas bioindicadoras de poluição aérea é uma prática comum para o monitoramento da qualidade do ar e para o diagnóstico dos efeitos da poluição aérea na vegetação. Entre as plantas bioindicadoras mais utilizadas, destaca-se a gramínea C3 de clima temperado Lolium multiflorum ssp. italicum Beck var. Lema, que acumula metais pesados, fluoretos e enxofre quando se desenvolve na presença desses elementos químicos no ar. Além disso, esta espécie acumula frutano, um carboidrato solúvel cuja produção pode ser alterada por um número variado de perturbações ambientais. O presente trabalho foi dividido em três experimentos com os objetivos de (1) verificar a partição de carboidratos solúveis em cinco partes distintas (lâmina em alongamento, lâmina expandida, porção superior do estolho, porção inferior do estolho e raízes) de plantas de L. multiflorum; (2) analisar as relações sazonais e diurnas entre concentrações de poluentes aéreos urbanos, principalmente de dióxidos de enxofre e de nitrogênio e de material particulado, entre outros fatores ambientais, concentrações foliares de metais e mudanças no acúmulo de frutanos em plantas de Lolium multiflorum Lam ssp. italicum expostas a diferentes condições de poluição do ar na cidade de São Paulo; (3) verificar se as variações sazonais e diurnas no conteúdo de frutanos são determinadas por acúmulos foliares de enxofre e metais pesados ou por variações nas concentrações de poluentes gasosos; e (4) verificar a eficiência bioacumuladora da variedade “Lema" de Lolium multiflorum para a região tropical. Para isso, as plantas foram expostas durante períodos consecutivos de 28 dias ou 2 meses, em cada estação do ano, em Congonhas (local com altos níveis de poluentes aéreos) e em casa de vegetação com ar filtrado (local controle). Após cada exposição, a frutose total foi quantificada pelo método de antrona e os frutanos analisados qualitativamente por HPAEC-PAD. As concentrações de enxofre foram determinadas por turbidimetria, enquanto que as concentrações de metais pesados (Al, Fe, Cu, Zn, Pb, Cd e Ba) foram medidas utilizando um espectrofotômetro de absorção atômica, após digestão ácida. Os resultados mostraram que o estolho contendo as bainhas foliares de L. multiflorum acumulou mais frutanos, enquanto que as raízes acumularam os menores níveis desse carboidrato. As variações sazonais no conteúdo dos carboidratos solúveis foram marcantes no presente trabalho e pareceram estar relacionadas com as condições climáticas, mas intensificadas pelas concentrações de poluentes aéreos, especialmente de metais pesados contidos no material particulado, e acumulados nas folhas de L. multiflorum expostas em Congonhas. Já as variações diurnas foram sutis, mas suficientes para refletir as variações de irradiância, temperatura e de alguns poluentes gasosos ao longo do dia. Tanto no experimento sazonal como no diurno, os frutanos de alto peso molecular pareceram ter sido hidrolisados sendo o acúmulo de frutanos de peso molecular intermediário preferencial nas plantas dos dois locais de estudo. As interferências no conteúdo e na composição de frutanos influenciaram a produção de biomassa e a partição desta entre as partes da plantas, o que ode ter alterado a capacidade acumuladora da espécie de elementos provenientes da poluição atmosférica. Assim, o uso da variedade Lema de L. multiflorum para fins de biomonitoramento da qualidade do ar em regiões tropicais deve ser cuidadoso. / The development of urban and industrial activities has caused a significant increase in the emissions of pollutants to the atmosphere. Air pollution, one of the most destructive factors of the environment, causes stress to plants, since certain compounds such as heavy metals are very reactive and dangerous to plant cells. The utilization of bioindicator plants is an usual practice to monitor the air quality and to diagnose the effects of air pollution on vegetation. The C3 temperate grass Lolium multiflorum ssp. italicum Beck var. Lema is usually employed as bioindicator of air pollution, due to its high capacity of accumulating heavy metals, fluorine and sulfur when growing in the presence of these chemical elements in the air. This species also accumulates fructan, a soluble carbohydrate, whose production can be altered by to many environmental perturbations. In the present work three experiments were performed aiming (1) to verify the partitioning of water total carbohydrates in five different parts of plants of L. multiflorum (upper stubble; under stubble; elongating leaf, expanded leaf and roots); (2) to analyze the seasonal and the diurnal relationships between the concentrations of urban air pollutants, mainly nitrogen and sulfur dioxides and particulate material, among other environmental factors, leaf concentrations of heavy metals and changes in fructan accumulation in plants of L. multiflorum exposed to different conditions of air pollution in São Paulo city; (3) to verify if the seasonal and diurnal variations in the content of fructans are determined by leaf accumulation of sulfur and heavy metals or by variations in the concentrations of gaseous pollutants or other environmental factors; and (4) to verify the bioindicator efficiency of the variety “Lema" of Lolium multiflorum in tropical areas. Plants were consecutively exposed during 28 days or two months in each season at Congonhas (site reached by high levels of air pollutants) and at a glasshouse with filtered air (reference situation). After each exposure, fructan composition as well as leaf concentrations of total fructose and of sulfur and metals (Al, Fe, Cu, Zn, Pb, Cd and Ba) were measured. Total fructose was quantified by the anthrone method and fructan composition was analyzed by HPAEC-PAD. Sulfur concentrations were determined by the turbidimetric method, whereas metal concentrations were measured using an atomic absorption spectrophotometer, after acidic digestion. Results showed that the stubble containing the leaf sheaths of L. multiflorum accumulated the highest levels of fructans, while the roots accumulated the lowest levels. Seasonal variations in the contents of soluble carbohydrates were evident in the present work and seemed to be related with the climatic conditions but intensified by the air pollutants, especially heavy metals contained in the particulate matter, and accumulated in the leaves of L. multiflorum exposed in Congonhas. However the diurnal variations were tenuous, but reflected the variations of irradiance, temperature and of some gaseous pollutants during daytime. In the both seasonal and diurnal experiments, the fructans of high molecular weight seemed to be hydrolyzed and the accumulation of fructans of intermediary molecular weigh was significant in the plants from both sites. The changes in the content and in the composition of fructans altered the biomass production and partition among roots and shoots, which might have been disturbed their characteristic of accumulating chemical elements from air pollution in the leaves. So, the use of the variety Lema of L. multiflorum for biomonitoring purposes in tropical areas should be carefully evaluated.

Genótipos de azevém (Lolium multiflorum Lam.) diplóides e tetraplóides são cultivados como forrageira na produção animal, no entanto, quando sistemas de produção que envolve o plantio direto são estabelecidos após o cultivo da forrageira é comum a dessecação com o herbicida glyphosate. Portanto, é importante conhecer se existe suscetibilidade diferencial a este herbicida entre os genótipos. Desta forma, foi desenvolvida a presente pesquisa como o objetivo de avaliar o grau de tolerância dos genótipos diplóides e tetraplóides de azevém ao herbicida glyphosate em quatro estádios fenológicos de desenvolvimento. Para isso, foram instalados quatro experimentos, sendo um para cada estádio fenológico do azevém (duas folhas, quatro perfilhos, pré-florescimento e formação de grãos). Os tratamentos consistiram da combinação dos dois genótipos e seis doses do herbicida glyphosate (240; 480; 960; 1.920; 3.840 e 7.680 g e.a. ha-1), e uma testemunha sem aplicação de glyphosate, em delineamento experimental de blocos ao acaso, quatro repetições. Os parâmetros analisados foram porcentagem de controle e fitomassa seca das plantas. Os resultados foram submetidos à análise de variância e em seguida ajustados para modelo de curva de dose-resposta do tipo logística, sendo destes modelos calculados valores de controle correspondestes a 50, 80, 90 e 99%. As conclusões principais obtidas nesta pesquisa foram de que os genótipos de azevém diplóide e tetraplóide apresentam suscetibilidade diferencial ao herbicida glyphosate, sendo o genótipo tetraplóide mais tolerante ao herbicida. O grau diferencial de tolerância, medido pelo fator de tolerância (FT) diferencial entre os genótipos, expresso pelo valor médio dos quatro estádios fenológicos estudados, utilizando como base o controle de 50% das plantas pelo glyphosate foi de 1,6 vezes a dose de glyphosate no genótipo tetraplóide em relação ao genótipo diplóide. Os estádios fenológicos de desenvolvimento das plantas de ambos os genótipos estudados afetam o grau de tolerância ao glyphosate. De maneira geral, em estádios mais avançados de desenvolvimento fenológico dos dois genótipos a suscetibilidade do azevém é menor ao glyphosate, exceto para o estádio de préflorescimento, no qual a planta é mais suscetível que o estádio de quatro perfilhos, quando o parâmetro de análise é a dose necessária para controle de 50% das plantas. O parâmetro de análise de suscetibilidade fitomassa seca das plantas apresentou a mesma tendência diferencial entre os fenótipos diplóides e tetraplóides que o parâmetro porcentagem de controle visual. / Diploid and tetraploid genotypes of Italian ryegrass (Lolium multiflorum Lam.) are cultivated as forage crop for animal production, however, when cropping systems that involve no tillage is established after the forage cultivation it is common the dessecation with the herbicide glyphosate. However, it is important to know if there is differential susceptibility between the genotypes to the herbicide, in four phenological stages of development. Therefore, it was developed this research with the objective of evaluating the degree of tolerance of the diploid and tetraploid genotypes of Italian ryegrass to the herbicide glyphosate. For that, four experiments were installed being one for each of the Italian ryegrass phenological stages (two leaves, four tillers, pre-flowering, and grain formation). The treatments consisted of the combination of the two genotypes and six rates of glyphosate (240; 480; 960; 1.920; 3.840 and 7.680 g a.e. ha-1) and a check plot without glyphosate application, in randomized complete blocks design, four replications. The parameters that were analyzed were control percentage and dry biomass. Results were submitted to analysis of variance and subsequently adjusted to non linear model of logistic dose-response curves, and from these models control values were calculated at 50, 80, 90 and 99%. The main conclusions obtained in this research were that genotypes of Italian ryegrass presented differential susceptibility to the herbicide glyphosate. The differential degree of tolerance, measured by the tolerance factor (TF) between the biotypes, expressed by the mean value of the four development stages studied, using the 50% Italian ryegrass control, was 1.6 times more glyphosate rate for the tetraploid genotype compared to the diploid genotype. The phenological stages of development of both genotypes affected the tolerance degree to glyphosate. In general, the more is the advanced development stages of both biotypes, the lower is the susceptibility of Italian ryegrass to glyphosate, except for the stage of pre-flowering, in which the plant is less susceptible than the stage of four tillers, when the analyzed parameter is the rate necessary to control 50% of the plants. The parameter of analysis of susceptibility plant dry biomass presented the same tendency of differential control between the diploid and tetraploid genotypes than the visual control percentage.