Swedish University of Agricultural Sciences, Department of Agricultural Research for Northern Sweden, S-901 83 Umeå, Sweden.
J Dairy Sci. 2013 Mar;96(3):1715-26. doi: 10.3168/jds.2012-5752. Epub 2013 Jan 9.
Indigestible neutral detergent fiber (NDF) predicts forage digestibility accurately and precisely when determined by a 288-h ruminal in situ incubation, and it is an important parameter in mechanistic rumen models. The long incubation time required is a disadvantage. Further, intrinsic cell wall characteristics of feeds should be determined under ideal conditions for fiber digestion. The objective of this study was to determine the effects of diet composition and rumen incubation time on the concentrations of indigestible NDF (iNDF) for a wide range of feeds in dairy cows. Additionally, predicted concentrations of unavailable NDF generated using the National Research Council (NRC) model and the Cornell Net Carbohydrate and Protein System (CNCPS) were evaluated. Indigestible NDF was evaluated in 18 feeds using 4 cows in a split-split plot design. Treatments were in a 3 × 3 factorial arrangement, consisting of different diets and incubation times. Diet composition was primarily varied by changing the level of concentrate supplementation between 190 (low), 421 (medium), and 625 (high)g/kg of diet dry matter (DM). Grass silage was used as the basal forage for all cows. The feeds were incubated for 144, 216, and 288 h. Indigestible NDF was determined from 2-g samples weighed into polyester bags with a pore size of 12 µm and a pore area equal to 6% of the total surface area, giving a sample size to surface ratio of 10mg/cm(2). Across all feeds, the measured iNDF concentrations ranged from 6 to 516 g/kg of DM. The feed iNDF concentration was not affected by the cow used, but diet composition had a significant effect. The mean measured iNDF concentrations for cows consuming low-, medium-, and high-concentrate diets were 178, 186, and 197 g/kg of DM, respectively. The incubation time also affected the feed iNDF concentrations, which averaged 199, 185, and 177 g/kg of DM for 144-, 216-, and 288-h incubations, respectively. We also observed significant interactions between incubation time and feed, and between diet composition and feed, with fiber-rich feeds being most sensitive to these factors. The evaluation of model predictions of unavailable NDF indicated poor precision with prediction errors of 56 (NRC) and 84 (CNCPS)g/kg of DM. Indigestible NDF should be determined based on 288-h ruminal in situ incubations in cows consuming diets with a low proportion of concentrate to represent the feed fraction that is unavailable to the animal.
不可消化中性洗涤纤维（NDF）通过 288 小时瘤胃原位培养准确且精确地预测饲料消化率，并且是机制瘤胃模型中的一个重要参数。但该方法需要较长的孵育时间，这是其缺点。此外，饲料的内在细胞壁特性应在纤维消化的理想条件下确定。本研究的目的是确定日粮组成和瘤胃孵育时间对奶牛多种饲料不可消化 NDF（iNDF）浓度的影响。此外，还评估了使用国家研究委员会（NRC）模型和康奈尔净碳水化合物和蛋白质系统（CNCPS）生成的不可利用 NDF 的预测浓度。在一项 4 头奶牛的裂区-裂区设计中，用 18 种饲料评估不可消化 NDF。处理采用 3×3 因子排列，包括不同的日粮和孵育时间。日粮组成主要通过改变精料补充料的水平来改变，精料补充料的水平分别为 190（低）、421（中）和 625（高）g/kg 日粮干物质（DM）。所有奶牛均以草青贮料作为基础饲草。饲料的孵育时间分别为 144、216 和 288 小时。通过将 2g 样品称重到聚酯袋中进行不可消化 NDF 的测定，聚酯袋的孔径为 12μm，孔面积等于总表面积的 6%，从而使样品面积与表面积的比值为 10mg/cm(2)。所有饲料的 iNDF 浓度范围为 6 至 516g/kg DM。牛的使用不会影响饲料的 iNDF 浓度，但日粮组成有显著影响。分别饲喂低、中、高精料日粮的奶牛的平均测量 iNDF 浓度分别为 178、186 和 197g/kg DM。孵育时间也影响饲料 iNDF 浓度，144、216 和 288 小时孵育的平均饲料 iNDF 浓度分别为 199、185 和 177g/kg DM。我们还观察到孵育时间与饲料、日粮组成与饲料之间存在显著的相互作用，纤维丰富的饲料对这些因素最为敏感。不可利用 NDF 的模型预测值评估表明，预测误差为 56（NRC）和 84（CNCPS）g/kg DM，精度较差。在奶牛日粮中精料比例较低时，应通过 288 小时瘤胃原位培养来确定不可消化 NDF，以代表动物无法利用的饲料部分。
