动物营养学报    2022, Vol. 34 Issue (9): 5852-5865    PDF    
夏季补饲蛋氨酸铬对干奶牛热应激状态、抗氧化能力和被动免疫转移能力的影响
马翊暄1 , 沈宜钊1 , 李妍2 , 冯维3 , 张海博1,3 , 郭刚3 , 王美美1 , 孙凤莉4 , 李秋凤1,4,5 , 李建国1,4,5,6 , 曹玉凤1,4,5 , 高艳霞1,4,5,6     
1. 河北农业大学动物科技学院, 保定 071001;
2. 河北农业大学动物医学院, 保定 071001;
3. 北京首农 畜牧发展有限公司, 北京 100076;
4. 河北省畜牧兽医研究所, 保定 071000;
5. 河北省牛羊 胚胎技术创新中心, 保定 071001;
6. 河北省乳制品产业技术研究院, 石家庄 050000
摘要: 本试验旨在研究蛋氨酸铬对热应激状态下干奶牛直肠温度、抗氧化能力和被动免疫转移能力的影响, 为蛋氨酸铬在干奶牛上的合理应用提供理论依据。试验选取72头胎次、预产期和体重相近的健康经产荷斯坦干奶牛, 采用单因素完全随机试验设计, 随机分为4组(C组、L组、M组、H组), 每组18个重复, 每个重复1头牛。对照组(C组)饲喂牧场基础饲粮, L组、M组、H组在基础饲粮中分别添加有效铬含量为3、6、9 mg/(头·d)的蛋氨酸铬。试验期从产前第60天至产犊当天。试验期间每10 d测定1次直肠温度。在干奶牛产前第60、28、10天和产犊当天尾静脉采血, 测定血清生化、抗氧化及免疫指标等。奶牛产犊后立即采集初乳样品, 犊牛出生2 h内灌服初乳, 24 h后颈静脉采血, 测定血清免疫指标。结果显示: 1)饲粮添加蛋氨酸铬对各时期干奶牛15:00的直肠温度无显著影响(P>0.05);干奶牛产前第30天09:00的直肠温度随蛋氨酸铬添加量的增加而线性降低(P=0.02);产前第10天09:00, L组和H组干奶牛的直肠温度显著低于对照组(P < 0.05);从全期来看, M组和H组干奶牛09:00的直肠温度显著低于对照组(P < 0.05)。2)产前第45天, 各补饲蛋氨酸铬组干奶牛的干物质采食量(DMI)显著高于对照组(P<0.05)。3)从全期来看, M组干奶牛血清总抗氧化能力(T-AOC)显著高于对照组(P < 0.05), L组和M组血清超氧化物歧化酶(SOD)活性显著高于对照组(P < 0.05);分娩当天, 干奶牛血清丙二醛(MDA)含量随蛋氨酸铬添加量的增加呈三次降低(P=0.04)。4)产前第10天, M组干奶牛血清甘油三酯(TG)含量显著低于其他各组(P < 0.05);产前第28天, M组干奶牛血清非酯化脂肪酸(NEFA)含量显著低于对照组(P < 0.05);产前第10天, L组和M组干奶牛血清NEFA含量显著低于对照组(P < 0.05), 且血清β-羟基丁酸(BHBA)含量随蛋氨酸铬添加量的增加呈二次降低趋势(P=0.08)。5)M组初乳中免疫球蛋白G(IgG)含量显著高于对照组(P < 0.05);初乳中免疫球蛋白A(IgA)含量随蛋氨酸铬添加量的增加有线性升高趋势(P=0.08);M组初乳中免疫球蛋白M(IgM)含量显著高于对照组和H组(P < 0.05)。产前第28天, 干奶牛血清IgA含量随蛋氨酸铬添加量的增加二次升高(P=0.04);L组、M组和H组干奶牛血清IgM含量显著高于对照组(P < 0.05)。产前第10天, 干奶牛血清IgM含量随蛋氨酸铬添加量的增加呈二次升高趋势(P=0.09);分娩当天, 干奶牛血清IgG含量随蛋氨酸铬添加量的增加呈线性升高趋势(P=0.07), 且IgM含量也呈二次升高趋势(P=0.09)。M组犊牛血清IgG含量显著高于对照组和H组(P < 0.05)。综上可知, 干奶牛饲粮中添加蛋氨酸铬可以有效缓解热应激, 提高抗氧化能力和被动免疫转移能力, 其中添加有效铬含量为6 mg/(头·d)的蛋氨酸铬效果最优。
关键词: 干奶牛    蛋氨酸铬    热应激    抗氧化    被动免疫转移    
Effects of Supplementary Feeding with Chromium Methionine in Summer on Heat Stress State, Antioxidant Capacity and Passive Immune Transfer Ability of Dry Cows
MA Yixuan1 , SHEN Yizhao1 , LI Yan2 , FENG Wei3 , ZHANG Haibo1,3 , GUO Gang3 , WANG Meimei1 , SUN Fengli4 , LI Qiufeng1,4,5 , LI Jianguo1,4,5,6 , CAO Yufeng1,4,5 , GAO Yanxia1,4,5,6     
1. College of Animal Science and Technology, Hebei Agricultural University, Baoding 071001, China;
2. College of Veterinary Medicine, Hebei Agricultural University, Baoding 071001, China;
3. Beijing Shounong Animal Husbandry Development Co., Ltd., Beijing 100076, China;
4. Hebei Institute of Animal Science and Veterinary Medicine, Baoding 071000, China;
5. Embryo Engineering and Technological Center of Cattle and Sheep of Hebei, Baoding 071001, China;
6. Hebei Dairy Industry Technology Research Institute, Shijiazhuang 050000, China
Abstract: This experiment was conducted to study the effects of chromium methionine on rectal temperature, antioxidant capacity and passive immune transfer ability of dry cows under heat stress, and to provide a theoretical basis for the rational application of chromium methionine in dry cows. Seventy-two healthy Holstein dry cows with similar parity, pre-natal period and body weight were selected in the experiment. The single-factor completely randomized experimental design was adopted and they were randomly divided into four groups (groups C, L, M and H), with 18 replicates in each group and one cow in each replicate. The dry cows in control group (group C) were fed a pasture-basal diet, and those in groups L, M and H were supplemented with chromium methionine (Cr-Met) at an effective chromium content of 3, 6 and 9 mg/(head·d) in the basal diet, respectively. The experimental period was from the 60th day before the expected calving to the day of calving. Rectal temperature was measured every 10 days during the experiment. Blood samples of dry cows were collected from tail vein on the 60th, 28th and 10th day of prenatal delivery and the day of calving, and the serum biochemical, antioxidant and immune indexes were determined. Colostrum samples were collected immediately after calving, and colostrum was infused within 2 hours after birth. Blood samples of cavies were collected from jugular vein 24 hours later, and blood immune indexes were determined. The results showed as follows: 1) the rectal temperature of dry cows at 15:00 on each period was not significantly affected by dietary Cr-Met supplementation (P > 0.05), but the rectal temperature at 09:00 on the 30th day before delivery decreased linearly with the increase of Cr-Met supplemental level (P=0.02). On the 10th day before delivery, the rectal temperature of dry cows at 15:00 in groups L and H was significantly lower than that in control group (P < 0.05). During the whole period, the rectal temperature of dry cows at 09:00 in groups M and group H was significantly lower than that in control group (P < 0.05). 2) On the 45th day before delivery, the dry matter intake (DMI) of dry cows in Cr-Met supplementation groups was significantly higher than that in control group (P < 0.05). 3) During the whole period, the serum total antioxidant capacity (T-AOC) of dry cows in group M was significantly higher than that in control group (P < 0.05), and the serum superoxide dismutase (SOD) activities in groups L and M was significantly higher than that in control group (P < 0.05). On the day of delivery, the serum malondialdehyde (MDA) content of dry cows decreased cubically with the increase of Cr-Met supplemental level (P=0.04). 4) On the 10th day before delivery, the serum triglyceride (TG) content of dry cows in group M was significantly lower than that in other groups (P < 0.05). On the 28th day before delivery, the content of serum non-esterified fatty acids (NEFA) in group M was significantly lower than that in control group (P < 0.05). On the 10th day before delivery, the content of serum NEFA in groups L and M was significantly lower than that in control group (P < 0.05), and the content of serum β-hydroxybutyrate (BHBA) shows a quadratic decrease trend with the increase of Cr-Met supplemental level (P=0.08). 5) The content of immunoglobulin G (IgG) in colostrum in group M was significantly higher than that in control group (P < 0.05). The content of immunoglobulin A (IgA) in colostrum showed a linear increase trend with the increase of Cr-Met supplemental level (P=0.08). The content of immunoglobulin M (IgM) in group M was significantly higher than that in the control group and group H (P < 0.05). On the 28th day before delivery, the serum IgA content of dry cows increased quadratically with the increase of Cr-Met supplemental level (P=0.04), and the IgM content of dry cows in groups L, M and H was significantly higher than that in control group (P < 0.05). On the 10th day before delivery, the serum IgM content of dry cows tended to increase quadratically with the increase of Cr-Met supplemental level (P=0.09). On the day of delivery, the serum IgG content of dry cows tended to increased linearly with the increase of Cr-Met supplemental level (P=0.07), and the IgM content tended to increase quadratically (P=0.09). The serum IgG content in group M was significantly higher than that in control group and group H (P < 0.05). In conclusion, dietary supplementation of Cr-Met in dry cows can effectively alleviate heat stress, improve antioxidant capacity and passive immune transfer, and the effect of Cr-Met with effective chromium content of 6 mg/(head·d) is the best.
Key words: dry cows    chromium methionine    heat stress    antioxidation    passive immune transfer    

热应激是造成奶牛养殖业经济损失的重要原因之一[1]。夏季高温高湿环境容易使奶牛遭受热应激的威胁,导致奶牛代谢紊乱、乳腺损伤、产奶量降低和生育能力下降[2]。因此,如何缓解奶牛热应激是我国奶牛业所面临的巨大挑战。由于干奶牛不产生直接经济效益,人们往往忽略其重要性,然而在此阶段,胎儿在母体迅速发育,加之奶牛生理代谢发生剧变,极易出现能量负平衡和免疫能力下降等问题,同时干奶期的管理会对胎儿及新生犊牛的生长发育和健康状况产生潜在影响[3]。前人研究证明热应激会破坏干奶牛乳腺细胞的自我调控,导致下一泌乳期产奶量下降[4],还会降低初乳中免疫球蛋白G(IgG)、免疫球蛋白A(IgA)以及总蛋白的含量[5],可能会损害被动免疫传递[6]。Laporta等[7]研究发现,热应激干奶牛子代存活率降低,淘汰率明显提高。

铬是动物营养的必需微量元素,三价铬离子是胰岛素辅助因子葡萄糖耐受因子(GTF)的主要成分。铬通过GTF来协助胰岛素在机体内发挥生化作用,参与蛋白质、碳水化合物、脂质和核酸代谢[8],进而影响动物的生长、繁殖、抗病力及产品品质。研究表明,热应激条件下补饲铬可以提高奶牛的血清中免疫球蛋白含量及抗氧化能力,缓解机体热应激[9]。刘影等[10]发现,在热应激奶牛饲粮中添加丙酸铬可降低奶牛直肠温度。刘琳珊等[11]发现,在围产期奶牛饲粮中添加酵母铬可显著提高血清中免疫球蛋白含量。Mousavi等[12]发现,在热应激犊牛饲粮中添加铬可有效提高抗氧化能力,防止脂质过氧化。蛋氨酸铬是由铬与蛋氨酸形成的螯合物,为有机铬,蛋氨酸铬可借助氨基酸的吸收途径直接穿过肠细胞膜被机体吸收利用,因此与其他有机铬相比蛋氨酸铬的吸收利用率更高[13]。然而,蛋氨酸铬在缓解干奶牛热应激上的研究较少。因此,本试验旨在研究蛋氨酸铬对热应激干奶牛直肠温度、抗氧化能力和被动免疫转移能力的影响,为蛋氨酸铬在干奶牛热应激上的防控提供科学依据。

1 材料与方法 1.1 试验材料

本试验所用蛋氨酸铬由某动物营养科技有限公司提供,为蛋氨酸与铬螯合形成的有机铬,其中铬含量≥1 000 mg/kg,蛋氨酸含量≥0.9%。

1.2 试验设计

饲养试验于2021年6月25日至2021年9月5日在北京首农畜牧河南分公司(新乡市)进行。选取72头胎次(2~4胎)、预产期(产前50~70 d)和体重相近且健康无病的经产荷斯坦干奶牛,采用单因素完全随机试验设计,将试验牛随机分为4组(C组、L组、M组、H组),每组18个重复,每个重复1头牛。对照组(C组)饲喂牧场常规基础饲粮,L组、M组、H组在基础饲粮中分别添加有效铬含量为3、6、9 mg/(头·d)的蛋氨酸铬。试验从产前第60天开始,至产犊当天结束。

1.3 饲养管理

试验牛进行大群饲养,每天08:00和17:00各饲喂1次全混合日粮,将蛋氨酸铬与基础饲粮混匀,基础饲粮按NRC(2001)配制,其组成及营养水平见表 1。保证奶牛自由饮水,牛舍定期消毒。干奶牛在预产期前第21天转群,由干奶前期饲粮换为干奶后期饲粮。犊牛出生后立即称重,转至犊牛岛,2 h内灌服4 L初乳。

表 1 基础饲粮组成及营养水平(干物质基础) Table 1 Composition and nutrient levels of basal diets (DM basis)  
1.4 样品采集 1.4.1 饲料样品

在2个时期连续3 d测定奶牛的TMR采食量,用四分法取饲粮样品,测定干物质(DM)含量,从而计算每头牛每天的干物质采食量(DMI)。饲粮样品65 ℃恒温烘48 h至恒重,将样品分2份处理,粉碎后分别过10和40目筛,装入封口袋,-20 ℃保存待测饲粮中营养物质含量。

1.4.2 血液及初乳样品

分别在产前第60天、第28天、第10天及产犊当天采集血液样品。每组随机选取8头牛,晨饲时尾静脉采血15 mL,37 ℃水浴30 min后3 000×g离心15 min,将上清液分装于0.5 mL离心管中,-20 ℃保存待测。母牛产后2 h内采集初乳100 mL,待测免疫球蛋白含量。犊牛出生后立即称重,记录初生重,2 h内灌服母体初乳,灌服初乳24 h后颈静脉采血,血清制备方法同上。

1.5 测定指标 1.5.1 饲粮中营养物质含量的测定

营养物质含量测定采用实验室常规分析法,其中粗蛋白质含量通过半自动凯氏定氮仪(Kjeltec 8400,丹麦FOSS)采用凯氏定氮法测定,参照GB/T 6432—2018;粗脂肪含量采用索氏提取法测定,参照GB/T 6433—2006;钙含量采用高锰酸钾法测定,参照GB/T 6436—2018;磷含量采用分光光度法测定,参照GB/T 6437—2018;中性洗涤纤维和酸性洗涤纤维含量通过全自动纤维仪(ANKOM-A2000i,美国ANKOM科技公司)采用范氏法测定,分别参照GB/T 20806—2006、NY/T 1459—2007。

1.5.2 牛舍温湿指数的测定

试验期间,将温湿度仪挂于距地面1.5 m处,每组挂1个。每天07:00、14:00、21:00记录温度及相对湿度,并计算温湿指数,计算公式[14]如下:

式中:THI为牛舍温湿指数;T为牛舍平均温度(℃);R为牛舍平均相对湿度(%)。

1.5.3 直肠温度的测定

试验期间,每10 d测定1次直肠温度,测定时间为09:00和15:00。利用专用兽用体温计在直肠停留3 min后取出,读取数据,记录直肠温度。

1.5.4 血清指标的测定

血清甘油三酯(TG)、葡萄糖(GLU)含量与谷草转氨酶(AST)、谷丙转氨酶(ALT)活性用深圳迈瑞BS-420型全自动生化分析仪检测,检测试剂为深圳迈瑞配套生化试剂盒。血清超氧化物歧化酶(SOD)活性、总抗氧化能力(T-AOC)及β-羟基丁酸(BHBA)、非酯化脂肪酸(NEFA)、丙二醛(MDA)、热休克蛋白70(HSP70)、IgA、IgG、免疫球蛋白M(IgM)含量采用酶联免疫吸附测定(ELISA)法测定,试剂盒购自北京华悦昌生物科技有限公司,所用设备为AMR-100酶标仪。

1.5.5 初乳指标的测定

初乳中IgG、IgA、IgM含量采用ELISA法测定,设备为ELx800全自动酶标仪。

1.6 统计分析

采用SAS 9.4软件的MIXED模型进行数据分析,试验处理是固定效应,奶牛是随机效应。用Tukey法进行多重比较检验。采用contrast语句对蛋氨酸铬添加量的线性、二次和三次效应进行分析。试验结果以平均值和均值标准误(SEM)表示,P < 0.05为差异显著,0.05≤P < 0.10表示差异有显著的趋势。

2 结果 2.1 牛舍温湿指数

试验期间,07:00、14:00、21:00,牛舍平均温度分别为24.86、31.69、27.18 ℃,平均相对湿度分别为94.97%、79.12%、89.14%。如图 1所示,牛舍早(07:00)、中(14:00)、晚(21:00)平均温湿指数分别为76.17、85.13、79.45,最高达到94.27,均高于温湿指数阈值68。

图 1 牛舍温湿指数折线图 Fig. 1 Barn temperature-humidity index line chart
2.2 蛋氨酸铬对热应激干奶牛直肠温度的影响

表 2所示,产前第30天09:00,干奶牛的直肠温度随蛋氨酸铬添加量的增加而线性降低(P=0.02);产前第10天09:00,L组和M组干奶牛的直肠温度显著低于对照组(P < 0.05);从全期来看,M组和H组干奶牛09:00的直肠温度显著低于对照组(P < 0.05)。各时期干奶牛15:00的直肠温度各组之间差异均不显著(P>0.05)。

表 2 蛋氨酸铬对热应激干奶牛直肠温度的影响 Table 2 Effects of chromium methionine on rectal temperature of heat stress dry cows  
2.3 蛋氨酸铬对热应激干奶牛DMI的影响

表 3所示,干奶前期,各补饲蛋氨酸铬组干奶牛的DMI均显著高于对照组(P<0.05);干奶后期,各组干奶牛DMI差异不显著(P>0.05)。

表 3 蛋氨酸铬对热应激干奶牛干物质采食量的影响 Table 3 Effects of chromium methionine on DMI of heat stressed dairy cows  
2.4 蛋氨酸铬对热应激干奶牛血清指标的影响 2.4.1 蛋氨酸铬对热应激干奶牛血清抗氧化指标与肝脏损伤指标的影响

表 4所示,从全期来看,M组血清T-AOC显著高于对照组(P < 0.05),L组和M组血清SOD活性显著高于对照组(P < 0.05)。分娩当天,血清MDA含量随蛋氨酸铬添加量的增加三次降低(P=0.04)。产前第28天,M组血清ALT活性显著低于对照组和H组(P < 0.05),M组血清AST活性显著低于对照组和H组(P < 0.05)。从全期来看,M组血清AST活性显著低于对照组和H组(P < 0.05)。产前第28天,M组血清HSP70含量显著高于对照组(P < 0.05)。其余指标各组间均无显著差异(P>0.05)。

表 4 蛋氨酸铬对热应激干奶牛血清抗氧化指标与肝脏损伤指标的影响 Table 4 Effects of chromium methionine on serum antioxidation indexes and liver injury indexes of heat stressed dry cows
2.4.2 蛋氨酸铬对热应激干奶牛能量代谢的影响

表 5所示,饲粮中添加蛋氨酸铬对干奶牛血清GLU含量未产生显著影响(P>0.05)。产前第10天,M组血清TG含量显著低于其他各组(P < 0.05);从全期来看,M组血清TG含量显著低于其他各组(P < 0.05)。产前第28天,M组血清NEFA含量显著低于对照组(P < 0.05);产前第10天,L组和M组血清NEFA含量显著低于对照组(P < 0.05);从全期来看,M组、L组和H组NEFA含量均显著低于其他各组(P < 0.05),且M组还显著低于L组和H组(P < 0.05)。产前第10天,干奶牛血清BHBA含量随蛋氨酸铬添加量的增加呈二次降低趋势(P=0.08)。

表 5 蛋氨酸铬对热应激干奶牛能量代谢的影响 Table 5 Effects of chromium methionine on energy metabolism of heat stress dry cows  
2.5 蛋氨酸铬对初乳、母子血清中免疫球蛋白含量和犊牛初生重的影响

表 6所示,M组初乳中IgG含量显著高于对照组(P < 0.05);初乳中IgA含量随蛋氨酸铬添加量的增加有线性升高趋势(P=0.08);M组血清IgM含量显著高于对照组、L组和H组(P < 0.05),L组和H组还显著高于对照组(P < 0.05)。分娩当天,干奶牛血清中IgG含量随蛋氨酸铬添加量的增加呈线性升高趋势(P=0.07)。产前第28天,干奶牛血清中IgA含量随蛋氨酸铬添加量的增加二次升高(P=0.04)。产前第28天,L组、M组和H组干奶牛血清中IgM含量均显著高于对照组(P < 0.05);产前第10天,干奶牛血清中IgM含量随蛋氨酸铬添加量的增加呈二次升高趋势(P=0.09);分娩当天,干奶牛血清IgM含量随蛋氨酸铬添加量的增加也呈二次升高趋势(P=0.09)。M组犊牛血清中IgG含量显著高于对照组和H组(P < 0.05),L组和M组犊牛血清中IgM含量显著高于对照组(P < 0.05)。饲粮中添加蛋氨酸铬对犊牛初生重未产生显著影响(P>0.05)。

表 6 蛋氨酸铬对初乳、母子血清中免疫球蛋白含量和犊牛初生重的影响 Table 6 Effects of chromium methionine on immunoglobulin contents in colostrum and serum of cows and calves and birth body weight of calves
3 讨论 3.1 蛋氨酸铬对热应激干奶牛直肠温度的影响

夏季高温环境容易引起奶牛热应激,温湿指数是衡量奶牛热应激程度的有效指标。研究表明,当温湿指数处于68~74时,奶牛出现轻度热应激迹象,当温湿指数≥75时将导致奶牛生产性能急剧下降[15]。热应激状态往往会导致奶牛体温升高,严重影响其生产性能和健康状态[16]。单强等[17]在热应激泌乳奶牛饲粮中添加富铬酵母,结果发现,随着饲粮铬含量的增加,奶牛直肠温度有降低趋势。梁茂文等[18]在热应激奶牛饲粮中添加吡啶羧酸铬,发现补饲铬能显著降低奶牛直肠温度。本研究发现,添加有效铬含量为6和9 mg/(头·d)的蛋氨酸铬同样能降低奶牛的直肠温度,与前人用不同形式铬添加剂的研究结果一致。Mousaie等[19]发现,铬降低直肠温度的可能原因是:应激状态下,皮质醇对胰岛素起拮抗作用,阻止葡萄糖进入肌肉和脂肪组织,使其不被吸收,添加铬可以降低皮质醇含量,从而发挥抗应激作用。

3.2 蛋氨酸铬对热应激干奶牛DMI的影响

当奶牛容易处于热应激状态时,其体温的升高会延长瘤胃内食糜的留滞时间,瘤胃壁上的胃伸张感应器作用于下丘脑的厌食中枢,使奶牛食欲下降[20],同时为避免营养物质消化和代谢产生额外的热负荷,所以奶牛采食量降低[21]。许多研究表明,饲粮中添加有机铬会提高奶牛的DMI。Yasui等[22]在围产期奶牛饲粮中添加丙酸铬,发现饲喂丙酸铬的奶牛产前DMI有增加的趋势。Hung等[23]在热应激杂交羊饲粮中添加纳米吡啶甲酸铬,结果发现提高了平均日采食量。本试验中同样发现各补饲蛋氨酸铬组干奶牛的DMI均高于对照组,说明补饲蛋氨酸铬可以提高热应激奶牛的DMI,这可能是因为蛋氨酸铬降低了干奶牛的体温,缓解了因为高温引起的食欲抑制,所以提高了DMI。

3.3 蛋氨酸铬对热应激干奶牛抗氧化能力及肝脏损伤的影响

在正常情况下,机体自由基的产生和消耗处于动态平衡[24],但热应激会导致自由基的产生量增加,超过机体的消耗能力,从而导致氧化应激[25]。机体SOD是清除自由基的主要物质,能抵抗和阻断氧自由基造成的损伤,并及时修复受损细胞,机体T-AOC代表抗氧化能力,而MDA是生物膜遭受氧自由基攻击形成的产物,是反映氧化应激的指标[26]。Seifalinasab等[27]发现热应激条件下饲喂蛋氨酸铬能提高羔羊血清T-AOC,降低MDA含量。Zhang等[28]在奶牛上的研究也发现,热应激条件下在饲粮中添加吡啶甲酸铬可以降低血清MDA含量,提高T-AOC和SOD活性。本研究中,添加有效铬含量为6 mg/(头·d)的蛋氨酸铬同样发现了血清T-AOC、SOD活性升高和MDA含量下降,说明添加蛋氨酸铬有提高奶牛抗氧化能力,降低氧化损伤的作用。

血清中ALT和AST的活性是反映代谢性肝脏损伤的指标。毛亚芳等[29]在绵羊上的研究发现,酵母铬能降低血浆ALT、AST活性,与本试验结果一致,说明补饲铬可能有降低肝脏损伤的作用。本研究发现,干奶牛血清AST和ALT活性的变化规律与T-AOC和SOD活性的变化规律一致,而赵娇[30]研究发现氧化损伤会导致机体的肝脏损伤,因此本研究中补饲蛋氨酸铬组干奶牛血清AST和ALT活性的下降可能与其的氧化损伤较低有关,且添加有效铬含量为6 mg/(头·d)的蛋氨酸铬,干奶牛血清中AST和ALT活性最低,对缓解奶牛肝脏损伤作用最好。

热休克蛋白普遍存在于动物体内,其中HSP70是一种胞质分子伴侣,能够缓解热损伤,并且能增强机体的耐热力[31],是一种公认的衡量奶牛热应激程度的标志蛋白。杨金泽[32]研究发现,随着丙酸铬添加量的增加,奶牛血清中HSP70含量有线性升高趋势,与本试验研究结果相似。本试验中,M组血清HSP70含量显著高于对照组,表明饲粮中添加有效铬含量为6 mg/(头·d)的蛋氨酸铬可提高机体热应激耐受力,可能是因为补铬后机体抗氧化能力提高,氧自由基减少,进而受损和变性的生物大分子减少,从而导致HSP70的消耗降低。

3.4 蛋氨酸铬对热应激干奶牛能量代谢的影响

血清BHBA、NEFA、GLU含量可反映奶牛糖、脂代谢中的能量变化情况,是衡量机体能量代谢的重要指标[33]。干奶期奶牛体脂动员加剧,产生大量NEFA,加重肝脏负担,未被清除的NEFA最终以TG形式储存在肝脏,造成肝脏损伤,损害奶牛健康[34]。热应激奶牛为维持能量需要,会加快体脂分解,导致血液中TG和NEFA含量增加。有研究表明,围产期奶牛饲粮中添加有机铬显著降低了产后血清TG和NEFA含量[35-36]。本研究在干奶牛饲粮中添加蛋氨酸铬同样发现血清TG和NEFA含量降低,并且M组显著低于对照组,代表着添加有效铬含量为6 mg/(头·d)的蛋氨酸铬对缓解能量负平衡有一定促进作用,也说明了蛋氨酸铬对维护肝脏功能起到了积极作用。

3.5 蛋氨酸铬对热应激干奶牛血清免疫蛋白含量、被动免疫转移能力的影响

初乳中的免疫球蛋白能抵抗细菌和病毒的侵袭,有效保护机体免受感染[37]。新生犊牛自身的免疫系统发育未成熟,不能有效应对外来抗原的侵袭,因此,犊牛出生后必须摄入足够的初乳免疫球蛋白来获得被动免疫[37]。妊娠后期的热应激会降低初乳中IgG和IgA的含量[38],损害新生犊牛的被动免疫转移[6]。已有试验证实,热应激通过降低血清免疫球蛋白含量来抑制牛的免疫功能[39]。范春玲等[40]在围产后期奶牛饲粮中添加酵母铬,发现血清中IgG、IgA和IgM含量提高。本试验结果与前人研究结果一致,说明饲粮中添加蛋氨酸铬可能通过缓解热应激提高血清免疫球蛋白含量进而增强奶牛的免疫性能。肖芳[41]在母羊饲粮中添加丙酸铬,发现初乳中IgA、IgG和IgM含量提高,子代羔羊血清IgG和IgM含量提高。本研究使用蛋氨酸铬同样发现了干奶牛血清及初乳中免疫球蛋白含量升高,同时饲粮中添加有效铬含量为6 mg/(头·d)的蛋氨酸铬的效果优于其他添加量,且L组和M组犊牛血清IgG和IgM含量显著提高,说明添加蛋氨酸铬可提高奶牛免疫力和产后被动免疫转移能力。

4 结论

在本试验条件下,干奶牛饲粮中添加适量的蛋氨酸铬可以有效缓解热应激,提高奶牛的抗氧化能力和产后被动免疫转移能力,其中添加有效铬含量为6 mg/(头·d)的蛋氨酸铬为宜。

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