Chinese Journal of Animal Nutrition >

2023 , Vol. 35 >Issue 7: 4420 - 4431

DOI: https://doi.org/10.12418/CJAN2023.411

RESEARCH PAPER

Effects of Periodic Fluctuation of Dietary Crude Protein Level on Performance, Nutrient Apparent Digestibility and Rumen Fermentation Parameters in Dairy Cows

  • LIU Yifan ,
  • ZHANG Chuankai ,
  • HE Liukuan ,
  • QUAN Yingkai ,
  • BU Ya’ou ,
  • ZHANG Liyang ,
  • GAO Tengyun ,
  • FU Tong ,
  • LIAN Hongxia , **
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  • International Joint Laboratory of Nutrition Regulation and Ecological Raising of Domestic Animal, College of Animal Science and Veterinary Medicine, Henan Agricultural University, Zhengzhou 450046, China
** associate professor, E-mail:

*Contributed equally

Received date: 2022-12-07

  Online published: 2023-07-11

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Abstract

This experiment was conducted to investigate the effects of periodic fluctuations of dietary crude protein (CP) levels on performance, apparent digestibility of nutrients and rumen fermentation parameters in dairy cows. Twenty-four Holstein cows weighing (569.23±47.17) kg, lactation days of (150.32±19.10) d, and milk yield of (22.23±2.80) kg/d were randomly divided into three groups: oscillating CP level diet (OP) group, which was fed high CP level diet (HP; CP level was 18.05%) and low protein diet (LP; CP level was 14.04%) at intervals of 48 h; HP group (CP level was 18.05%); stable CP level diet (SP) group (CP level was 16.05%), with eight replicates per group, with one cow per replicate. The experiment lasted for 60 days. The results showed as follows: 1) the periodic fluctuation of dietary CP level had no significant effect on the performance of dairy cows (P>0.05). 2) On the first day of the fluctuation cycle, the apparent digestibility of ether extract (EE) in OP group was extremely significant lower than that in HP group (P<0.01); on the fourth day, the apparent digestibility of EE in OP group was significantly lower than that in HP group (P<0.05), and significantly higher than that in SP group (P<0.05); the apparent digestibility of acid detergent fiber (ADF) was extremely significant higher than that in SP group (P<0.01). 3) On the first day of the fluctuation period, the concentration of ammonia nitrogen in OP group was significantly lower than that in HP group (P<0.05). On the second day, the concentration of ammonia nitrogen in OP group was extremely significant higher than that in SP group (P<0.01). The ammonia nitrogen concentration in OP group was extremely significantly lower than that in HP group on the third and the fourth day (P<0.01). 4) On the first day of the fluctuation cycle, the concentration of butyric acid in OP group was extremely significant lower than that in HP group (P<0.01) and extremely significant higher than that in SP group (P<0.01), while the concentration of propionic acid in OP group was significantly higher than that in HP group and SP group (P<0.05). On the second day, the concentration of propionic acid in OP group was extremely significant lower than that in SP group (P<0.01), but there was no significant difference between OP group and HP group (P>0.05). On the third day, the concentration of butyric acid in OP group was significantly lower than that in HP group (P<0.05). On the fourth day, the concentration of acetic acid in OP group was significantly higher than that in SP group (P<0.05), and the concentration of butyric acid was significantly higher than that in HP group (P<0.05). In conclusion, the periodic fluctuation of dietary CP level can improve the apparent digestibility of EE and ADF of dairy cows during the period of feeding low CP level diet, as well as the concentration of acetic acid and butyric acid in the rumen, which is conducive to promoting the absorption and utilization of CP in the diet by rumen microorganisms and increasing the urea nitrogen cycle, thereby promoting the absorption and utilization of nitrogen by dairy cows and reducing nitrogen emissions.

Key words： periodic fluctuation of crude protein; cows; performance; apparent digestibility of nutrient; rumen fermentation parameter

Cite this article

LIU Yifan , ZHANG Chuankai , HE Liukuan , QUAN Yingkai , BU Ya’ou , ZHANG Liyang , GAO Tengyun , FU Tong , LIAN Hongxia . Effects of Periodic Fluctuation of Dietary Crude Protein Level on Performance, Nutrient Apparent Digestibility and Rumen Fermentation Parameters in Dairy Cows[J]. Chinese Journal of Animal Nutrition, 2023 , 35(7) : 4420 -4431 . DOI: 10.12418/CJAN2023.411

由于现代社会中畜牧业养殖多采用集约化的形式,导致在环境、经济和可持续发展等方面面临许多困难,而在这其中有关于氮排放对空气、水质、生物多样性以及人类健康造成的影响已成为了一个重要的问题[1-2]。奶牛能够利用植物资源为人类提供优质的乳产品,但是奶牛仅能将其中有限的氮进行转化及利用,饲粮中粗蛋白质(CP)水平和奶牛对CP的消化率能够影响奶牛排出的氮[3]。在产奶业中,人们大多将饲粮中CP水平提高,从而能够使奶牛的产奶量和乳品质得到改善,但是这会导致奶牛的粪便和尿液中排出的氮增多,较多的氮渗入到水、土地和空气中,会导致生态环境受到破坏[4-5]。因此,增加奶牛的氮利用效率,实现蛋白质高效利用与氮素减排迫在眉睫。
目前,养殖业为了保护环境、减少环境污染,多采用调整饲粮的饲喂途径以及饲粮管理等方法来实现提高反刍动物的氮利用率、减少氮排放。在自然界中,我们发现生物在生长发育中会经历一种波动性的营养过程,这种波动性的营养过程似乎是以代偿性的方式来影响生物的稳态。当羔羊饲喂以48 h为基础的CP水平波动饲粮,与连续饲喂稳定CP水平饲粮的羔羊相比,氮存留率更高[6]。当给肉牛饲喂以48 h为基础的CP水平波动饲粮,与连续饲喂高CP水平的饲粮相比,肉牛尿氮排泄量降低,与连续饲喂稳定CP水平的饲粮相比,肉牛氮存留率提高[7]。Zhang等[8]在荷斯坦公犊上的试验也表明,以48 h为间隔的饲粮CP水平波动,促进肝脏合成更多的尿素氮,从而增加胃肠道尿素氮的合成量,有利于促进再合成代谢,提高犊牛的氮利用率。因此,本试验通过探讨饲粮CP水平周期性波动对奶牛生产性能、养分表观消化率和瘤胃发酵参数的影响,以期为减少奶牛饲养过程中的氮排放提供理论依据。

1 材料与方法

1.1 试验设计

试验于河南农业大学教学试验基地(开封)完成。选取体重为(569.23±47.12) kg、泌乳天数为(150.32±19.10) d、产奶量为(22.23±2.80) kg/d的健康荷斯坦奶牛24头,随机分为3组:波动CP水平饲粮(OP)组,即以48 h间隔饲喂高CP水平饲粮(HP,CP水平为18.05%)和低CP水平饲粮(LP,CP水平为14.04%);HP组(CP水平为18.05%);稳定CP水平饲粮(SP)组(CP水平为16.05%),每组8个重复,每个重复1头奶牛。
首先对所有奶牛进行预试验,为期16 d,以确保正式试验期开始前所有奶牛均处于相同的生理状态。预试期结束后,进入为期60 d(OP组15个波动周期;每个波动周期4 d)的正式试验。于试验第53天准备开展代谢试验,时间为8 d,测定奶牛养分消化和瘤胃发酵的动态变化。

1.2 试验饲粮与饲养管理

3组试验奶牛均单栏饲喂,每日饲喂2次(07:00、15:00),挤奶3次(06:00、12:00、18:00)。试验期间奶牛自由采食并保证试验牛槽内有5%左右的剩料量。于每日早晨饲喂前收集剩余的饲粮。基础饲粮为全混合日粮(TMR),试验饲粮组成及营养水平见表1。除CP水平外,HP、LP和SP饲粮的营养水平尽可能一致。饲喂期间,观察和统计3组奶牛每天的平均采食量。试验奶牛自由活动,自由饮水。
表1 试验饲粮组成及营养水平(干物质基础)

Table 1 Composition and nutrient levels of experimental diets (DM basis) %

项目
Items		 饲粮Diets
HP LP SP
原料Ingredients
玉米Corn 23.51 31.49 27.96
干酒糟及其可溶物DDGS 6.53 11.13 7.74
豆粕Soybean meal 12.09 1.45 7.88
棉籽粕Cottonseed meal 2.90 0.97 1.45
磷酸氢钙CaHPO4 0.48 0.48 0.48
石粉Limestone 0.97 0.97 0.97
食盐NaCl 0.48 0.48 0.48
预混料Premix1) 0.53 0.53 0.53
小苏打NaHCO3 0.73 0.73 0.73
氧化镁MgO2 0.15 0.15 0.15
糖蜜Molasses 3.29 3.29 3.29
玉米青贮Maize silage 18.09 18.09 18.09
大豆皮Soybean hull 6.13 6.13 6.13
全棉籽Whole cottonseed 4.03 4.03 4.03
苜蓿干草Alfalfa hay 14.83 14.83 14.83
花生秧Peanut hay 5.24 5.24 5.24
合计Total 100.00 100.00 100.00
营养水平Nutrient levels2)
干物质DM 79.41 79.30 79.33
泌乳净能NEL/(MJ/kg) 6.61 6.61 6.61
粗蛋白质CP 18.05 14.04 16.05
粗脂肪EE 3.02 3.19 2.97
中性洗涤纤维NDF 29.10 28.79 28.71
酸性洗涤纤维ADF 17.28 16.37 16.78
粗灰分Ash 9.12 7.93 8.11
钙Ca 1.06 1.04 1.05
磷P 0.42 0.37 0.39

1)每千克预混料含有 Per kg of premix contained the following:VA 150 000 IU,VD3 35 000 IU,VE 3 000 IU,烟酰胺 niacinamide 2 000 mg,生物素 biotin 200 mg,β-胡萝卜素 β-carotin 300 mg,Mn 1 500 mg,Cu 650 mg,Zn 2 800 mg,I 35 mg,Co 23 mg,Se 25 mg。

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 指标测定与方法

1.3.1 生产性能

于试验期每天记录每头奶牛的产奶量,在前52 d中的第3(13~16 d)、7(29~32 d)、11波动周期(45~48 d),采集每头奶牛连续4 d的奶样,用于分析3组奶牛的生产性能,在代谢期第14波动周期(53~56 d),采集每头奶牛连续4 d的奶样,用于分析3组奶牛生产性能的动态变化。采样时按照早、中、晚4∶3∶3的比例采取奶样50 mL,混匀后添加0.06%重铬酸钾作防腐剂放于4 ℃保存,送于河南省奶牛生产性能测定中心测定乳成分。
4%乳脂校正乳产量(4% FCM,kg)=(0.4×产奶量)+(15×乳脂产量)。

1.3.2 养分表观消化率

在代谢期第14(53~56 d)、15波动周期(56~60 d),采用直肠取粪的方法连续取8 d粪样(采样时弃掉第1把粪),每天共采集2次。采样需在饲喂后6 h进行,采集重量为500~1 000 g的鲜粪,并在鲜粪中加入10%的稀硫酸用以固氮,鲜粪与稀硫酸比例为1∶10。于65 ℃烘箱中烘至恒重,将其粉碎,每头奶牛样品1∶1混匀备测。测定饲粮和粪样中干物质(DM)、CP、粗脂肪(EE)、中性洗涤纤维(NDF)、酸性洗涤纤维(ADF)以及氮的含量,计算养分表观消化率。
待测样品的DM、EE含量分别参照GB/T 6435—2014、GB/T 6433—2006中方法测定;CP、氮含量使用全自动凯氏定氮仪(K9860,济南海能仪器有限公司)测定;NDF、ADF含量采用滤袋法(ANKOM A200i型半自动纤维分析仪,美国Ankom公司)测定。
内源指示剂回收率(%)=(排出指示剂含量/进食指示剂含量)×100;
饲粮养分表观消化率(%)=100-[(饲粮中指示剂含量/粪样中指示剂含量)×(粪样中养分含量/饲粮中养分含量)]×100。

1.3.3 瘤胃发酵参数

在代谢期第14波动周期(53~56 d),利用注射器从口腔采集瘤胃液,时间为每日早晨饲喂2 h后,每次采集完瘤胃液后,需立刻测pH,然后将瘤胃液倒入4层纱布,目的是除去样品中的瘤胃内容物,在2 500 r/min下离心15 min,弃去剩余的残渣,取上清液20 mL平均分成2份,其中一份加入2 mL 25%的偏磷酸,于-20 ℃冷冻保存,备测。
pH:使用SG2-ELK型便捷式pH计测定。
氨态氮(NH3-N)浓度:参照冯宗慈等[9]的比色法测定。
挥发性脂肪酸(VFAs)浓度:采用SykamS150离子色谱仪测定。测定前对样品进行预处理,从-40 ℃冰箱中取出瘤胃液样品解冻后放入离心机,先以4 000 r/min低速离心10 min后取出1 mL加入0.2 mL的偏磷酸,再以10 000 r/min高速离心10 min取出,稀释50倍后振荡,用注射器装玻璃小瓶,放入S5300自动进样器(Sykam GmbH)内,样品在提前设置好的离子色谱系统中被注入Dionex AS11-HC柱(4 mm×250 mm;Thermo Fisher Scientific Inc,美国),用0.5和50 mmol/L 2种浓度的NaOH分别洗脱,共40 min。

1.4 数据统计与分析

利用Excel 2021对试验数据进行整理与计算,采用SPSS 24.0对数据进行单因素方差分析,采用Tukey检验法进行各组间的多重比较。试验结果以平均值±均值标准误表示。P≤0.05为差异显著,P≤0.01为差异极显著。

2 结果与分析

2.1 饲粮CP水平周期性波动对奶牛生产性能的影响

由表2知,在试验期内,饲粮CP水平周期性波动对奶牛的干物质采食量、产奶量、4% FCM、乳脂产量、乳蛋白产量、乳脂率及乳蛋白率均没有显著影响(P>0.05)。
表2 饲粮CP水平周期性波动在试验期内对奶牛生产性能的影响

Table 2 Effects of periodic fluctuation of dietary CP level on performance of dairy cows during experiment period

项目
Items		 组别Groups SEM P值
P-value
HP OP SP
干物质采食量Dry matter intake/(kg/d) 18.66 18.62 18.86 0.20 0.61
产奶量Milk yield/(kg/d) 24.83 26.57 25.70 1.40 0.47
4%乳脂校正乳产量4% FCM/(kg/d) 22.08 23.31 22.91 1.30 0.64
乳脂率Milk fat rate/% 3.20 3.15 3.33 0.18 0.61
乳脂产量Milk fat yield/(kg/d) 0.79 0.83 0.85 0.06 0.64
乳蛋白率Milk protein rate/% 3.30 3.27 3.38 0.08 0.53
乳蛋白产量Milk protein yield/(kg/d) 0.82 0.86 0.87 0.04 0.44

同行数据肩标无字母或相同字母表示差异不显著(P>0.05),不同小写字母表示差异显著(P<0.05),不同大写字母表示差异极显著(P<0.01)。下表同。

In the same row, values with no letter or the same letter superscripts mean no significant difference (P>0.05), while with different small letter superscripts mean significant difference (P<0.05), and with different capital letter superscripts mean significant difference (P<0.01). The same as below.

由表3可知,饲粮CP水平周期性波动对奶牛的产奶量、4% FCM、乳脂产量、乳脂率、乳蛋白产量及乳蛋白率均没有显著影响(P>0.05)。
表3 饲粮CP水平周期性波动对奶牛生产性能的影响

Table 3 Effects of periodic fluctuation of dietary CP level on performance of dairy cows

项目
Items		 时间
Time		 组别Groups SEM P值
P-value
HP OP SP
D1 24.20 25.90 23.60 1.91 0.47
产奶量 D2 23.78 26.00 22.13 1.77 0.11
Milk yield/(kg/d) D3 21.38 24.35 21.68 2.22 0.36
D4 21.55 22.30 20.18 1.69 0.46
D1 20.24 22.58 21.90 1.91 0.46
4%乳脂校正乳产量 D2 20.50 22.63 20.86 2.12 0.40
4% FCM/(kg/d) D3 18.77 21.79 20.27 2.12 0.38
D4 20.00 21.00 18.15 1.82 0.30
D1 2.93 3.16 3.50 0.25 0.16
乳脂率 D2 3.08 3.14 3.54 0.27 0.65
Milk fat rate/% D3 3.23 3.33 3.54 0.33 0.65
D4 3.54 3.61 3.31 0.26 0.49
D1 0.70 0.81 0.83 0.09 0.30
乳脂产量 D2 0.73 0.82 0.80 0.10 0.44
Milk fat yield/(kg/d) D3 0.68 0.80 0.77 0.10 0.44
D4 0.76 0.81 0.67 0.08 0.29
D1 3.45 3.36 3.41 0.11 0.71
乳蛋白率 D2 3.45 3.51 3.40 0.13 0.66
Milk protein rate/% D3 3.45 3.48 3.40 0.14 0.84
D4 3.53 3.48 3.25 0.15 0.18
D1 0.83 0.86 0.81 0.06 0.71
乳蛋白产量 D2 0.82 0.91 0.76 0.06 0.09
Milk protein yield/(kg/d) D3 0.73 0.84 0.74 0.08 0.31
D4 0.76 0.78 0.66 0.07 0.19

D1表示第53天,D2表示第54天,D3表示第55天,D4表示第56天。其中OP组D1、D2表示饲喂高CP水平饲粮,D3、D4表示饲喂低CP水平饲粮。下表同。

D1 represents day 53, D2 represents day 54, D3 represents day 55, and D4 represents day 56. D1, D2 of the OP group were fed high CP level diets, and D3, D4 were fed low CP diets. The same as below.

2.2 饲粮CP水平周期性波动对奶牛养分表观消化率的影响

由表4可知,在波动周期4 d中,OP组与HP组和SP组的DM表观消化率、CP表观消化率和氮表观消化率均没有显著差异(P>0.05)。OP组EE表观消化率与HP组相比,在波动周期第1天,OP组极显著低于HP组(P<0.01),在波动周期第2天和第3天,没有显著差异(P>0.05),在波动周期第4天,OP组显著低于HP组(P<0.05)。OP组EE表观消化率与SP组相比,在波动周期第1天、第2天和第3天均没有显著差异(P>0.05),在波动周期第4天OP组显著高于SP组(P<0.05)。在波动周期4 d中,OP组NDF表观消化率与HP组和SP组没有显著差异(P>0.05)。在波动周期4 d中,OP组ADF表观消化率与HP组没有显著差异(P>0.05)。在波动周期前3天OP组ADF表观消化率与SP组相比也没有显著差异(P>0.05),但在波动周期第4天OP组显著高于SP组(P<0.05)。
表4 饲粮CP水平周期性波动对奶牛养分表观消化率的影响

Table 4 Effects of periodic fluctuation of dietary CP protein level on nutrient apparent digestibility of dairy cows %

项目
Items		 时间
Time		 组别Groups SEM P值
P-value
HP OP SP
D1 94.31 93.02 94.00 0.62 0.12
干物质 D2 94.11 94.61 93.94 0.82 0.70
DM D3 93.68 94.16 93.82 0.68 0.77
D4 94.72 93.53 93.22 0.71 0.11
D1 72.08Aa 59.19Ba 58.11Ba 4.06 <0.01
粗脂肪 D2 81.13 81.84 76.86 2.88 0.20
EE D3 82.12 81.12 76.49 4.22 0.38
D4 88.50Aa 81.94ACb 72.66Cc 2.65 <0.01
D1 75.41 72.23 71.11 2.68 0.27
粗蛋白质 D2 72.85 76.05 76.34 2.35 0.28
CP D3 72.12 69.03 70.91 3.48 0.68
D4 73.71 67.67 71.10 2.68 0.10
D1 49.39 41.93 48.94 4.52 0.21
中性洗涤纤维 D2 48.28 50.41 49.96 3.81 0.84
NDF D3 44.94 50.28 43.50 6.86 0.59
D4 48.20Aa 40.37ABa 30.90Ba 4.76 <0.01
D1 53.29 43.60 49.05 4.89 0.16
酸性洗涤纤维 D2 48.49 55.09 52.44 3.98 0.27
ADF D3 39.75 45.71 44.74 7.50 0.70
D4 51.62Aa 45.07ABa 33.24Bb 4.49 <0.01
D1 69.70 64.26 62.62 4.01 0.21
氮 D2 66.05 69.54 67.41 3.25 0.57
N D3 65.43 61.92 60.07 4.38 0.48
D4 67.56Aa 59.91ABa 53.82Ba 4.10 0.01

2.3 饲粮CP水平周期性波动对奶牛瘤胃发酵参数的影响

由表5可知,在波动周期4 d中,饲粮CP水平波动对奶牛瘤胃液pH没有显著影响(P>0.05)。在波动周期第1天,OP组氨态氮浓度显著低于HP组(P<0.05),与SP组相比差异不显著(P>0.05)。在波动周期第2天,OP组氨态氮浓度极显著高于SP组(P<0.01),与HP组相比差异不显著(P>0.05)。在波动周期第3天和第4天,OP组氨态氮浓度极显著低于HP组(P<0.01),与SP组相比差异不显著(P>0.05)。在波动周期第1天、第2天和第3天,OP组乙酸浓度与HP组和SP组相比差异不显著(P>0.05);在波动周期第4天,OP组乙酸浓度显著高于SP组(P<0.05),与HP组相比差异不显著(P>0.05)。在波动周期第1天,OP组丙酸浓度显著高于HP组和SP组(P<0.05);在波动周期第2天,OP组丙酸浓度极显著低于SP组(P<0.01),与HP组相比差异不显著(P<0.05);在波动周期第3天和第4天,OP组丙酸浓度与HP组和SP组相比差异不显著(P>0.05)。在波动周期第1天,OP组丁酸浓度极显著低于HP组(P<0.01),极显著高于SP组(P<0.01);在波动周期第2天,OP组丁酸浓度与HP组和SP组相比差异不显著(P>0.05);在波动周期第3天,OP组丁酸浓度显著低于HP组(P<0.05),与SP组相比差异不显著(P>0.05);在波动周期第4天,OP组丁酸浓度显著高于HP组(P<0.05),与SP组相比差异不显著(P>0.05)。
表5 饲粮CP水平周期性波动对奶牛瘤胃发酵参数的影响

Table 5 Effects of periodic fluctuation of dietary CP level on rumen fermentation parameters of dairy cows

项目
Items		 时间
Time		 组别Groups SEM P值
P-value
HP OP SP
D1 6.29 5.95 6.15 0.17 0.16

pH		 D2 6.28 6.17 6.34 0.20 0.67
D3 6.18 6.27 6.38 0.23 0.69
D4 6.27 6.35 6.50 0.17 0.39
D1 41.26a 28.24b 27.08b 5.01 0.02
氨态氮 D2 41.23Aa 43.19Aa 25.72Ba 4.25 <0.01
Ammoniacal nitrogen/(mg/dL) D3 43.35Aa 29.90Ba 24.71Ba 2.63 <0.01
D4 38.36Aa 24.11Ba 24.44Ba 1.43 <0.01
D1 61.53 63.89 56.82 5.96 0.49
乙酸 D2 69.74 71.51 60.15 9.69 0.46
Acetic acid/(mmol/L) D3 72.13a 66.64ab 52.22a 7.69 0.04
D4 72.43a 70.52a 55.38b 6.60 0.03
D1 12.06a 15.81b 11.94a 1.33 0.02
丙酸 D2 9.18ABa 7.70Aa 12.39Bb 1.46 0.02
Propionic acid/(mmol/L) D3 10.40 8.23 7.12 1.42 0.09
D4 8.11 9.95 8.48 0.94 0.14
D1 4.89Aa 3.74Ba 2.89Ca 0.29 <0.01
丁酸 D2 3.17 3.04 3.30 0.40 0.81
Butyric acid/(mmol/L) D3 4.40Aa 3.36ABb 2.52Bb 0.46 <0.01
D4 2.26a 3.09b 2.84ab 0.32 0.04

3 讨论

3.1 饲粮CP水平周期性波动对奶牛生产性能的影响

Tebbe等[10]证明了饲喂波动CP水平饲粮(24 h)的奶牛在乳蛋白和FCM产量方面与饲喂稳定CP水平饲粮的奶牛表现相似。Rauch等[11]报道了饲喂CP水平由低到高波动的饲粮的奶牛与饲喂稳定CP水平饲粮的奶牛表现相似,产奶量和乳成分不受饲粮CP水平波动的影响。据已报道的研究结果来看,饲喂波动CP水平饲粮的奶牛相较于饲喂稳定CP水平饲粮的奶牛在泌乳性能方面似乎没有得到明显的改善,这与本试验的结果相似。OP组在饲喂高CP水平饲粮时期第2天乳蛋白产量相比于SP组有提高趋势,这可能是伴随着高CP水平饲粮的饲喂,代谢蛋白质(MP)供应量在体内增加的结果。MP的供应是产奶量的限制性营养因素[12],泌乳净能(NEL)和MP供应对产奶量和乳成分产量的影响是累加的。SP组在饲喂低CP水平饲粮时期的泌乳性能与HP组没有显著差异。在对绵羊的几项研究报告中,基于48 h阶段饲喂波动CP水平饲粮时,绵羊的氮保留或平均日增重有所改善[6]。这可能是在饲喂高CP水平饲粮时期体内滞留的氮弥补了饲喂低CP水平饲粮时期氮供应的不足。

3.2 饲粮CP水平周期性波动对奶牛养分表观消化率的影响

从试验结果看,在波动周期4 d内,DM的表观消化率不受波动的影响,在波动周期第4天,也就是OP组饲喂低CP水平饲粮时期的最后阶段,OP组的EE、NDF、ADF表观消化率显著高于SP组。据Broderick等[13]和Lee等[14]的报道,当饲粮CP水平降低时会降低NDF的消化率,也有研究表明,当增加饲粮中CP的水平时,NDF、ADF表观消化率也会伴随着提高[15],主要是由于反刍动物利用纤维的途径,反刍动物的瘤胃中含有一些微生物,其能够分解纤维从而促进反刍动物的吸收和利用。而瘤胃微生物需要通过肽、氨基酸和氨获取能量,从而加速生长发育,这些能量来源则是饲粮中的蛋白质。也有研究发现,在西藏白绒山羊妊娠后期,随着饲粮中CP水平的增加,山羊对CP的表观消化率也随之增加,与低CP水平饲粮组相比差异显著[16]。但本试验中并没有观察到这种结果,OP组的NDF、ADF表观消化率的提高可能与瘤胃微生物对纤维的利用有关。Panadi等[17]给杜泊羔羊饲喂CP水平分别为14%、16%、18%和20%的饲粮,研究结果表明,粗灰分、CP和EE的摄入量随着饲粮CP水平的升高而呈线性增加。Katongole等[18]在泌乳期奶牛中研究表明,当饲粮中CP的水平增加时,DM、NDF和ADF的表观消化率没有随之增加或减少,而CP的表观消化率则随之增加。然而在波动周期第4天,本试验并没有观察到OP组与SP组相比CP、氮表观消化率的变化,OP组在饲喂低CP水平饲粮时期处于氮平衡状态,这可能是瘤胃氮循环的增加,促进了微生物对氮的利用率。我们知道瘤胃微生物生长及合成蛋白所依赖肽、氨基酸和氨主要来自饲粮中的氮供应,增加饲粮中氮的水平有助于提高微生物的活性。伴随着微生物对氮利用率的提高,微生物对纤维的消化速率和程度也会随着提高[19⇓⇓-22]。由于CP水平较高的饲粮可能会降低将ME转化为NEL的效率[22-23],这在一定程度上能够解释OP组EE表观消化率提高的原因。

3.3 饲粮CP水平周期性波动对奶牛瘤胃发酵参数的影响

饲粮中的氮在瘤胃降解过程中产生了大量的氨。氨是大多数瘤胃细菌菌株所需的氮形式,瘤胃发酵过程中产生的氨可对瘤胃pH起到缓冲的作用,还可用于瘤胃微生物的生长。瘤胃pH与瘤胃微生物能否维持平衡状态息息相关,瘤胃pH的变化能够反映其发酵的程度和方式[24]。牛骁麟[25]研究表明,瘤胃pH与饲粮CP水平呈正相关关系,原因可能是随着饲粮CP水平的增加,瘤胃VFAs浓度具有下降的趋势,从而导致了瘤胃pH的增加。这与徐俊等[26]用体外培养法研究CP水平变化对瘤胃发酵的影响试验研究结果相同。王雅倩等[27]给湖羊饲喂CP水平分别为8.4%、11.2%和14.0%的饲粮,研究结果表明,不同组湖羊的瘤胃pH变化与饲粮CP水平不相关,组间差异不显著。这与本试验结果相同,这可能是瘤胃尿素氮或氨的缓冲作用。
NH3-N浓度能够表示动物降解饲料中CP的情况和合成微生物蛋白质所需氨是否达到平衡。即NH3-N的浓度能够表示动物能否降解饲料中含氮物质及其体内瘤胃微生物能否吸收利用氨,瘤胃微生物能够合成NH3-N,然后进一步合成瘤胃微生物蛋白质。在瘤胃微生物区系中,约80%的细菌可以将NH3-N作为直接氮源[28]。郭庆河等[29]研究表明,提高浓缩料的CP水平,可以显著提高瘤胃NH3-N浓度,王雅倩等[27]也得出相同结论,当饲粮中CP水平增加时,NH3-N浓度也会增加。Zhou等[30]试验结果显示,当饲粮中CP水平从111 g/kg增加为136 g/kg,动物瘤胃中的NH3-N浓度也会增加。Bahrami-Yekdangi等[31]表明饲粮CP水平的降低不会对瘤胃pH和NH3-N的浓度产生影响。瘤胃NH3-N浓度已成为微生物对饲粮CP降解和利用双重指标。NH3-N浓度的变化与饲粮CP的摄入量呈正相关关系[32]。OP组NH3-N浓度在饲喂低CP水平饲粮时期与SP组相比没有显著差异,这可能是尿素氮向瘤胃循环的增加,提高了氮存留率,维持了饲喂低CP水平饲粮时期的氮平衡,在一定程度上反映了瘤胃微生物能抵抗以48 h为间隔的波动的饲粮CP水平的刺激。
瘤胃中含有一些微生物,微生物能够通过发酵功能把淀粉等碳水化合物转化为丙酮酸,丙酮酸又能通过一些代谢途径生成VFAs,在此过程中也会释放出ATP。ATP能够作为能量来源,促进瘤胃微生物的生长以及微生物蛋白质的合成。OP组在波动周期第4天出现的乙酸浓度变化可能是由于NDF、ADF表观消化率改善的原因。Doranalli等[33]观察到,相对于波动饲粮CP水平处理的高饲粮CP水平阶段,低饲粮CP水平处理的瘤胃pH和瘤胃氨水平较低,与静态处理相比,波动处理的瘤胃VFA浓度总体增加。尽管尿素氮向瘤胃循环可以通过简单的扩散进行,但已有研究证明尿素转运蛋白B(UT-B)在促进尿素转运中是关键的存在[34-35]。瘤胃中的VFAs能够刺激并促进尿素转运,尤其丁酸的影响效果更好。有研究结果显示,将利木赞杂交牛分别饲喂青贮型和精料型的饲粮,后者UT-B2 mRNA和蛋白的表达水平均得到提高,此外瘤胃中丁酸的浓度也有所增加[36]。丁酸可能会对UT-B2的表达产生影响,从而进一步影响瘤胃的尿素氮循环。因此,OP组丁酸浓度的提高可能是尿素氮向瘤胃循环增加所诱导的。

4 结果

本试验条件下,适当调节饲粮中CP水平能够在饲喂低CP水平饲粮时期提高奶牛的EE、ADF表观消化率,以及提高瘤胃中乙酸和丁酸的浓度,有利于促进瘤胃微生物吸收利用饲粮中的CP以及增加尿素氮向瘤胃循环,从而促进奶牛吸收利用氮,减少氮排放。因此,调节饲粮中CP水平可以为提高饲粮氮利用和减少氮排放提供新的可能性。

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