Palo R Thomas, Sunnerheim Kerstin, Theander Olof
Department of Animal Physiology, Swedish University of Agricultural Sciences, S-75007, Uppsala, Sweden.
Department of Chemistry and Molecular Biology, Swedish University of Agricultural Sciences, S-75007, Uppsala, Sweden.
Oecologia. 1985 Feb;65(3):314-318. doi: 10.1007/BF00378904.
Birch twigs of diameter ≦1.5 mm exhibit seasonal trends in ruminant in vitro organic matter digestibility (IVOMD), and in the contents of crude protein, cell walls (neutral detergent fibre, NDF), and phenolic compounds. The IVOMD is low in winter twigs, increases in spring, and reaches a maximum in early summer. Crude protein behaves similarly. On the other hand, the proportion of hydrophilic phenols and cell walls (cellulose, hemicellulose, and lignin) to dry weight decreases dramatically in spring when leaves start emerging and growth is initiated. This reduction of phenols is reflected by concomitant changes in concentration of catechin, a major phenolic compound of birch. The concentration of phenolic acids are low in winter and spring but increase after leafing.The biological activity of an extract containing the phenolic compounds, measured as reduction of IVOMD, also decreases concomitantly with the decline of the total phenolic concentration and catechin. It is notable that catechin when tested alone at natural concentrations does not depress IVOMD. It is possible, however, that the amount of catechin reflects the level of condensed tannins, which may be responsible for IVOMD depression. The results strongly indicate that the decline of NDF and phenolic constituents is important for an improved food quality. Phenols may constitute the major chemical defense of birch in winter against browsing vertebrates by reducing digestibility and having toxic properties.
直径小于等于1.5毫米的桦树枝条在反刍动物体外有机物消化率（IVOMD）、粗蛋白含量、细胞壁（中性洗涤纤维，NDF）和酚类化合物含量方面呈现出季节性变化趋势。冬季枝条的IVOMD较低，春季升高，在初夏达到最大值。粗蛋白的变化趋势与之相似。另一方面，当春天叶子开始长出并开始生长时，亲水性酚类和细胞壁（纤维素、半纤维素和木质素）占干重的比例会急剧下降。酚类物质的这种减少反映在桦树主要酚类化合物儿茶素浓度的相应变化上。酚酸的浓度在冬季和春季较低，但在叶子长出后会增加。以IVOMD降低来衡量的含酚类化合物提取物的生物活性也会随着总酚浓度和儿茶素的下降而相应降低。值得注意的是，当单独以天然浓度测试儿茶素时，它不会降低IVOMD。然而，儿茶素的量可能反映了缩合单宁的水平，而缩合单宁可能是导致IVOMD降低的原因。结果有力地表明，NDF和酚类成分的下降对提高食物质量很重要。酚类物质可能是桦树在冬季抵御食草脊椎动物的主要化学防御手段，通过降低消化率和具有毒性来实现。
