Effects of Tannin Extracts on Performance, Nutrient Apparent Digestibility and Blood Indicators of Early-Lactating Holstein Cows

REN Shuai; SHEN Yizhao; XU Hongjian; LI Yan; ZHAO Xiaojing; HUO Zihan; WANG Zhiyuan; WANG Meimei; SUN Fengli; LI Jianguo; GAO Yanxia

doi:10.12418/CJAN2023.530

2023 , Vol. 35 >Issue 9: 5755 - 5764

DOI: https://doi.org/10.12418/CJAN2023.530

RESEARCH PAPER

Effects of Tannin Extracts on Performance, Nutrient Apparent Digestibility and Blood Indicators of Early-Lactating Holstein Cows

REN Shuai ^,¹ ,
SHEN Yizhao ¹^,² ,
XU Hongjian ¹^,² ,
LI Yan ³ ,
ZHAO Xiaojing ⁴ ,
HUO Zihan ¹ ,
WANG Zhiyuan ¹ ,
WANG Meimei ¹^,² ,
SUN Fengli ⁵ ,
LI Jianguo ^,¹^,²^,⁶^,⁷^,^* ,
GAO Yanxia ^,¹^,²^,⁶^,⁷^,^*

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¹ College of Animal Science and Technology, Hebei Agricultural University, Baoding 071001, China
² Key Laboratory of Healthy Breeding in Dairy Cattle (Co-Construction by Ministry and Province), Ministry of Agriculture and Rural Affairs, Baoding 071001, China
³ College of Veterinary Medicine, Hebei Agricultural University, Baoding 071001, China
⁴ Baoding Vocational and Technical College, Baoding 071001, China
⁵ Hebei Institute of Animal Sciences and Veterinary Medicine, Baoding 071001, China
⁶ Hebei Technology Innovation Center of Cattle and Sheep Embryo, Baoding 071001, China
⁷ Hebei Dairy Industry Technology Research Institute, Shijiazhuang 050000, China

* LI Jianguo, professor, E-mail: jgli@hebau.edu.cn;

GAO Yanxia, professor, E-mail: yxgaohebau@126.com

Received date: 2023-03-17

Online published: 2023-09-13

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Abstract

This study was conducted to examine the effects of diets supplemented with different levels of tannin extracts on performance, nutrient apparent digestibility and blood indicators of early-lactating Holstein cows. Sixty Holstein cows in early lactation [(77.85±3.23) days in milk, (2.54±0.21) parities, milk yield was (46.83±1.92) kg/d] were divided into 4 groups by a randomized complete design, and each group had 15 cows. Cows in control group were fed a basal diet, and those in experimental groups were fed the basal diet supplemented with 10, 20 and 30 g/(d·head) tannin extracts (It was composed of chestnut tannin extract and white wood tannin extract, and the total content of tannin was 73%) for 56 d. The results showed as follows: 1) although dry matter intake, fat- and protein-corrected milk yield and feed efficiency were not significantly affected by tannin extracts supplementation (P>0.05), the milk yield, milk protein percentage and milk protein yield increased linearly (P<0.05), the milk urea nitrogen content decreased linearly (P<0.05), and the somatic cell count tended to decrease linearly with increasing tannin extracts supplementation (P=0.10). 2) The apparent digestibility of nutrients was not significantly affected by tannin extracts supplementation (P>0.05). 3) Supplementation of tannin extracts did not significantly affect the contents of serum glucose, β-hydroxybutyric acid, non-esterified fatty acid, total protein, albumin, creatinine, and the activities of serum aspartate aminotransferase and alanine aminotransferase (P>0.05), whereas serum total antioxidant capacity and activities of superoxide dismutase and glutathione peroxidase increased linearly (P<0.05), and the contents of serum urea nitrogen (P<0.05) and malondialdehyde (P=0.05) decreased linearly with increasing tannin extracts supplementation. In conclusion, feeding tannin extracts to early-lactating Holstein cows can improve the performance by increasing antioxidative status and nitrogen efficiency without affecting dry matter intake and nutrient apparent digestibility. The optimal supplemental level of tannin extracts under this experimental condition should be 30 g/(d·head).

Key words： tannin extracts; dairy cows; performance; nutrient apparent digestibility; antioxidant capacity

Cite this article

REN Shuai , SHEN Yizhao , XU Hongjian , LI Yan , ZHAO Xiaojing , HUO Zihan , WANG Zhiyuan , WANG Meimei , SUN Fengli , LI Jianguo , GAO Yanxia . Effects of Tannin Extracts on Performance, Nutrient Apparent Digestibility and Blood Indicators of Early-Lactating Holstein Cows[J]. Chinese Journal of Animal Nutrition, 2023 , 35(9) : 5755 -5764 . DOI: 10.12418/CJAN2023.530

在泌乳早期,由于采食高峰期滞后于产奶高峰期^[1],易出现摄入的能量不能满足高产奶牛机体维持需要和泌乳需要的情况,使奶牛出现能量负平衡^[2]。能量负平衡可能导致增加的氧化产物不能被及时清除,产生氧化应激。氧化应激会降低动物生产性能,诱发疾病,甚至导致动物死亡。因此,奶牛需要一些抗氧化剂来防止氧化应激。单宁是一种广泛存在于乔木、灌木、豆科植物、禾本科植物和谷物中的天然多酚类物质,是一类天然的抗氧化剂,有着较强的抗氧化活性^[3]。单宁根据化学结构不同分为水解单宁和缩合单宁^[4-5],可与蛋白质、氨基酸、金属离子和多糖等形成复合物。近些年来,通过对单宁提取物进行深入研究,发现单宁提取物对蛋白质有较强的亲和力,可提高蛋白质的过瘤胃水平^[6]以及机体对氮的利用率^[7-8]。另有报道称,单宁提取物具有抗氧化、抑菌、抗炎症和改善肠道健康的功效^[9]。研究发现,饲粮中添加0.1%的单宁能够提高奶牛生产性能、改善乳品质,提高奶牛健康水平和对养分的消化率^[10]。但也有研究表明,高水平的单宁提取物对奶牛的生产性能^[7,11]和营养物质表观消化率^[12-13]产生了负面影响,但这些研究单宁提取物的添加水平大多在0.45% DM(约115 g/d)以上,部分研究^[8,14]甚至高达6.0% DM(约270 g/d)或750 g/d。

目前使用单宁这类植物次生代谢物降低瘤胃蛋白质降解率是提高氮利用率的一种策略^[15],但单宁的化学结构、植物来源和添加水平会对反刍动物产生不同的影响。前人研究发现,高水平的单宁提取物可能会对反刍动物造成一定的负面影响,而目前添加低水平[10、20和30 g/(d·头)]缩合单宁和水解单宁提取物的组合对于泌乳早期奶牛生产性能和体内抗氧化特性的潜在有益作用的报道还较少。因此,本试验旨在研究添加低水平缩合单宁和水解单宁提取物对泌乳奶牛生产性能、抗氧化能力的影响,以期确定适宜添加水平,降低单宁对反刍动物的负面影响。

1 材料与方法

1.1 试验材料

试验用单宁提取物购于意大利某公司,由栗木单宁提取物与白坚木单宁提取物按照1:2的比例混合而成,其中栗木单宁为水解单宁,白坚木单宁为缩合单宁^[1],该单宁提取物中单宁总含量≥73%。

1.2 试验设计

选取60头健康且乳产量[(46.83±1.92) kg/d]、泌乳天数[(77.85±3.23) d]、胎次[(2.54±0.21)胎]相近的荷斯坦奶牛,采用完全随机试验设计分为4组,每组15头牛。对照组(CON组)饲喂基础饲粮,试验组在基础饲粮基础上分别添加10(TE₁₀组)、20(TE₂₀组)和30 g/d的单宁提取物(TE₃₀组)。具体添加方法为:每天将10、20和30 g单宁提取物与200 g精料混合,平均分成2份,分别在提供全混合日粮(total mixed ration,TMR)之后立即饲喂给每头奶牛,保证完全摄入。试验期为56 d。

1.3 试验饲粮与饲养管理

试验奶牛每日饲喂2次(09:00和17:00)TMR,挤奶3次(08:00、16:00和00:00)。奶牛自由采食与饮水,散栏饲养,每日挤奶时对圈舍消毒和填充垫料,保持卧床的干燥和卫生,各组饲养管理条件一致。基础饲粮组成及营养水平见表1。

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 the basal diet (DM basis)%

项目 Items	含量 Content
原料 Ingredients
全株玉米青贮 Whole corn silage	34.40
苜蓿干草 Alfalfa hay	12.30
燕麦草 Oat hay	4.38
全棉籽 Whole cottonseed	6.25
蒸汽压片玉米 Steam flaked corn	18.40
豆粕 Soybean meal	10.60
甜菜颗粒 Dried beet pellet	7.29
小麦麸 Wheat bran	1.38
膨化大豆 Extruded soybean	1.74
脂肪粉 Fat powder	0.65
氧化镁 MgO	0.12
磷酸氢钙 CaHPO₄	0.64
石粉 Limestone	0.34
小苏打 NaHCO₃	0.43
食盐 NaCl	0.17
碳酸钾 K₂CO₃	0.07
过瘤胃蛋氨酸 Rumen-protected methionine	0.04
预混料 Premix¹⁾	0.78
脱霉剂 Mold remover	0.02
合计 Total	100.00
营养水平 Nutrient levels²⁾
产奶净能 NE_L/(MJ/kg)	7.29
粗蛋白质 CP	17.20
粗脂肪 EE	4.53
中性洗涤纤维 NDF	38.30
酸性洗涤纤维 ADF	22.90
钙 Ca	0.81
磷 P	0.45

1)每千克预混料含有 One kilogram of premix contained the following:VA 700 000 IU,VD₃ 250 000 IU,VE 10 000 mg,Cu 1 700 mg,Fe 1 860 mg,Zn 7 512 mg,Mn 4 030 mg,Co 62 mg,Se 50 mg,I 100 mg。

2)产奶净能为计算值,将每种原料的产奶净能分别乘以各自在TMR中所占比例,再相加求和即为饲粮的产奶净能, 其余营养水平为实测值。NE_L was a calculated value, NE_L of each raw material is multiplied by its respective proportion in the TMR, and the sum is added to obtain NE_L of the diet, while the other nutrient levels were measured values.

1.4 样品采集与指标测定

1.4.1 营养成分含量的测定

从试验第56天开始,连续3 d采用四分法采集饲粮样品;在试验第56天采用直肠采粪法采集粪便样品。

根据AOAC(2005)^[16]的方法测定饲粮与粪便样品中的干物质(DM)(方法930.15)、粗蛋白质(CP)(方法996.11)、粗脂肪(EE)(方法920.39)、粗灰分(Ash)(方法942.05)、酸性洗涤纤维(ADF)(方法973.18)含量,根据Van Soest等^[17]的方法测定饲粮与粪便样品中的中性洗涤纤维(NDF)含量,根据国标《饲料中盐酸不溶灰分的测定》(GB/T 23742—2009)测定饲粮与粪便样品中的盐酸不溶灰分含量。有机物(OM)、半纤维素(HEMID)、非纤维碳水化合物(NFC)含量采用如下公式计算:

OM=1-Ash;

HEMID=NDF-ADF;

NFC=OM-NDF-CP-EE。

1.4.2 营养物质表观消化率的计算

营养物质表观消化率计算公式如下:

某营养物质表观消化率(%)=[(A/C-B/D)/(A/C)]×100。

式中:A为饲粮中某营养物质的含量(%);B为粪便中某营养物质的含量(%);C为饲粮中盐酸不溶灰分的含量(%);D为粪便中盐酸不溶灰分的含量(%)。

1.4.3 干物质采食量(DMI)的测定

在试验第1、28和56天以组为单位连续3 d测定采食量,每天每组总的投料量减去剩料量,除以该组牛只总数即为每头牛每天的采食量,取样鲜料TMR和剩料TMR测定DM含量,计算各组的DMI。

1.4.4 乳产量和乳成分测定

试验期内人工记录每头奶牛的乳产量,以周为单位计算每头奶牛的日平均乳产量,并计算各组奶牛乳产量的全期均值。在试验第56天采集乳样,将早、中、晚乳样按照4:3:3比例混合均匀至加有重铬酸钾防腐剂的50 mL离心管中,于河北省奶牛生产性能测定(DHI)中心进行乳蛋白率、乳脂率、乳糖率、体细胞数(SCC)、乳尿素氮(MUN)含量、总固形物含量等指标的测定。参照如下公式计算脂蛋校正乳产量(FPCM)、饲料效率和体细胞评分(SCS):

FPCM=乳产量(kg/d)×[0.122 6×乳脂率(%)+0.077 6×乳蛋白率(%)+0.253 4]^[18];

饲料效率=乳产量(kg/d)或FPCM(kg/d)/DMI(kg/d);

SCS=log₂(SCC/100)+3^[19]。

1.4.5 血液指标的测定

在试验第56天,每组随机选择8头牛在晨饲前进行尾根无菌采血,于水浴锅37 ℃水浴10 min后1 500 r/min离心15 min,收集上清于0.5 mL离心管中,-20 ℃保存。

血清总抗氧化能力(T-AOC)和超氧化物歧化酶(SOD)、谷胱甘肽过氧化物酶(GSH-Px)活性以及丙二醛(MDA)、非酯化脂肪酸(NEFA)和β-羟丁酸(BHBA)含量均采用酶联免疫吸附试验(ELISA)法测定,试剂盒购自北京华悦昌生物科技有限公司。血清谷草转氨酶(AST)、谷丙转氨酶(ALT)活性与葡萄糖(GLU)、总蛋白(TP)、白蛋白(ALB)、尿素氮(UN)、肌酐(Cr)含量亦采用北京华悦昌生物科技有限公司生产的试剂盒测定。

1.5 统计分析

试验数据用SAS 9.4统计软件的MIXED程序进行分析,试验处理为固定效应,奶牛为随机效应,用Tukey法进行多重比较检验。试验结果以最小二乘均数和最小二乘均数的标准误表示。采用contrast语句对单宁提取物添加水平的线性和二次效应进行分析。P≤0.05表示差异显著,0.05<P≤0.10表示差异有显著趋势。

2 结果与分析

2.1 饲粮中添加不同水平单宁提取物对奶牛生产性能的影响

由表2可知,随着饲粮中单宁提取物添加水平的增加,虽然由乳产量和FPCM计算的2种饲料效率均未发生显著变化(P>0.05),但是乳产量线性提高(P<0.05),TE₃₀组显著显著高于CON组、TE₁₀组和TE₂₀组(P<0.05),而MUN含量线性降低(P<0.05),SCS有线性降低的趋势(P=0.10)。

表2 饲粮中添加不同水平单宁提取物对奶牛泌乳性能和饲料效率的影响

Table 2 Effects of diets supplemented with different levels of tannin extracts on lactation performance and feed efficiency of dairy cows¹⁾

项目 Items	组别 Groups				标准误 SE	P值 P-value
项目 Items	CON	TE₁₀	TE₂₀	TE₃₀	标准误 SE	处理 Treatment	线性 Linear	二次 Quadratic
干物质采食量 DMI/(kg/d)²⁾	27.9	28.2	27.8	28.0	0.17	0.59	0.81	0.78
乳产量 Milk yield/(kg/d)²⁾	45.5^b	45.6^b	45.7^b	46.3^a	0.25	0.04	0.01	0.30
脂蛋校正乳产量 FPCM/(kg/d)	43.0	43.9	43.3	45.9	1.25	0.43	0.11	0.81
乳尿素氮含量 MUN content/(mg/dL)	11.0	10.1	9.5	9.6	0.46	0.10	0.02	0.32
体细胞评分 SCS	10.6	10.7	10.4	10.1	0.25	0.30	0.10	0.39
饲料效率 Feed efficiency
乳产量/干物质采食量 Milk yield/DMI	1.63	1.62	1.64	1.68	0.03	0.53	0.21	0.43
脂蛋校正乳产量/干物质采食量 FPCM/DMI	1.63	1.62	1.64	1.68	0.07	0.78	0.31	0.99

1)同行数据肩标不同小写字母表示差异显著(P<0.05),相同或无字母表示差异不显著(P>0.05)。下表同。In the same row, values with different small letter superscripts mean significant difference (P<0.05), while with the same or no letter superscripts mean no significant difference (P>0.05). The same as below.

2)干物质采食量和乳产量为全期均值。DMI and milk yield were the mean of the whole period.

由表3可知,随着饲粮中单宁提取物添加水平的增加,乳蛋白率线性增加(P<0.05),TE₃₀组显著高于CON组和TE₁₀组(P<0.05),TE₂₀组显著高于CON组(P<0.05);乳蛋白产量线性升高(P<0.05)。不同处理对乳脂率、乳脂产量、乳糖率、乳糖产量和总固形物含量无显著影响(P>0.05)。

表3 饲粮中添加不同水平单宁提取物对奶牛乳品质的影响

Table 3 Effects of diets supplemented with different levels of tannin extracts on milk quality of dairy cows

项目 Items	组别 Groups				标准误 SE	P值 P-value
项目 Items	CON	TE₁₀	TE₂₀	TE₃₀	标准误 SE	处理 Treatment	线性 Linear	二次 Quadratic
乳脂率 Milk fat percentage/%	3.96	4.13	4.13	3.91	0.24	0.87	0.89	0.42
乳脂产量 Milk fat yield/(kg/d)	1.72	1.82	1.83	1.89	0.09	0.57	0.19	0.80
乳蛋白率 Milk protein percentage/%	2.99^c	3.04^bc	3.18^ab	3.22^a	0.05	0.02	0.01	0.91
乳蛋白产量 Milk protein yield/(kg/d)	1.31	1.35	1.37	1.45	0.04	0.10	0.02	0.52
乳糖率 Lactose percentage/%	5.25	5.18	5.23	5.17	0.05	0.68	0.40	0.93
乳糖产量 Lactose yield/(kg/d)	2.24	2.22	2.26	2.24	0.09	0.99	0.88	0.98
总固形物含量 Total solids content/%	12.6	12.4	12.5	12.4	0.26	0.97	0.70	0.99

2.2 饲粮中添加不同水平单宁提取物对奶牛营养物质表观消化率的影响

由表4可知,不同处理对DM、OM、CP、EE、NDF、ADF、HEMID和NFC的表观消化率均无显著影响(P>0.05)。

表4 饲粮中添加不同水平单宁提取物对奶牛营养物质表观消化率的影响

Table 4 Effects of diets supplemented with different levels of tannin extracts on nutrient apparent digestibility of dairy cows%

项目 Items	组别 Groups				标准误 SE	P值 P-value
项目 Items	CON	TE₁₀	TE₂₀	TE₃₀	标准误 SE	处理 Treatment	线性 Linear	二次 Quadratic
干物质 DM	69.9	69.5	69.1	68.9	1.39	0.95	0.55	0.95
有机物 OM	72.5	72.2	71.4	71.8	1.31	0.93	0.60	0.78
粗蛋白质 CP	76.1	74.3	74.0	74.3	1.16	0.51	0.26	0.33
粗脂肪 EE	89.6	90.7	89.4	89.3	0.71	0.48	0.48	0.40
中性洗涤纤维 NDF	50.4	49.1	47.1	48.2	2.17	0.72	0.37	0.55
酸性洗涤纤维 ADF	43.4	43.3	41.1	42.4	2.25	0.85	0.58	0.73
半纤维素 HEMID	59.2	56.4	54.6	55.6	2.32	0.55	0.23	0.42
非纤维碳水化合物 NFC	90.5	91.9	92.1	91.8	1.08	0.61	0.32	0.37

2.3 饲粮中添加不同水平单宁提取物对奶牛血液指标的影响

由表5可知,随着单宁提取物添加水平的增加,血清UN含量线性降低(P<0.05),TE₃₀组和TE₂₀组显著低于CON组(P<0.05);血清T-AOC线性升高(P<0.05),TE₃₀组和TE₂₀组显著高于CON组(P<0.05);血清SOD和GSH-Px活性线性升高(P<0.05),MDA含量线性降低(P=0.05)。不同处理对血清GLU、BHBA、NEFA、TP、ALB、Cr含量及AST和ALT活性均无显著影响(P>0.05)。

表5 饲粮中添加不同水平单宁提取物对奶牛血液指标的影响

Table 5 Effects of diets supplemented with different levels of tannin extracts on blood indicators of dairy cows

项目 Items	组别 Groups				标准误 SE	P值 P-value
项目 Items	CON	TE₁₀	TE₂₀	TE₃₀	标准误 SE	处理 Treatment	线性 Linear	二次 Quadratic
尿素氮 UN/(mmol/L)	6.46^a	5.45^ab	5.02^b	4.70^b	0.39	0.01	0.01	0.34
葡萄糖 GLU/(mmol/L)	6.32	6.85	6.71	6.59	0.32	0.69	0.64	0.31
β-羟丁酸 BHBA/(mmol/L)	0.66	0.57	0.60	0.68	0.06	0.54	0.72	0.17
非酯化脂肪酸 NEFA/(mmol/L)	0.47	0.48	0.51	0.48	0.04	0.90	0.69	0.66
谷草转氨酶 AST/(U/L)	54.8	53.8	52.1	60.1	4.70	0.66	0.50	0.35
谷丙转氨酶 ALT/(U/L)	32.2	34.2	37.8	37.3	2.70	0.43	0.13	0.65
总蛋白 TP/(g/L)	77.1	77.7	73.0	74.5	2.37	0.47	0.25	0.85
白蛋白 ALB/(g/L)	52.5	51.0	49.8	50.8	2.52	0.89	0.57	0.61
肌酐 Cr/(μmol/L)	111	108	107	101	9.89	0.92	0.49	0.88
总抗氧化能力 T-AOC/(U/mL)	5.82^b	6.23^ab	6.97^a	7.01^a	0.32	0.04	0.01	0.58
丙二醛 MDA/(nmol/L)	6.69	6.40	5.99	5.43	0.45	0.24	0.05	0.77
超氧化物歧化酶 SOD/(U/mL)	143	160	161	174	7.91	0.08	0.01	0.84
谷胱甘肽过氧化物酶 GSH-Px/(U/L)	549	569	576	616	21.30	0.17	0.04	0.65

3 讨论

3.1 饲粮中添加不同水平单宁提取物对奶牛生产性能的影响

DMI、乳产量和饲料效率是衡量泌乳奶牛生产性能的重要指标。有关添加单宁提取物对奶牛生产性能影响的研究结果不尽一致。Prodanovi

c'

等^[20]研究发现,添加20 g/(d·头)单宁提取物对奶牛DMI无显著影响,这与Liu等^[21]的报道一致。另有研究发现低水平的单宁提取物显著提高了泌乳奶牛的乳产量^[22],这与本研究结果一致。相反,Dschaak等^[8]观察到,当给荷斯坦奶牛饲喂3.0% DM(约750 g/d)缩合单宁提取物时,DMI下降;Henke等^[7]报道,饲粮添加3.0% DM(约750 g/d)白坚木单宁提取物降低了泌乳奶牛的乳产量、乳脂率和乳蛋白率,表明高水平白坚木单宁提取物对乳产量及乳品质产生了负面影响。本研究中,饲粮中添加低水平的单宁提取物并未显著影响DMI,但线性提高了乳产量,这可能是低水平的单宁提取物与蛋白质或纤维结合形成的复合体并未造成瘤胃填充^[23],同时增加了蛋白质的过瘤胃率,提高了蛋白质的利用率^[24],因此在不影响DMI的情况下提高了乳产量。而高水平的单宁提取物产生的负面效应可能是其与唾液蛋白结合沉淀^[25],产生涩味,降低饲粮的适口性,或是形成的单宁复合物降解速度较慢,造成了瘤胃的填充,导致DMI降低。

单宁提取物能与蛋白质结合形成复合体的性质可能会影响乳蛋白率和乳蛋白产量。Broderick等^[15]研究显示,饲喂单宁提取物提高了泌乳奶牛的乳产量和乳蛋白产量;Bhatta等^[26]报道,在饲粮中添加罗望子壳单宁提取物显著提高了泌乳中奶牛的乳蛋白率。Aguerre等^[11,27]曾2次报道低水平的单宁提取物(0.45% DM,约115 g/d)对乳蛋白率产生了有益影响,但高水平单宁提取物(0.90% DM和1.80% DM,约230和460 g/d)对DMI、乳蛋白产量和营养物质表观消化率有不利影响。本研究中,饲粮中添加单宁提取物线性提高了奶牛的乳产量、乳蛋白率和乳蛋白产量等指标,最佳添加水平为30 g/(d·头)。Dey等^[28]和Woodward等^[29]也发现饲喂添加单宁的饲粮可以提高奶牛的乳产量,并且不影响DMI;刘立成等^[30]研究发现,饲喂40 g/(d·头)的单宁在不影响采食量的情况下提高了奶牛的乳产量和乳蛋白率;回海勇等^[31]研究发现,饲喂20 g/(d·头)的单宁有降低DMI的趋势,但显著提高了乳产量和乳蛋白率。这与本试验结果一致,表明低水平的单宁或单宁提取物能够在不影响DMI的情况下提高乳产量和乳蛋白率。研究表明,提高饲粮中过瘤胃蛋白质水平可对反刍动物的生产性能产生有益效果^[32],乳蛋白的合成主要依赖于氨基酸的供应^[33],而本试验中MUN与BUN(即血清UN)含量随单宁提取物添加水平的增加线性降低表明单宁提取物提高了机体对游离氮的利用^[7-8],提高了小肠氨基酸的供应^[34-35]。

SCS在许多国家被用作监测乳腺炎症和评价乳品质的指标。Suriyasathaporn等^[36]报道,SCS与MDA含量呈正相关,而MDA是脂质过氧化产物^[37],这表明SCS与氧化应激之间存在一定关系。本研究中,SCS和血清MDA含量随单宁提取物添加水平的增加均线性降低,这表明单宁提取物缓解了泌乳奶牛的氧化应激和炎症,与Liu等^[21]的研究结果一致。刘立成等^[30]研究发现,饲喂40 g/(d·头)的单宁时体细胞数较对照组降低近50%,这也表明了低水平的单宁或单宁提取物在改善奶牛炎症上的功效。

综上可知,在饲粮中添加单宁提取物提高了蛋白质的过瘤胃水平和流向小肠的氨基酸数量,增加了小肠对营养物质的消化与吸收,提高了氮利用率,并且单宁提取物改善了机体的抗氧化状态,这可能是机体在DMI和营养物质消化率没有变化的基础上乳产量、乳蛋白率和乳蛋白产量增加的原因。

3.2 饲粮中添加不同水平单宁提取物对奶牛营养物质表观消化率的影响

Aguerre等^[11]发现低水平的单宁提取物不影响奶牛的DM、OM和CP表观消化率,这与本研究结果一致。但杨凯^[14]研究发现,高水平的单宁提取物(2.6% DM,约140 g/d)会显著降低肉牛对DM、OM和CP的表观消化率;Ahnert等^[13]研究发现,饲喂高水平的白坚木单宁提取物(4%和6% DM,约180和270 g/d)降低了荷斯坦育成母牛的全肠道养分表观消化率。这可能是高水平的单宁提取物降低了瘤胃微生物活性,从而引起瘤胃消化减缓,造成瘤胃填充,导致DMI和营养物质的消化率降低,或是单宁提取物与蛋白质或纤维形成的复合物在皱胃中没有完全解离,导致蛋白质和纤维的营养物质消化率降低。本试验中,饲喂10、20和30 g/(d·头)的单宁提取物对奶牛营养物质表观消化率没有产生显著影响,未发生饲粮蛋白质过度保护的情况。

3.3 饲粮中添加不同水平单宁提取物对奶牛血液指标的影响

NEFA和BHBA是衡量奶牛能量状态的重要指标,本研究中奶牛处于能量平衡状态,单宁提取物未对能量平衡和肝脏功能产生影响,与前人研究结果^[38]一致。Prodanovi

c'

等^[20]和Aguerre等^[11]研究表明,单宁提取物可以显著降低血液中UN含量,与本试验结果相符。BUN含量与MUN含量呈密切正相关关系^[11],本研究中BUN和MUN的含量均随单宁提取物添加水平的增加线性降低,这表明机体处于氨基酸平衡状态^[39],游离氮保持着动态平衡^[40]。本试验中,奶牛血清T-AOC与SOD和GSH-Px活性的升高表明了机体抗氧化能力提高^[41],清除了大量多余的自由基^[42],降低了机体脂质的过氧化程度^[43]及其细胞损伤程度^[44],血清MDA含量和SCS降低也证实了这一结论。Prodanovi

c'

等^[20]发现,在围产期奶牛饲粮中添加20 g/(d·头)的栗木单宁提取物显著提高了血清T-AOC,并改善了初乳质量;Liu等^[21]发现,给围产期奶牛饲喂栗木单宁提取物能够显著降低脂质过氧化,提高血浆中SOD、GSH-Px活性和T-AOC,上述研究结果与本试验结果相一致。泌乳奶牛可能是通过提升机体抗氧化状态和氮利用率提高乳产量、乳蛋白率和乳蛋白产量。本试验条件下,单宁提取物的最佳添加水平为30 g/(d·头)。

4 结论

在泌乳早期荷斯坦奶牛饲粮中添加10、20和30 g/(d·头)单宁提取物线性提高了乳产量、乳蛋白率及乳蛋白产量,改善了奶牛的抗氧化能力,降低了MUN和BUN含量,提高了氮利用率。因此,给泌乳早期荷斯坦奶牛饲喂单宁提取物可以在不影响DMI和营养物质表观消化率的情况下,通过提高抗氧化状态和氮利用效率来改善奶牛的生产性能。本试验条件下,单宁提取物的最佳添加水平为30 g/(d·头)。

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