日糧uNDF對(duì)反芻動(dòng)物生長和代謝的作用

蘭貴生，閆佰鵬，李發(fā)弟,2，李 飛

(1.蘭州大學(xué)草地農(nóng)業(yè)生態(tài)系統(tǒng)國家重點(diǎn)實(shí)驗(yàn)室，蘭州大學(xué)草地農(nóng)業(yè)科技學(xué)院，甘肅 蘭州730030；2.甘肅省肉羊繁育生物技術(shù)工程實(shí)驗(yàn)室，甘肅 民勤 733300)

反芻動(dòng)物生產(chǎn)中，粗飼料的品質(zhì)、供給量直接影響其生產(chǎn)效益[1]，而作為粗飼料重要品質(zhì)之一的纖維消化特性是評(píng)價(jià)粗飼料品質(zhì)和配制日糧的重要因素。中性洗滌纖維(neutral detergent fiber，NDF)的消化特性影響動(dòng)物的采食行為、反芻行為、粒徑的破碎率、過腹率、填充、干物質(zhì)采食量(dry matter intake，DMI)及總的牛奶成分產(chǎn)量的效率[2]。以前，營養(yǎng)學(xué)家只注重NDF中可消化部分的研究，后來發(fā)現(xiàn)NDF中不可消化部分在一定程度上影響瘤胃纖維發(fā)酵率，人們才開始關(guān)注它。最初Mertens等[3]、Huhtanen等[4]把瘤胃中未消化的中性洗滌纖維稱為iNDF(indigestible neutral detergent fiber)。然而，為了提高描述纖維發(fā)酵動(dòng)力學(xué)術(shù)語的準(zhǔn)確性，Mertens[5]提出uNDF(undigested neutral detergent fiber)的概念，即在特定發(fā)酵時(shí)間內(nèi)發(fā)酵后未消化的NDF。對(duì)于uNDF和iNDF的認(rèn)識(shí)，人們之前錯(cuò)誤地認(rèn)為uNDF=iNDF，其實(shí)它們是兩個(gè)不同概念。uNDF是指在特定發(fā)酵時(shí)間內(nèi)發(fā)酵后未消化的NDF，現(xiàn)在多指在發(fā)酵240 h后未消化的NDF。iNDF是指在無限長的發(fā)酵時(shí)間后仍不能消化的NDF，是一個(gè)理論值，實(shí)際生產(chǎn)中不能達(dá)到?？傊?，以前報(bào)道的iNDF實(shí)際上就是現(xiàn)在的uNDF。

uNDF是一種功能性纖維成分，能夠影響粗飼料的物理效能、瘤胃填充、纖維的流通及消化。Nousiainen等[6]報(bào)道，uNDF與有機(jī)物(organic matter，OM)消化率之間有一定的相關(guān)性，因此可以用uNDF的含量來預(yù)測粗飼料質(zhì)量。Raffrenato和van Amburgh[7]報(bào)道可以用uNDF和早期發(fā)酵時(shí)間點(diǎn)來評(píng)估纖維的快速和慢速發(fā)酵池以及它們的發(fā)酵率。Cotanch等[2]報(bào)道，uNDF的最低攝入量應(yīng)為體重的0.39%～0.48%時(shí)才能維持瘤胃內(nèi)環(huán)境的穩(wěn)定。因此，正確評(píng)估日糧中uNDF對(duì)反芻動(dòng)物的營養(yǎng)調(diào)控作用，可以為合理的配制日糧提供理論依據(jù)。

1 uNDF測定方法的演變過程

為了準(zhǔn)確評(píng)估粗飼料和日糧中uNDF的含量，科學(xué)家們進(jìn)行了大量探究?？的藸杻籼妓衔锖偷鞍踪|(zhì)體系(CNCPS)、Fox等[8-9]、Tylutki等[10]用2.4倍ADL(ADL×2.4/NDF；其中，ADL為酸性洗滌木質(zhì)素，2.4，固定因子)來評(píng)估粗飼料uNDF含量。這種方法是由Chander等[11]在1980年提出的，且用此方法測定結(jié)果與Weiss等[12]測定的結(jié)果一致。近年來，營養(yǎng)學(xué)家們利用飼料長時(shí)間在體外發(fā)酵或體內(nèi)降解的方法測定uNDF的含量，該方法測定的結(jié)果與Chander等[11]提出的方法測定結(jié)果不一致。Basle等[13]、Huhtanen等[4]、Raffrerato[14]采用飼料在體外發(fā)酵240 h或在尼龍袋降解288 h測定纖維的消化率與Chander等[11]測定結(jié)果不一致，這表明木質(zhì)素和纖維消化率之間的相關(guān)關(guān)系不確定，測定的結(jié)果受物候條件、水分、溫度、光照和纖維素的影響。目前，微生物發(fā)酵240 h已經(jīng)替代2.4倍ADL成為最新uNDF測定方法。最近發(fā)現(xiàn)，飼料樣品中含有較高灰分可以抑制NDF洗滌液溶解礦物質(zhì)的能力，殘留的礦物質(zhì)污染NDF，導(dǎo)致NDF測量值偏高，同時(shí)也影響uNDF測定[15]。為了試驗(yàn)的準(zhǔn)確性，對(duì)測定的NDF做了一定的處理，并定義為去除灰分和有機(jī)物的NDF(aNDFom)，是指用淀粉酶和亞硫酸鈉處理，并在有機(jī)物為基礎(chǔ)上測定，排除了礦物質(zhì)的影響，使測定uNDF含量更加準(zhǔn)確。Apalmonari等[16]多位營養(yǎng)學(xué)家在aNDFom的基礎(chǔ)上，用體外發(fā)酵240 h方法來測定飼料中uNDF的含量。這種方法已廣泛應(yīng)用于生產(chǎn)實(shí)踐和科學(xué)研究，是目前測定反芻動(dòng)物日糧和粗飼料uNDF含量最常用的方法。

2 日糧uNDF對(duì)反芻動(dòng)物營養(yǎng)調(diào)控作用

2.1 日糧uNDF對(duì)采食量影響

采食量是影響動(dòng)物生產(chǎn)性能的主要因素。瘤胃填充會(huì)使動(dòng)物產(chǎn)生飽腹感反饋刺激動(dòng)物從而影響采食量[17]。填充瘤胃的物質(zhì)主要是瘤胃中難以消化的纖維物質(zhì)，由快速發(fā)酵NDF池、慢速發(fā)酵NDF池和uNDF池組成。它們在瘤胃中的流通速度影響瘤胃空間，流速越快、瘤胃空間越大、采食量就越高[2]。日糧uNDF不能被微生物利用，需在總消化道停留較長的時(shí)間[18]。日糧uNDF長時(shí)間停留在瘤胃，占據(jù)瘤胃空間使動(dòng)物產(chǎn)生飽腹感從而降低采食量。Detmann等[19]用meta分析方法得出肉牛在育肥期間隨日糧uNDF含量的增加，干物質(zhì)(dry matter，DM)采食量呈線性下降。Lippke[20]報(bào)道，當(dāng)日糧中iNDF含量超過總DM含量的15%時(shí)，日糧iNDF的含量與采食量之間有明顯的負(fù)相關(guān)關(guān)系。William等[21]報(bào)道，妊娠后期奶牛飼喂高uNDF240含量(約10.7%DM)苜蓿(Medicagosativa)干草與飼喂低uNDF240含量(約8.3%DM)苜蓿干草相比有更高DM采食量，并能改善奶牛的健康狀態(tài)；但在后期試驗(yàn)中，奶牛飼喂高含量uNDF240苜蓿干草與低含量uNDF240苜蓿干草相比，DM采食量明顯下降。Fustin等[22]報(bào)道，奶牛飼喂高含量uNDF240苜蓿干草與低含量uNDF240相比DM采食量和反芻時(shí)間沒有明顯變化，并得出DM采食量主要受粗飼料纖維的消化率而不僅僅是uNDF240攝入量的影響，并推薦飼喂干燥的全混合日糧時(shí)，uNDF240攝入量約為活體重的0.48%才能維持健康的瘤胃環(huán)境。綜上所述，日糧uNDF對(duì)瘤胃的填充能夠影響反芻動(dòng)物采食量，且隨著日糧uNDF的含量增加，反芻動(dòng)物的采食量下降，控制日糧uNDF水平是保證反芻動(dòng)物較高采食量的重要營養(yǎng)調(diào)控手段。

2.2 日糧uNDF對(duì)瘤胃功能的影響

瘤胃草墊層是指過于成熟的牧草，經(jīng)過反芻動(dòng)物的物理嚼碎，漂浮在瘤胃液中形成的一層草墊。它的形成是反芻動(dòng)物維持正常瘤胃功能的前提和重要指標(biāo)，主要有兩個(gè)物理功能。一個(gè)是通過物理刺激反芻、唾液分泌和瘤胃運(yùn)動(dòng)來優(yōu)化瘤胃微環(huán)境，特別是調(diào)節(jié)瘤胃pH；另一個(gè)是延長纖維顆粒滯留時(shí)間，提高纖維在瘤胃中的消化率[23-25]。Sutherland[26]將瘤胃草墊層稱為非常有效的第一階段的分離器，其可以通過選擇性地滯留不可消化小顆粒飼料(“過濾床”效應(yīng))來調(diào)節(jié)固體食糜在瘤胃中停留的時(shí)間。瘤胃中的uNDF墊可作為瘤胃內(nèi)容物的過濾層，在uNDF墊下面未消化營養(yǎng)物質(zhì)隨瘤胃液移出瘤胃進(jìn)入后消化道。正常的瘤胃草墊層可以選擇性地滯留飼料中的纖維成分，能夠提高纖維的消化率和產(chǎn)奶量。相對(duì)較小的食糜團(tuán)通過瘤胃的速度較快，飼料效率及瘤胃消化率較低；相對(duì)較大的食糜團(tuán)通過瘤胃的速度較慢，瘤胃中飼料消化率較高[27]。粒徑較大的纖維食糜團(tuán)會(huì)對(duì)瘤胃壁施加壓力，引起動(dòng)物產(chǎn)生反復(fù)反芻，從而降低動(dòng)物采食量。在瘤胃消化能力較強(qiáng)的條件下，消化過多的淀粉使瘤胃淀粉分解菌的數(shù)量增加，纖維分解菌的數(shù)量減少，降低纖維的消化率，從而降低牛奶的產(chǎn)量，提高動(dòng)物瘤胃丙酸及總揮發(fā)性脂肪酸含量，同時(shí)也提高了反芻動(dòng)物患瘤胃酸中毒的風(fēng)險(xiǎn)[27]。Fustini等[22]報(bào)道，與低uNDF日糧相比，奶牛飼喂高uNDF日糧瘤胃pH在5.8以下持續(xù)的時(shí)間更短。因此,可以通過在日糧中配制一定比例富含uNDF的飼料原料調(diào)節(jié)淀粉在瘤胃中的降解速度以及增加動(dòng)物的反芻時(shí)間，中和淀粉過度發(fā)酵產(chǎn)生的揮發(fā)性脂肪酸及乳酸，防止亞急性瘤胃酸中毒，維持瘤胃健康。

2.3 uNDF對(duì)NDF消化率的影響

Combs和Eastridge[28]總結(jié)了影響NDF消化率的3個(gè)主要因素，一是潛在可消化中性洗滌纖維(potentially digestible neutral detergent fiber，pdNDF)在日糧纖維中所占的比例。日糧中的纖維包含兩部分，pdNDF和iNDF，通常用日糧中總的NDF減去iNDF來確定pdNDF。日糧纖維中不可消化的部分只能通過瘤胃蠕動(dòng)移出瘤胃[29]。與潛在可消化部分相比，不可消化部分在瘤胃滯留的時(shí)間更長，容易在瘤胃中堆積[30-31]。長時(shí)間滯留在瘤胃會(huì)使動(dòng)物產(chǎn)生飽腹感從而降低干物質(zhì)采食量[32]。二是pdNDF在消化道中的流通速度。影響瘤胃纖維食團(tuán)在瘤胃中消化率的主要因素是瘤胃食團(tuán)在瘤胃中的流通速度，流速越快，消化率就越高。van Soest等[33]提出瘤胃食糜中NDF包含兩部分，且這兩部分消化順序不同，其中一部分消化速度快，另一部分消化速度慢。Raffrenato和Amburgh[7]根據(jù)兩部分NDF的消化速度不同將其定義為快速消化NDF和慢速消化NDF。Raffrenat[14]將瘤胃食糜中的NDF分為三部分，快速發(fā)酵NDF池、NDF池及uNDF(圖1)?？焖侔l(fā)酵NDF在瘤胃中的發(fā)酵速度較快，且易被微生物消化，可以快速在瘤胃中消失；慢速發(fā)酵NDF在瘤胃中發(fā)酵的速度較慢，容易造成uNDF在瘤胃中堆積，致使瘤胃中滯留大量食糜不能及時(shí)排出，瘤胃空間減小，DM采食量及NDF的消化率降低。而Lund等[34]研究發(fā)現(xiàn)，瘤胃食糜NDF中的pdNDF與iNDF相比，pdNDF流通的速度更慢。這與Raffrenato[14]的研究結(jié)果不一致。這可能與他們所研究粗飼料粒徑的均一性和粒徑的大小有關(guān)。除此之外，其他因素，例如飼料粒徑的大小、粒徑的重力和浮力以及其他的物理特征(飼料的韌性和咀嚼過程中飼料破碎率)也能影響飼料顆粒在瘤胃流通速率。瘤胃內(nèi)容物含有兩層，上層為顆粒較大固體層，該層作用主要是通過調(diào)節(jié)和刺激咀嚼來改變飼料顆粒的大小，這部分食團(tuán)的流通速率極慢；下層是顆粒較小的液體層，該層食糜的流通速度較快,這種模式是由Mertens和Ely[35]提出的。飼料中uNDF的含量較高，在瘤胃中不易消化，滯留在固體層，占據(jù)一定的瘤胃空間，阻礙了可消化纖維的流通速度，影響采食量，從而降低了纖維的消化率。三是pdNDF在瘤胃中的降解率。飼糧中90%～95%纖維在瘤胃中降解[36]。飼料纖維降解速率主要取決于纖維固有的特征及瘤胃微生物與纖維接觸面積[29]。植物表面被角質(zhì)層覆蓋，抵制微生物發(fā)酵并限制微生物進(jìn)入發(fā)酵位點(diǎn)。通過破碎、研磨和咀嚼等方式使飼料顆粒斷裂，微生物由斷裂部位滲入并降解[37]。日糧中uNDF不能被微生物降解，覆蓋pdNDF表面，對(duì)瘤胃食團(tuán)中的pdNDF形成包被阻擋了纖維分解菌與pdNDF接觸，從而降低NDF消化率。

圖1 飼料在體外發(fā)酵30、120和240 h NDF的消化率及NDF池Fig. 1 The digestibility of NDF and NDF pool in vitro at 30, 120 and 240 h

2.4 日糧uNDF對(duì)反芻動(dòng)物生產(chǎn)性能的影響

uNDF在瘤胃中不能被微生物降解成有機(jī)酸，為機(jī)體提供生長所需的能量，但日糧uNDF含量會(huì)影響動(dòng)物的生產(chǎn)性能(表1)。Thoney和Hinge[38]用易消化性豆殼和難消化性燕麥(Avenafatua)殼飼喂羔羊發(fā)現(xiàn)，iNDF含量越高采食量越低，NDF消化率越高采食量越高。Oba和Allen[39]在體外發(fā)酵和尼龍袋降解粗飼料試驗(yàn)中報(bào)道，NDF消化率每增加1%，DM采食量增加0.17 kg，4%的脂肪校正乳(fat correction milk，F(xiàn)CM)增加0.25 kg。Oba等[40]體外發(fā)酵11種不同的青貯玉米(Zeamays)，得出NDF每增加1%，DM采食量增加0.12 kg，4%FCM增加0.22 kg。Jung等[41]發(fā)現(xiàn)，奶牛飼喂至少包含40%玉米青貯，NDF消化率每增加1%，DM采食量增加0.14 kg，F(xiàn)CM增加0.12 kg。Bernard等[42]研究發(fā)明，給奶牛飼喂高纖維、低消化率青貯玉米，牛奶中的乳脂濃度更高，能量校正乳的產(chǎn)量較低。Kr?mer-Schmid等[43]報(bào)道，產(chǎn)奶量和日增重與aNDFom的消化率正相關(guān)，瘤胃中pdNDF池減少會(huì)導(dǎo)致細(xì)胞壁結(jié)合蛋白和微生物蛋白產(chǎn)量以及奶?？偟拇x蛋白降低。綜上所述，提高NDF的消化率可以提高動(dòng)物的生產(chǎn)性能。

表1 不同粗飼料來源uNDF含量對(duì)反芻動(dòng)物生產(chǎn)性能的影響Table 1 Effect of undigested neutral detergent fiber (uNDF) content from different forage sources on the growth performance of ruminants

↑表示增加；↓表示降低。

↑, increase;↓, decrease. FCM, fat correction milk.

3 如何使用uNDF配制反芻動(dòng)物日糧

目前，高產(chǎn)反芻動(dòng)物日糧配方的焦點(diǎn)在于評(píng)估日糧中NDF的含量，以實(shí)現(xiàn)攝入更多的高能日糧。同種飼料的NDF和ADF含量越高，飼料營養(yǎng)品質(zhì)越差[45]，然而飼料中的NDF水平較低會(huì)造成反芻瘤胃功能障礙，因此需要一個(gè)既滿足瘤胃健康又滿足動(dòng)物高產(chǎn)的最低NDF水平。NRC推薦在DM的基礎(chǔ)上，奶牛日糧中應(yīng)該包含30%～35%的NDF才能夠?qū)崿F(xiàn)產(chǎn)奶量和DM采食量之間的平衡，但這并未考慮乳脂的含量[46]。在肉牛生產(chǎn)飼養(yǎng)方面，動(dòng)物飼喂高淀粉日糧，NDF含量經(jīng)常低于16%[46]。粗飼料中的NDF含量變異性很大。當(dāng)粗飼料中的NDF的范圍在30%～80%時(shí)，會(huì)給反芻動(dòng)物日糧的配制提供困難[47]。而粗飼料中的NDF含量隨著植物成熟度、生長環(huán)境、陽光、水分的變化而變化[48]。據(jù)報(bào)道，隨著玉米秸稈收割時(shí)間的延遲，鈣、粗灰分、粗脂肪、NDF、酸性洗滌纖維和ADL的含量呈增加趨勢，而粗蛋白、磷、水溶性碳水化合物和總糖的含量都呈減少趨勢[49]。即使日糧總NDF含量相同，pdNDF也不相同。當(dāng)反芻動(dòng)物飼喂粗飼料基礎(chǔ)日糧時(shí)，增加纖維消化率，就能提高動(dòng)物的生產(chǎn)性能。在許多模型中，iNDF是影響飼料利用率的主要因素[50]。康奈爾凈碳水化合物和蛋白質(zhì)系統(tǒng)(CNCPS)[10,51]和北歐奶牛代謝模型[52]中利用iNDF計(jì)算OM、NDF消化率、瘤胃NDF池及微生物N的流量，為合理地配制日糧提供了依據(jù)。牧草中iNDF含量對(duì)采食量的影響在亞熱帶和熱帶反芻動(dòng)物放牧系統(tǒng)中尤為顯著。降低日糧中的iNDF含量可以提高奶牛的生產(chǎn)性能而不是降低日糧中的NDF含量[53]。奶牛飼喂苜蓿干草和小麥(Triticumaestivum)秸稈為基礎(chǔ)日糧時(shí)，uNDF的攝入量為活體重的0.48%，最低攝入量不能低于體重的0.4%，才能維持正常瘤胃健康[2]。羔羊和母羊飼喂豆殼和燕麥殼為基礎(chǔ)日糧時(shí)，推薦日糧中iNDF最大含量為17%，并推薦泌乳母羊日糧中iNDF的含量最大不能超過10%[38]。因此在配制反芻動(dòng)物日糧時(shí)，準(zhǔn)確評(píng)估牧草uNDF的含量，將高uNDF牧草與低uNDF牧草混合使用實(shí)現(xiàn)日糧配方最佳的uNDF水平,使之既能滿足動(dòng)物的能量需求，又能維持瘤胃的健康。

4 小結(jié)

uNDF對(duì)反芻動(dòng)物的采食量、纖維消化率、瘤胃健康及動(dòng)物生產(chǎn)性能方面有重要的作用。準(zhǔn)確評(píng)估日糧uNDF的含量，保證日糧中最佳的uNDF含量，既能滿足動(dòng)物的能量需求，又能維持瘤胃內(nèi)環(huán)境穩(wěn)定和動(dòng)物健康。目前，uNDF對(duì)反芻動(dòng)物營養(yǎng)調(diào)控作用的研究較少，其很多營養(yǎng)作用還不清楚，需日后進(jìn)一步研究uNDF在反芻動(dòng)物瘤胃內(nèi)環(huán)境中的調(diào)節(jié)機(jī)制及營養(yǎng)調(diào)控作用。

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