畜牧兽医学报  2023, Vol. 54 Issue (9): 3745-3757. DOI: 10.11843/j.issn.0366-6964.2023.09.015    PDF    
猪饲粮纤维的利用及其对养分消化的影响
陈莹, 钟儒清, 陈亮, 张宏福     
中国农业科学院北京畜牧兽医研究所/畜禽营养与饲养全国重点实验室, 北京 100193
摘要:我国粮食安全主要压力在饲料粮, 破解粮食安全的重要潜力也在饲料粮。饲料资源短缺、人畜争粮矛盾日益加剧。开发非粮饲料资源和提高饲料养分利用率是缓解我国粮食安全矛盾的重大战略需求。自1864年Henneberg与Stohmann首倡的概略成分分析方法以来, 饲料养分的测试方法已沿用一个半世纪, 基本保持原体系未变。而针对饲料原料碳水化合物组分的复杂性和多样性, 其分析方法和分析层次从Weende proximate粗纤维(概略养分分析)——van Soest洗涤纤维(范式洗涤纤维法)——总饲粮纤维法不断地演进。饲粮纤维黏性、溶解性和持水力等理化特性限制了本身以及饲粮中其它养分被单胃动物消化、吸收和利用, 其抗营养作用受到饲粮结构、畜禽品种、生理阶段、环境条件等种种复杂因素的影响, 同时饲粮纤维的分解产物具有重要的营养健康功能。因此, 剖析饲粮碳水化合物组分和解析饲粮纤维对养分消化、吸收、利用规律至关重要。本文从饲粮纤维的定义和内涵入手, 剖析饲粮碳水化合物组分的分析方法和层次的演进, 分析饲粮纤维的物理化学特性及其抗营养机制, 总结饲粮纤维在猪消化道利用特点, 其中侧重分析了饲粮纤维水平和类型对猪饲粮能量、蛋白质、氨基酸、脂肪等主要养分消化率的影响规律, 并对其影响的机理做出相应的阐述, 旨在为非粮饲料资源开发与饲粮纤维高效利用提供重要参考。
关键词饲粮纤维    养分    消化率    饲料        
Utilization of Dietary Fiber and Its Impact on Nutrient Digestion of Pigs
CHEN Ying, ZHONG Ruqing, CHEN Liang, ZHANG Hongfu     
State Key Laboratory of Animal Nutrition and Feeding, Institute of Animal Sciences, Chinese Academy of Agricultural Sciences, Beijing 100193, China
Abstract: The main pressure of food security in China is feed safety, and the important potential of solving food security is also feed safety. Due to the shortage of feed resources, the conflict between human and animals for food is increasingly intensified. Development of non-grain feed resources and improvement of feed nutrient utilization are important strategic to alleviate the contradiction of food security in China. Since the outline nutrient analysis method first invented by Henneberg and Stohmann in 1864, the test method of feed nutrients has been used for a century and a half, and the original system has basically remained unchanged. In view of the complexity and diversity of carbohydrate components in feed materials, the analysis methods and levels of analysis have evolved from Weende proximate crude fiber to van Soest fiber and total dietary fiber. The physical and chemical properties of dietary fiber such as viscosity, solubility and water holding capacity limit the digestion, absorption and utilization of itself and other nutrients in the diet by monogastric animals. The anti-nutritional effect of dietary fiber is affected by various complex factors such as diet structure, livestock and poultry breeds, physiological stages of animals, and environmental conditions. At the same time, the decomposition products of dietary fiber have important nutritional and health functions. Therefore, it is very important to analyze dietary carbohydrate composition and dietary fiber for nutrient digestion, absorption and utilization. This paper starts with the definition and connotation of dietary fiber, dissects the evolution of the analysis methods and levels of dietary carbohydrate components, analyzes the physicochemical properties of dietary fiber and its anti-nutritional mechanism, and summarizes the utilization characteristics of dietary fiber in pig digestive tract. Among them, the influence of dietary fiber level and type on the digestibility of main nutrients such as energy, protein, amino acids, and fat in pig diets was focused on analysis, and the mechanism of its influence was explained. Aiming to provide important reference for the development of non-grain feed resources and efficient utilization of dietary fiber.
Key words: dietary fiber    nutrient    digestibility    feed    pigs    

我国粮食安全主要压力在饲料粮,破解粮食安全的重要潜力也在饲料粮。2021年我国粮食总产量6.83亿吨,产量连年丰收,但进口量仍达1.65亿吨,且粮食供应总量一半左右用作饲用。粮食安全的矛盾日渐凸显,其主要原因是畜禽生产消耗量越来越大[1]。“畜禽为能而食”,碳水化合物是猪饲粮中最主要的供能物质,占总能量摄入的60%~70%,然而15%~30%未被猪胃肠道消化[2],未消化的碳水化合物主要为饲粮纤维,特别是非淀粉多糖(non-starch polysaccharides, NSP),不仅造成巨大浪费,还污染环境。因此,深入了解饲粮纤维在猪体内的消化、吸收和利用规律为非粮饲料资源开发和饲料原料利用率提升具有重要的战略意义。本文通过剖析国内外大量文献,综述饲料原料中饲粮纤维的分析方法和手段以及饲粮纤维重要的理化特性,侧重分析饲粮纤维在猪体内的消化、降解、转移和利用以及饲粮纤维与其他饲料养分的互作,并从纤维水平和类型两方面阐述其作用的途径,旨在为非粮饲料资源开发与饲粮纤维高效利用提供重要参考。

1 饲粮纤维的定义和内涵

剖析饲料原料中碳水化合物组分具有重要的意义,由于绝大部分的畜禽饲粮成分来源于植物材料,且饲粮中能量绝大部分来自于植物性饲料原料中的碳水化合物。如图 1所示,植物中碳水化合物可分成细胞壁和细胞内容物,细胞壁化合物包括木质素、纤维素、半纤维素、β-葡聚糖及果胶;细胞内容物化合物包括抗性淀粉、果聚糖、寡糖、二糖和淀粉[3]。随着对饲料成分的不断剖析研究,饲粮纤维组分的分析方法不断完善和发展,其经历了Weende proximate粗纤维(概略养分分析)——van Soest洗涤纤维(范式洗涤纤维法)——总饲粮纤维(total dietary fiber,TDF)分析法三个发展过程。概略养分分析法至今沿用了一个半世纪,粗纤维被认为是酸和碱处理后不溶解的有机物残渣,包括数量上具有较大变异的纤维素(40%~100%),少量的半纤维素(15%~20%)以及木质素(5%~90%)[4]。概略养分分析法缺少饲料原料中纤维素和半纤维素确切的含量,粗纤维含量不能准确地表述饲料原料的营养价值,但因其稳定性和可重复性较好,目前仍然在猪饲料中设置粗纤维水平[5]。van Soest洗涤分析法将饲粮纤维进一步剖分为中性洗涤纤维(NDF)、酸性洗涤纤维(ADF)和木质素[6]。纤维素含量使用ADF和木质素的差值表示,半纤维素含量用NDF和ADF的差值表示。目前van Soest洗涤分析法被广泛应用,但该方法缺乏对果胶、中性糖类和β-葡聚糖等可溶性组分的有效分析[7]。因而,当测定饲料原料中总纤维含量时,可溶性纤维含量越高,van Soest分析法测定的结果准确度就越差。在谷物类饲料(如玉米和干酒槽及其可溶物(DDGS))中,由于其具有高含量的不溶性纤维,常较少考虑可溶性纤维的含量。但在大豆皮、甜菜渣等可溶性纤维含量较高的饲料原料中应当考虑可溶性纤维含量[8]。另外,在van Soest分析时,可能由于淀粉和蛋白质残渣等的污染导致测试稳定性和重复性降低[4]。且van Soest分析法难以对饲料纤维的化学组分和生物学功能进行精确的分析[9]。而TDF分析法克服了上述两种分析方法的缺点,TDF分析法可定量饲料原料中所有的纤维组分,包括可溶性饲粮纤维(soluble dietary fiber,SDF)和不溶性饲粮纤维(insoluble dietary fiber,IDF)[10]。TDF分析法可作为揭示饲粮纤维(dietary fiber,DF)对饲料养分营养价值的一个有力工具。然而,目前TDF分析法最大的挑战是可重复性低于van Soest分析法。TDF分析法需要进一步改进,将低分子量不可消化碳水化合物纳入TDF体系,并校正不可消化残渣的含量[11]。本实验室系统研究了饲料中NSP组分测定的适宜称样量、组分解离等关键参数,建立了乙酸酐衍生气相色谱分析饲料NSP组分的方法,测定值变异在2.2%以内[12-13],建立的NSP组分分析方法为全国31个省、62个试验站、226个监测点开展“饲料营养价值与畜禽营养需求”长期监测提供统一方法与数据规范。尽管目前TDF分析法还没有广泛在营养实验室应用,但饲料原料碳水化合物组分已经纳入最新版NRC(2012)[14]饲料原料营养成分表中。

虚线箭头表示成分含量不确定 Dotted arrows indicate the component content are uncertain 图 1 饲粮纤维定义和内涵的发展图[1-2] Fig. 1 Development diagram of definition and connotation of dietary fiber[1-2]

饲粮纤维包括丰富的内涵。根据2009年食品法典委员会(Codex Alimentarius Commission, CAC)广泛被接受的定义,饲粮纤维被认为是具有十个或更多单体单元的可食用碳水化合物聚合物,可抵抗内源性消化酶,因此在小肠中既不会被水解也不会被吸收[15]。2010年EFSA专家小组进一步将膳食纤维定义为不可消化的碳水化合物加木质素[16]。Davani-Davari等[17]认为,饲粮纤维根据来源主要包括三个亚组:1)天然存在于可食用植物中的碳水化合物聚合物,作为蔬菜、水果、种子、谷类和块茎食用;2)通过物理、酶和化学手段从生食物中获得的可食用碳水化合物聚合物,并具有已证实的生理益处(如抗性低聚糖、菊粉和车前草);3)具有已证实生理益处的人工合成的碳水化合物聚合物(如甲基纤维素)。医学上认为饲粮纤维的定义包括三层内涵:饲粮纤维、功能纤维和总纤维。饲粮纤维是指存在于植物中的不可消化的碳水化合物和木质素;功能纤维包括有益于人类生理健康的不可消化的碳水化合物;总纤维则包含饲粮纤维和功能纤维两部分[18]

2 饲粮纤维的理化属性

饲粮纤维具有很多物化属性,如:溶解性、持水力、黏性和发酵性等[19]。这些特性会对猪胃肠道的结构和功能产生重要的作用。它们不仅会影响饲粮纤维的消化与利用,而且影响饲料其他养分的消化与吸收。

2.1 溶解性

溶解性(solubility)是指饲粮纤维溶解于水的能力,主要是由碳水化合物聚合物的结构决定[20]。线型结构可增加非共价键的强度,具有稳定的构象,从而不溶于水,而非线型结构则相反。如:纤维素是由成千上万的D-葡萄糖分子通过β(1,4)糖苷键连接形成的线型高分子化合物,故其为不溶性纤维,而β-葡聚糖因其特殊的键连接方式和分子内氢键的存在,形成螺旋型的分子结构,故其为可溶性纤维[2]。除此之外,溶解度还取决于温度和pH等外部因素[21]。根据在水中的溶解度,饲粮纤维可分为可溶性饲粮纤维(SDF)和不溶性饲粮纤维(IDF)[22]。将饲粮纤维分为可溶和不溶的两部分,有助于深入理解饲粮纤维的营养功能。如:SDF影响脂类和葡萄糖的吸收;而IDF影响养分在肠内的流动[23]

2.2 持水力

持水力(water holding or water binding capacity)指饲料中纤维结构对水的吸收能力。纤维可通过离子作用、氢合力或者毛细管作用等方式与水作用[24]。纤维与水作用的强度和结合水的数量与纤维的形态结构及组成成分有关[24]。饲粮纤维结合水的能力有两种表现形式:持水力(water holding capacity)指在没有外力作用下,可结合纤维的水的数量;结合水能力(water binding capacity)指在外力作用下,滞留在含水纤维中水的数量[25]。离心、pH变化和减少粒度等压力因素可增加饲粮纤维的持水力。SDF和IDF都具有一定的持水力[26]。一般来说,SDF比IDF的持水力强,例如果胶和甜菜的持水力高于小麦和大麦的种子残渣及豆皮的持水力[27]

2.3 黏性

黏性(viscosity)是与物质流动性有关的属性。饲粮纤维的黏性主要取决于溶解性、相对分子量和粒度,溶解性又取决于纤维的化学结构和与其连接的其他细胞壁化合物。如:燕麦中的β-葡聚糖在很大程度上是解聚的[28],因此对黏度的影响较小;相反可溶性阿拉伯木聚糖较难降解,导致较高的肠腔黏度[29-30]。推测由于饲粮纤维特别是SDF具有高的黏性,可改变饲料养分通过动物消化道中的流通时间,从而影响饲料养分的吸收能力。与低分子量的瓜尔豆胶相比,大分子量的瓜尔豆胶黏度较高[31]。粒度较大的饲粮纤维可增加猪盲肠食糜黏度[32]。可见,溶解度越大、相对分子量越高及粒度越大,饲粮纤维的黏性就越强。

2.4 发酵性

单胃动物内源消化酶无法消化分解饲粮纤维,但其肠道微生物菌群可对饲粮纤维进行发酵利用同时产生挥发性脂肪酸(volatile fatty acid,VFA)[33]。饲粮纤维的发酵性受纤维类型的影响。如燕麦麸和大豆皮中SDF的表观回肠消化率(apparent ileal digestibility,AID)分别为38.61%和48.93%,而IDF的表观回肠消化率仅分别为3.67%和1.93%[34]。可见,SDF的发酵性优于IDF。不同类型的纤维在动物肠道内的发酵部位亦不相同。Jaworski和Stein[35]测量了猪不同肠道段DDGS、次粉和大豆皮的非淀粉多糖的消化率,发现SDF的主要发酵部位在小肠和盲肠,而IDF的发酵部位在结肠。

3 饲粮纤维的利用

饲粮中的各成分在回肠食糜及粪中的占比如图 2图 3所示。饲粮纤维本质上为多糖,是碳水化合物的一种[36]。它不能直接被胃肠道消化吸收,但却可以被肠道微生物发酵,分解成小分子化合物从而被机体吸收利用。Müller[37]研究发现,猪肠道微生物可将多糖通过水解、氧化还原、磷酸化等一系列反应解聚成较小的碳水化合物组分。Urriola等[38]试验发现,原料类型不同,其纤维素的全消化道消化率(apparent total tract digestibility,ATTD)也不尽相同。其中,大麦为23%~65%,小麦以及小麦副产品为24%~60%,黑麦以及黑麦成分为10%~ 84%。玉米DDGS中总饲粮纤维(TDF)的ATTD变异范围为29.3%~57.0%,平均值为47.5%,其中SDF的ATTD为92.0%高于IDF的ATTD(41.3%)。Knudsen等[39]发现,淀粉回肠消化率均在98%以上,且淀粉来源和加工工艺均会影响淀粉的消化。Knudsen和Jørgensen[40]试验发现,当猪采食高纤维饲料原料时,总饲粮纤维的表观回肠消化率AID变异范围为:10%~62%。Knudsen等[39]还通过51个消化试验证明:在小肠末端,干物质、有机物、粗蛋白质、粗脂肪表观回肠消化率均在70%以上;而总碳水化合物表观回肠消化率为81.1%,其中单糖和寡糖(接近100%)和淀粉(97%)在通过小肠过程中几乎全部被消化掉,NSP表观回肠消化率仅为21.7%。从小肠运输到大肠的有机物中,NSP几乎占未消化残渣的一半,而到达大肠的有机物中,约有50%在通过大肠的过程中被发酵。综上所述,饲粮纤维主要靠肠道微生物发酵生成VFA进而被机体吸收利用,且SDF的发酵利用率大于IDF,少数无法发酵的则随粪便一起排出体外。

图 2 生长猪养分消化 Fig. 2 Nutrient digestion of growing pigs
图 3 生长猪碳水化合物消化 Fig. 3 Carbohydrate digestion of growing pigs
4 饲粮纤维对饲料养分消化率的影响

猪饲料中饲粮纤维的消化率为40%~50%,而其他养分的消化率(如:蛋白质、脂肪或淀粉)常在80%以上[41]。DF浓度是影响饲粮养分和能量消化率最重要的因素,DF对饲料养分的消化率的作用受DF来源和水平以及猪的生理年龄的影响[42-43]

4.1 饲粮纤维对能量消化率的影响

增加饲粮纤维水平可线性地减少表观能量消化率(表 1)。随着来源于棕榈仁粕的饲粮纤维添加量的增加(10%、20%、30%及40%),育肥猪总能的表观全消化道消化率(apparent total tract digestibility,ATTD)显著线性减少,同时也伴随着干物质和有机物ATTD的显著线性降低[44]。在生长猪日粮基础上分别添加15%、30%及45%的DDGS,总能的ATTD、表观回肠消化率(apparent ileal digestibility,AID)均随纤维含量的增加而显著下降[45]。与饲喂基础日粮的猪相比,饲喂含有大豆皮或次粉的日粮的生长猪和育肥猪能量的ATTD均下降[46]。相似的报道还有一些[38, 47-54]。推测可能是因为纤维在后肠中发酵,其中能量以挥发性脂肪酸(volatile fatty acid,VFA)的形式被吸收,其能量效率低于以小肠葡萄糖形式吸收的能量,故饲粮纤维降低了能量消化率[46]

表 1 饲粮纤维对猪饲料能量消化率的影响 Table 1 Effect of dietary fiber on feed energy digestibility of diets by pigs

纤维类型亦能影响能量的消化率,饲喂添加SDF甜菜渣饲粮的生长猪总能、消化能、代谢能及净能的ATTD均高于添加IDF大豆皮的。推测是由于SDF易被肠道微生物发酵利用并产生短链脂肪酸供能,而IDF既不能被内源酶消化, 又无法有效地被微生物发酵利用[55]。大豆皮和豆腐渣饲粮总能的ATTD显著高于苹果渣和发酵苹果渣饲粮[56]。可能与大豆皮和豆腐渣中富含易于发酵的低聚糖有关[56]

4.2 饲粮纤维对蛋白质和氨基酸消化率的影响

表 2显示了饲粮纤维对饲粮粗蛋白质和氨基酸消化率的影响。纤维水平可降低蛋白质的消化率。在猪日粮中添加DDGS、麦麸、菜籽粕、油菜籽壳、地瓜秧粉、甜菜渣和大豆皮粉均可显著降低CP的AID或ATTD(P<0.05)[45, 49, 53-54, 58]。也有报道显示,饲粮纤维对蛋白质和氨基酸消化率的影响作用有一定的阈值。如Li和Sauer[59]发现,增加纤维素会减少CP消化率,但当纤维含量超过10%时却不影响CP消化率。纤维类型不同对CP的消化率影响亦不相同。Owusu-Asiedu等[50]和Renteria-Flores等[43]试验发现,在猪日粮中添加SDF对CP的AID或ATTD影响不大,而添加IDF则会显著降低CP的AID或ATTD。可见IDF对CP消化率影响更大。SDF主要通过增加消化物的黏度,限制营养物质和酶之间的相互作用,促进肠表面未搅动水层的形成,产生物理屏障来影响CP的消化率;IDF则主要通过持水力实现对CP的影响[60]

表 2 饲粮纤维对猪饲料蛋白质和氨基酸消化率的影响 Table 2 Effects of dietary fiber on protein and amino acid digestibility of diets by pigs

纤维水平也可影响氨基酸的消化率,有试验证明,在猪日粮中添加DDGS、麸皮和地瓜秧粉、发酵玉米胚芽粕和ADF等纤维,可使大多数必需氨基酸及少数非必需氨基酸回肠消化率下降[42, 45, 58, 61-63]

4.3 饲粮纤维对淀粉和葡萄糖消化率的影响

研究发现,99%的淀粉在小肠中被消化[64]。日粮中添加饲粮纤维对淀粉消化率的影响目前并不确定(表 3),有研究发现回肠淀粉消化率会随着纤维含量的增加而显著减少(P<0.05)[45, 64-65]。也有试验发现谷物基础饲粮中添加饲粮纤维并不影响淀粉的ATTD[39, 53],说明饲粮纤维对淀粉消化率没有影响。故饲粮纤维是否会影响淀粉消化率还有待验证。Owusu-Asiedu等[50]研究发现,在饲粮中添加瓜尔胶可减少葡萄糖50%空肠的吸收,推测可能是因为瓜尔胶可增大食糜黏度,减少葡萄糖从肠道扩散到上皮细胞,从而引起葡萄糖吸收减少[66]。说明添加饲粮纤维可降低葡萄糖的消化。

表 3 饲粮纤维对猪饲料淀粉消化率的影响 Table 3 Effect of dietary fiber on starch digestibility of diets by pigs
4.4 饲粮纤维对脂肪消化率的影响

纤维饲粮会稀释饲料的营养,因此高纤维含量的猪饲粮常加入油脂来提高能量含量。饲粮纤维影响脂肪的消化率已经广泛地被报道(表 4),吕知谦等[55]发现,在生长猪玉米-豆粕基础日粮中添加22%的甜菜渣和20%的大豆皮,可分别显著减少22%和15%粗脂肪的ATTD。李娟花[68]发现,随着麸皮水平(7%、14%)的增加,育肥猪粗脂肪AID显著下降,且14%麸皮组粗脂肪AID显著低于7%麸皮组。Wilfart等[49]也通过试验得到同样结论。可见,在饲粮中添加纤维可减少脂肪消化率,且纤维添加水平与粗脂肪消化率呈负相关关系。

表 4 饲粮纤维对猪饲料脂肪消化率的影响 Table 4 Effect of dietary fiber on fat digestibility of diets by pigs

不同纤维类型对脂肪的消化也有不同的影响,如吕知谦等[55]分别用SDF甜菜渣和IDF大豆皮替代生长猪部分基础日粮,饲喂28 d后发现,SID甜菜渣组脂肪的ATTD低于IDF大豆皮组。Lee等[65]亦通过试验证明,饲喂SDF饲粮的生长猪脂肪ATTD和AID均低于饲喂IDF饲粮的。如前所述的饲粮纤维的理化属性——黏性可知,纤维的溶解度越大,其黏度越大。故SDF黏度大于IDF,而脂肪对高黏度纤维特别敏感[65],因为高黏度纤维可降低脂肪乳化作用[67],因此,SDF饲粮脂肪的ATTD低于IDF饲粮。

4.5 饲粮纤维对排泄物的影响

饲粮中添加纤维可降低总氮的排泄量,且主要是降低尿氮的排泄。尿氮主要以尿素的形式(90%)进入环境,当遇到尿素酶时可快速转换为氨气,造成空气污染[69]。粪中的氮多为有机氮(80%),在环境中分解缓慢,危害相对较小[70]。如表 5所示,饲粮纤维不仅能将挥发性较大的尿氮排泄向更稳定的粪氮排泄转移,还可增加氮沉积。这样既有利于改进畜牧生产中氨态氮在环境中的释放,同时还能促进氮的利用[71]。尿氮转化到粪氮受饲粮纤维水平的影响。如在妊娠母猪日粮中添加18.0 g ·kg-1菊粉与18.9 g ·kg-1的纤维素,可使尿氮的排泄量减少6%,粪氮排泄量增加6%,氮的净利用率提高6.9%[72]。后备母猪日粮中添加15%甜菜渣可显著减少尿氮的排泄量,增加粪氮排泄量,并提高氮沉积率[73]。李娟花[68]、Yang等[74]也得到类似结论。可见猪日粮中添加饲粮纤维可减少尿氮的排泄,增加粪氮排泄量,增加氮沉积。同时发现尿氮、粪氮排泄量与纤维水平呈线性关系。如将5种不同浓度梯度的饲粮纤维(80、160、240、320和400 g ·kg-1)添加到生长育肥猪的日粮中,饲喂4周后发现,随着纤维饲料添加量的增加,试验猪的尿氮排泄量显著降低,粪氮排泄量显著线性增加[75]。随着燕麦麸添加量(5%、12.5%及20%)的增加,生长猪尿氮排泄量和尿氮/粪氮比线性减少,粪氮排泄量逐渐增加[70]。饲粮纤维使尿氮排泄量减少,粪氮排泄量增加的原因可能与饲粮纤维在猪后肠内被微生物发酵,产生短链脂肪酸供能并促进微生物生长,进而使微生物对氨的需要量增长,导致原本用于合成尿素的氨被用来合成微生物蛋白并随粪便排出有关[76]

表 5 饲粮纤维对猪饲料氮排泄量的影响 Table 5 Effect of dietary fiber on nitrogen excretion of diets by pigs

饲粮纤维也可影响猪的排粪量。研究发现,在生长育肥猪日粮中分别添加80、160、240、320和400 g ·kg-1饲粮纤维,饲喂4周后发现,随着纤维水平增加,排粪量呈线性增加[75]。朱丽媛[70]在研究中也得到类似结论。可见排粪量与饲粮纤维添加水平呈正相关关系。排粪量还受纤维类型的影响,饲喂高纤维饲粮(禾本科牧草、玉米秸秆和葵花籽壳)生长育肥猪的粪便排泄量高于饲喂低纤维饲粮的(玉米芯和苜蓿干草)[75]。因为与低纤维饲粮相比,高纤维饲粮中含有较多的ADF和NDF等高度不可消化成分,这些成分不能在肠道中发酵,只能随粪便排出[75]

5 小结

饲料中饲粮纤维不为动物的内源消化酶降解,故影响动物的生理。纤维组分可从不同层次进行分析,总饲粮纤维分析方法是最准确地反映饲料原料中纤维组分的方法。饲粮纤维不能直接被胃肠道消化吸收,主要是通过胃肠道微生物发酵,从而被机体吸收利用。增加饲粮纤维水平可减少能量、蛋白质与氨基酸、葡萄糖及脂肪的消化率,降低总氮的排泄量,增加排粪量。纤维类型不同,对能量、蛋白质、脂肪的消化影响亦不同。且纤维类型对消化率差异与纤维溶解性和黏性等理化属性密切相关[20]。尽管饲粮纤维降低了饲料养分的消化,但是可促进尿氮向粪氮转化,从而减少氨态氮的合成,有利于降低畜牧生产中氨态氮在环境中的释放,从而有助于节能减排。

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