O presente estudo foi realizado na Fazenda Areão, unidade da Escola Superior de Agricultura "Luiz de Queiroz", em Piracicaba - SP (22°42'30" Latitude Sul ; 47°38'30" Longitude Oeste) e teve por objetivo avaliar a dinâmica do sistema radicular do capim Tanzânia (Panicum maximum, Jacq.) quando submetido a três intensidades de manejo, correspondidas pelos resíduos pós pastejo de 1000, 2500 e 4000 kg de matéria seca verde por hectare (MSV.ha-1). A área experimental foi irrigada por pivô central, sendo pastejada por novilhos da raça nelore e manejada de forma rotacionada com ciclo de 36 dias, sendo 3 dias de ocupação e 33 dias de descanso. Foram realizadas análises quantitativas da densidade radicular através de sua massa, superfície e comprimento. As análises qualitativas se relacionaram à separação entre as raízes vivas e mortas, de maneira que pudéssemos verificar mais efetivamente o real potencial de absorção para os diferentes tratamentos, assim como o estresse imposto pelo pastejo em cada situação. O desenvolvimento do sistema radicular do capim Tanzânia mostrou comportamento inversamente proporcional às intensidades de pastejo impostas, relacionando pastejos intensos a uma retomada mais lenta na atividade de raízes. Nas condições de irrigação e adubação impostas, o resíduo 1000 kg MSV.ha-1 mostrou menor atividade radicular em relação aos tratamentos de 2500 e 4000 kg MSV.ha-1 de resíduo pós pastejo. Até 12 dias após o pastejo não houve crescimento de novas raízes, indicando que a planta fica sob condição de estresse até pelo menos 2 semanas após ser colhida, independente do manejo imposto. No entanto, para o pastejo mais intenso (resíduo 1000 kg MSV.ha-1), o desenvolvimento radicular ficou comprometido até 21 dias após o pastejo. Para as profundidades analisadas (0-20 e 0-40 cm), na média dos 3 tratamentos, em torno de 76% das raízes vivas e 82% das raízes mortas foram encontradas nos primeiros 20 cm de solo. Estes resultados indicam a possibilidade de novas pesquisas utilizarem apenas esta profundidade para amostragens, uma vez que os valores são relevantes e a dificuldade de coleta em maiores profundidades é fator limitante para trabalhos com raízes. A separação de raízes vivas pareceu ser uma boa metodologia para melhorar a acuracidade na análise dos resultados, já que amostragens do sistema radicular de plantas forrageiras apresentam variabilidade alta. Estudos que relacionem a decomposição e renovação do sistema radicular de forrageiras tropicais com fatores bióticos e abióticos deveriam ser intensificados, buscando melhor entendimento dos fluxos de tecidos da planta inteira em diferentes estações do ano. Metodologias de amostragens associadas a conceitos de interpretação estatística em processos biológicos deveriam ser revistos, uma vez que os estudos existentes sobre sistema radicular de plantas forrageiras tropicais poucas vezes são conclusivos quando interpretados pela estatística ortodoxa. / The present study was carried out at Escola Superior de Agricultura "Luiz de Queiroz", Piracicaba - SP, SP (22°42'30" S; 47°38'30" W) to evaluate the root system dynamics of irrigated Tanzania grass (Panicum maximum cv. Tanzania) pasture managed under three grazing intensities, determinated by adjusting the post grazing residues to 1000, 2500 and 4000 kg of green dry matter per hectare (GDM.ha-1). The experimental area was irrigated by a central pivot system. Nelore steers were used in a 36 days rotational grazing system, represented by an occupation period of 3 days and resting period of 33 days. Quantitative and qualitative evaluations were both performed. In the former case the variables measured were root mass, surface and length. In the latter case, collected samples were separated into live and dead roots, providing a reliable way to assess grazing intensity effects upon root system dynamics. Root system development responded inversely to grazing intensity. Hard grazing (1000 kg GDM.ha-1) was associated to slower root system regrowth compared to moderate (2500 kg GDM.ha-1) and lenient (4000 kg GDM.ha-1) grazing. In the 1000 kg GDM.ha-1 post-grazing residue no root development was observed up to three weeks, whereas for the other two treatments root regrowth was reactivated after 12 days. The three treatments averaged, 76% and 82% of live and dead roots, respectively, were found in the 0-20 cm soil layer. This information might suggest further research on root system dynamics could focus on sampling in the range of 0-20 cm depth. This seems reasonable, since the results obtained in this soil layer were representative and also because root samplings in depth greater than 20 cm (e.g. 20-40 cm) are not so easy to be performed as compared to samplings taken from 0-20 cm. Sample separation into live and dead roots provided a reliable and more accurate method to analyze the data. In part, this can be attributed to lower variability among samples. It is important to stress that additional work has to be undertaken to study decomposition and turnover of the root system of tropical pastures, as well as the relationship between these variables with biotic and abiotic factors. This effort should provide a better understanding on tissue turnover of the plant as a whole (shoots and root system). Finally, improved methods on root samplings should be developed. Also, new concepts on statistical interpretation for biological processes should be emphasized, due to difficulties in interpreting data on root system by conventional statistical methods.

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O planejamento e o manejo do pastejo em sistemas de produção animal em pastagens utiliza como informação essencial valores de massa, produção e taxas de acúmulo de forragem. Esses valores podem ser determinados de maneira rápida e ágil através de uma simples régua graduada em centímetros ou equipamentos mais sofisticados como o prato ascendente, embora resultados experimentais utilizando esses procedimentos sejam bastante escassos para plantas de ambiente tropical. O presente trabalho avaliou esses dois métodos de estimativa de massa de forragem em área de pastagem formada pelos cultivares Florakirk, Tifton 85 e Coastcross de Cynodon spp.. Pastos formados por cada um dos cultivares foram mantidos a 5, 10, 15 e 20 cm de altura através de lotação contínua e taxa de lotação variável, seguindo um delineamento experimental de blocos completos ao acaso com 4 repetições. Durante 12 meses foram geradas curvas de calibração entre altura do pasto, medida por cada um dos dois métodos, e a massa de forragem correspondente. Como resultado foram geradas quatro curvas de calibração do tipo y = a + bx para cada cultivar (y = massa de forragem em Kg MS/ha e x = altura do pasto), uma para cada época do ano (primavera, verão, outono e inverno). De forma geral os valores de intercepto (a) foram os mais influenciados por cultivar e época do ano, com uma variação apenas modesta nos valores de coeficiente angular (b). Essa variação esteve relacionada com modificações da estrutura do dossel dos pastos, particularmente a proporção de material morto. Os métodos não diferiram em precisão, porém o prato ascendente mostrou-se um método mais rápido e ágil. Concluiu-se que apesar das similaridades entre os cultivares estudados, a calibração dos métodos deve ser específica para cada cultivar e realizada frequentemente para que as equações geradas sejam precisas e robustas, de forma a permitir a obtenção de valores confiáveis de estimativas de massa de forragem. / Grazing management and planning of animal production systems from pastures are essentially based on estimates of herbage mass, production and accumulation rates. These values can be rapidly determined with a sward stick and rising plate meter, although experimental results generated this way are scarce for tropical forage plants. The present experiment evaluated both methods for estimating herbage mass in pasture areas formed by Florakirk, Tifton 85 and Coastcross Cynodon spp. cultivars. Paddocks of each of the cultivars were maintained at 5, 10, 15 and 20 cm sward surface height through continuous stocking and variable stocking rate, according to a complete randomised block design with four replications. Over twelve months, calibration equations of stick and plate sward height versus the correspondent herbage mass were generated. As a result, four y = a + bx type calibration equations (y = herbage mass - kg DM/ha and x = sward height) were obtained, one for each season of the year (spring, summer, autumn and winter). Generally, most of the variation among cultivars and seasons occurred in intercept values (a) and less with slope values (b). This variation was related to modifications in sward structure, particularly in dead material content. The methods did not differ in precision, although the rising plate meter proved to be faster and more agile. Despite the similarities among cultivars, calibration of both methods must be specific for each cultivar and carried out regularly in order to result in precise and robust equations capable of generating reliable estimates of herbage mass.

No description available.

Bibliography: leaves 213-229. / xxiii, 229, 26 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 Plant Science, 2000

Bibliography: leaves 115-130.

Bibliography: leaves 209-254. Experiments detailed in this thesis, evaluate the impact of various herbicides and herbicide mixtures on the growth, nodulation and nitrogen fixation of annual pasture legumes, the efficacy of the herbicides for weed control, and the potential role of mechanical defoliation to replace herbicides, leading to lower cost and more sustainable farming systems.

Thesis (Ph. D.)--University of Wisconsin--Madison, 2001. / Includes bibliographical references (p. 210-218). Also available on the Internet.

Our objective was to identify the major barriers to natural regeneration of tree species in abandoned Panamanian pastures as a first step in formulating management strategies to facilitate forest recovery. We tested whether fire, seed dispersal, and the presence of an introduced grass, Saccharum spontaneum L., were barriers to forest regeneration. We examined growth, survival, and density of both experimentally-introduced and naturally-regenerating tree seedlings using a series of multifactorial experiments. / We found that seed dispersal limits forest regeneration. Large-seeded species have the highest performance in the Saccharum spontaneum, but were found in the lowest abundance in natural conditions. Small-seeded species were most frequently observed, but they have the lowest survival. We conclude that dispersal limitations preclude entry of the larger-seeded species. Distance from the forest limits dispersal of many small-seeded species. Fire is a major barrier to natural regeneration because it lowers species diversity.

In traditional forage breeding programmes, breeders have spent decades improving the agronomic characteristics of grasses, such as herbage yield, persistence and resistance to diseases, without considering the nutrient requirements of the grazing animal. In an attempt to improve the nutritive value of Italian ryegrass, which is widely utilised for intensive dairy, lamb and beef production in South Africa, Enhancer ryegrass was developed from predominantly Italian types of Lolium multiflorum, with a minor Westerwolds component, by selecting for a higher concentration of total nonstructural carbohydrate (TNC) and lower moisture content than that currently available in commercial cultivars. The nutritional value of Enhancer was compared with Midmar ryegrass in a controlled environment study and in a grazing trial with weaned lambs; and with Dargle ryegrass in a grazing trial with Holstein dairy cows. Neutral detergent fibre, acid detergent fibre, lignin, nitrogenous compounds, mineral content and in vitro digestibility were also investigated as parameters of nutritive value. The anatomical features of Enhancer and Midmar were studied to determine possible structural differences. Weaned lambs grazed Enhancer and Midmar in an eight-paddock rotational grazing system, with 3.5 days spent in each paddock, allowing a 24.5 day regrowth period for the pastures. Holstein dairy cows grazed Enhancer and Dargle which were established on 16 and 19 hectare pastures, respectively. The n-alkane technique was used to estimate dry matter intake (DMI) in both grazing trials. Results from the controlled environment study suggest that the differences in the dry matter and TNC concentration of Enhancer are not positively linked to anti-quality factors associated with forage species, but can be attributed to genetic differences between the two grasses. Despite the significantly higher (P < 0.01) DMI of weaned lambs grazing Midmar compared with Enhancer, the lambs on Enhancer outperformed those on Midmar in terms of liveweight gain and carcass quality. The superior animal performance on Enhancer is likely due to an improvement in the readily digestible energy to protein ratio as a result of its significantly higher (P < 0.001) concentration of TNC compared with Midmar. Milk yield for cows grazing Enhancer in period 1 of the cross-over study was significantly higher (P < 0.05) than for cows grazing Dargle, despite the significantly lower (P < 0.05) DMI of animals on Enhancer. The higher TNC concentration relative to the true protein content of Enhancer would suggest that the protein metabolism in the rumen can be enhanced. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 2003.