Chinese Journal of Animal Nutrition >

2024 , Vol. 36 >Issue 5: 3363 - 3372

DOI: https://doi.org/10.12418/CJAN2024.288

RESEARCH PAPER

Effects of Different Proportions of Whole Rape Silage Replacing Whole Corn Silage on Simulated Rumen Fermentation Parameters in Vitro

  • WANG Le , 1, 2 ,
  • GAO Shuai 3 ,
  • ZHANG Xiumin 2 ,
  • WANG Min 2 ,
  • ZHENG Chen , 1, * ,
  • WANG Rong , 2, *
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  • 1 College of Animal Science and Technology, Gansu Agricultural University, Lanzhou 730070, China
  • 2 Institute of Subtropical Agriculture, Chinese Academy of Sciences, Changsha 410125, China
  • 3 Hunan Institute of Animal Husbandry and Veterinary Medicine, Changsha 410131, China
* ZHENG Chen, professor, E-mail: ;
WANG Rong, assistant professor, E-mail:

Received date: 2023-11-15

  Online published: 2024-05-15

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Abstract

This study was conducted to investigate the effects of different proportions of whole rape silage replacing whole corn silage on total gas, hydrogen, methane and volatile fatty acid production of simulated rumen fermentation in vitro. A single-factor experimental design was employed, with whole rape silage replacing whole corn silage at proportion of 0 (control), 25%, 50%, 75%, 75% and 100%, respectively. Three Xiangdong black goats with permanent rumen fistula were selected as rumen fluid donors for a 72 h simulated rumen fermentation experiment in vitro. The results showed as follows: 1) the organic matter degradation rate, gas production per gram of substrate, gas production per gram of degraded substrate, potential maximum gas production, gas production rate and initial substrate degradation rate, and the gas production per gram of substrate, gas production per gram of degraded substrate, potential maximum gas production and gas production rate of methane, as well as the hydrogen consumption rate, total volatile fatty acid content and propionic acid ratio were linearly decreased with the increase of whole rape silage proportion (P<0.05). The gas production per gram of substrate, gas production per gram of degraded substrate, potential maximum gas production and gas production rate of hydrogen, the pH, ammonia nitrogen content, acetic acid, isobutyric acid, valeric acid, isovaleric acid ratios and acetic acid to propionic acid ratio were all linearly increased with the increase of whole rape silage proportion (P<0.05). 2) Compared with the control group, when the whole rape silage proportion≥50%, the organic matter degradation rate, gas production per gram of substrate, gas production per gram of degraded substrate and gas production rate, and the gas production per gram of substrate, potential maximum gas production and gas production rate of methane as well as propionic acid ratio were significantly decreased (P<0.05); the gas production per gram of degraded substrate, potential maximum gas production and gas production rate of hydrogen, the pH, ammonia nitrogen content, acetic acid, isobutyric acid, valeric acid, isovaleric acid ratios and acetic acid to propionic acid ratio were significantly increased (P<0.05). In conclusion, although the whole rape silage replacing corn silage can inhibit methane production of simulated rumen fermentation in vitro, when the proportion is 50%, the organic matter degradation rate significantly decreases, indicating that the proportion of whole rape silage replacing whole corn silage should not exceed 50%.

Key words： simulated rumen fermentation in vitro; whole rape silage; whole corn silage; methane; volatile fatty acids

Cite this article

WANG Le , GAO Shuai , ZHANG Xiumin , WANG Min , ZHENG Chen , WANG Rong . Effects of Different Proportions of Whole Rape Silage Replacing Whole Corn Silage on Simulated Rumen Fermentation Parameters in Vitro[J]. Chinese Journal of Animal Nutrition, 2024 , 36(5) : 3363 -3372 . DOI: 10.12418/CJAN2024.288

粗饲料是反刍动物的主要能量来源,也是机体维持健康的重要基础。随着我国畜牧业的快速发展,粗饲料供求关系紧张、优质粗饲料短缺等成为限制畜牧业发展的主要因素之一[1]。近年来,我国推出的“粮改饲”、“油改饲”等农业供给侧结构调整政策,能够有效缓解养殖业中的冬春饲料资源短缺问题,有力地促进了粮食作物、油料作物等非常规饲料替代传统优质粗饲料[2]。因此,充分利用当地非常规饲料资源,挖掘其潜在利用价值,已经成为缓解我国畜牧业优质粗饲料资源短缺问题的重要举措,也是目前反刍动物营养研究的热点。
油菜(Brassica napus L.)为十字花科、芸薹属植物,其因具有营养价值高、生长快、产量高,可在冬季利用闲田种植,不与粮食作物争地等优点,在中国很多地区开始大规模栽培及推广,尤其是在湖南省,种植面积达138.87万hm2以上,播种面积稳居全国前2位,并有持续增长的趋势,是一种极具开发潜力的优质粗饲料资源[3]。殷雨洋等[4]研究发现,油菜青贮饲喂湖羊可达到与玉米青贮相近的生长效果。Zhou等[5]研究发现,油菜青贮替代乳水牛饲粮中的部分玉米青贮,不仅不会对乳水牛产奶性能和瘤胃发酵模式产生不利影响,还会显著提高乳品质(如乳蛋白、乳脂含量等),以及促进乳水牛干物质采食量的提高。此外,在哺乳母羊基础饲粮中额外添加1 kg/d油菜,可以改善母羊哺乳期体质状况,促进羔羊生长发育,提高羔羊成活率;全株油菜做成发酵全混合日粮对羔羊强度育肥效果较好[6-7]。前期研究发现,油菜籽中富含植物油脂,菜籽油能够减少肉牛甲烷排放[8]。笔者推测,利用全株油菜青贮替代全株玉米青贮也能够减少甲烷产生,然而目前相关研究报道较少,还需要进一步证实。因此,本试验旨在利用体外模拟瘤胃发酵技术,探究全株油菜青贮替代全株玉米青贮对体外模拟瘤胃发酵总产气、氢气、甲烷和挥发性脂肪酸生成的影响,为全株油菜青贮在牛羊养殖生产中的应用提供理论依据。

1 材料与方法

1.1 试验材料

选用湖南省常德市德人牧业种植基地的饲用全株油菜和湖南省株洲攸县的全株玉米作为试验材料。全株油菜收割于2023年5月,全株玉米收割于2023年7月,收割后用揉丝机切碎至2~5 cm后,收回饲料加工车间,做成约500 kg的青贮包,进行发酵加工;发酵50 d后,分别取发酵品质良好的全株油菜青贮和全株玉米青贮样品各1 kg带回实验室,先在105 ℃条件下烘干0.5 h,然后在65 ℃条件下烘干48 h至恒重,接着将全株油菜青贮与全株玉米青贮用粉碎机粉碎后过40目筛,筛上样品再粉碎再过筛,直至样品全部过筛,最后装于密封袋中保存待测。

1.2 试验动物及饲粮

选取3只体重相近且安装永久性瘤胃瘘管的健康成年黑山羊作为瘤胃液供体,每天饲喂精粗比为1∶1的饲粮,其中粗饲料为花生秧,精饲料组成为47.00%玉米、24.00%豆粕、22.00%麦麸、0.77%食盐、2.23%石粉和4.00%预混料。试验期间,每天饲喂2次(07:00和17:00各1次),每次饲喂精饲料和花生秧各300 g(干物质基础),自由饮水。

1.3 体外模拟瘤胃发酵试验

本试验采用单因素试验设计,设置5个组,分别以全株油菜青贮替代0(对照)、25%、50%、75%和100%的全株玉米青贮,并确保每个发酵瓶中的发酵底物重量一致。各组发酵底物的干物质(DM)、粗灰分(Ash)、粗蛋白质(CP)和淀粉含量参照张丽英[9]的方法测定;中性洗涤纤维(NDF)和酸性洗涤纤维(ADF)含量参照Van Soest等[10]的滤袋法测定;总能(GE)采用等温式全自动热量仪(5E-AC8018,长沙开元仪器有限公司)进行测定。各组发酵底物原料组成及营养水平见表1。
表1 各组发酵底物原料组成及营养水平(干物质基础)

Table 1 Composition and nutrient levels of fermentation substrate in each group (DM basis) %

项目
Items		 全株油菜青贮比例Proportions of whole rape silage/%
0 (对照Control) 25 50 75 100
原料Ingredients
全株玉米青贮Whole corn silage 100 75 50 25
全株油菜青贮Whole rape silage 25 50 75 100
营养水平Nutrient levels
干物质DM 94.54 94.58 94.62 94.65 94.69
粗蛋白质CP 7.51 9.29 11.06 12.84 14.62
中性洗涤纤维NDF 53.93 53.97 54.01 54.06 54.10
酸性洗涤纤维ADF 26.18 29.35 32.52 35.70 38.87
半纤维素Hemicellulose 27.75 24.62 21.49 18.36 15.23
总能GE/(MJ/kg) 16.82 17.33 17.83 18.34 18.84
淀粉Starch 27.65 24.16 20.66 17.17 13.68

干物质含量为风干基础。

DM content was air-dry basis.

参照Wang等[11]的方法进行体外模拟瘤胃发酵试验。精确称取1 g发酵物底物于150 mL厌氧发酵瓶中,置于振荡频率为50 r/min、39.5 ℃的恒温培养箱中预热2 h,取出发酵瓶,通入纯二氧化碳以保证发酵瓶中为厌氧环境。参照Menke等[12]提供的方法配制人工瘤胃培养液,用注射器迅速向每个发酵瓶中加入60 mL人工瘤胃培养液,立即盖严瓶塞,并使培养液与发酵底物混合均匀,然后将发酵瓶转移至39 ℃恒温培养箱进行体外模拟瘤胃发酵。每个组设置3个平行,每个发酵瓶为1个平行,试验重复3次,每次选2头不同的瘘管羊作为瘤胃液供体。
试验采用的全自动体外模拟瘤胃发酵设备主要包括恒温培养箱、发酵瓶、三通电磁阀、压力传感器、计算机和气相色谱仪等几部分组成。发酵瓶通过导管与三通电阀及压力传感器连接,压力传感器与计算机连接,对发酵瓶中的压力进行实时检测,根据压力与气体容积间的关系计算气体生成量。三通电阀在计算机的控制下,当发酵瓶中压力超过9 kPa时,电磁阀会自动开启,将瓶内气体释放并经过导管进入气相色谱仪,测定排出气体中的氢气和甲烷。参考Wang等[13]提供的方法对发酵每克底物的产气参数、氢气和甲烷产量进行计算。

1.4 样品采集与测定

72h发酵结束后,依次取下发酵瓶,采用pH计(S210,Mettler Toledo)测定发酵液的pH。取4 mL发酵液,在4 ℃条件下,7 992×g离心10 min,取1.5 mL上清液,加入0.15 mL 25%的偏磷酸固定,于-20 ℃条件下保存过夜。参考Wang等[14]的方法测定各个挥发性脂肪酸组分的浓度,氨态氮浓度根据Weatherburn[15]的方法测定。参照程景等[16]的方法,计算有机物降解率。

1.5 数据统计分析

采用NLREG软件程序[17],计算潜在最大产气量、产气速率以及起始底物降解速率(FRD0,/h)等[18-19]。采用SPSS 23.0软件中的一般线性模型程序对数据进行方差分析,并采用Duncan氏法对各组平均值进行多重比较,然后使用正交多项式比较分析各个指标随全株油菜青贮和全株玉米青贮组合比例变化的趋势。以P<0.05作为差异显著的判断标准。

2 结果与分析

2.1 不同比例全株油菜青贮替代全株玉米青贮后的常规营养成分含量

由表1可知,不同比例全株油菜青贮替代全株玉米青贮后,发酵底物中DM含量介于94.54%~94.69%,CP含量介于7.51%~14.62%,NDF含量介于53.93%~54.10%,ADF含量介于26.18%~38.87%,GE介于16.82~18.84 MJ/kg,半纤维素含量介于15.23%~27.75%,淀粉含量介于13.68%~27.65%。

2.2 不同比例全株油菜青贮替代全株玉米青贮对体外模拟瘤胃发酵产气参数的影响

如图1所示,各组发酵底物之间的产气曲线存在明显差异。由表2可知,不同比例全株油菜青贮替代全株玉米青贮显著影响有机物降解率、每克底物的产气量、降解每克底物的产气量、潜在最大产气量、产气速率和起始底物降解速率(P<0.05),其中有机物降解率、每克底物的产气量、降解每克底物的产气量、潜在最大产气量、产气速率和起始底物降解速率均随全株油菜青贮替代比例的提高而呈线性降低(P<0.05)。与对照组相比,当全株油菜青贮替代比例≥25%时,每克底物的产气量和产气速率显著降低(P<0.05);当全株油菜青贮替代比例≥50%时,有机物降解率和降解每克底物的产气量显著降低(P<0.05);当全株油菜青贮替代比例为100%时,潜在最大产气量和起始底物降解速率显著降低(P<0.05)。
图1 不同比例全株油菜青贮替代全株玉米青贮对体外模拟瘤胃发酵产气曲线的影响

Fig.1 Effects of different proportions of whole rape silage replacing whole corn silage on gas production curve of simulated rumen fermentation in vitro

2.3 不同比例全株油菜青贮替代全株玉米青贮对体外模拟瘤胃发酵氢气和甲烷生成的影响

如图1所示,各组发酵底物之间的氢气生成曲线存在明显差异。由表3可知,不同比例全株油菜青贮替代全株玉米青贮显著影响氢气的每克底物的产气量、潜在最大产气量、降解每克底物的产气量和产气速率(P<0.05),其中氢气的每克底物的产气量、潜在最大产气量、降解每克底物的产气量和产气速率均随全株油菜青贮替代比例的提高而呈线性提高(P<0.05),氢气的消耗速率随全株油菜青贮替代比例的提高而呈现线性降低(P<0.05)。与对照组相比,当全株油菜青贮替代比例≥25%时,氢气的产气速率显著提高(P<0.05);当全株油菜青贮替代比例≥50%时,氢气的潜在最大产气量和降解每克底物的产气量显著提高(P<0.05);当全株油菜青贮替代比例≥75%时,氢气的每克底物的产气量显著提高(P<0.05)。
表2 不同比例全株油菜青贮替代全株玉米青贮对体外模拟瘤胃发酵产气参数的影响

Table 2 Effects of different proportions of whole rape silage replacing whole corn silage on gas production parameters of simulated rumen fermentation in vitro

项目
Items		 全株油菜青贮比例Proportions of whole rape silage/% 均值
标准误
SEM		 P值P-value
0 (对照
Control)		 25 50 75 100 处理
Treatment		 线性
Linear		 二次
Quadratic
有机物降解率
Organic matter degradation
rate/(g/kg)		 681.94a 669.74ab 650.83b 571.89c 571.13c 0.411 <0.01 <0.01 0.31
产气量Gas production/(mL/g)
每克底物的产气量
Gas production per
gram of substrate		 271.24a 248.63b 225.45c 196.59d 158.73e 2.560 <0.01 <0.01 0.12
降解每克底物的产气量
Gas production per gram
of degraded substrate		 397.88a 372.48ab 347.88b 345.57b 278.34c 5.512 <0.01 <0.01 0.21
潜在最大产气量
Potential maximum gas
production		 300.47a 297.77a 294.14a 278.22a 241.81b 3.391 <0.01 <0.01 0.01
产气速率
Gas production rate/
(×10-2/h)		 5.97a 5.54b 5.09c 4.75d 4.59d 0.200 <0.01 <0.01 0.06
起始底物降解速率
Initial substrate
degradation rate/(×10-2/h)		 1.96a 2.01a 1.93a 2.05a 1.61b 0.300 0.04 0.03 0.03

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)。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). The same as below.

表3 不同比例全株油菜青贮替代全株玉米青贮对体外模拟瘤胃发酵氢气和甲烷生成的影响

Table 3 Effects of different proportions of whole rape silage replacing whole corn silage on hydrogen and methane production of simulated rumen fermentation in vitro

项目
Items		 全株油菜青贮比例Proportions of whole rape silage/% 均值
标准误
SEM		 P值P-value
0 (对照
Control)		 25 50 75 100 处理
Treatment		 线性
Linear		 二次
Quadratic
氢气Hydrogen/(mL/g)
每克底物的产气量
Gas production per
gram of substrate		 0.10b 0.11b 0.12ab 0.14a 0.13a 0.003 0.01 <0.01 0.76
潜在最大产气量
Potential maximum
gas production		 0.10c 0.11c 0.14b 0.18a 0.19a 0.004 <0.01 <0.01 0.96
降解每克底物的产气量
Gas production per
gram of degraded substrate		 0.15c 0.16bc 0.19b 0.24a 0.25a 0.005 <0.01 <0.01 0.50
产气速率
Gas production rate/(/h)		 0.15c 0.36b 0.58a 0.68a 0.69a 0.029 <0.01 <0.01 0.06
消耗速率
Consumption rate/(/h)		 2.93 1.74 1.93 0.87 0.50 0.287 0.08 0.02 0.88
甲烷Methane/(mL/g)
每克底物的产气量
Gas production per
gram of substrate		 26.81a 25.64ab 24.64b 21.94c 15.85d 0.304 <0.01 <0.01 <0.01
潜在最大产气量
Potential maximum
gas production		 32.80a 31.19b 31.07b 27.93c 24.86d 0.141 <0.01 <0.01 <0.01
降解每克底物的产气量
Gas production per
gram of degraded substrate		 39.18a 38.30a 38.83a 38.85a 29.24b 0.511 <0.01 <0.01 <0.01
产气速率
Gas production rate/
(×10-2/h)		 7.12a 6.63ab 5.85bc 5.61bc 5.48c 0.200 <0.01 <0.01 0.41
如图1所示,各组发酵底物之间的甲烷生成曲线存在明显差异。由表3可知,不同比例全株油菜青贮替代全株玉米青贮显著影响甲烷的每克底物的产气量、潜在最大产气量、降解每克底物的产气量和产气速率(P<0.05),其中甲烷的每克底物的产气量、潜在最大产气量、降解每克底物的产气量和产气速率均随全株油菜青贮替代比例的提高而呈现线性降低(P<0.05)。与对照组相比,当全株油菜青贮替代比例≥25%时,甲烷的潜在最大产气量显著降低(P<0.05);当全株油菜青贮替代比例≥50%时,甲烷的每克底物的产气量和产气速率显著降低(P<0.05);当全株油菜青贮替代比例为100%时,甲烷的降解每克底物的产气量显著降低(P<0.05)。

2.4 不同比例全株油菜青贮替代全株玉米青贮对体外模拟瘤胃发酵挥发性脂肪酸生成的影响

由表4可知,不同比例全株油菜青贮替代全株玉米青贮显著影响体外模拟瘤胃发酵pH、氨态氮和总挥发性脂肪酸含量以及乙酸、丙酸、异丁酸、戊酸、异戊酸比例和乙酸/丙酸比值(P<0.05),其中pH、氨态氮含量以及乙酸、异丁酸、戊酸、异戊酸比例和乙酸/丙酸比值均随全株油菜青贮替代比例的提高而呈现线性提高(P<0.05),而总挥发性脂肪酸含量和丙酸比例随全株油菜青贮替代比例的提高而呈现线性降低(P<0.05)。与对照组相比,当全株油菜青贮替代比例≥25%时,pH、氨态氮含量以及乙酸、异丁酸、戊酸、异戊酸比例和乙酸/丙酸比值显著提高(P<0.05),丙酸比例显著降低(P<0.05);当全株油菜青贮替代比例为100%时,总挥发性脂肪酸含量显著降低(P<0.05)。
表4 不同比例全株油菜青贮替代全株玉米青贮对体外模拟瘤胃发酵挥发性脂肪酸生成的影响

Table 4 Effects of different proportions of whole rape silage replacing whole corn silage on production of volatile fatty acids of simulated rumen fermentation in vitro

项目
Items		 全株油菜青贮比例Proportions of whole rape silage/% 均值
标准误
SEM		 P值P-value
0 (对照
Control)		 25 50 75 100 处理
Treatment		 线性
Linear		 二次
Quadratic
pH 6.25d 6.41c 6.53bc 6.65ab 6.72a 0.023 <0.01 <0.01 0.38
氨态氮
Ammonia nitrogen/
(mmol/L)		 9.21b 14.72a 13.82a 15.19a 16.86a 0.481 <0.01 <0.01 0.20
总挥发性脂肪酸
Total VFA/
(mmol/L)		 99.85a 98.33a 93.94a 98.32a 79.80b 1.504 <0.01 <0.01 0.06
各挥发性脂肪酸摩尔比例Molar proportions of individual VFA/%
乙酸Acetate 60.64e 62.87d 65.03c 67.68b 70.49a 0.277 <0.01 <0.01 0.49
丙酸Propionate 29.91a 27.11b 24.25c 20.92d 17.28e 0.219 <0.01 <0.01 0.26
丁酸Butyrate 6.95 7.10 7.22 7.28 6.90 0.161 0.93 0.98 0.38
异丁酸Isobutyrate 0.68e 0.77d 0.90c 1.06b 1.30a 0.014 <0.01 <0.01 <0.01
戊酸Valerate 0.82e 0.98d 1.18c 1.44b 1.86a 0.019 <0.01 <0.01 <0.01
异戊酸Isovalerate 1.00e 1.16d 1.36c 1.67b 2.17a 0.026 <0.01 <0.01 <0.01
乙酸/丙酸
Acetate/propionate		 2.03e 2.32d 2.69c 3.24b 4.09a 0.036 <0.01 <0.01 <0.01

3 讨论

3.1 不同比例全株油菜青贮替代全株玉米青贮对体外模拟瘤胃发酵产气参数的影响

饲粮经牛羊瘤胃发酵后产生氢气、甲烷和二氧化碳等,产气量可以反映饲粮被瘤胃微生物消化利用的情况,通常情况下,饲粮被微生物消化利用的越彻底,总产气量越高[20]。马绍楠等[21]报道,饲粮在肉羊瘤胃中的干物质降解率与体外产气量之间存在正相关关系,因此,产气量也可以作为衡量饲料可发酵程度的重要指标[22]。在本试验中,当全株油菜青贮替代比例≥50%时,每克底物的产气量和有机物降解率显著降低,并且随着替代比例的提高,呈现线性降低。研究表明,瘤胃有机物降解率取决于饲粮本身的结构和组成,饲粮木质化程度越低,表明越容易被降解[23]。有研究发现,结实期的全株油菜青贮含有10.23% CP、49.78% NDF和41.15% ADF[24],这与本试验结果略有差异。这可能是由于油菜的品种、种植地域或收割时间不同所导致的。本试验结果表明,全株油菜青贮中的纤维含有更多难以降解的结构性碳水化合物(ADF),而相对比较容易降解的半纤维素和非结构性碳水化合物(淀粉)含量较低,这可能是导致其降解率降低的主要原因。

3.2 不同比例全株油菜青贮替代全株玉米青贮对体外模拟瘤胃发酵氢气和甲烷生成的影响

饲粮中的可发酵碳水化合物在瘤胃微生物的作用下,生成挥发性脂肪酸,并且往往伴随着氢气的产生[25]。为了维持瘤胃内氢气分压稳定和碳水化合物降解,瘤胃内甲烷菌会直接将一部分氢气转化为甲烷排出体外。研究结果表明,碳水化合物类型会明显影响瘤胃内溶解态氢的浓度[26]。与淀粉相比,纤维的降解和发酵速率较低,这会导致瘤胃氢浓度处于较低水平[27]。因此,利用非结构性碳水化合物含量低的粗饲料替代非结构性碳水化合物高的粗饲料,对反刍动物瘤胃氢气和甲烷生成具有调节作用[28]。除此之外,氢的产生还会受到多种因素影响,如瘤胃发酵模式、瘤胃内饲粮降解程度以及甲烷菌对氢的利用程度等[29]。在本研究中,当全株油菜青贮替代比例≥50%时,氢气的潜在最大产气量和降解每克底物的产气量显著提高。笔者推测,这可能与瘤胃内甲烷生成受到抑制有关,也与甲烷的每克底物的产气量和产气速率显著降低的结果相一致。研究结果表明,当瘤胃甲烷生成路径受到抑制后,会导致瘤胃内氢的积累,使得氢气产量增加[30]。有研究发现,在羔羊饲粮中用不同水平的饲用油菜代替黑麦草可减少甲烷排放[31]。这与本试验结果相一致,在本试验中,当全株油菜青贮替代比例≥50%时,甲烷的每克底物的产气量、潜在最大产气量和产气速率显著降低。这有可能是全株油菜中含有某种可以抑制甲烷生成的物质。有研究表明,饲粮中添加植物油可抑制甲烷生成[32]。Zhang等[8]研究发现,在反刍动物饲粮中添加菜籽油,可能有助于减少甲烷排放。全株油菜中含有菜籽,含有丰富的油脂,这可能会对甲烷菌产生直接的抑制作用,减少甲烷菌对氢气的利用,从而减少甲烷产生和氢气积累。

3.3 不同比例全株油菜青贮替代全株玉米青贮对体外模拟瘤胃发酵挥发性脂肪酸生成的影响

pH是影响瘤胃微生物生长代谢的重要因素,也是反映瘤胃发酵的关键指标。在本试验中,各组发酵液pH介于6.25~6.72,均在适宜范围。研究表明,瘤胃挥发性脂肪酸含量及组成主要由饲粮碳水化合物类型决定[33],其中结构性碳水化合物发酵产物主要是乙酸[34],而非结构性碳水化合物发酵会产生更多的丙酸[35]。在本试验中,随着全株油菜青贮替代比例的提高,乙酸比例和乙酸/丙酸比值呈现线性提高,而丙酸比例呈现线性降低。这主要是因为全株油菜青贮替代比例的提高导致发酵底物结构性碳水化合物含量升高,非结构性碳水化合物含量降低,从而促使更多的乙酸生成。异丁酸和异戊酸均属异构酸,瘤胃内异构酸主要来源于蛋白质降解成氨基酸后在微生物作用下经氢化脱氨基或脱羧基后的产物[36]。有研究发现,用饲用油菜青贮和全株玉米青贮饲喂山羊,饲用油菜青贮组的异丁酸比例显著高于全株玉米青贮组[37],这与本试验结果相一致。本试验中,异丁酸和异戊酸比例随全株油菜青贮替代比例的提高呈线性提高,这可能是因为随着全株油菜青贮替代比例的提高,发酵底物中CP含量得以提高,进而促进了微生物对蛋白质的降解,导致氨基酸分解增加,异丁酸和异戊酸比例升高[38]。此外,全株油菜青贮替代全株玉米青贮后,氨态氮含量显著提高,这可能是因为蛋白质被消化降解后,会代谢产生氨态氮[29],从而导致瘤胃内氨态氮含量显著提高。除此之外,蛋白质在瘤胃降解过程中也会产生异丁酸、戊酸和异戊酸,这也与本试验结果相一致。

4 结论

全株油菜青贮替代全株玉米青贮虽然会对体外模拟瘤胃发酵甲烷生成产生抑制作用,但是当替代比例为50%时,发酵底物的有机物降解率显著降低,这说明全株油菜青贮替代全株玉米青贮的比例不宜超过50%。

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