Chinese Journal of Animal Nutrition >

2024 , Vol. 36 >Issue 2: 995 - 1002

DOI: https://doi.org/10.12418/CJAN2024.090

RESEARCH PAPER

Effects of Folic Acid and Betaine on Lactation Performance, Milk Fatty Acid Composition, Nutrient Digestion and Rumen Fermentation of Dairy Cows during Perinatal Period

  • YAN Guojun , 1 ,
  • LI Fugang 2 ,
  • LIU Zefeng 2 ,
  • HUANG Shi 2 ,
  • LIU Qiang 2 ,
  • GUO Gang 2 ,
  • WANG Cong , 2, *
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  • 1 Development Center of Rural Industry Integration, Department of Agriculture and Rural Affairs of Shanxi Province,Taiyuan 030001, China
  • 2 College of Animal Science, Shanxi Agricultural University, Taigu 030801, China
* professor, E-mail:

Received date: 2023-08-22

  Online published: 2024-02-01

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Abstract

This experiment was conducted to study the effects of folic acid (FA) and betaine (BT) on lactation performance, milk fatty acid composition, nutrient digestion and rumen fermentation of dairy cows during perinatal period. Forty Holstein dairy cows during perinatal period with similar expected delivery date, parity, body weight and average milk yield during the previous lactation period were randomly divided into 4 groups with 10 cows in each group. Cows in the control group were fed a basal diet, and others in FA, BT and FA+BT groups were fed basal diets supplemented with 135 mg/d FA, 100 g/d BT and 135 mg/d FA+100 g/d BT, respectively. The pre-experimental period lasted for 10 days, and the experimental period lasted for 42 days. The results showed as follows: 1) the actual milk, 4% fat-corrected milk, milk fat and milk protein yields, and milk fat rate, feed efficiency (FE) of FA and FA+BT groups were significantly higher than those of the control group (P<0.05). The 4% fat-corrected milk, milk fat and milk protein yields, and milk fat rate, FE of BT group were significantly higher than those of the control group (P<0.05). 2) The contents of C10:0, C12:0, C14:0, C16:1, C18:3 and total 4 to 15 carbon fatty acid in milk fat of FA group were significantly higher than those of the control group (P<0.05), and the C18:0 content was significantly lower than that of the control group (P<0.05). The contents of C10:0, C12:0, C14:0, C16:1, C18:2, C18:3 and total 4 to 15 carbon fatty acid in milk fat of BT and FA+BT groups were significantly higher than those of the control group (P<0.05), and the C18:0 and total >16 carbon fatty acid contents were significantly lower than those of the control group (P<0.05). 3) The apparent digestibilities of dry matter (DM), organic matter (OM), neutral detergent fiber (NDF) and acid detergent fiber (ADF) of FA, BT and FA+BT groups were significantly higher than those of the control group (P<0.05). 4) The rumen total volatile fatty acids (TVFA), acetate and butyrate contents and the ratio of acetate to propionate of FA, BT and FA+BT groups were significantly higher than those of the control group (P<0.05). In conclusion, the supplementation of FA and BT for dairy cows during perinatal period can improve the lactation performance and nutrient digestion, and the combined supplementation of FA and BT is more effective in improving lactation performance compared with than it addition alone.

Key words： folic acid; betaine; lactation performance; feed efficiency; milk fatty acid; dairy cows during perinatal period

Cite this article

YAN Guojun , LI Fugang , LIU Zefeng , HUANG Shi , LIU Qiang , GUO Gang , WANG Cong . Effects of Folic Acid and Betaine on Lactation Performance, Milk Fatty Acid Composition, Nutrient Digestion and Rumen Fermentation of Dairy Cows during Perinatal Period[J]. Chinese Journal of Animal Nutrition, 2024 , 36(2) : 995 -1002 . DOI: 10.12418/CJAN2024.090

一碳循环中,叶酸(FA)是一碳基团的载体,甜菜碱(BT)是一碳基团的供体,二者共同参与脂质、碳水化合物和蛋白质代谢[1]。围产期奶牛泌乳早期处于能量负平衡状态,乳中4~14碳脂肪酸含量减少,>16碳脂肪酸含量增加[2]。乳中长链脂肪酸来源于体脂分解,其含量增加表明体脂分解增多、能量负平衡加剧,易诱发营养代谢病[2]。提高养分利用率能改善围产期奶牛能量供应,调控乳脂肪酸组成。饲粮中添加的FA,约97%在瘤胃中被微生物降解或破坏,因此,生产中使用包被叶酸(CFA)添加剂[3]。研究发现,饲粮中添加FA可提高牛瘤胃总挥发性脂肪酸(TVFA)产量和养分表观消化率[4-5];改善泌乳早期奶牛能量平衡,增加4%乳脂校正乳和乳脂肪产量[4];上调公牛肌肉组织中脂肪酸合成代谢关键基因表达[5]。但是,FA对围产期奶牛乳脂肪酸组成影响的研究还未见报道。泌乳中期奶牛饲粮添加100 g/d的BT,改善了养分表观消化率和饲料效率(FE),增加了4%乳脂校正乳(FCM)、乳脂肪产量[6-7],减少了乳中长链多不饱和脂肪酸含量[8];泌乳早期羊饲喂BT,增加了乳中短链脂肪酸和亚麻酸(C18:3)含量[9]。另外,Cheng等[7]试验发现,饲粮添加BT降低了泌乳奶牛血液FA含量,表明BT能提高FA的利用率。基于FA和BT在一碳代谢中的作用和现有研究结果提出假设,围产期奶牛添加FA或/和BT,能通过改善养分消化和瘤胃发酵,提高泌乳性能和FE,改变乳脂肪酸组成,而且二者共同添加比单独添加效果更好。因此,本试验旨在研究围产期奶牛添加FA或/和BT对泌乳性能、乳脂肪酸组成、养分消化和瘤胃发酵的影响,明确FA和BT对围产期奶牛的营养调控特点及机制,为营养调控技术在生产中的应用提供参考。

1 材料与方法

1.1 试验材料

试验用FA添加形式为CFA,依据Wang等[10]的方法制备,FA含量为2%,用尼龙袋法和装有瘤胃和十二指肠瘘管的奶牛测定CFA在瘤胃和小肠的降解率分别为28.3%和69.7%。试验用BT为饲料级,纯度98%。饲粮FA和BT添加量依据Zhang等[4]和Wang等[6]的试验结果确定。

1.2 试验设计

试验于2022年6—9月在山西省晋中市祁县泓润牧业有限公司完成。选取胎次[(2.15±0.73)胎]、体重[(659±53) kg]、上一泌乳期平均产奶量[(9 958±407) kg]和预产日期相近的围产期奶牛40头,依据随机区组设计分为4组,每组10头牛。试验采用2×2因子设计,对照组饲喂基础饲粮,FA组、BT组和FA+BT组分别在基础饲粮中添加135 mg/d的FA、100 g/d的BT和135 mg/d的FA+100 g/d的BT。基础饲粮依据NRC(2001)[11]配制,其组成及营养水平见表1。奶牛产前31 d进入试验,预试期10 d,正试期42 d,包括产前21 d和产后21 d。
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)%

项目
Items		 产前
Prepartum		 产后
Postpartum
原料 Ingredients
玉米青贮 Corn silage 36.00 26.00
苜蓿干草 Alfalfa hay 13.00
燕麦草 Oat hay 24.00 11.00
玉米 Corn grain 15.10 24.00
麸皮 Wheat bran 6.00 6.00
豆粕 Soybean meal 10.00 10.60
菜籽粕 Rapeseed meal 2.40 2.50
棉籽粕 Cottonseed meal 5.20 5.00
碳酸氢钙 Ca(HCO3)2 0.70 0.60
食盐 NaCl 0.60 0.50
磷酸氢钙 CaHPO4 0.40 0.30
预混料 Premix1) 0.60 0.50
合计 Total 100.00 100.00
营养水平 Nutrient levels2)
有机物 OM 94.52 94.48
粗蛋白质 CP 15.96 17.17
粗脂肪 EE 3.16 3.18
中性洗涤纤维 NDF 35.29 31.13
酸性洗涤纤维 ADF 21.47 19.35
钙 Ca 0.70 0.73
磷 P 0.47 0.48
泌乳净能 NEL/(MJ/kg) 6.41 6.68

1)每千克预混料含有 One kg premix contained the following:Fe 20 000 mg,Cu 1 600 mg,Mn 8 000 mg,Zn 7 500 mg,I 120 mg,Se 60 mg,Co 20 mg,VA 820 000 IU,VD 300 000 IU,VE 10 000 IU。

2)泌乳净能为计算值,其余为实测值。NEL was a calculated value, while the others were measured values.

1.3 饲养管理

试验牛饲养于同一圈舍,每日07:00和19:30饲喂,05:30、13:00和18:00挤奶。试验牛自由采食和饮水,早晨饲喂前,将FA和BT与约500 g全混合日粮(TMR)混匀,待牛全部采食后再喂剩余饲粮。

1.4 样品采集与指标测定

1.4.1 饲粮营养水平

饲粮中性洗涤纤维(NDF)和酸性洗涤纤维(ADF)含量依据Van Soest等[12]的方法测定,其余养分含量依据AOAC(2006)[13]的方法测定。泌乳净能依据NRC(2001)[10]计算:
泌乳净能(MJ/kg)=(0.024 5×饲粮可消化总养分-0.12)×4.184。

1.4.2 产奶性能和乳脂肪酸组成

奶牛产后第4天开始,上颈枷饲喂,记录每头牛每天喂料量和剩料量,计算干物质采食量(DMI);记录每头牛每天产奶量,采集产后第7、14和21天奶样。奶样分2份,一份保存于装有2-溴-2-硝基-1,3-丙二醇的100 mL奶样瓶中,用丹麦MK-120型红外乳品分析仪测定乳脂率、乳蛋白率和乳糖率;一份装于50 mL EP管中,-20 ℃保存,用气相色谱仪测定乳脂肪酸含量。用实际乳产量乘以乳成分含量计算乳脂肪、乳蛋白质和乳糖产量,4% FCM产量和FE的计算公式如下:
4%FCM产量=(0.4+15×乳脂率)×实际乳产量;
FE=实际乳产量/DMI。

1.4.3 养分表观消化率

奶牛产后,每周采集TMR样1次;第18~21天,以6 h为间隔,每天通过直肠采集每头牛粪便样品4次,每次约100 g。TMR样和粪样-20 ℃保存,试验结束后,粪样以牛为单位按湿重比制成混合样,加入湿粪重25%的10%酒石酸溶液。TMR样和粪样65 ℃烘至恒重,粉碎过1 mm筛,依据AOAC(2006)[13]方法测定酸不溶灰分(AIA)、干物质(DM)、有机物(OM)、粗蛋白质(CP)和粗脂肪(EE)含量;依据Van Soest等[12]方法测定NDF和ADF含量。
某养分表观消化率(%)=100-[(TMR中AIA含量×粪中某养分含量)/(粪中AIA含量×TMR中某养分含量)×100]。

1.4.4 瘤胃发酵参数

奶牛产后第19、20和21天,晨饲后3~4 h,用胃管采样器采集每头牛瘤胃液样品约100 mL。为避免唾液污染,先采集的约200 mL瘤胃液弃掉。瘤胃液样品立即用PHS-3C酸度计测定pH,然后用纱布(4层)过滤,滤液保存于-20 ℃,用Trace 1300气相色谱仪,采用内标法测定挥发性脂肪酸(VFA)含量[13]。

1.5 数据处理与统计分析

奶牛产后采食量和产奶量数据按照每头牛全期平均值进行统计分析。每周乳成分测定值、产后瘤胃液成分测定值均按照每头牛平均值进行统计分析。所有数据均采用SPSS 22.0软件中的ANOVA程序进行方差分析,采用Duncan氏法进行组间多重比较,结果用平均值和均值标准误(SEM)表示,P<0.05为差异显著。

2 结果

2.1 FA和BT对奶牛泌乳性能的影响

由表2可知,各组之间奶牛DMI、乳糖产量、乳蛋白率和乳糖率无显著差异(P>0.05)。FA组和FA+BT组的实际乳、4%乳脂校正乳、乳脂肪、乳蛋白质产量以及乳脂率和FE显著高于对照组(P<0.05)。BT组的实际乳产量与对照组无显著差异(P>0.05),4%乳脂校正乳、乳脂肪、乳蛋白质产量以及乳脂率和FE显著高于对照组(P<0.05)。FA+BT组的4%乳脂校正乳和乳蛋白产量显著高于FA组和BT组(P<0.05)。
表2 FA和BT对奶牛泌乳性能的影响

Table 2 Effects of FA and BT on lactation performance of dairy cows

项目
Items		 组别 Groups SEM P值
P-value
对照 Control FA BT FA+BT
干物质采食量 DMI/(kg/d) 20.83 21.72 21.24 22.61 1.007 0.092
产量 Yield/(kg/d)
实际乳 Actual milk 40.14b 43.18a 41.56ab 44.44a 1.032 0.047
4%乳脂校正乳 4%FCM 39.42c 44.22b 42.74b 45.97a 0.578 0.016
乳脂肪 Milk fat 1.56b 1.80a 1.74a 1.88a 0.071 0.041
乳蛋白 Milk protein 1.30c 1.39b 1.38b 1.46a 0.030 0.024
乳糖 Lactose 1.88 1.98 1.97 2.08 0.078 0.407
乳成分 Milk component/%
乳脂率 Milk fat rate 3.88b 4.16a 4.19a 4.23a 0.065 0.046
乳蛋白率 Milk protein rate 3.23 3.21 3.31 3.29 0.032 0.094
乳糖率 Lactose rate 4.69 4.58 4.73 4.69 0.044 0.261
饲料效率 FE 1.93b 1.98a 1.96a 1.97a 0.002 0.001

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.

2.2 FA和BT对奶牛乳脂肪酸组成的影响

由表3可知,各组之间奶牛乳脂肪中丁酸(C4:0)、己酸(C6:0)、辛酸(C8:0)、十一烷酸(C11:0)、十三烷酸(C13:0)、肉豆蔻一稀酸(C14:1)、十五烷酸(C15:0)、棕榈酸(C16:0)、十七烷酸(C17:0)、十七烷一稀酸(C17:1)、油酸(C18:1)、其他脂肪酸、总16碳脂肪酸、总饱和脂肪酸和总不饱和脂肪酸含量无显著差异(P>0.05)。FA组乳脂肪中葵酸(C10:0)、月桂酸(C12:0)、肉豆蔻酸(C14:0)、棕榈一稀酸(C16:1)、C18:3和总4~15C脂肪酸含量显著高于对照组(P<0.05),硬脂酸(C18:0)含量显著低于对照组(P<0.05),亚油酸(C18:2)和总>16碳脂肪酸含量与对照组无显著差异(P>0.05)。BT组和FA+BT组乳脂肪中C10:0、C12:0、C14:0、C16:1、亚油酸(C18:2)、C18:3和总4~15碳脂肪酸含量显著高于对照组(P<0.05),C18:0和总>16碳脂肪酸含量显著低于对照组(P<0.05)。FA+BT组奶牛乳脂肪酸组成与FA组和BT组无显著差异(P>0.05)。
表3 FA和BT对奶牛乳脂肪酸组成的影响

Table 3 Effects of FA and BT on milk fat composition of dairy cows%

项目
Items		 组别 Groups SEM P值
P-value
对照 Control FA BT FA+BT
丁酸 C4:0 6.81 6.42 6.63 6.62 0.172 0.165
己酸 C6:0 2.70 2.64 2.65 2.81 0.091 0.621
辛酸 C8:0 1.21 1.25 1.35 1.30 0.065 0.145
葵酸 C10:0 2.49b 2.87a 3.18a 2.87a 0.174 0.033
十一烷酸 C11:0 0.02 0.03 0.02 0.09 0.013 0.342
月桂酸 C12:0 2.68b 3.13a 3.58a 3.37a 0.222 0.027
十三烷酸 C13:0 0.07 0.09 0.08 0.10 0.017 0.133
肉豆蔻酸 C14:0 9.47b 9.96a 10.42a 10.21a 0.043 0.049
肉豆蔻一稀酸 C14:1 0.96 1.02 1.07 1.05 0.114 0.153
十五烷酸 C15:0 1.00 1.00 1.10 1.03 0.065 0.083
棕榈酸 C16:0 25.47 25.05 25.70 25.47 0.742 0.112
棕榈一稀酸 C16:1 2.40b 2.81a 2.71a 2.71a 0.113 0.047
十七烷酸 C17:0 0.69 0.59 0.57 0.53 0.032 0.632
十七烷一稀酸 C17:1 0.40 0.64 0.66 0.63 0.027 0.975
硬脂酸 C18:0 11.43a 10.40b 9.50b 10.14b 0.561 0.041
油酸 C18:1 24.85 24.44 23.51 23.81 1.374 0.117
亚油酸 C18:2 2.36b 2.71ab 3.05a 2.98a 0.011 0.042
亚麻酸 C18:3 0.96b 1.14a 1.21a 1.20a 0.056 0.027
其他脂肪酸 Other FA 4.02 3.90 3.30 3.25 0.351 0.168
总4~15碳脂肪酸 Total 4 to 15C FA 27.41b 28.41a 30.08a 29.45a 0.886 0.001
总16碳脂肪酸 Total 16C FA 27.87 27.86 28.41 28.18 0.698 0.134
总>16碳脂肪酸 Total >16C FA 44.72a 43.82ab 41.80b 42.54b 1.076 0.041
总饱和脂肪酸 Total SFA 64.04 63.43 64.78 64.54 1.013 0.571
总不饱和脂肪酸 Total UFA 31.93 32.76 32.21 32.38 0.689 0.423

其他脂肪酸为≥20碳脂肪酸。

Other FA was ≥20C FA.

2.3 FA和BT对奶牛养分表观消化率的影响

由表4可知,各组之间奶牛CP和EE表观消化率无显著差异(P>0.05)。FA组、BT组和FA+BT组的DM、OM、NDF和ADF表观消化率显著高于对照组(P<0.05)。FA+BT组的各养分表观消化率与FA组和BT组无显著差异(P>0.05)。
表4 FA和BT对奶牛养分表观消化率的影响

Table 4 Effects of FA and BT on nutrient apparent digestibilities of dairy cows%

项目
Items		 组别 Groups SEM P值
P-value
对照 Control FA BT FA+BT
干物质 DM 70.74b 73.64a 73.64a 74.61a 1.484 0.033
有机物 OM 72.67b 74.61a 74.61a 75.58a 1.058 0.041
粗蛋白质 CP 75.12 76.35 76.56 76.14 1.087 0.074
粗脂肪 EE 88.43 89.01 88.72 88.34 1.035 0.335
中性洗涤纤维 NDF 58.14b 62.98a 62.02a 62.92a 1.388 0.036
酸性洗涤纤维 ADF 51.36b 56.20a 55.23a 56.08a 1.775 0.014

2.4 FA和BT对奶牛瘤胃发酵参数的影响

由表5可知,各组之间奶牛瘤胃pH、丙酸、戊酸、异丁酸和异戊酸含量无显著差异(P>0.05)。FA组、BT组和FA+BT组的瘤胃TVFA、乙酸和丁酸含量及乙酸/丙酸显著高于对照组(P<0.05)。FA+BT组的奶牛瘤胃发酵参数与FA组和BT组无显著差异(P>0.05)。
表5 FA和BT对奶牛瘤胃发酵参数的影响

Table 5 Effects of FA and BT on rumen fermentation parameters of dairy cows

项目
Items		 组别 Groups SEM P值
P-value
对照 Control FA BT FA+BT
pH 6.55 6.60 6.43 6.41 0.081 0.425
总挥发性脂肪酸 TVFA/(mmol/L) 134.0b 143.4a 147.3a 145.1a 2.463 0.037
乙酸 Acetate/(mmol/L) 81.44b 88.77a 91.47a 91.91a 1.573 0.028
丙酸 Propionate/(mmol/L) 33.30 32.70 34.47 32.66 1.015 0.339
丁酸 Butyrate/(mmol/L) 14.98b 17.35a 16.35a 17.41a 0.627 0.009
戊酸 Valerate/(mmol/L) 1.85 2.09 2.08 2.19 0.028 0.850
异丁酸 Isobutyrate/(mmol/L) 0.74 0.87 0.96 0.94 0.058 0.188
异戊酸 Isovalerate/(mmol/L) 1.74 1.65 2.02 1.99 0.083 0.125
乙酸/丙酸 Acetate/propionate 2.45b 2.71a 2.65a 2.81a 0.037 0.025

3 讨论

3.1 FA添加效果

本试验中,饲粮中添加FA对奶牛DMI无显著影响,与其他研究结果[4,14]一致,即围产期奶牛饲粮中添加FA后,泌乳早期DMI无显著变化。实际乳、乳脂肪和乳蛋白质产量提高,归因于养分表观消化率和瘤胃TVFA含量增加,表明饲粮中添加FA提高了饲粮养分利用率,表现为FE增加。另外,奶牛泌乳性能的提高可能与FA在一碳代谢中的作用有关。作为甲基载体,FA参与DNA、RNA和蛋白质的合成[1]。已有报道,泌乳牛肌肉注射补充FA,乳腺组织蛋白质合成代谢基因表达上调[15]。其他研究同样发现,奶牛产前补充FA,泌乳早期产奶性能和FE改善[4,14]。乳脂肪产量和乳脂率以及乳中C10:0、C12:0、C14:0、C16:1和总4~15碳脂肪酸含量增加,归因于瘤胃乙酸和丁酸含量增加,表明饲粮中添加FA促进了乳腺组织乳脂肪酸的从头合成。乳腺组织利用乙酸和丁酸合成乳中全部的4~15碳脂肪酸和约50%的16碳脂肪酸;乙酸和丁酸作为信号分子,能上调乳脂合成相关转录因子和酶的表达,促进乳脂肪酸合成[16]。研究发现,奶牛乳脂肪产量和乳脂率与乙酸和丁酸的供应呈正相关[17]。乳中长链脂肪酸来源于消化道吸收的饲粮脂肪酸和体脂分解产生的脂肪酸[2]。乳脂肪酸组成可用于衡量奶牛产后体脂动员程度和能量平衡状态[2]。奶牛体脂肪分解,乳中4~15碳脂肪酸含量降低,16碳和>16碳脂肪酸,尤其是C18:1和C18:0含量增加[2,18]。饲粮中添加FA后,乳中16碳脂肪酸、C18:1和总>16碳脂肪酸含量无显著变化,C18:0含量显著下降,而且EE表观消化率无显著变化,表明奶牛产后实际乳和乳脂肪产量增加,没有导致体脂分解,即补充FA改善了能量利用效率。前期试验发现,围产期补充FA,泌乳早期奶牛能量利用率改善,表现为体重无显著变化,血液非酯化脂肪酸和β-羟丁酸含量显著降低[4]。
本试验中,饲粮中添加FA提高了DM、OM、NDF和ADF表观消化率,与Zhang等[4]的试验结果一致。FA是瘤胃微生物的必需营养因子,参与微生物一碳代谢的调控[1,19],也是维持小肠和胰腺结构和功能的必需因子[1]。因此,补充FA有助于养分在瘤胃和小肠的消化。瘤胃TVFA、乙酸和丁酸含量及乙酸/丙酸提高,表明FA在瘤胃释放的FA刺激了纤维类降解微生物的生长,提高了养分瘤胃降解率。早期体外试验发现,5-甲基四氢叶酸缺乏,黄色瘤胃球菌生长速度降低[19]。近期试验发现,补充FA可提高肉牛瘤胃纤维类分解菌数量、DM和NDF有效降解率[10]。另外,CFA中约69.7%的FA能到达小肠被吸收。已有报道,饲粮中添加FA可提高公牛小肠胰淀粉酶、胰蛋白酶和糜蛋白酶活性[20],提高体外DM小肠消化率[21]。

3.2 BT添加效果

本试验中,饲粮中添加FA对奶牛DMI无显著影响,但是4%乳脂校正乳、乳脂肪和乳蛋白产量提高,与Monteiro等[22]在产前60 d奶牛饲粮中添加BT的研究结果一致。试验结果归因于饲粮养分表观消化率和瘤胃TVFA含量提高。其中,乳脂肪产量和乳脂率提高,与瘤胃乙酸和丁酸含量增加有关。乳蛋白产量提高则与BT对一碳循环的调控有关[22]。一碳代谢中,BT作为甲基供体,参与蛋氨酸合成,蛋氨酸是奶牛乳蛋白质合成的限制性氨基酸[1,13]。另外,BT能调节细胞渗透压,维持乳腺细胞的结构和活力[23]。饲粮中添加BT后,乳中总4~15碳脂肪酸含量增加,C18:0和总>16碳脂肪酸含量降低,表明产奶量的增加没有导致奶牛体脂分解。已有报道,饲粮中添加BT后产奶量的增加,对泌乳早期奶牛能量平衡[22]和泌乳中期奶牛体重变化[6-7]无显著影响。乳中总4~15碳脂肪酸、C16:1、C18:2和C18:3含量增加,与瘤胃乙酸和丁酸浓度增加有关。纤维类物质在瘤胃降解产生的乙酸和丁酸是部分4~15碳脂肪酸和C16:1合成的前体[16]。研究发现,奶牛乳中C16:1和多不饱和脂肪酸含量与瘤胃乙酸产生菌数量呈正相关[24]。类似地,Fernández等[9]发现,围产期羊产前饲喂BT,泌乳早期乳中短链脂肪酸和C18:3含量增加。
DM、OM、NDF和ADF表观消化率提高,与瘤胃TVFA、乙酸和丁酸含量增加有关,表明饲粮中添加BT促进了瘤胃微生物生长和养分在瘤胃的降解。饲粮中添加BT为瘤胃微生物的生长提供了甲基和氮源,同时BT能有效调节消化道细胞和微生物细胞的渗透压,调节消化道食糜的黏性[23,25]。前期研究发现,奶牛饲粮中添加BT提高了瘤胃总细菌、真菌和原虫数量[7],提高了养分表观消化率和瘤胃TVFA含量[6-7]。

3.3 FA和BT共同添加效果

本试验中,FA和BT共同添加后,奶牛泌乳早期4%乳脂校正乳和乳蛋白产量较单独添加FA或BT高,但是,FA+BT组乳脂肪酸组成、养分表观消化率和瘤胃TVFA含量与FA组或BT组无显著差异。因此,FA+BT组产奶性能较高的试验结果与FA和BT对一碳代谢的调控有关,即FA和BT共同添加对一碳单位转移效率和乳蛋白合成的促进作用可能比二者单独添加更有效。一碳循环中,同型半胱氨酸接受BT的甲基形成蛋氨酸,BT转化为二甲基甘氨酸,四氢叶酸接受二甲基甘氨酸的甲基形成5,10-亚甲基四氢叶酸,后者及其还原物10-甲酰基四氢叶酸和5-甲基四氢叶酸,共同参与蛋白质、DNA和RNA的合成[1]。前期试验发现,泌乳牛单独添加BT,血液FA含量降低;采食含FA饲粮的泌乳牛补充BT,血液FA含量增加[7],表明FA的利用效率受BT添加的影响。但是,目前的研究还无法阐明FA和BT的互作机制。

4 结论

围产期奶牛饲粮中添加FA和BT提高了泌乳性能和乳中短链脂肪酸含量,改善了养分消化和瘤胃发酵,而且FA和BT共同添加对改善泌乳性能比二者单独添加更有效。

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