Desempenho nutricional de bovinos em pastejo durante os períodos de seca e de águas suplementados com compostos nitrogenados e/ou amido

Abstract

A presente tese foi baseada em dois experimentos com bovinos de corte em pastagem composta por capim-braquiária (Uruchloa decumbens), sendo um conduzido durante o período da seca e o outro durante o período das águas. Objetivou-se avaliar os efeitos da suplementação protéica e/ou com amido sobre o desempenho nutricional de bovinos em pastejo. No primeiro experimento, conduzido no período da seca entre julho e setembro de 2008, foram utilizados cinco novilhos mestiços, Holandês x Zebu, com peso corporal médio (PC) de 396,8±13,4 kg, fistulados no rúmen e no abomaso. Foram avaliados os seguintes tratamentos: 1.controle (somente forragem basal); 2.suplementação com compostos nitrogenados supridos na proporção de 1 g de proteína bruta (PB) por kg de PC via administração ruminal; 3.suplementação com amido na proporção de 2,5 g/kg de PC via administração ruminal; 4.suplementação com compostos nitrogenados e amido na forma como descrito para os tratamentos 1 e 2; e 5.suplementação similar ao descrito para o tratamento 4 no qual o amido foi substituído por mistura de amido e compostos nitrogenados de modo que a porção energética do suplemento apresentasse 15% de PB. O experimento foi conduzido segundo delineamento em quadrado latino 5 x 5, adotando-se esquema fatorial 2 x 2 +1 (adição ou não de suplemento nitrogenado, adição ou não de amido, com tratamento adicional correspondente ao tratamento 5 descrito anteriormente). O consumo de pasto não foi afetado (P>0,10) por nenhum dos fatores avaliados. O consumo de nutrientes digestíveis totais (NDT) foi elevado (P<0,10) tanto pela suplementação com compostos nitrogenados, como pela suplementação com amido. O consumo de NDT foi também incrementado (P<0,10) pelo suprimento adicional de compostos nitrogenados para os animais suplementados com nitrogênio e amido. A suplementação com compostos nitrogenados elevou (P<0,10) o nível dietético de NDT não afetando os coeficientes de digestibilidade total (P>0,10), com exceção da PB, no qual se observou efeito interativo (P<0,10) com a suplementação com amido. O suprimento adicional de compostos nitrogenados para os animais suplementados com nitrogênio e amido elevou (P<0,10) o nível dietético de NDT e os coeficientes de digestibilidade total da matéria seca (MS), matéria orgânica (MO) e fibra em detergente neutro (FDN). O balanço aparente de compostos nitrogenados (BN) foi afetado (P<0,10) de forma interativa pela suplementação. As estimativas de BN somente foram incrementadas (P<0,10) pela suplementação conjunta com compostos nitrogenados e amido. Diante destes resultados, concluiu-se que a suplementação com amido e compostos nitrogenados para bovinos mantidos a pasto durante o período seco confere efeito interativo potencializando a retenção de nitrogênio no organismo animal. No segundo experimento, realizado no período das águas entre janeiro e março de 2009, foram utilizados cinco novilhos Nelore, com PC de 211,0±17,1 kg, fistulados no rúmen. Foram avaliados os mesmos tratamentos, e utilizado o mesmo delineamento e o mesmo esquema experimental descritos para o primeiro experimento. A suplementação com compostos nitrogenados ampliou (P<0,10) o consumo de PB, porém não afetou isoladamente (P>0,10) nenhuma das demais variáveis associadas ao consumo voluntário. A suplementação com amido incrementou (P<0,10) o consumo total de MS, mas não afetou (P<0,10) o consumo de pasto. Adicionalmente, efeitos isolados da suplementação com amido foram verificados (P<0,10) sobre os consumos de PB, MS digerida e MO digerida, havendo incremento na ingestão destes componentes na presença de amido. Verificou-se efeito de interação da suplementação com compostos nitrogenados e amido (P<0,10) sobre os consumos de CNF e de NDT. O desmembramento deste efeito indicou que o consumo de NDT não foi ampliado (P>0,10) em relação ao controle quando nitrogênio ou amido foram fornecidos isoladamente, mas foi incrementado (P<0,10) quando ambos os componentes foram fornecidos conjuntamente. Em nenhuma das variáveis associadas ao consumo voluntário se verificou efeito (P>0,10) do fornecimento adicional de nitrogênio aos animais suplementados com compostos nitrogenados e amido. Os coeficientes de digestibilidade da MS e da MO e o nível dietético de NDT foram afetados de forma interativa (P<0,10), verificando-se maiores estimativas quando amido e compostos nitrogenados foram fornecidos conjuntamente. O BN foi incrementado (P<0,10) tanto pela suplementação com compostos nitrogenados, como pela suplementação com amido. Contudo, mesmo sem efeito interativo (P>0,10), a estimativa de BN se mostrou superior com a suplementação conjunta com amido e compostos nitrogenados em comparação à suplementação isolada com estes compostos. Diante dos resultados obtidos, concluiu-se que a suplementação com amido e compostos nitrogenados para bovinos mantidos a pasto durante o período das águas, considerando-se forragem com alto teor de proteína bruta, confere efeito interativo potencializando a digestão da matéria seca e o consumo de energia. Efeitos indiretos envolvendo aspectos metabólicos são observados, entre os quais se destaca a ampliação da retenção de nitrogênio no organismo animal.

This study was prepared based upon two experiments with beef cattle in pasture constituted by Brachiaria grass (Uruchloa decumbens). One of them performed during the dry season, and the other during the rainy season. This work was carried out aiming to evaluate the effects of protein and/or starch supplementation relative to the nutritional performance of bovine under pasture during the dry and rainy seasons. The first experiment, performed during the months of July and September, five Dutch x Zebu half-breed young bulls with average body weight (BW) of 396.8 ± 13.4 kg, cannulated in the rumen and in the abomasum were used. The following experimental treatments were evaluated: 1. Control (for basal forage only). 2. Supplementation with nitrogen compounds supplied in the proportion of 1g of crude protein (CP) per kg of BW by ruminal infusion; 3. Supplementation with starch in the proportion of 2.5 g/kg of BW by ruminal infusion. 4. Supplementation with nitrogen compounds and starch as described for the treatments 1 and 2; and 5. Supplementation similar to the one described for the treatment 4 in which the starch was replaced by a mixture of starch and nitrogen compounds in such a way that the energetic portion of the supplement was 15% of the CP. The experiment was performed following the delineation in a 5 x 5 latin square, adopting a factorial scheme 2 x 2 +1 (addition or not of nitrogen supplement, addition or not of starch, and addition of energetic supplement containing 15% of the CP). The intake of pasture DM, pasture OM, NDFap, digested NDFap and NDFi were not affected (P>0.10) by any of the factors evaluated in this study. The CP consumption showed an effect of interaction of the supplement with nitrogen compounds and starch (P<0.10), the evolution of this interaction showed that both in the absence or in the presence of starch, the supplementation of nitrogen compounds increased (P<0.10) the intake of CP. However, in the presence of supplements with nitrogen compounds, it was noticed a lower intake of CP (P<0.10) in the lack of supplement with starch compared to the animals that have received both types of supplements. The absence of supplementation with nitrogen compounds, the supplementation with starch did not affect (P>0.10) the CP consumption, while the additional supply of nitrogen compounds to animals supplemented with nitrogen and starch increased (P<0.10) the CP consumption. The intake of total digestible nutrients (TDN) was greater (P<0.10) both by the supplementation with nitrogen compounds as well by the supplementation with starch. The TDN consumption was also increased (P<0.10) by the additional supply of nitrogen compounds to the animals supplemented with nitrogen and starch. The supplementation with nitrogen compounds increased (P<0.10) the dietary TDN level, not affecting the total digestibility coefficient (P>0.10), except for the CP, in which the interactive affect was observed (P<0.10) with the supplementation with starch. The additional supply of nitrogen compounds to animals supplemented with nitrogen and starch raised (P<0.10) the dietary TDN level and the total digestibility coefficients of DM, OM and NDFap. As a probable reflection of the consumption of nitrogen, the apparent balance of nitrogen (NB) compounds was affected (P<0.10) in an interactive form by the supplementation. The NB estimates were incremented only (P<0.10) by the supplementation with nitrogen compounds in the presence of starch. As a consequence, the supplementation with starch could only increase (P<0.10) the NB in the presence of supplementary nitrogen compounds. Considering these results, we may conclude that the supplementation with starch and nitrogen compounds for bovines kept in pasture during the dry season causes an interactive effect potentiating the nitrogen retention in the animal organism. Besides, the additional supply of nitrogen compounds to animals supplemented simultaneously with starch and nitrogen may increase the ruminal degradation of the fiber and increase the production of microbial nitrogen compounds in the rumen, possibly due to the reduction of the effects of competition between fibrolitic and non-fibrolitic species. In the second experiment, performed from January to March, the rainy season, five Nellore young bulls were used, with 211 ± 17.12 kg BW, cannulated in the rumen. The animals were submitted to the same treatment described before. So being, this experiment was performed as per the above mentioned delineation and, as the prior experiment, the 2 x 2 +1 factorial scheme was adopted. The supplementation with nitrogen compounds increased (P<0.10) the CP consumption, but did not affect, separately (P>0,10). Any of the other variables associated to the voluntary consumption. The supplementation with starch increased (P<0.10) the DM and OM consumptions, but did not affect (P<0.10) the intake of pasture DM and pasture OM. Additionally, isolated effects of the supplementation with starch were noticed (P<0.10) on the intake of CP, digested DM and digested OM, with an increase in the ingestion of such components in the presence of starch. An interaction effect was noticed in the supplementation with nitrogen compounds and starch (P<0.10) on the consumptions of CNF and of total digestible nutrients (TDN). The unfolding of this effect on the TDN consumption was not increased (P>0.10) in respect to the control when nitrogen or starch were supplied separately, but was increased (P<0.10) when both components were supplied jointly. In none of the variable associated to the voluntary consumption there was noticed the effect (P>0.10) of the additional supply of nitrogen to the animals supplemented with nitrogen compounds and starch. The NB was increased (P<0.10) both by the supplementation with nitrogen compounds, as well as by the supplementation with starch. However, the relative nitrogen balance (RNB) was only increased (P<0.10) by the supplementation with nitrogen compounds. The effect of the additional supply of nitrogen to animals supplemented with starch and nitrogen compounds were only noticed on the NAR concentration and on the NB. In both cases, the additional supply of nitrogen increased (P<0.10) the obtained estimates. The effectively degraded portion of the NDF (FED) remained practically unchanged in respect to the control with the isolated supplementation with nitrogen compounds. However, it was depressed by the supplementation with starch. The joint supplementation with starch and nitrogen compounds resulted also in the DEF depression. However, in a less intense way when compared with the supplementation made only with starch. Considering these evidences, it can be concluded that the supplementation with starch and nitrogen compounds to beef cattle kept in pasture during the rainy season, considering high crude protein content in the forage, causes an interactive effect potentiating the digestion of dry matter and the consumption of energy. Indirect effects involving metabolic aspects are noticed, amongst which may point out the larger retention of nitrogen in the animal organism.