植物精油(EOs)是通过水蒸气蒸馏法、有机溶剂萃取法、微波辅助蒸馏法、超临界二氧化碳萃取法等方法从植物组织(花、叶和果实等)内提取的植物次生代谢产物，其中含有大量醛、酮、酯、萜烯、酚及苯丙素类化合物。随着欧盟(2003年)与我国(2020年)禁止在动物饲粮中添加促生长类抗生素，植物提取物作为新型绿色饲料添加剂受到人们的广泛关注，其中EOs因具有多种生物活性而引起了研究者极大的研究兴趣。EOs中的活性成分通常可以通过调节机体代谢影响动物的免疫性能，同时促进动物生长[1-3]，还通过减少畜产品中病原微生物数量提高畜产品品质、延长货架期[4]等，因而被认为具有良好的替抗潜力。近年来EOs在反刍动物上的应用逐渐成为热点，许多研究先后报道了EOs通过调节瘤胃微生物改善瘤胃发酵、提高反刍动物生产性能等方面的作用[5-6]。本文综述了EOs在反刍动物生产中的应用效果及其作用机制。
1 EOs的生物学活性EOs的生物学活性主要包括抗菌、抗氧化及抗炎活性。EOs的抗菌活性主要体现为减少微生物数量/丰度、抑制微生物生命活动。EOs可通过亲脂性吸附到细菌细胞膜上，改变膜通透性或通过直接作用对细胞膜结构产生影响，从而干扰部分维持细胞代谢必需的蛋白质/酶依赖性反应(如电子传递、离子梯度和/或细胞膜电位的维持、蛋白质转运、磷酸化和ATP的产生)，或引起细胞质凝聚和细胞裂解，最终抑制细菌的生长和增殖[7]。在动物生产中，EOs可通过改变细胞膜的通透性/结构来调节瘤胃微生物群落[5, 8]、减少畜产品中的食源性致病菌[9-10]，并减少动物细菌性疾病的发生[11-13]。
EOs的抗氧化活性表现为减少由自由基及其氧化反应带来的细胞损伤，EOs中富含多羟基和双键等还原性官能团的活性化合物，可以提供还原氢或电子来结合自由基，或提高机体内超氧化物歧化酶(SOD)、谷胱甘肽过氧化物酶(GSH-Px)、过氧化氢酶(CAT)等抗氧化酶活性，强化机体抗氧化防御系统[14]。在动物体内外研究中，EOs可通过螯合过渡金属离子来提高体内某些抗氧化酶的活性[15-17]、减少细胞和组织内丙二醛(MDA)及活性氧自由基(ROS)的含量[18-21]等方式达到体内外抗氧化的效果。
EOs的抗炎活性指EOs中的活性化合物帮助机体抵御炎症的发生和蔓延，组织体内各类因子对刺激进行抵御。炎症一般是由生物、化学和物理等伤害性刺激引起的组织和机体对损伤的正常反应，炎症的发生常伴随着炎性、氧化通路的过度激活和炎性因子、氧化因子的过表达[16]。在动物体内外研究中，EOs可通过抑制核因子-κB(NF-κB)和丝裂原激活蛋白激酶(MAPK)信号转导、调节血液及细胞内免疫因子及炎性因子的表达量等方式缓解细胞的炎性及应激反应，从而提高机体免疫和抵抗力[16, 22-24]。
2 EOs在反刍动物生产中的应用研究 2.1 EOs对反刍动物瘤胃发酵的影响及其作用机制反刍动物瘤胃微生物包括细菌、真菌和原虫，这些瘤胃微生物可利用底物发酵产生挥发性脂肪酸(VFA)和氨态氮(NH3-N)，合成微生物蛋白(MCP)，为动物提供能量和基本营养物质[25]。不同的瘤胃发酵模式，对反刍动物生产性能的影响不同。对于肉牛而言，瘤胃VFA中丙酸含量高有利于增重；而对奶牛来说，乙酸含量高更有利于乳脂的合成。研究显示，EOs可以改变瘤胃发酵模式，如饲粮中添加23 g/kg DM的百里香精油可以在不影响总短链脂肪酸(SCFA)含量的前提下降低荷斯坦犊牛瘤胃丙酸和丁酸含量，提高乙酸/丙酸比例[26]；Silva等[5]在荷斯坦奶牛饲粮中添加0.044 g/kg DM的混合EOs后瘤胃总VFA、乙酸、丁酸及支链脂肪酸含量得到提高。而在内洛尔肉牛饲粮中添加0.09 g/kg DM的混合EOs对总VFA含量无显著影响，但降低了乙酸含量[27]；EOs降低肉牛瘤胃乙酸含量、提高丙酸含量的效果也有报道[28]。关于绵羊的研究显示，饲喂7 g/d的牛至油钴混合物可显著降低瘤胃总VFA和乙酸含量[29]。另有研究显示，EOs对犊牛、肉牛和奶牛的瘤胃发酵没有明显的调节作用[8, 28, 30]。由此可见，EOs的添加对瘤胃发酵模式产生不同的影响，可能与动物品种、EOs的种类和剂量有关。瘤胃中的NH3-N是合成MCP的重要氮源，在一定范围内瘤胃NH3-N含量的提高可能有利瘤胃蛋白质代谢。研究显示，在荷斯坦奶牛饲粮中添加0.044 g/kg DM的混合EOs降低了NH3-N含量[5]；0.09 g/kg DM的混合EOs可降低肉牛瘤胃NH3-N含量[27]；7 g/d DM的牛至油钴混合物可显著降低绵羊瘤胃NH3-N含量[29]。然而，也有研究报道EOs会提高肉牛和奶牛的瘤胃NH3-N含量[25, 31-32]。EOs降低瘤胃NH3-N的含量可能与瘤胃氮元素的吸收及MCP合成的提高有关；一些EOs没有影响/提高瘤胃的NH3-N含量，可能是瘤胃产氨微生物(HAP)对EOs产生了适应性，使HAP的正常生理代谢不再受到影响[25]。
EOs可通过影响反刍动物的瘤胃微生物区系从而影响瘤胃发酵。据报道，拟杆菌门(Bacteroidetes)微生物可降解饲粮中半纤维素、蛋白质和淀粉，产生VFA、NH3-N及其他小分子化合物，而添加牛至油可增加荷斯坦奶牛瘤胃中普雷沃氏菌属(Prevotella)、小杆菌属(Dialister)及纤维杆菌门(Fibrobacteres)的丰度，影响乙酸含量及其他各VFA的比例[33]；而牛至油、大蒜油和薄荷油可通过降低溶纤维丁酸弧菌属(Butyrivibrio fibrisolvens)、Syntrophococcus sucromutans和Acetanaerobacterium的丰度降低乙酸、丁酸的含量[34]。也有研究显示，饲喂混合EOs对奶牛瘤胃微生物区系及VFA含量没有显著影响[35]。瘤胃NH3-N含量与HAP的脱氨作用相关，HAP主要通过斯提柯兰氏反应进行氨基酸的脱氨基作用并产生氨[36]。研究显示，混合EOs可通过对细胞膜的作用抑制不同的蛋白水解菌(Megasphaera elsdenii等)的数量，从而导致瘤胃蛋白质的脱氨反应被抑制及NH3-N含量的降低[37]。除HAP的作用外，瘤胃原虫的活动也会增加氨产量，EOs也可以通过降低瘤胃产氨相关原虫的数量来降低NH3-N含量[38]。
2.2 EOs对反刍动物瘤胃甲烷(CH4)产量的影响及其作用机制反刍动物的CH4有87%来自瘤胃、13%来自大肠，产甲烷菌可利用瘤胃发酵中产氢细菌、厌氧真菌和原虫产生的氢气(H2)生化还原二氧化碳(CO2)产生CH4，造成营养物质中2%~15%的能量损失[25]。据报道，EOs可影响反刍动物的CH4产量。如体外研究显示，0.5 g/L的丁香油、桉叶油和牛至油均可降低体外瘤胃发酵液中的CH4产量[31]；Kouazounde等[39]研究表明，添加0.4 g/L的香樟精油、罗勒精油及柑橘精油可降低发酵液中的CH4产量。体内研究中，Castro-Montoya等[40]饲喂0.2 g/d混合EOs 6周后肉牛CH4排放量降低了20%；Kolling等[41]的研究报道了饲喂10 g/d的牛至油降低奶牛瘤胃CH4产量。也有报道显示，饲喂0.15 g/kg DM的混合精油对肉牛瘤胃CH4产量没有显著影响[41]；饲粮中添加0.05 g/kg DM的牛至油对奶牛瘤胃及肠道CH4的排放量也无显著影响[42]。但大多数学者认为，EOs可通过影响瘤胃甲烷菌数量/区系降低CH4产量[25, 28, 30]。
EOs影响瘤胃CH4产量主要有2种机制：一是通过直接作用影响甲烷菌的数量及丰度从而减少CH4产生；二是通过减少合成CH4的底物H2和CO2来降低CH4产量。体外研究报道，桉叶油、肉桂油和莳萝籽精油混合可通过降低产甲烷古菌的丰度来降低CH4产量[8]。Khorrami等[43]的研究显示，荷斯坦公牛饲粮中添加百里香精油和肉桂油均可降低瘤胃产甲烷菌的相对丰度，从而降低CH4产量。EOs还可通过直接作用于甲烷菌的细胞膜或影响甲烷菌在瘤胃上皮及瘤胃液中的定植来调控甲烷菌的数量及丰度[43]。一些研究认为，瘤胃乙酸和丁酸含量的增加可促进CH4的产生，而丙酸含量的增加会竞争性地抢夺CH4的合成底物H2而抑制CH4的产生[33, 44]。据相关研究报道，EOs可通过影响瘤胃琥珀酸弧菌科(Succinivibrionaceae)、白色瘤胃球菌(Ruminococcus albus)、黄色瘤胃球菌(Ruminococcus flavefaciens)及原虫的数量改变乙酸、丙酸、丁酸的含量，从而改变3种VFA与CH4间的H2竞争关系[8, 33, 44]，最终影响CH4的产生。CO2可在瘤胃微生物及多种酶的联合作用下被还原为C1基团，供给合成CH4的碳源[45]；EOs可能通过抑制CO2向CH4的流动或促进CO2向VFA的流动来抑制CH4的产生，但具体的作用机制还需要更多的研究。
2.3 EOs对反刍动物生产性能的影响及其作用机制据研究报道，EOs在改善反刍动物生产性能上具有一定的效果，如在颗粒料中添加44.1 ppm的混合EOs可提高荷斯坦犊牛的平均日增重(ADG)、干物质采食量(DMI)及饲料转化率[2]；饲喂0.09 g/kg DM的混合EOs可提高内洛尔肉牛的DMI、ADG及胴体重[46]；饲喂1 g/d的混合EOs可提高荷斯坦奶牛DMI及产奶量[47]；饲喂4和7 g/d的牛至油钴混合物可显著提高绵羊瘤胃ADF和中性洗涤纤维(NDF)消化率[29]；而饲喂0.3 g/kg DM的柑橘精油可提高泌乳母羊的DMI和产奶量[48]。然而，也有研究显示，犊牛代乳中添加1 g/d的混合精油对断奶前后犊牛的ADG、DMI、各营养物质采食量及体况评分均无显著影响[1]；饲喂8 mL/d的百里香精油或4 g/d的丁香精油对内洛尔肉牛的DMI、ADG、各营养物质的采食量及消化率或改善胴体性状无显著影响[32, 49]；饲喂0.05 g/kg DM的牛至油或百里香精油对荷斯坦奶牛的DMI、营养物质消化率及产奶量没有显著提升作用[42, 50]。由此可见，EOs对动物生产性能的影响效果不一致。EOs对犊牛采食量的不同影响可能与其添加方式有关[1-2, 13, 51]，EOs对奶牛或肉牛的不同作用效果可能受到EOs种类和剂量的影响，从而影响动物的采食量与瘤胃微生物区系，进而影响营养物质的消化，最终影响动物生产性能[32, 42, 50]。
EOs主要通过影响营养物质的利用调控反刍动物的生产性能。EOs影响营养物质消化率的可能机制主要有酶促机制、细胞机制及微生物机制等。研究显示，EOs可通过提高α-淀粉酶、β-葡萄糖苷酶和纤维素酶的活性提高瘤胃营养物质的降解率[6, 38]；EOs也可通过提高瘤胃乳头长度、调节瘤胃上皮细胞的发育来提高瘤胃对营养物质的消化和吸收能力[6]；EOs还可以通过调节瘤胃营养代谢相关微生物的数量及区系而影响瘤胃对营养物质的消化与利用。因为瘤胃对不同营养物质的消化能力有赖于强大的微生物系统，如拟杆菌门中的普雷沃氏菌属包含更多的瘤胃蛋白降解菌[34]，而厚壁菌门(Firmicutes)中的丁酸弧菌属(Butyrivibrio)、瘤胃球菌属(Ruminococcus)和毛螺菌科(Lachnospiraceae)具有更强的多糖及半纤维素降解能力[34]。研究显示，牛至油可提高瘤胃中编码碳水化合物活性酶基因相关拟杆菌门微生物Parabacteroides、坦纳菌属(Tannerella)和Coprobacter的丰度，从而促进瘤胃对植物结构性多糖的降解能力[6]。而Patra等[34]通过RumenBactArray分析得出牛至油和薄荷油可抑制瘤胃中琥珀酸弧菌属(Succinivibrio)、瑞丁放线菌属(Actinomyces ruminicola)的丰度, 提高普雷沃氏菌属的丰度，从而改变瘤胃对纤维、淀粉及蛋白质的降解能力；另有研究显示，艾草精油和桉叶油可通过提高瘤胃球菌属的相对丰度促进瘤胃纤维的降解[52-53]。综上所述，精油可通过调控瘤胃微生物数量及区系而影响反刍动物对营养物质的消化利用，从而影响动物的生产性能。
2.4 EOs对反刍动物畜产品品质的影响及其作用机制研究发现，一些EOs具有改善肉品质的作用，如饲喂牛至油或丁香油的肉牛皮下脂肪厚度、牛肉嫩度及肉中的多不饱和脂肪酸含量更高[21, 54]；饲喂1.33 g/d的迷迭香和1.33 g/d的丁香精油可提高牛肉肌节长度、可溶性胶原蛋白含量及肉的嫩度[54]。饲喂0.3 g/kg DM的牛至油也可提高羊肉中多不饱和脂肪酸的含量及肉的抗压强度，并改善肉色[55]。但也有研究显示，饲喂EOs对牛肉的pH、肉色及蛋白质含量无显著影响[28]。脂质氧化会对牛肉的口感与存放时间产生不利影响，EOs可通过降低牛肉中ROS和MDA含量提高肉的氧化稳定性[19, 21]。乳的风味和营养价值与乳中的乳脂、乳蛋白、乳糖、微量元素及风味物质含量有关。一些研究报道了EOs改善乳品质的效果，如饲喂1 g/kg DM的香菜精油或牛至油可使EOs中的柠檬烯、ρ-伞花烃、香芹酚经奶牛的呼吸和肠道消化系统进入乳中，给牛奶带来特殊香味[56]；饲喂0.044 g/kg DM的混合EOs可提高乳蛋白含量[5]；而饲喂5 g/d的大蒜油或2 g/d杜松子油可提高牛奶中共轭亚油酸的含量，提高牛奶的营养价值与风味[57-58]；饲喂0.3 g/kg DM的柑橘精油可提高羊奶中乳蛋白的含量[48]。也有一些研究显示，饲喂0.044 g/kg DM的混合精油或含有4.21%牛至油的牛至叶对乳蛋白、乳糖、乳脂含量均没有显著影响[5, 57]。
肉的嫩度受肌肉脂肪酸与蛋白质等的含量与分布影响。研究显示，饲喂EOs后牛肉嫩度与脂肪含量的提高可能与肉牛增重速率与饲粮蛋白质转化率的提高导致肌间脂肪沉积增加与肌肉蛋白质转化速率加快有关[59]。EOs中的含氧萜类可通过清除牛肉中的ROS抑制蛋白酶的活性，提高肉的嫩度[59]。肉色与肌肉中肌红蛋白的含量有关，肌肉中ROS对肌红蛋白的氧化作用可导致肉色变化，而EOs中的含氧萜类化合物可以结合肉中的ROS，从而降低肌红蛋白的氧化程度，延缓肉色的氧化损失[54]。研究报道，瘤胃丁酸弧菌属、Anaerovoax、毛螺菌属(Lachnospiraceae incertae sedis)、梭菌目(Clostridiales)和瘤胃球菌属可通过生物加氢反应利用营养物质降解得到还原氢并用于合成肉与奶中的多不饱和脂肪酸[34]；添加牛至油和薄荷油可通过增加上述微生物的相对丰度促进生物加氢反应，解释了EOs改善乳和肉中脂肪酸含量的机制[34, 59]。乳中蛋白质来源于瘤胃中微生物的代谢氮，普雷沃氏菌属被认为在利用NH3-N方面具有较强的能力[34]，研究发现，牛至油对瘤胃其他微生物的抑制作用可能强于普雷沃氏菌属，因此在其作用下普雷沃氏菌属的相对丰度增大，奶牛瘤胃氮的利用率提高，MCP的合成及氮向乳的转移效率也更高，因此牛乳蛋白含量也会提高[34]。
2.5 EOs对反刍动物机体免疫性能和抗炎反应的影响及其作用机制外界刺激引起的炎症、感染、组织损伤等机体反应可导致动物免疫力及健康状态下降，从而影响生产性能[28]。一些研究报道了EOs在改善反刍动物免疫性能上的潜力，如饲粮加入0.3 g/kg DM或初乳中加入0.4 g/d的混合EOs可降低荷斯坦犊牛血液γ-谷氨酰转移酶(GGT)、谷草转氨酶(AST)、谷丙转氨酶(ALT)、SOD活性和MDA的含量，增强犊牛的抗氧化应激和抗肝损伤能力[3, 13]；犊牛颗粒料中添加44.1 ppm的混合EOs与益生元复合物可提高血清免疫球蛋白G(IgG)、免疫球蛋白M(IgM)、免疫球蛋白A(IgA)和总蛋白含量，缓解犊牛腹泻并提高其免疫力[2]。在肉牛研究中，饲粮中添加EOs可降低血液触珠蛋白(HP)浓度以及嗜碱性粒细胞和中性粒细胞的比例，并增强肉牛对疾病的抵抗力，降低瘤胃亚急性酸中毒(SARA)的风险[28]。在奶牛研究上，体外细胞研究显示75 μg/mL的薄荷精油可加强乳腺细胞抵御乳房炎的能力[60]；此外EOs还可以通过抗菌机制抑制奶牛乳房炎致病菌从而预防和缓解乳房炎[61]。另外，饲喂0.45 g/kg DM的柑橘精油可提高母羊血浆抗氧化酶的活性[48]。另有部分研究显示，饲喂0.3 g/kg DM的混合EOs或23 g/kg DM的百里香精油或桉叶油没有显著改变犊牛血液免疫球蛋白、MDA含量和肝酶活性[3, 26]；饲喂EOs对肉牛肝脓肿的预防作用不明显[62]；饲喂1 g/d的混合EOs对围产期奶牛肝脏的炎性反应、血液肝酶活性与HP、瘤胃内脂多糖(LPS)含量均无显著影响[63]。
体内过量ROS导致的脂质氧化可导致细胞膜结构破坏、生理功能改变，动物体可通过血液释放抗氧化酶(SOD、CAT、GSH-Px等)来抵御机体的氧化损伤[64]。在犊牛试验中发现，EOs可通过清除细胞内的ROS、调节动物代谢及激素水平而提高抗氧化酶活性[13, 64]。犊牛肝脏组织及功能的损伤会引起血液中肝酶活性的升高，EOs可通过清除自由基缓解肝脏的氧化损伤并降低肝酶活性[65]。EOs对血液免疫球蛋白含量的提升效果可追溯至EOs促进免疫细胞分化和分泌抗体的作用，但相关细胞机制目前尚不明确。在肉牛的研究中发现，EOs可通过降低血浆触珠蛋白浓度缓解SARA引起的炎性反应[28]；肉牛肝脓肿的发生与饲喂高精料饲粮(>70%)及牧场的管理有关，然而，饲喂EOs对预防及缓解肝脓肿并无显著效果[62]。在奶牛体内，乳房炎和肝脏炎症的发生受到诸多因素的影响，且炎性反应会引起细胞中NF-κB等信号通路及免疫机制的激活，血液白细胞介素-1β(IL-1β)、肿瘤坏死因子-α(TNF-α)、红系衍生的核因子2相关因子(Nrf2)等细胞因子的表达量上升[66]，EOs可通过激活巨噬细胞吞噬机制、调节细胞Balb/c免疫机制的应答、降低多态核中性粒细胞的浸润及IL-1β和TNF-α的mRNA表达来加强奶牛乳腺细胞抵御乳房炎的能力[60]。然而，EOs似乎无法通过降低肝细胞NF-κB相关因子的mRNA表达量来缓解奶牛肝脏的炎症，也无法通过降低Nrf2相关因子的mRNA表达量来提高肝细胞的抗氧化损伤能力[63]。综上所述，EOs可通过清除体内ROS、提高抗氧化酶活性而提高反刍动物的抗氧化能力，且在缓解肉牛SARA和奶牛乳房炎上具有一定的功效，但对肉牛及奶牛的肝脏炎症的缓解作用目前的研究还未见成效。
3 小结与展望EOs利用其特有的生物活性，在调节反刍动物瘤胃发酵、减少CH4产量、提高动物生产性能、改善乳和肉品质及提高动物免疫性能上具有很大潜力。EOs可通过抗微生物活性调控瘤胃微生物数量和区系从而影响瘤胃发酵模式、降低CH4的产量并提高营养物质的消化率，提升反刍动物的生产性能，同时提高动物的免疫力，促进动物健康。近年来，随着一些细胞分子技术的蓬勃发展，关于EOs在动物体内抗炎、抗氧化的作用及机制的研究也更加广泛。未来EOs在反刍动物生产上的应用还存在诸多亟需解决的问题，如长期对动物使用时会导致瘤胃微生物的适应性，可能需要调整EOs的剂量或剂型；不同种类EOs间的混合作用效果如何，体内抗炎、抗氧化作用的细胞机制如何等仍需要更多研究来验证。
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