动物营养学报 >

2024 , Vol. 36 >Issue 5: 2793 - 2803

DOI: https://doi.org/10.12418/CJAN2024.241

综述

犊牛断奶应激的研究进展及营养调控措施

  • 孔晓丽 ,
  • 马煜斌 ,
  • 张力莉 ,
  • 徐晓锋 , *
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  • 宁夏大学动物科技学院,银川 750021
* 徐晓锋,教授,博士生导师,E-mail:

孔晓丽(1998—),女,甘肃永靖人,硕士研究生,从事反刍动物胃肠道健康与营养调控研究。E-mail:

Office editor: 陈鑫

收稿日期: 2023-12-11

  网络出版日期: 2024-05-15

基金资助

宁夏回族自治区重点研发项目——奶公犊成活保健及其育肥关键技术研究与示范应用(2023BCF01034)

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Progress of Research on Weaning Stress in Calves and Nutritional Control Measures

  • KONG Xiaoli ,
  • MA Yubin ,
  • ZHANG Lili ,
  • XU Xiaofeng , *
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  • College of Animal Science and Technology, Ningxia University, Yinchuan 750021, China
* professor, E-mail:

Received date: 2023-12-11

  Online published: 2024-05-15

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摘要

断奶在奶牛场的管理策略中起着至关重要的作用,会影响未来犊牛生长性能和健康。断奶会引起犊牛应激,影响机体生长性能、胃肠道健康及免疫功能。本文从生长性能、胃肠道健康和免疫功能等方面阐述了断奶应激对犊牛的影响及其作用机理,并提出了一些科学的营养调控措施,旨在为缓解犊牛断奶应激、促进现代规模化奶牛养殖下犊牛的健康发育提供参考。

关键词： 犊牛; 断奶应激; 生长性能; 胃肠道健康; 免疫功能; 营养调控

本文引用格式

孔晓丽 , 马煜斌 , 张力莉 , 徐晓锋 . 犊牛断奶应激的研究进展及营养调控措施[J]. 动物营养学报, 2024 , 36(5) : 2793 -2803 . DOI: 10.12418/CJAN2024.241

Abstract

Weaning plays a crucial role in the management strategy of dairy farms and can affect future calf performance and health. Weaning causes calf stress, which affects growth performance, gastrointestinal health and immune function. This paper describes the effects of weaning stress on calves and its mechanism from the aspects of growth performance, gastrointestinal health and immune function, and puts forward some scientific nutritional control measures, aiming to provide reference for alleviating calf weaning stress and promoting the healthy development of calves under modern large-scale dairy farming.

Key words： calf; weaning stress; growth performance; gastrointestinal health; immune function; nutritional regulation

在规模化奶牛养殖场,犊牛约2月龄时断奶[1],但此时的犊牛身体机能尚未发育完全,尤其是瘤胃,正处于发育阶段,容易受到环境变化的影响[2],导致犊牛易发生断奶应激。断奶应激不利于犊牛的健康生长,处理不当会影响其后期生长性能[3],再加上环境等其他因素的影响,犊牛应激反应增强,采食量下降,生长发育缓慢,免疫力下降,导致呼吸系统及消化系统疾病发生[4],降低犊牛的成活率,给养牛业造成巨大的经济损失[5]。断奶犊牛易受到致病菌的侵害,这些病原菌会引起肠道功能障碍,严重影响犊牛的健康生长[6]。断奶也会引发犊牛生理反应,包括血清皮质醇和急性期蛋白浓度增加。急性期蛋白浓度增加可能是身体对断奶过程中营养不足的一种应激反应。应激状态的持续可能会影响犊牛免疫系统功能,使其更容易受到疾病侵袭。因此,专家和学者一直致力于了解断奶对犊牛生理反应的影响,并寻求方法来减轻这些负面影响,以提高犊牛生长性能和健康水平,这涉及到改进断奶实践管理、提供科学合理的营养调控等措施。断奶后犊牛血浆皮质醇水平升高、中性粒细胞数量增多和晚期淋巴细胞数量减少,这导致中性粒细胞:淋巴细胞比值更大,影响了犊牛免疫功能,这些生理变化与犊牛生长性能和健康呈负相关,这一直是该领域的研究热点[7-8]。本文综述了犊牛断奶应激的产生及其对犊牛生长性能、胃肠道健康和免疫功能影响的研究进展及缓解断奶应激的一些营养调控措施,为促进养殖场犊牛发育提供理论依据。

1 断奶应激的产生及其对犊牛的影响

1.1 断奶应激的产生

断奶期犊牛胃肠系统发育不全、肠道微生态系统不完整、免疫和抗氧化能力低下、消化吸收能力弱[9],因此,犊牛在此期间将面临心理、生理和身体上的一定压力,产生断奶应激[10]。断奶应激主要包括营养和心理上的应激,涉及神经、免疫以及内分泌系统的一系列活动[11]。营养上的应激包括从断奶前主要以牛奶为基础饲粮(高脂肪、低纤维)转变为断奶后主要以粗饲料和额外的浓缩物(低脂肪、高纤维)为基础饲粮,导致了胃肠道微生物的重大变化,新生犊牛的消化器官发育尚不完全,难以快速适应这种改变。心理上的应激包括犊牛对母牛的依赖和分离焦虑的产生,经历情绪上的不适,也可能经历运输、受伤、冷热应激、饮水不足及发生疾病等各种因素的影响[4]。由于犊牛各种器官发育尚不完全,机体体温调节能力很弱,对外部恶劣的环境温度非常敏感,需要适应新的生活环境和社群关系[12]。

1.2 断奶应激对犊牛的影响

1.2.1 对生长性能的影响

断奶是犊牛生命中压力最大的关键时期之一,会导致犊牛采食量减少,体重和平均日增重(ADG)降低[13],腹泻甚至死亡等现象,这是造成动物饲料利用率及生长性能低的重要因素。研究表明,断奶日龄会影响后期犊牛的生长性能,如体重、胸围和肩高。Ferronato等[14]发现断奶1周内,45日龄犊牛的ADG比60日龄犊牛低。断奶后45和60日龄犊牛ADG没有显著差异,但是肩高和胸围随断奶时间延长而增加;45和60日龄犊牛在采食量方面存在差异,60日龄犊牛的采食量高于45日龄犊牛[15]。

1.2.2 对胃肠道健康和免疫功能的影响

断奶应激会损害胃肠道结构和功能,并严重影响动物后续生长[16]。小肠是一个重要的多功能器官,在免疫应答的调节以及各种酶和免疫球蛋白的分泌中起着至关重要的作用[17]。绒毛高度、隐窝深度、绒毛高度/隐窝深度(V/C)是评价小肠完整性和消化能力的主要指标。犊牛断奶时,由于犊牛消化系统不完善,饲粮结构改变,导致犊牛肠绒毛收缩、隐窝增生和V/C降低,犊牛消化吸收酶活性下降。肠绒毛高度降低和隐窝深度增加是肠道屏障功能受损的标志[18]。研究发现,断奶前补充罗伊氏乳杆菌L81[2.0 g/(d·头)]可显著降低回肠隐窝深度,增加断奶后回肠V/C[19]。胃肠道的正常发育是犊牛营养沉积和生长的基础,而幼年动物易受病原菌侵害,导致胃肠道屏障功能受损和炎症反应[20]。肠道疾病是犊牛断奶前发病率和死亡率很高的最常见原因之一[1],断奶应激破坏了胃肠道屏障完整性并导致细胞旁胃肠道通透性增加[21]。犊牛断奶可诱发犊牛腹泻[22],犊牛腹泻不仅影响自身生长发育,还严重影响后备牛群质量。紧密连接蛋白是评估胃肠道屏障功能的重要指标,保持紧密连接蛋白的完整性有助于降低犊牛腹泻的发生率[23]。
血液生化指标可以反映动物的免疫力以及生理状态,并且可以反映营养物质在动物体内的代谢情况,为饲养管理提供理论支撑。犊牛血清生化指标变化可以反映其机体的代谢变化[24],血清中白蛋白(ALB)水平反映机体的营养状况和蛋白质吸收和代谢水平,其水平升高表明肝脏功能和蛋白质合成代谢能力增强[25]。在断奶应激的影响下,下丘脑-垂体-肾上腺(HPA)轴被激活,导致体内激素水平改变,如皮质醇的分泌增加[26]。HPA轴是一种应激反应性神经内分泌系统,它的激活是为了响应应激源并恢复体内平衡。断奶应激影响犊牛外周血液中性粒细胞:淋巴细胞比值以及急性期蛋白、乳铁蛋白、皮质醇和炎性细胞因子的水平[27]。免疫功能生物标志物包括血清淀粉样蛋白A(SAA)和触珠蛋白(Hp),通常在犊牛中测量SAA和Hp水平评估炎症程度,当SAA和Hp水平上升时表明犊牛炎症更加严重[28]。血清生理应激生物标志物为皮质醇,吸收后代谢生物标志物包括葡萄糖、非酯化脂肪酸(NEFA)和胰岛素。肾上腺皮质激素水平的急剧升高引发了各种组织的能量代谢,并向循环中供应足够的葡萄糖,以维持体内稳态并应对应激反应[29]。血清免疫球蛋白G(IgG)、免疫球蛋白M(IgM)和免疫球蛋白A(IgA)水平反映了体液免疫的功能状态[30],断奶后,血清中IgG、IgM和IgA水平都发生了变化,说明犊牛的免疫状态发生了变化。

2 缓解犊牛断奶应激的营养措施

2.1 营养素的过渡

出生后2~3 h内,初乳供应是成功饲养犊牛的先决条件。初乳中高水平的生长因子和细胞调节因子可以刺激犊牛小肠黏膜绒毛生长[31]。初乳期过后,犊牛依靠摄入牛奶或优质代乳料(MR)形式的液体饲料来提供营养。过渡乳(TM,初乳之后的前几次挤奶)营养水平较高,饲喂TM的犊牛体重增加了0.64 kg/d,绒毛长度、绒毛宽度、黏膜厚度、V/C升高,十二指肠、空肠和回肠的上皮细胞增殖,且提高了血液中IgG水平[32]。在出生后的前4天以TM或代乳粉(0.8 kg DM/d)饲喂额外的营养物质和生物活性化合物可促进犊牛断奶前的生长,同时降低血清Hp水平[33]。研究表明,从出生后第5天到第14周荷斯坦母犊牛饲喂高水平MR(10 L/d)可改善断奶前和断奶后的体重和ADG,但不会影响固体发酵饲料摄入量[34]。犊牛在出生后,不同阶段所需的营养物质不同。
犊牛饲养中初乳、TM以及不同饲料形式对犊牛生长和瘤胃发育有重要作用。瘤胃发育是调节早期固体采食量的重要因素,从根本上影响犊牛生长性能和饲料效率。提供一定量优质粗饲料会刺激犊牛对固体饲料的采食,有助于反刍行为的建立,降低异常挑食情况发生[35]。为了最大限度提高犊牛采食量,可选择中性洗涤纤维(NDF)含量相对较低(16%~18%)的开食料[36]。给犊牛饲喂充足的饲草可以减少瘤胃乳头并发症,增加肌肉厚度,改善瘤胃发酵环境,并减轻瘤胃上皮角化症和斑块形成[37],促进瘤胃发育。研究表明,优质干草作为唯一的固体饲料,可以替代富含精料的犊牛开食料,而不会影响犊牛采食量和生长性能,尤其是在犊牛断奶的高度敏感期[38]。断奶前,为犊牛提供代乳品和干草或犊牛发酵剂饲粮以引入断奶。断奶需要循序渐进,应继续饲喂哺乳期饲粮一段时间后再逐渐换料,换料和断奶不能同时进行[39]。优质粗饲料对幼龄反刍动物瘤胃发育、生长和健康具有关键作用,研究发现,花生藤可作为断奶犊牛饲粮中主要的粗饲料来源,提高荷斯坦断奶公犊牛ADG、饲料利用率[40]。有研究表明,饲喂饲草与精料比为20∶80的饲料可提高犊牛断奶后的采食量和生长性能,并减少反刍行为[41]。在断奶前(7周龄)、断奶过渡期(8周龄)和断奶后(9~11周龄)阶段,补充苜蓿干草可以增加拟杆菌群(一种重要的纤维素分解细菌家族)在瘤胃的丰度,并通过增加瘤胃pH促进瘤胃上皮发育和瘤胃健康[42]。
在断奶期间,犊牛从牛奶或代乳品的液体饲料过渡到固体饲料,转录因子过氧化物酶体增殖物激活受体α(PPARA)和过氧化物酶体增殖物激活受体δ(PPARD)的活性发生了转变。断奶后,PPARA被上调并激活,PPARA基因产物是一种配体激活的转录因子,调节炎症、免疫、营养代谢和能量稳态过程,同样PPARD在断奶后被上调和激活,PPARA和PPARD在调节瘤胃上皮细胞发育过程中发挥关键作用[43]。在断奶后期(10周龄),真核翻译起始因子2(eIF2)信号通路被激活和上调,eIF2是蛋白质合成的重要因子[44]。这些基因的调节与犊牛瘤胃上皮细胞的发育、营养代谢、免疫功能等密切相关,有助于犊牛适应新的饲料类型,促进犊牛健康生长。

2.2 益生菌添加剂

研究表明,枯草芽孢杆菌可改变犊牛瘤胃发酵模式,提高犊牛的生长性能[45]。补充酵母对反刍动物营养也有多种好处,最常用的酵母菌株是酿酒酵母,它可以调节微生物群落组成,增加营养物质的消化率,提高增重和饲料效率[46]。Klopp等[47]通过在断奶后的犊牛饲粮中添加酿酒酵母发酵产物(SCFP)(第57~84天添加0.44% SCFP;第85~112天添加0.275% SCFP),发现断奶后犊牛体重和ADG发生了变化,且改善了犊牛健康状况,这与Maina等[48]研究的试验结果一致:补充SCFP有益于犊牛健康和生长性能。
为了开发更有效的益生菌来缓解犊牛腹泻策略,一项研究从健康犊牛正常粪便中分离出乳酸菌菌株,其中罗伊氏乳杆菌和约翰逊乳杆菌对引起犊牛腹泻的病原体表现出强大的抗菌活性[49]。这作为一种潜在益生菌可以改善断奶前犊牛胃肠道健康。有研究表明,补充益生菌[2.0 g/(d·头)]可增加断奶后犊牛ADG,降低断奶腹泻指数,提高血清IgA、IgM和IgG水平,并从牛粪便中分离得到2种菌株在低浓度粪便菌群移植后改善了断奶犊牛的生长性能、免疫性能和肠道屏障功能,表明它们具有缓解断奶犊牛腹泻的潜力[19]。
饲喂含有优质成分的饲粮对犊牛断奶前生长、免疫功能和瘤胃发育至关重要[50]。益生菌作为抗生素生长促进剂的替代品之一,在改善动物生长性能、增强免疫功能、改善肠道菌群方面发挥着重要作用[51]。因此,补充益生菌有助于提高犊牛生长性能,改善免疫功能和肠道屏障,降低犊牛腹泻率。

2.3 植物活性物质添加剂

当归、党参和甘草是传统、广泛使用的中草药,它们的根部含有生物活性化合物,可以提高免疫力和抗氧化能力。据报道,食用党参根提取物可增加早期断奶犊牛ADG,分别饲喂80 mL/kg的3种草药根提取物会增加瘤胃丙酸盐和异戊酸酯的比例,并改变瘤胃微生物群,党参被证明是对断奶犊牛最有益的中草药[52]。研究表明,每天补充50和100 mg/kg BW葡萄籽产品可以缓解炎症反应并增强抗氧化性,可以提高犊牛生长性能[53]。
紫锥菊(EP)是一种草本植物,已经证明有提高动物免疫力、健康和生产性能的潜力。早期报道,补充250 mg EP和62.5 mg天竺葵5 d会引起犊牛免疫反应,表现为血清IgG水平增加,血液中γ-干扰素(γ-IFN)、白细胞介素-2(IL-2)和肿瘤坏死因子-α(TNF-α)的基因表达水平提高[54]。后来,Ayrle等[55]提出EP可能会激活局部肠道免疫系统。有研究给断奶前的犊牛饲喂EP(3.0 g/次),结果表明EP可降低血浆结合珠蛋白水平、减少分段中性粒细胞计数和降低中性粒细胞和淋巴细胞比值,同时增加了淋巴细胞计数[56],以此证明了EP与免疫调节和减少炎症有关。研究发现,添加的葡萄籽提取物(GSE)[4.0 g/(d·头)]可以促进断奶肉犊牛的生长[57],另外,犊牛饲粮中添加GSE[4.0 g/(d·头)]可降低小肠黏膜促炎细胞因子白细胞介素-1β(IL-1β)和TNF-α的表达,增加抗炎细胞因子白细胞介素-10(IL-10)的mRNA表达,通过抑制核转录因子-κB(NF-κB)信号通路来调节细胞炎症反应[58]。因此,GSE可能通过增强屏障功能和缓解炎症反应来改善断奶肉犊牛胃肠道结构。相关研究在断奶前给犊牛注射4.0 mL蜂胶乙醇提取物(EEP),发现EEP对犊牛生长性能和粪便结构有积极影响,并减少了腹泻[59]。辣木多糖(MOP)是一种安全高效的天然活性物质,在奶牛养殖业中可以替代抗生素缓解新生犊牛肠炎。研究发现,MOP调控犊牛肠道中丝裂原活化蛋白激酶(MAPK)信号通路、转化生长因子-β(TGF-β)信号通路、磷脂酰肌醇3激酶(PI3K)-丝氨酸/苏氨酸蛋白激酶(Akt)信号通路、肿瘤坏死因子(TNF)信号通路等的相关基因的表达,减少肠道炎症发生[60]。
近年来,植物来源的天然活性化合物在畜牧业中得到了广泛的研究应用。这些化合物包括多酚、精油、儿茶素和类黄酮,已被证明可以调节犊牛胃肠道屏障功能和营养转运,对消化器官的健康发育具有重要功能[61]。

2.4 矿物质添加剂

锰是所有动物必需的微量矿物质,在骨骼系统发育、碳水化合物和脂质代谢中具有重要的生理作用。研究发现,饲粮中添加20 mg/kg DM锰,以赖氨酸和谷氨酸螯合物(LGM)的形式按1∶1混合可增加犊牛干物质采食量、ADG和胸围;LGM可以改变犊牛粪便菌群结构,使断奶前和断奶后犊牛纤维降解菌密螺旋体菌属和阿克曼菌属的相对丰度增加,使粪便菌群更加稳定[62]。断奶期会导致犊牛免疫功能下降,使犊牛更容易生病,很多研究发现补充矿物质添加剂可以改善免疫系统,这对断奶期的犊牛有营养调控作用。断奶后,犊牛身体会处于一种分解代谢状态,这会导致大量氧反应物质的产生,从而影响犊牛的抗氧化能力[63]。在这种情况下,氧化平衡状态改变会导致氧化应激并诱发免疫功能障碍,从而间接影响犊牛生长。
总抗氧化状态(TAS)是用于评估抗氧化防御能力的参数,谷胱甘肽过氧化物酶(GPx)是主要的抗氧化酶之一[64],Mattioli等[65]通过肠外注射补充矿物质(10 mg/mL铜、40 mg/mL锌和10 mg/mL锰,以乙二胺四乙酸盐计;5 mg/mL硒,以亚硒酸钠计)和维生素(3.5%维生素A和5.0%维生素E)补充剂,发现可以防止犊牛血清TAS和GPx活性降低,在断奶前和断奶当天连续2次补充矿物质和维生素补充剂使犊牛体液免疫反应增强,犊牛ADG也有所增加。据报道,10 g/d的蛋白质-铁复合物可提高犊牛血清TAS和GPx活性,对犊牛的抗氧化状态有积极影响[66]。断奶应激可能会消耗犊牛血清中的铬,增加尿液中矿物质的排泄,增加犊牛对铬的需求。有研究表明,在暴露于夏季的断奶犊牛饲粮中添加0.05 mg/kg BW铬时,会通过增加血清过氧化氢酶活性来提高犊牛抗氧化能力[67]。蛋氨酸铬(Cr-Met)是铬的天然来源,在断奶前后向犊牛的饲粮中补充2.0 g/(d·头)益生菌和2.0 g/(d·头) Cr-Met后,发现犊牛血清皮质醇、腺苷脱氨酶(ADA)、球蛋白和白细胞介素等一些应激指标水平较低[68]。锌是一种必需的营养元素,参与体内的许多生理功能。蛋白锌是一种潜在的有机锌来源,对于断奶犊牛来说,其生物利用度高于氧化锌。有研究表明,在犊牛出生后的前2周内补充蛋白锌或氧化锌可减轻从第1天到第28天的腹泻,并提高断奶前犊牛的干物质采食量、ADG和饲料转化率。此外,添加蛋白锌可增加血清IgG水平、GPx活性和总抗氧化能力(T-AOC)以及血浆锌含量,降低血清丙二醛(MDA)水平。早期补充蛋白锌不影响直肠微生物多样性,而氧化锌降低了厚壁菌门相对丰度,同时增加了拟杆菌门相对丰度,并提高了普雷沃氏菌属、亚颗粒菌属和臭杆菌属的相对丰度[69],这有助于改善犊牛的肠道健康。
综上所述,补充适量的矿物质提高了犊牛生长性能和免疫功能,并改善犊牛肠道健康,可缓解犊牛断奶应激。

2.5 维生素添加剂

维生素A是必需营养素,在体内许多组织的形成及免疫功能中发挥重要作用。Harris等[70]向安格斯犊牛注射维生素A发现肌内脂肪沉积增加,而且发现对新生犊牛注射2次150 000 IU维生素A可使断奶时体重增加13.1%,在308日龄时体重增加8.8%,表明维生素A在犊牛肌肉发育和生长中起作用,锌指蛋白423(ZFP423)和过氧化物酶体增殖物激活受体γ(PPARG)是调节脂肪生成的2个关键转录因子。研究发现,犊牛出生后45 d,补充维生素A(45 000 IU/d)可以增加犊牛体重,促进前脂肪细胞发育,并提高肌肉发育相关mRNA表达标志物的水平[71]。研究表明,犊牛补充40 000 IU维生素A可上调背最长肌中配对盒转录因子7(PAX7)的mRNA表达,促进了断奶犊牛肌肉中卫星细胞的储存[72],这为未来的肌肉发育提供了基础。PAX7是一种关键的转录因子,对调节卫星细胞的增殖和分化至关重要。
Xu等[73]通过饲喂含25-羟基维生素D3(300 mg/kg 25-羟基维生素D3和1.2 mg/kg维生素D)的代乳品发现可提高犊牛的生长性能,使犊牛血清皮质醇和Hp水平降低,并通过增强抗氧化能力、降低炎症和应激指标来减缓断奶应激,这是改善犊牛生长和胃肠道健康的一种潜在营养调控策略。维生素E对动物生产性能、繁殖和健康具有重要的作用。维生素E又名生育酚,包括α-、β-、γ-和δ-生育酚及其相应的生育三烯酚,是一种必需营养素,不能由动物合成,应通过饲粮提供。研究发现,在犊牛发酵剂中补充245或490 mg/kg的天然维生素E,可显著提高断奶和断奶后血浆维生素E水平,此外,较高的血浆维生素E水平会使犊牛的血浆皮质醇和SAA水平降低[74]。在犊牛浓缩料中添加天然形式的维生素E会使血浆维生素E水平保持在推荐水平,并且200 mg/kg天然维生素E是在犊牛饲粮中补充α-生育酚最有效的来源,也是将断奶后犊牛血浆维生素E水平维持在推荐水平的唯一有效剂量,使犊牛断奶后的α-生育酚水平与断奶前的水平保持相近[75]。维生素C是血浆中主要的水溶性抗氧化剂,在应激发生前进行肌肉注射,可以减轻犊牛应激反应对生长和免疫功能的影响。研究发现,在断奶或转运前对早期断奶的肉犊牛注射5.0 g维生素C可增加血浆抗坏血酸水平,但并未改善生长性能[76]。B族维生素和胆碱混合物补充剂,无论是非保护还是瘤胃保护形式,都改善了断奶后犊牛血浆维生素B12状态,但叶酸水平没有改变,该补充剂没有影响犊牛在高营养摄入水平下的生长、干物质摄入量、饲料利用率或其他指标,但是在管理和营养供应条件允许、生长速度为1.0 kg/d的情况下,犊牛可以从饲粮和瘤胃合成中获得足够的B族维生素,从肝脏合成中获得胆碱[77]。
因此,补充维生素可以促进犊牛的生长性能、肌肉发育、抗氧化能力以及免疫功能,为缓解犊牛断奶应激提供了营养调控策略。

2.6 其他营养素

研究发现,单独添加1.0%谷氨酰胺(Gln)或5.0%腐植酸钠(HNa)和1.0% Gln联合添加可提高断奶犊牛ADG,并显著降低了断奶犊牛血清中促炎细胞因子TNF-α和IL-10的水平,提高了断奶犊牛血清中T-AOC和GSH活性,降低了MDA水平,表明HNa和Gln具有潜在的抗炎活性,该研究还发现HNa和Gln联合补充增加了断奶犊牛直肠中有益微生物(瘤胃丙酸杆菌、双歧杆菌和乳杆菌)的丰度[78]。因此,联合补充HNa和Gln改善了犊牛免疫功能、抗氧化能力和改变肠道有益微生物群,提高断奶犊牛生长性能并降低了腹泻率。Wang等[79]通过在饲粮中添加L-谷氨酰胺(L-Gln),发现1.0%和2.0%的L-Gln可提高犊牛ADG,使十二指肠和空肠绒毛高度更长,隐窝深度更短,V/C更大,且降低了犊牛血清皮质醇、Hp和白细胞介素-8(IL-8)含量,这有利于犊牛的健康生长。Ceja等[80]在断奶前向荷斯坦母犊牛的代乳品中补充1.0% L-Gln(以干物质计)可改善胃肠道完整性,降低生理应激的生物标志物水平。因此,L-Gln可提高犊牛免疫力和抗氧化能力,改善肠道健康,减轻断奶对犊牛的负面影响,在实际生产中可以考虑在犊牛饲粮中添加适量的L-Gln。
研究发现,糖蜜基液补充剂增加了犊牛瘤胃丁酸盐浓度,改变了胃肠道屏障功能,降低了血清Hp和脂多糖结合蛋白水平,改善了炎症状态[81]。许多研究表明,丁酸盐对于肠上皮细胞增殖、分化和绒毛发育以及在改善肠道防御系统有重要作用。丁酸钠(SB)是动物重要的营养添加剂,其改善生长性能的潜力备受关注。据报道,在断奶前犊牛起始饲粮中补充SB可促进犊牛生长和胃肠道发育,SB下调了白细胞介素-17(IL-17)、NF-κB信号通路因子的表达,抑制趋化因子和细胞因子的分泌,减少了瘤胃和空肠上皮的炎症。SB还影响瘤胃微生物群落和活化营养物质,激活碳水化合物代谢途径[82]。研究表明,断奶前犊牛补充SB的最佳剂量为8.7 g/d,并表明补充SB可以改善犊牛生长性能和肠道菌群[83]。有研究发现,犊牛断奶前起始料中淀粉和SB补充剂的相互作用显著影响犊牛瘤胃发育和生长性能[84]。总得来说,丁酸盐通过抑制炎症、促进免疫和激活瘤胃微生物代谢调节犊牛生长和胃肠道发育。

3 小结

综上所述,断奶被认为是畜牧生产中的关键时期,影响犊牛生理、代谢、内分泌等过程,导致犊牛后期生长性能下降,胃肠道功能受损,发生肠道疾病如腹泻,引起一些炎症反应,这增加了断奶期犊牛的发病率和死亡率,影响了犊牛的福利,对养殖场造成经济损失,影响了优质后备牛群的构建。相关研究人员已经开发了许多饲料添加剂来缓解犊牛断奶应激,如益生菌、植物活性物质、矿物质添加剂、维生素添加剂和其他营养素等,通过科学营养的饲喂方式调控犊牛生长性能、免疫力、抗氧化能力和胃肠道健康。尽管如此,还有很多饲料添加剂值得去研究和开发,以此促进现代规模化养殖场犊牛的健康生长。

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