Parâmetros de degradabilidade potencial da matéria seca e da proteína bruta das silagens de seis genótipos de sorgo (Sorghum bicolor (L.) Moench), com e sem tanino no grão, avaliados pela técnica in situ

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Abstract

Para testar o efeito da presença de tanino no grão sobre os parâmetros de degradabilidade potencial da matéria seca e da proteína bruta de seis genótipos de sorgo para silagem, foi conduzido um experimento utilizando a técnica da degradabilidade in situ. Quatro novilhos canulados no rúmen, alimentados individualmente, duas vezes ao dia ad libitum com feno de Tifton 85, foram utilizados para a incubação de silagens de sorgo no estádio de grão leitoso, pertencentes a seis genótipos: BR 303, BR 304, BR 601e AG 2006 (sem tanino no grão); BR 700 e BR 701 (com tanino). Foram incubadas 5 g de amostra nos tempos de incubação: 6, 12, 24, 48, 72 e 96 horas. O tempo zero (t0) foi utilizado para cálculo da fração solúvel. A presença do tanino reduziu a extensão de degradação da matéria seca e da proteína bruta nas silagens de sorgo BR 700 e BR 701. As degradabilidades potenciais destes nutrientes foram influenciada pelo tanino. Não foi observado efeito depressivo do tanino sobre as taxas de degradação para nenhum nutriente, em nenhuma silagem testada.

Translated abstract

The aim of this experiment was to evaluate the effect of tannin on grain over potential degradability parameters of the dry matter and crude protein of six sorghum silage genotypes by in situ technique. It was used four crossbred steers, canulated in rumen, fed ad libitum twice at day with a diet comprised by Cynodon dactylon hay. Six sorghum silage genotypes, harvested at milk stage, were incubated in the rumen: BR 303, BR 304, BR 601, and AG 2006 (without tannin on grain); BR 700, and BR 701 (with tannin on grain). Five grams of samples were incubated at times: 6, 12, 24, 48, 72, and 96 hours. The time zero (t0) was used to calculate the soluble fraction. Tannin decrease the extent of degradation of dry matter and crude protein of BR 700 and BR 701 sorghum silages. The potential degradabilities of these nutrients were reduced by tannin presence on grain. There was not effect of tannin on degradation rate of the nutrient compounds in anyone tested sorghum silage.

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Nutritional toxicology of tannins and related polyphenols in forage legumes.

R. Reed (1995)

Proanthocyanidins (PA) (condensed tannins) and hydrolyzable tannins (HT) are the two major classes of tannins. Proanthocyanidins are flavonoid polymers. Hydrolyzable tannins are polymers of gallic or ellagic acid esterified to a core molecule, commonly glucose or a polyphenol such as catechin. Proanthocyanidins are the most common type of tannin found in forage legumes. Problems in the analysis of tannins are that sample processing and drying decrease extraction and reactivity, suitable standards are unavailable, and quantitative analytical methods are poorly correlated with enzyme inhibition, protein precipitation, and nutritional effects. Hydrolyzable tannins are potentially toxic to ruminants. Pyrogallol, a hepatotoxin and nephrotoxin, is a product of HT degradation by ruminal microbes. Proanthocyanidins are considered to be non-toxic because they are not absorbed, but they are associated with lesions of the gut mucosa. Research on tannins in forage legumes has determined their effects on protein digestion and metabolism but more research on tannin structure in relation to digestion of specific proteins is needed. The widely accepted explanation for positive effects of PA on protein digestion and metabolism is that PA-protein complexes escape ruminal degradation and the protein is available in the lower tract. This proposed mechanism may be incorrect because PA also complex carbohydrates, endogenous proteins, and microbial products and the degradability of PA-protein complexes by ruminal microbes has not been adequately studied. Several alternative hypotheses (to escape protein) that explain the effect of PA on protein digestion and metabolism in ruminants are also consistent with experimental results on forage legumes. These include increased microbial protein synthesis, increased use of endogenous nitrogen in the rumen, and increased secretion of salivary glycoproteins. Research on manipulating the content and type of PA in forage legumes is justified because they are associated with non-bloating legumes, lower soluble non-protein nitrogen in silage, and improved efficiency of protein utilization. Research on the biosynthesis, molecular genetics, and cell biology of PA in forage legumes needs to be integrated with research on toxicology and nutrition.