Hydrolysis of dietary lipids to free fatty acids (FFA) is a prerequisite for ruminal biohydrogenation, a bacterially mediated process that extensively saturates unsaturated FFAs thus limiting the absorption and ultimate assimilation of these healthy nutrients into ruminant produced foods. Three experiments were conducted to learn how to better enrich, isolate and study lipolytic bacteria from the rumen while providing further characterization of four prominent lipase-producing bacteria that are known to be major contributors of lipolysis in the rumen. In experiment one the effects of various physical treatments on ruminal lipase activity were investigated by comparing incubation positions, glass bead levels, transfer techniques and combinations of headspace gasses. Based on results from this experiment an incubation system was established as a standard for subsequent studies for culturing and transferring mixed and pure cultures of ruminal bacteria. In experiment two the effect of glycerol on lipolysis by Anaerovibrio lipolyticus 5S, Butyrivibrio fibrisolvens 49, Propionibacterium avidum, and Propionibacterium acnes was examined. Two levels of glycerol were examined on lipase activity and results showed that glycerol inhibited rates of FFA accumulation at both levels. In addition the mechanism behind glycerol inhibition was also examined by culturing and assaying activity of the four bacteria to determine if glycerol inhibition is a result of equilibrium displacement or lipase gene expression inhibition. Results indicated that higher and constitutively expressed lipase activity of A. lipolyticus 5S and P. avidum probably contribute more to lipolysis in ruminants than P. acnes and B. fibrisolvens 49. In the case of P. acnes and B. fibrisolvens 49 cells, results suggest that lipase gene expression is down-regulated in these bacteria. Experiment three was conducted to further characterize the lipase activity of the four different bacteria by growing them with four different energy substrates and measuring enzyme activity at early logarithmic and stationary phase. Results from this study showed that diets containing a high content of oleic acid and linolenic acid promoted higher rates of lipolysis in the rumen. In accordance with findings in experiment two these results support that P. avidum may contribute to a higher amount of lipolysis than previously considered.

The main objective of this work was to investigate the expression of xylanase and cellulose genes from <I>Ruminococcus flavefaciens</I> when introduced into related Gram-positive bacteria including the rumen bacterium <I>Streptococcus bovis</I>. In addition the discovery, and isolation of the β(1,3-1,4)-glucanase gene of <I>S. bovis</I> in the course of this work may provide new possibilities to express foreign genes in <I>S. bovis</I> or in other Gram-positive bacteria. The main findings of this work are summarised below: 1. Genes encoding polysaccharidase activity from the strictly anaerobic rumen bacterium <I>R. flavefaciens</I> can be transferred by electroporation into the ruminal bacterium <I>S. bovis</I> and into other Gram-positive bacteria from different habitats, including <I>Lactococcus lactis, Enterococcus faecalis </I>and <I>Streptococcus sanguis</I>. 2. <I>XynD </I>and <I>endA</I> genes of <I>R. flavefaciens </I>can be expressed from their own promoters in these species. Among the bacteria used as hosts for gene expression, <I>S. bovis </I>gave the highest yields of active enzyme. The expression levels of both gene products were found to be higher in <I>S. bovis</I> than in <I>E. coli</I>. 3. The <I>R. flavefaciens </I>enzymes were mainly secreted in the culture medium of <I>S. bovis</I>; however in <I>E. coli</I> they were mainly cell-associated. 4. The full-length enzyme of <I>xynD</I> was detected in several Gram-positive bacteria, suggesting the effect of proteases may be less than in <I>E. coli</I>. 5. The general rearrangement of the introduced plasmid and genes were not found in Gram-positive bacteria and the genes seem to be stable in these organisms. However rearrangement of <I>xynD</I> was observed in some transformants of <I>S. bovis </I>JB1, although non-rearranged transformants were also obtained. 6. Expression of <I>endA</I> and <I>xynD</I> activity was affected by energy sources supplied to <I>S. bovis</I> cultures, reflecting the accumulation of lactic acid.

No description available.

This report describes a new group of anaerobic bacteria that degrade oxalic acid. The new genus and species, Oxalobacter formigenes, are inhabitants of the rumen and also of the large bowel of man and other animals where their actions in destruction of oxalic acid may be of considerable importance to the host. Isolates from the rumen of a sheep, the cecum of a pig, and from human feces were all similar Gram-negative, obligately anaerobic rods, but differences between isolates in cellular fatty acid composition and in serologic reaction were noted. Measurements made with type strain OxB indicated that 1 mol of protons was consumed per mol of oxalate degraded to produce approximately 1 mol of CO2 and 0.9 mol of formate. Substances that replaced oxalate as a growth substrate were not found.

Consumer awareness of the link between dietary fats and health outcomes has led to increased demand for food products enriched with bioactive fatty acids (FA). Ruminant-derived fats, such as dairy fats, contribute significantly to the American diet and contain many unique beneficial FA, such as short- and medium-chain FA, n-3 FA, conjugated linoleic acids (CLA), vaccenic acid (VA), as well as odd-and branched-chain FA (OBCFA). Increasing these FA in dairy products by altering farm management practices, such as breed, lactation stage, and nutrition, may improve human health without a change to the diet. The overarching goal of this dissertation was to evaluate on-farm strategies to increase the content of bioactive FA in bovine milk. The first objective was to enrich milk fat with bioactive FA via supplementation with echium oil, a terrestrial oil rich in n-3 FA. Treatments were 1.5% and 3.0% dry matter as lipid encapsulated echium oil (EEO) which were compared to a control (no EEO). Milk fat contents of n-3 FA increased with EEO supplementation but the transfer of n-3 FA from EEO into milk fat was rather low (< 5%). In a subsequent trial, ruminal protection of EEO and post-ruminal release of EEO-derived FA was examined. EEO-derived FA were preferentially incorporated into plasma lipid fractions unavailable to the mammary gland. Moreover, fecal excretion of EEO-derived FA ranged from 7-14% of intake, and VA and CLA, the biohydrogenation and metabolism products of n-3 FA, increased in milk and feces with EEO supplementation. Therefore, lipid-encapsulation provided inadequate digestibility and low transfer efficiency of n-3 FA into milk. The second objective was to compare the bacterial community structure and unique bioactive FA in bacterial membranes and milk fat between Holstein (HO), Jersey (JE), and HO x JE crossbreeds (CB) across a lactation. Lactation stage had a prominent effect on rumen bacterial taxa, with Firmicutes being most abundant during early lactation. The FA composition of bacterial cells was affected by both lactation stage and genetics, and OBCFA in bacterial cells were positively correlated with several bacteria of the Firmicutes phylum. HO and CB exhibited greater contents of various bioactive FA in milk than JE. The highest content of all bioactive FA occurred at early lactation, while OBCFA were highest at late lactation. The third objective was to determine the effects of grazing a monoculture vs. a diverse pasture on rumen bacterial and protozoal taxa, their membrane FA composition, and milk FA. Microbial communities shifted in response to grazing regime accompanied with changes in their membrane FA profiles. Rumen microbiota from cows grazing a diverse pasture had higher contents of n-3 FA and VA, but lower contents of OBCFA. Microbial membrane FA correlated with microbial taxa, the contents of ALA and n-3 FA were positively correlated with the bacterial genus Butyrivibrio and the protozoal genus Eudioplodinium. Milk contents of CLA and n-3 FA increased when cows grazed a diverse pasture, while grazing a monoculture led to greater milk contents of OBCFA. In conclusion, grazing cows on a diverse pasture, when compared to genetic effects and lipid supplementation, was the most efficacious strategy to increase the content of bioactive FA in milk.

The effect of increased neutral detergent fibre digestibility (NDFD) of sugarcane silage on intake, rumen kinetics and metabolism, microbial population, performance, and body fatness of beef cattle was evaluated through two trials. In the metabolism trial, eight ruminal-cannulated Nellore steers were used in a replicated 4×4 Latin square design. In the performance trial, forty-eight Nellore young bulls were used in a complete randomized block design with a 2×2 factorial arrangement of treatments. Two sugarcane genotypes with high or low NDFD harvested for silage in two maturity stages were used (2 genotypes and 2 maturity stages). Diets were formulated with 40% of sugarcane on dry matter (DM) basis. Intake of DM was likely limited by metabolic feedback. The DM intake (DMI) of cannulated steers was reduced when late-maturity sugarcane was fed, but only with low-NDFD genotype. Turnover and passage rate of NDF was accelerated when high-NDFD genotype was fed. Rumen DM, NDF and iNDF pools were greater when diets with the low-NDFD genotype were fed. There was a tendency for greater relative populations of F. succinogenes and R. albus when animals were fed with diets containing low-NDFD genotype. Performance and carcass characteristics were not affected by treatments. Excess of NFC and fermentation end products of sugarcane silage can limit intake by metabolic pathway. High-NDFD genotype can accelerated NDF turnover and passage rate in the rumen. Late-maturity ensiled sugarcane can reduce DMI of beef steers fed with low-NDFD genotype. The effect of live yeast (LY - Saccharomyces cerevisiae CNCM I-1077) on in situ fiber digestibility, rumen fermentation, and rumen cellulolytic bacteria population of grazing Nellore cattle was evaluated throughout the year. Eight rumen cannulated steers were used in a 2x4 factorial arrangement of treatments: with or without LY, and the four seasons of the year. Yeast was given daily to provide 8x109 CFU per animal. Animals were kept on a rotational grazing system with mineral supplementation. Every 45 d, in situ rumen NDF degradability of 5 reference forages was determined after 24 and 48h incubation. Rumen fluid was collected to measure yeast count and fermentation parameters, and rumen contents collected for quantitative PCR quantification of cellulolytic bacteria. Although pasture composition varied greatly throughout the year, there was no LY*season interaction. Yeast supplementation increased NDF degradability in all seasons. Among the four rumen bacteria evaluated, R. flavefaciens was the most prevalent and LY increased R. flavefaciens population in the summer and spring. Supplementing LY can benefit fibre-degrading bacteria and increase fibre degradability of grazing animals, independently of the season. / O efeito do aumento da digestibilidade da fibra detergente neutro (DFDN) da silagem de cana-de-açúcar sobre consumo, cinética e metabolismo ruminal, população microbiana, desempenho e características de carcaça de bovinos de corte foram avaliados através de dois experimentos. No experimento de metabolismo, oito novilhos Nelore canulados no rúmen foram utilizados em delineamento experimental com dois quadrados latinos 4×4 contemporâneos. No experimento de desempenho, quarenta e oito novilhos nelore foram usados em delineamento em blocos inteiramente casualizados com arranjo de tratamento em fatorial 2×2. Dois genótipos de cana-de-açúcar com alta e baixa-DFDN colhidos para ensilagem em dois estágios de maturidade (2 genótipos e 2 estágios de maturidade). As dietas foram formuladas contendo 40% de silagem de cana-de-açúcar na matéria seca. O consumo de matéria seca (CMS) foi limitado pela via metabólica. O CMS dos novilhos canulados no rúmen foi reduzido quando foram alimentados com a cana-de-açúcar colhida na maturidade tardia, mas somente para o genótipo de baixa-DFDN. A taxa de passagem e o turnover de FDN foram acelerados quando o genótipo de alta-DFDN foi fornecido. O pool ruminal de MS, FDN e FDNi foram maiores para as dietas contendo o genótipo de baixa-DFDN. Houve tendência para maior população relativa de F. succinogenes e R. albus quando os animais foram alimentado com dietas contendo o genótipo de baixa-DFDN. O desempenho e as características de carcaça não foram afetados pelos tratamentos. O excesso de carboidratos não fibrosos (CNF) e os produtos de fermentação das silagens podem limitar o consumo pela via metabólica. O genótipo de alta-DFDN pode acelerar a taxa de passagem e o turnover de FDN no rúmen. O CMS pode ser reduzido quando o genótipo de baixa-DFDN é colhido para ensilagem com maturidade tardia. O feito da suplementação com levedura viva (LV - Saccharomyces cerevisiae CNCM I-1077) sobre a digestibilidade in situ, fermentação ruminal e população de bactérias celulolíticas no rúmen de bovinos Nelore em pastejo foi avaliado ao longo de um ano. Oito novilhos canulados no rúmen foram usados em arranjo fatorial 2×4: com ou sem LV e quantro estações do ano. O tratamento com LV foi realizado diariamente para promover uma suplementação de 8x109 UFC por animal. Os animais foram mantidos em sistema de pastejo rotativo com suplementação mineral. A cada 45 dias, foi realizada a degradação de FDN n situ de 5 forragens referencia, determinada após 24 e 48h de incubação ruminal. O fluido ruminal foi coletado para avaliar a contagem de leveduras e os parâmetros de fermentação, e ainda para a quantificação de bactérias celulolíticas por PCR. Mesmo com grande variação da qualidade do pasto ao longo do ano, não foi observado interação entre LV*estação do ano. A suplementação com LV aumentou a degradabilidade da FDN em todas as estações do ano. Entre as quatro bactérias ruminais avaliadas, a R. flavefaciens foi a mais prevalente e o tratamento com LV aumentou sua população relativa no verão e na primavera. A suplementação com LV pode beneficiar bactérias degradadoras de FDN e aumentar a digestibilidade ruminal de fibra em animais em pastejo, independentemente da estação do ano.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In vitro assays were conducted to develop a method to estimate activity of bacterial enzymes in the rumen and the effect of changes in pH (5.5, 6.0, 6.5 and 7.0) and concentration of soluble carbohydrates (0, 100, 200 and 300 mg / dl) on adhesion to forage particles and ruminal bacterial fibrolytic activity. To evaluate the enzymatic extraction methods, samples of ryegrass (Lolium multiflorum) were incubated in a bovine rumen during 24-h and used to obtain enzymatic extracts through different extraction methods: sonication, sonication + lyzozyme, lyzozyme and without previous treatment (control). Then, the enzymatic extract was incubated with carboxymethylcelullose (CMC) and measured the reducing liberated sugars. The enzymatic extraction method did not affect significantly the enzymatic activity, but the standard deviation was lower when extraction only was through sonication. In another methodological assay, the effect of the reaction time between enzymatic extract and substrate on relative enzymatic activity (nmol of reducing sugars/ g DM of sample residue/ min) was tested. The relative activity decreased exponentially with time and stabilized after 240-min of incubation. To evaluate the effects of pH and concentration of sugars on bacterial attachment to feed particles and his degradability, samples of ryegrass (Lolium multiflorum) or cynodon (Cynodon dactilon) were incubated in vitro for 24-h in mediums with different pH (5,5; 6,0; 6,5 or 7,0) or different initial concentrations of glucose (0, 100, 200 and 300 mg/dl). The DM and phosphorus content (phosphorus was assumed be of bacterial origin) were determined in the fermentation residue. In ryegrass samples degradability and the residual phosphorus content were higher than in cynodon (P<0.01) and both factors were affected linearly and positively by pH (<0.01). The increase of soluble carbohydrates concentration exerted linear and negative effects on bacterial attachment (P<0.01), but it did not affected forage degradability. The effect of pH and sugar concentration was evaluated on fibrolityc enzyme activity in residues of the samples incubated in the rumen for 24-h and extracted through sonication. The enzymatic extract was incubated for 240-min with xylan or CMC in reaction mediums with different pH or sugar concentrations. In average, the activity of the enzymes extracted from the ryegrass residue was superior to the cynodon ones (P<0.01). On the other hand, the effect of both factors varied among assays and with the sample type, e.g. in different assays was observed that the pH effect was linear (P<0.05), quadratic (P<0.05) or even do not adjusted to none of the regressions. In the same way, the effect of presence of different concentrations of sugars in the medium on enzyme activity was quadratic mainly (P<0.01). In conclusion, the bacterial attachment to the forage particles was related directly to pH and inversely with concentration of soluble sugars in the incubation medium. However, these effects were less evident on enzymatic fibrolytic activity. / Ensaios in vitro foram conduzidos com o objetivo de desenvolver um método para estimar atividade de enzimas bacterianas do rúmen e avaliar os efeitos da alteração no pH e na concentração de carboidratos solúveis sobre a aderência às partículas de forragem e sobre a atividade fibrolítica bacteriana ruminal. Para avaliação de métodos de extração enzimática, amostras de azevém (lolium multiflorum) foram incubadas no rúmen de um bovino durante 24 horas e utilizadas para obtenção de extratos enzimáticos por diferentes métodos de extração: sonicação, sonicação + lisozima, lisozima e sem tratamento prévio (controle). O extrato enzimático foi então incubado com carboximetilcelulose (CMC) e medido a liberação de açúcares redutores. O método de extração de enzimas não afetou significativamente a atividade enzimática, mas o desvio padrão foi menor quando a extração foi somente por sonicação. Em outro ensaio metodológico foi testado o efeito do tempo de reação entre extrato enzimático e substrato sobre a atividade relativa das enzimas (nmol de açúcares redutores/g de matéria seca de resíduo de amostra/minuto). A atividade relativa diminuiu exponencialmente com o tempo, estabilizando-se a partir de 240 minutos de incubação. Para avaliar o efeito do pH e da concentração de açúcares sobre a aderência bacteriana às partículas de alimento e sobre a sua degradabilidade, amostras de azevém (Lolium multiflorum) ou cynodon (Cynodon dactilon) foram incubadas in vitro durante 24h em meios com diferentes pHs (5,5; 6,0; 6,5 ou 7,0) ou diferentes concentrações iniciais de glicose (0, 100, 200 e 300 mg/dl). No resíduo da fermentação foi determinado o teor de matéria seca e analisado o teor de P, assumindo-se ser de origem bacteriana. A degradabilidade e o teor de P residual foram mais altos nas amostras de azevém que de cynodon (P<0,01) e ambos fatores foram linearmente e positivamente afetados pelo pH (P<0,01). O aumento da concentração de carboidratos solúveis exerceu efeito linear e negativo sobre a aderência das bactérias (P<0,01), mas não afetou a degradação da forragem. O efeito do pH e da concentração de açúcares sobre a atividade enzimática fibrolítica foi avaliada no resíduo destas amostras forrageiras incubadas ruminalmente durante 24h e extraídas por sonicação. O extrato enzimático foi incubado durante 240 minutos com xilana ou CMC em meios de reação a diferentes pHs ou concentração de açúcares. Em média, a atividade de enzimas extraídas de resíduo de azevém foi superior à atividade daquelas oriundas de cynodon (P<0,01). No entanto, o efeito de ambos fatores variou entre ensaios e com o tipo de amostra. Por exemplo, em diferentes ensaios foi observado que o efeito de pH foi linear (P<0,05), quadrático (P<0,05) ou mesmo não foi ajustado à nenhuma destas regressões. Da mesma forma, a presença de diferentes concentrações de açúcares no meio exerceu efeito, na maioria das vezes quadrático, sobre a atividade enzimática (P<0,01). Em conclusão, a aderência bacteriana às partículas de forragem foi diretamente relacionada com o pH e inversamente relacionada com a concentração de açúcares solúveis no meio de incubação. Estes efeitos, contudo, foram menos evidentes sobre a atividade enzimática fibrolítica.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In vitro assays were conducted to evaluate the use of potential markers of bacterial adherence to particles of forage, in which samples of ryegrass (Lolium multiflorum), Cynodon spp., alfalfa (Medicago sativa) and perennial peanut (Arachis pintoi) were incubated for 24 hours. In the residue of incubation were analyzed the digestibility of dry matter and concentrations of markers of adherence: N15, purines, phosphorus (P) and enzymatic activity and its variables. Incubations were performed to evaluate the degree of phosphorus solubilization from the forage samples over 24 hours and samples of ruminal digesta were collected in order to isolate rumen bacteria and analyzing the chemical composition of the liquid phase associated bacteria (LAB) and bacteria adhered to particle (BAP). It was also assessed whether methylcellulose, used in the extraction of the BAP process, interfere with the chemical composition of bacteria extracted. Data were analyzed using a model that included the type of sample or bacteria as a block (randomized blocks). The results of the solubility test indicated that at the first 3 hours of incubation the concentration of phosphorus in the samples dropped sharply and thereafter tended to remain constant. There was difference (P <0.05) in the chemical composition of the BAP and BAL in relation to concentration of nitrogen, purine, DAPA and phosphorus. The data obtained from the trials evaluating the effect of methylcellulose on the chemical composition of bacteria associated with the liquid phase showed that the levels of nitrogen and purine when expressed in proportion of organic matter were not affected by treatment with methylcellulose (P> 0.05). However, a difference was observed in relation to the content of P (P <0.05). No correlation was observed between digestibility and bacterial adhesion markers and between the markers indicated correlation between N15 and reducing sugars with the substrate carboxymethylcellulose (P = 0.001), and N15 with reducing sugars substrate xylan (P <0.0001). In conclusion, the chemical composition of the liquid phase bacteria associated differs from the adherent bacteria and particulate material in general was not affected by treatment with methylcellulose. The concentration of the tested markers showed no consistent correlation between themselves or with the degradability of forage samples. The exception was the relationship between N15 and enzymatic activity, indicating that the isotope was incorporated mainly in bacterial enzymatic proteins / Foram conduzidos ensaios de digestibilidade in vitro com o objetivo de avaliar o uso de potenciais marcadores de aderência bacteriana a partículas de forragens, nos quais foram incubadas amostras de azevém (Lolium multiflorum), Cynodon spp., alfafa (Medicago sativa) e amendoim forrageiro (Arachis pintoi) por 24 horas. Nos resíduos da incubação foram analisados a digestibilidade da matéria seca e a concentração dos marcadores de aderência: N15, purinas, fósforo (P) e atividade enzimática e suas variáveis. Foram realizadas incubações para avaliar o grau de solubilização do fósforo de amostras de forragem ao longo de 24 horas e coletas de digesta ruminal com o objetivo de isolar bactérias ruminais e analisar a composição química das bactérias associadas à fase líquida (BAL) e bactérias aderidas à partícula (BAP). Também foi avaliado se a metilcelulose, utilizada no processo de extração das BAP, interfere na composição química das bactérias extraídas. Os dados foram analisados utilizando modelos que incluíram o tipo de amostra ou de bactéria como bloco (blocos ao acaso). Os resultados do teste de solubilização do fósforo indicam que nas primeiras 3 horas de incubação as concentrações de fósforo nas amostras diminuíram de forma acentuada e após esse período tenderam a permanecer constantes. Verificou-se diferença (P<0,05) na composição química das BAP e BAL em relação aos teores de nitrogênio, purinas, DAPA e fósforo. Os dados obtidos com os ensaios avaliando o efeito da metilcelulose na composição química das bactérias associadas à fase líquida demonstraram que os teores de nitrogênio e purinas quando expressos na proporção de matéria orgânica não foram afetados pelo tratamento com metilcelulose (P>0,05). No entanto, observou-se diferença em relação ao teor de P (P<0,05). Não foi observada correlação entre digestibilidadede e os marcadores de aderência bacteriana e entre os marcadores de aderência indicou correlação entre N15 e açúcares redutores com substrato carboximetilcelulose e (P=0,001), N15 e açúcares redutores com substrato xilano (P<0,0001). Concluiu-se que a composição química das bactérias associadas a fase líquida difere das bactérias aderidas a partículas de forragem e, em geral, não é afetada pelo tratamento com metilcelulose. A concentração dos marcadores testados não apresentou correlação consistente entre si ou com a degradabilidade das amostras de forragem. Exceção foi a relação entre N15 e atividade enzimática, indicando que o isótopo foi incorporado principalmente em proteínas enzimáticas bacterianas.

The effect of increased neutral detergent fibre digestibility (NDFD) of sugarcane silage on intake, rumen kinetics and metabolism, microbial population, performance, and body fatness of beef cattle was evaluated through two trials. In the metabolism trial, eight ruminal-cannulated Nellore steers were used in a replicated 4×4 Latin square design. In the performance trial, forty-eight Nellore young bulls were used in a complete randomized block design with a 2×2 factorial arrangement of treatments. Two sugarcane genotypes with high or low NDFD harvested for silage in two maturity stages were used (2 genotypes and 2 maturity stages). Diets were formulated with 40% of sugarcane on dry matter (DM) basis. Intake of DM was likely limited by metabolic feedback. The DM intake (DMI) of cannulated steers was reduced when late-maturity sugarcane was fed, but only with low-NDFD genotype. Turnover and passage rate of NDF was accelerated when high-NDFD genotype was fed. Rumen DM, NDF and iNDF pools were greater when diets with the low-NDFD genotype were fed. There was a tendency for greater relative populations of F. succinogenes and R. albus when animals were fed with diets containing low-NDFD genotype. Performance and carcass characteristics were not affected by treatments. Excess of NFC and fermentation end products of sugarcane silage can limit intake by metabolic pathway. High-NDFD genotype can accelerated NDF turnover and passage rate in the rumen. Late-maturity ensiled sugarcane can reduce DMI of beef steers fed with low-NDFD genotype. The effect of live yeast (LY - Saccharomyces cerevisiae CNCM I-1077) on in situ fiber digestibility, rumen fermentation, and rumen cellulolytic bacteria population of grazing Nellore cattle was evaluated throughout the year. Eight rumen cannulated steers were used in a 2x4 factorial arrangement of treatments: with or without LY, and the four seasons of the year. Yeast was given daily to provide 8x109 CFU per animal. Animals were kept on a rotational grazing system with mineral supplementation. Every 45 d, in situ rumen NDF degradability of 5 reference forages was determined after 24 and 48h incubation. Rumen fluid was collected to measure yeast count and fermentation parameters, and rumen contents collected for quantitative PCR quantification of cellulolytic bacteria. Although pasture composition varied greatly throughout the year, there was no LY*season interaction. Yeast supplementation increased NDF degradability in all seasons. Among the four rumen bacteria evaluated, R. flavefaciens was the most prevalent and LY increased R. flavefaciens population in the summer and spring. Supplementing LY can benefit fibre-degrading bacteria and increase fibre degradability of grazing animals, independently of the season. / O efeito do aumento da digestibilidade da fibra detergente neutro (DFDN) da silagem de cana-de-açúcar sobre consumo, cinética e metabolismo ruminal, população microbiana, desempenho e características de carcaça de bovinos de corte foram avaliados através de dois experimentos. No experimento de metabolismo, oito novilhos Nelore canulados no rúmen foram utilizados em delineamento experimental com dois quadrados latinos 4×4 contemporâneos. No experimento de desempenho, quarenta e oito novilhos nelore foram usados em delineamento em blocos inteiramente casualizados com arranjo de tratamento em fatorial 2×2. Dois genótipos de cana-de-açúcar com alta e baixa-DFDN colhidos para ensilagem em dois estágios de maturidade (2 genótipos e 2 estágios de maturidade). As dietas foram formuladas contendo 40% de silagem de cana-de-açúcar na matéria seca. O consumo de matéria seca (CMS) foi limitado pela via metabólica. O CMS dos novilhos canulados no rúmen foi reduzido quando foram alimentados com a cana-de-açúcar colhida na maturidade tardia, mas somente para o genótipo de baixa-DFDN. A taxa de passagem e o turnover de FDN foram acelerados quando o genótipo de alta-DFDN foi fornecido. O pool ruminal de MS, FDN e FDNi foram maiores para as dietas contendo o genótipo de baixa-DFDN. Houve tendência para maior população relativa de F. succinogenes e R. albus quando os animais foram alimentado com dietas contendo o genótipo de baixa-DFDN. O desempenho e as características de carcaça não foram afetados pelos tratamentos. O excesso de carboidratos não fibrosos (CNF) e os produtos de fermentação das silagens podem limitar o consumo pela via metabólica. O genótipo de alta-DFDN pode acelerar a taxa de passagem e o turnover de FDN no rúmen. O CMS pode ser reduzido quando o genótipo de baixa-DFDN é colhido para ensilagem com maturidade tardia. O feito da suplementação com levedura viva (LV - Saccharomyces cerevisiae CNCM I-1077) sobre a digestibilidade in situ, fermentação ruminal e população de bactérias celulolíticas no rúmen de bovinos Nelore em pastejo foi avaliado ao longo de um ano. Oito novilhos canulados no rúmen foram usados em arranjo fatorial 2×4: com ou sem LV e quantro estações do ano. O tratamento com LV foi realizado diariamente para promover uma suplementação de 8x109 UFC por animal. Os animais foram mantidos em sistema de pastejo rotativo com suplementação mineral. A cada 45 dias, foi realizada a degradação de FDN n situ de 5 forragens referencia, determinada após 24 e 48h de incubação ruminal. O fluido ruminal foi coletado para avaliar a contagem de leveduras e os parâmetros de fermentação, e ainda para a quantificação de bactérias celulolíticas por PCR. Mesmo com grande variação da qualidade do pasto ao longo do ano, não foi observado interação entre LV*estação do ano. A suplementação com LV aumentou a degradabilidade da FDN em todas as estações do ano. Entre as quatro bactérias ruminais avaliadas, a R. flavefaciens foi a mais prevalente e o tratamento com LV aumentou sua população relativa no verão e na primavera. A suplementação com LV pode beneficiar bactérias degradadoras de FDN e aumentar a digestibilidade ruminal de fibra em animais em pastejo, independentemente da estação do ano.

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