Higher stocking density under seasonal-rotation grazing is expected to increase plant abundance because expanded animal distribution and reduced selective grazing on forage species will reduce the spatial variation and competitive advantage of non-forage species compared to yearlong grazing. Rangeland scientists struggle with how long rangeland experiments must continue in order to detect grazing treatment effects, particularly in semi-arid ecosystems with slow responses and high spatio-temporal variability. My first study investigated grazing system effects on plant abundance (cover and density) over the short-term (12yrs) or long-term (22 or 34yrs) after accounting for covariates (mesquite and precipitation gradients). My second study assessed how grazing systems affected spatial variation in grass abundance over 22 or 34 years after accounting for precipitation gradient. The first study was a course resolution approach, looking at grazing impacts on plant abundance. The second study was a finer resolution assessment of the underlying assumption that rotational grazing systems reduce selective grazing. Using split-plot analysis of variance, with year as the split, changes in mean plant abundance and variance in grass abundance were compared between two grazing systems (yearlong vs. seasonal rotation), after accounting for covariate(s). Variance of grass abundance among sample locations within an experimental pasture was the measure of spatial variability and was expected to increase with selective grazing. Grazing systems did not influence plant abundance or spatial variation of grasses. The absence of grazing effect may be due to overriding influences of grazing intensity, large pasture sizes, temporal variation in precipitation, and few replicates. Specific to spatial variation, the absence of grazing system effect may be due to discrepancies in transect representation across ecological sites and spatial variation of grasses occurring at scales different than the 30-m transect size. Like earlier research, my studies failed to substantiate the fundamental premise for implementing rotational grazing systems. This exposes challenges that confront rangeland scientists when implementing grazing studies in spatio-temporally heterogeneous ecosystems. I recommend that research shift from comparing rigid schedule-driven grazing systems to more adaptive management approach where there are comparisons between different levels or different designs of flexible systems.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Animal performance was evaluated in bovine cattle livestock systems using tropical pasture based diets through a meta-analysis process, which included a database of scientific articles, dissertations and theses. Data was tabulated and categorized identifying the scientific work, the experiment, animals, animal performance, pasture, supplements and grazing. The conversion efficiency of supplement use was evaluated in the studies containing control (no supplementation), and calculated as the change in average daily gain of individual weight (CoEfADGan) or area (ha, CoEfADGha) per kg of supplement offered. Data distribution from a general database was assessed. To analyze the factors affecting animal performance variables the general database was sub-divided into two sub-bases, rainy and dry season. To evaluate the factors affecting animal performance variables data was weighted by n treatments and analysis of variance and covariance was used. The average daily gain of body weight per animal (ADGan, 0.870 vs. 0.611 kg / animal / day) and per hectare (ADGha vs. 5.76. 4.59 kg / ha / day), as well as daily animal load per hectare (DALha, 1483 vs. 1211 kg / ha / day) were higher (P <0.05) during the rainy season than during the dry season. During the rainy season, ADGan was lower in the genus Brachiaria and superior in the Panicum (P <0.05), but the DALha and ADGha were higher (P <0.05) in Cynodon. In dry season, ADGan was lower (P <0.05) in the Brachiaria than in Cynodon and Panicum, and ADGha was higher (P <0.05) in Panicum. In both dry and rainy seasons, ADGan, ADGha and DALha increased linearly (P <0.01) with increasing levels of supplementation, which ranged from zero to 1.6% of live weight (LW). The ADGan increased linearly (P <0.05) and DALha decreased linearly (P <0.05), while ADGha was quadratically (P <0.05) related to the increased availability of herbage. Animal performance was linear and positively correlated with leaf: stem proportion from pastures (P <0.05), both in the rainy and dry seasons. During the rainy season the best responses in ADGan and ADGha were obtained from the use of protein supplement compared with supplemental energy or proteinated salt (P <0.05), while the use of energy supplement enabled a higher DALha (P <0,05). During the dry season, ADGha was not affected by supplement type, but the best responses were obtained for ADGan with energy supplement (P <0.05) while protein supplementation, either through proteinated salt or protein supplement, was associated with a higher DALha (P <0.05). During the rainy season animal performance was quadratically (P <0.05) related to live weight (LW) of animals, and ADGan maximized in animals weighing about 380 kg and ADGha maximized in animals weighting about 400 kg. In the dry season, ADGan and ADGha were linearly and positively related animal LW (P <0.05). Pasture neutral detergent fiber (NDF) content ranged from 46 to 82% in dry matter (DM) and was quadratically related to ADGan (P <0.05) being that maximum gain was obtained with NDF content around 66%. Crude protein (CP) of pasture ranged from 1.9 to 21.6% in DM and was linear and positively correlated with ADGan (P <0.05). The ADGan was linear and negatively associated with the value obtained by the ratio between the content (% DM) of total digestible nutrients (TDN) and CP of pasture (P <0.05). Nitrogen fertilizer was linearly (P <0.05) associated with increased DALha, both in the rainy and dry seasons. The CoEfADGan and the CoEfADGha decreased lineally (P <0.05) with increasing animal LW during the rainy season, becoming negative from around 360 kg. In the rainy season as well as in the dry season, both efficiencies were also negatively (P <0.05) related to the CP content of pasture, being also negative from 10% CP, and positive and quadratically (P <0.05) related to the leaf: stem ratio of the pastures. Highest efficiencies were observed with leaf: stem ratio 3.7:1. During the dry season EfCoGMD and EfCoGAD decreased (P < 0.05) with the increasing the value obtained by the TDN: CP supplement ratio, becoming negative from 4,3:1. In conclusion, the meta-analysis enabled the identification of beef cattle performance in tropical pastures showing that it is influenced in an integrated manner by factors associated with several features such as climate, pasture, animal, handling and supplementation. However, the degree to which each factor influences animal performance could be more accuratelly assessed if publications were standardized and contained essential information regarding more detailed methodological descriptions and basic statistical information. / Foi avaliado o desempenho animal em sistemas pecuários bovinos utilizando dietas à base de pastagens tropicais através de processo meta-analítico, que incluiu na base de dados artigos científicos, dissertações e teses. Foram tabulados e categorizados dados de identificação do trabalho científico, do experimento, dos animais, do desempenho animal, da pastagem, dos suplementos e do método de pastejo. A eficiência de conversão do uso do suplemento foi avaliada nos estudos contendo tratamento testemunha (sem suplementação), e calculada como a variação do ganho médio diário de peso individual (EfCoGMD) ou por área (EfCoGAD) por kg de suplemento ofertado. Foi avaliada a distribuição dos dados da base geral e, para analisar os fatores que afetam as variáveis de desempenho animal, a base geral foi subdividida nas sub-bases águas e secas. Para avaliação dos fatores que afetam as variáveis de desempenho animal os dados foram ponderados pelo n dos tratamentos e utilizado análise de variância e covariância. O ganho médio de peso individual (GMD, 0,870 vs. 0,611 kg/animal/dia) e o ganho por área (GAD, 5,76 vs. 4,59 kg/ha/dia), assim como a carga animal (CAD, 1483 vs. 1211 kg/ha/dia) foram maiores (P<0,05) no período das águas que no período das secas. No período das águas o GMD foi menor na Brachiaria e superior no gênero Panicum (P<0,05), mas a CAD e GAD foram superiores (P<0,05) no Cynodon. Nas secas, o GMD foi menor (P<0,05) na Brachiaria que no Cynodon ou Panicum, e o GAD foi maior (P<0,05) no Panicum. Independentemente do período do ano, o GMD, o GAD e a CAD aumentaram linearmente (P<0,01) com o aumento do nível de suplementação, o qual variou de zero a 1,6% do peso vivo (PV). O GMD aumentou linearmente (P<0,05) e a CAD diminuiu linearmente (P<0,05), enquanto o GAD foi quadraticamente (P<0,05) relacionado com o aumento da oferta de forragem total. O desempenho animal foi linear e positivamente relacionado com proporção folha:colmo das pastagens (P<0,05), tanto no período das águas quanto no período das secas. No período das águas as melhores respostas em GMD e GAD foram obtidas com o uso de suplemento protéico quando comparado com suplemento energético ou sal proteinado (P<0,05), enquanto o uso de suplemento energético possibilitou maior CAD (P<0,05). No período das secas o GAD não foi afetado pelo tipo de suplemento, mas as melhores respostas para GMD foram obtidas com suplemento energético (P<0,05) enquanto que a suplementação proteica, seja através de sal proteinado ou suplemento proteico, foi relacionada com maior CAD (P<0,05). No período das águas o desempenho animal foi quadraticamente (P<0,05) relacionado com o peso vivo (PV) dos animais, sendo o GMD maximizado com animais pesando em torno de 380 kg e o GAD maximizado com animais de 400 kg. Nas secas o GMD e o GAD foram linear e positivamente relacionados com o PV dos animais (P<0,05). O teor de fibra em detergente neutro (FDN) da pastagem variou de 46 a 82 % na matéria seca (MS) e foi quadraticamente relacionado com o GMD (P<0,05) sendo que o máximo GMD foi obtido com teor de FDN em torno de 66%. O teor de proteína bruta (PB) da pastagem variou de 1,9 a 21,6 % na MS e foi linear e positivamente relacionado com GMD (P<0,05). O GMD foi linear e negativamente associado com o valor obtido pela razão (relação) entre o teor (% na MS) de nutrientes digestíveis totais (NDT) e PB da pastagem (P<0,05). A adubação nitrogenada foi linearmente (P<0,05) relacionada com aumento da CAD tanto no período das águas como no período das secas. A EfCoGMD e a EfCoGAD diminuíram linearmente (P<0,05) com o aumento do PV dos animais no período das águas, passando a serem negativas a partir de em torno de 360 kg. Tanto nas águas como nas secas, ambas as eficiências também foram negativamente (P<0,05) relacionadas com o teor de PB da pastagem, passando a serem negativas a partir de 10% de PB, e positiva e quadraticamente (P<0,05) relacionadas com a relação folha:colmo das pastagens. Máximas eficiências foram observadas com relação folha:colmo em torno de 3,7:1. No período das secas a EfCoGMD e a EfCoGAD diminuíram linearmente (P<0,05) com o aumento do valor observado na relação NDT:PB do suplemento, passando a serem negativas a partir de 4,3:1. Em conclusão, a meta-análise possibilitou identificar que o desempenho de bovinos de corte em pastagens tropicais é influenciado de forma integrada por fatores associados às características do clima, das pastagens, dos animais, do manejo e da suplementação. Contudo, o grau de influência de cada fator seria melhor definido se as publicações nessa área de conhecimento fossem padronizadas e contivessem informações mínimas tanto em relação à descrição da metodologia como em relação à presença de informações estatísticas básicas.