动物营养学报    2020, Vol. 32 Issue (7): 3012-3030    PDF    
放牧家畜采食量的测定
金有顺 , 侯扶江     
兰州大学草地农业生态系统国家重点实验室, 兰州大学农业农村部草牧业创新重点实验室, 兰州大学草地农业科技学院, 兰州 730020
摘要: 采食是动物-植物相互作用的重要途径,是维持生态系统物质和能量循环的主要过程。动物的生产水平和草地的可持续性取决于动物的采食量,明确放牧家畜采食量是草地管理的关键之一。目前,直接观测法、指示剂法、粪氮指数法和近红外光谱法等现有技术被广泛运用到采食量的测定中。直接观测法利用较低成本对采食量进行计算,但主观性较强;食道瘘管法能够把握采食量却忽略了动物福利;近红外光谱法和饱和链烷烃法在测定采食量的同时能够认识家畜的择食特征。为此,本文对现有常规测定方法进行综述,分析影响测定放牧家畜采食量的主要因素,并提出放牧行为与无人机技术相结合等新思路,为今后草原精准化管理提供技术支撑。
关键词: 草食家畜    干物质采食量    外源指示剂    放牧行为    草地    瘤胃瘘管    NIRS    
Determination of Feed Intake of Grazing Livestock
JIN Youshun , HOU Fujiang     
State Key Laboratory of Grassland Agro-Ecosystems, Key Laboratory of Grassland Livestock Industry Innovation, Ministry of Agriculture and Rural Affairs, College of Pastoral Agriculture Science and Technology, Lanzhou University, Lanzhou 730020, China
Abstract: Intake is an important way for animal-plant interaction, and also constitute the main process of maintaining substances and energy cycle of the ecosystem. The level of animal production and the sustainability of grassland depend on the intake of livestock, accurate measurement of feed intake of grazing livestock is one of the keys of grassland management. At present, existing techniques such as direct observation method, indicator method, fecal nitrogen index method and near infrared spectrum method are widely used in the measurements of feed intake. For example, direct observation method is capable of completing the measurements by using lower cost but is characterized by the strong subjectivity. The esophageal fistula method is able to measure accurately feed intake while it ignores animal welfare. The near infrared spectroscopy method and saturate alkane method can measure intake, and at the same time, they can observe some features of food selection of livestock. For those reasons, this thesis discussed the existing conventional measurement methods, and also analyzed the main factors that influenced the results of measurement, and proposed some new conceptions, such as the combination of grazing behavior and unmanned aerial vehicle technology, which aimed to provide technical support for accurate grassland management in the future.
Key words: grass-feeding livestock    dry matter intake    exogenous indicator    grazing behavior    meadow    esophagus fistula    NIRS    

放牧为人类社会提供健康草地、生态安全保障、绿色畜产品和传统草畜文化等多功能服务[1],是动物在人类管理下采食植物等活动的总称,是全球生态系统最重要的管理方式之一[2]。全球69%的农用地为永久性放牧地,其中大洋洲、非洲撒哈拉、南美洲和东亚放牧的农业土地分别为89%、83%、82%和80%[3]。放牧给人类提供1/2以上的肉类、1/3以上的奶制品以及毛皮等畜产品[4]。我国草原牧区牛羊肉、牛奶、毛绒产量分别占全国总产量的45.4%、49.7%和75.2%,在全国家畜生产迅猛增长的形势下,草原牧区的畜产品占比从2003年的20%增长到2013年的25%,目前呈上升趋势。采食量是反映家畜营养状况的直接因素,同时也是放牧生态系统中制定放牧率和合理利用草地的主要指标。总之,采食量是放牧的基本参数,既是系统测度家畜生产性能的基础,也是及时评估草地放牧压力的关键。因此,准确、简易地测定采食量是合理放牧,保障人类生态安全和食物安全的根本途径之一。

人类对环境、家畜本身行为状态与采食量关系的观察由来已久。春秋时期的《诗经·无羊》描写牛羊“或降于阿,或饮于池,或寝或讹”,注意到家畜放牧行为的空间格局;南北朝时期的《齐民要术》有“寒温饮饲,适其天性”,指出家畜营养需求的季节动态。19世纪中叶,各国学者开始系统研究家畜采食量。1932年,Erizian[5]提出利用体重相减法计算家畜的采食量。1955年,Smith等[6]利用三氧化二铬(Cr2O3)作为外源指示剂测定了放牧条件下奶牛的采食量。1956年,任继周等[7]采用模拟采食法估测了放牧家畜的采食量,目前仍然广泛使用。1963年,Tilley等[8]采取体外发酵测得了成年绵羊采食量。1986年,Mayes等[9]用植物皮层蜡质中奇数链烷烃为内源指示剂测定了羊的采食量以及明确了其择食特征。随着光谱技术兴起,近红外光谱(near-infrared spectroscopy,NIRS)技术被用于估测放牧家畜采食量[10]。近年来,开始利用传感技术进行自动化监测家畜采食量[11],同时利用定位技术和无人机(unmanned aerial vehicle,UAV)等结合观测家畜放牧行为[12]

草原的放牧管理要求家畜采食量的测定精度和即时性,需要统一、便捷、准确、低廉的测定方法,各种测定技术之间有定量关系以相互校正。为此,本文分析了放牧家畜采食量的测定技术及其优势和需要改进之处,为放牧研究和管理提供方法支撑。

1 直接测定法 1.1 直接观测法

用肉眼,或同时借助望远镜、照相机、摄像机等器具记录家畜的放牧行为[13]。一般从出牧到归牧全天观测,连续或分时段记录家畜的采食时间、采食速率、采食口数、行走时间、卧息时间、反刍时间、排粪次数等(图 1)行为特征,并模拟家畜的采食部位,估测采食量(表 1)。直接观测法简单、成本低廉、易操作,但是在生物多样性较高的草原或混播草地较难把握家畜采食的牧草种类[14],且主观性强,费时耗力。

图 1 直接观测法 Fig. 1 Direct observation method
表 1 直接观测法测定家畜采食量 Table 1 Determination of livestock feed intake by direct observation method

直接观测法通过定时观测家畜的单口采食量(intake bite,IB)和采食速率(intake rate,IR),计算家畜某一时段的采食量,计算公式如下:

式中:I为时刻1(t1)与时刻2(t2)之间家畜的采食量,一般为1头家畜在数小时或1 d中的采食量;IR常用单位时间(min或h)的采食口数表示(口/min或口/h),也有用每步采食口数表示(口/步)[15-16]

1.2 指示剂法 1.2.1 内源指示剂法

内源指示剂主要包括木质素、色素类、盐酸不溶灰分和饱和链烷烃等,是牧草生理活动的次生代谢产物,反刍家畜较难消化,绝大多数会通过肛门排出体外。

1.2.1.1 饱和链烷烃法

利用植物表皮蜡质中的烷烃作为内源指示剂,添加偶数碳原子烷烃,利用回收率相近的原理测定家畜采食量(表 2),采食牧草成分利用最小平方优化程序Eatwhat进行计算[28-31]。它具有以下特点:1)植物饱和奇数链烷烃技术和偶数链烷烃技术相结合,能够较为精确地估测放牧家畜牧草采食量;2)估测家畜个体采食牧草的差异;3)估测动物食物组成,甚至采食的植物部位;4)对动物没有创伤,重复性好[32-33]

表 2 饱和链烷烃法测定家畜采食量 Table 2 Determination of livestock feed intake by saturated alkane method

饱和链烷烃法计算公式如下:

式中:DMI为干物质采食量(kg/d);Dj为家畜日食入偶数链烷烃含量;Fi为粪样中天然奇数链烷烃浓度;Fj为粪样中偶数链烷烃浓度;Hi为牧草中奇数链烷烃浓度;Hj为牧草偶数链烷烃浓度。

1.2.1.2 盐酸不溶灰分法

常用的酸不溶灰分法主要有4 mol/L盐酸不溶灰分法和2 mol/L盐酸不溶灰分法。牧草中通常含有一种不溶于盐酸的次生代谢产物——盐酸不溶灰分,反刍动物较难利用,绝大多数会从肛门排出,以酸不溶灰分做指示剂,能够测得放牧动物的采食量(表 3)。盐酸不溶灰分回收率高,操作简单省力(只需收集粪样和草样),得到了广泛应用[43-45]。但事实上,在放牧生态环境中,放牧动物在采食牧草的同时难免会误食泥土或从沙土中摄取微量元素,这一择食特征使得粪中排出的酸不溶灰分回收率超过100%[46]。张晓庆等[47]研究表明,利用酸不溶灰分高估了放牧羔羊的采食量。酸不溶灰分法计算公式如下:

表 3 盐酸不溶灰分法测定家畜采食量 Table 3 Determination of livestock feed intake by AIA method

式中:I为采食量;F为家畜日排粪干物质含量;a为粪干物质中盐酸不溶灰分含量(%);b为采食牧草干物质中盐酸不溶灰分含量(%)。

1.2.2 外源指示剂法

外源指示剂包括Cr2O3、二氧化硅(SiO2)、二氧化钛(TiO2)等,不是牧草次生代谢产物,需人为添加饲喂家畜。原理与内源指示剂类似,通常将放牧家畜进行一定时间的训练,每天在饲喂牧草的同时并且添加一定量的外源指示剂,待家畜排出带有外源指示剂(如Cr2O3)的粪便时,收取具有代表性的粪样,计算采食量(图 2)。外源指示剂不被动物消化吸收或吸收量很少,但目前并没有研究发现回收率达到100%的指示剂。虽然外源指示剂法是目前较为准确测量放牧家畜采食量的方法之一[55-56],但仍然有诸多缺点,此法最大的限制性因素为粪便组分的日变化和指示剂的不完全吸收。各国学者采取外源指示剂法对多种放牧家畜开展了研究[57-59](表 4)。

图 2 指示剂法 Fig. 2 Indicator method
表 4 外源指示剂法测定家畜采食量 Table 4 Determination of livestock feed intake by exogenous indicator method
1.3 食道瘘管法

通过食道瘘管安装集样袋进行取样,测定家畜日采食量(图 3)。通过食道瘘管法能够更符合实际放牧情况采集家畜所食牧草,且收集的样品具有一定的代表性,同时可以准确把握家畜的择食特征。较指示剂法和木质素法等,食道瘘管法的结果更为准确,可行性更强。但是食道瘘管需借助外科手术,对动物有较大的创伤,同时,应激反应对动物的正常采食及消化会产生一定的影响。

图 3 食道瘘管法 Fig. 3 Esophagus fistula method
1.4 粪氮指数法

粪氮指数法(图 4)能够较为理想地估计牧草采食量(表 5),有2种关系模型:1)基于粪中所含粗蛋白质的含量和有机物摄取量;2)基于粪中粗蛋白质含量和有机干物质消化率。近年来,国外学者做了大量相关研究[66-67]。粪氮指数法所需分析仪器设备为常规饲料分析设备,对硬件设施要求较低且对试验动物不会产生较大的创伤,在未来放牧家畜动物营养中具有较广阔的研究前景。Kennedy等[68]得出了粪氮指数法的最佳函数式,具体如下:

图 4 粪氮指数法 Fig. 4 Fecal nitrogen index method
表 5 粪氮指数法测定家畜采食量 Table 5 Determination of livestock feed intake by fecal nitrogen index method

式中:Y表示饲料和粪中有机物比值;X1表示粪中的色素(415 nm);X2表示鲜粪中氮含量;X3表示干粪中氮含量。

1.5 NIRS法

利用有机物在NIRS区特定的波长吸收特性进行分析的一种方法,具有简易、准确、安全、成本低(不需要化学试剂)等特点[13]。在过去的20年里,NIRS法已经发展到可以表征饲料的营养价值。NIRS标准是从牧草或粪便中发展出来的光谱,目前可用于预测采食量(表 6)。NIRS法能够在任何条件下不干涉放牧家畜的采食特征进行采食量测定[75-78],同时通过提供快速、经济和客观的营养摄入来反映家畜营养摄入、补充物的反应以及不同环境下放牧家畜的生产力。NIRS法对放牧家畜营养监测有速度和成本优势,但准确性受到诸多影响因子限制,如标样的准确性选择以及标样中待测成分含量、分布情况会直接影响数据的准确性[13]。但是只要建立准确的数据分析模型,NIRS法就会发挥独特的优势。

表 6 NIRS法测定家畜采食量 Table 6 Determination of livestock feed intake by NIRS method
2 间接测定法 2.1 生物量差异法

通过放牧前、放牧后各物种生物量的差异变化间接计算采食量(表 7),是测量放牧家畜采食量的传统方法。此方法操作简单,但并不适用于连续放牧地区,因为牧草是持续生长的,并且差异法仅限在较短时期的测定,才能得到准确的采食量。此外,使用生物量差异法时,应当选择植被类型较为均匀的样地,陈友慷[82]的试验就因放牧样地植被类型差异过大在高寒草甸测定放牧牦牛采食量并未取得成功。同时,在明确牧草产量时,首先选择具有代表性的样地以及牧草的刈割高度一致[83]。采用生物量差异法测定采食量的计算方法如下:

表 7 生物量差异法测定家畜采食量 Table 7 Determination of livestock feed intake by different biomass method

式中:I为采食量;M为牧前草地质量;Mf为牧后草地余量;G为牧草积累系数;△Me为保护样方牧草积累量。

2.2 公式法

利用能量摄入需求来计算采食量的原理是动物采食的根本在于满足自身的维持代谢(能量需求),将影响采食量的各种因素与能量摄取建立线性回归方程,对家畜的采食量进行预测(表 8)。在不同生理阶段、牧草组成和放牧环境下家畜采食量存在明显差异,其代谢调控机理十分复杂[89],利用传统采食调控理论无法解释复杂的调控模式[90-91],因此运用基因组学、蛋白质组学等先进生物技术预测干物质采食量模型指导实践生产。Huhtanen等[93]引入了青贮干物质采食量[92-94]与精饲料干物质采食量变量对不同饲粮之间的采食量差异性进行区别,同时将饲粮对采食量的影响以及动物机体对采食量的影响进行分析,认为多元复合模型具有更高的预测精度[95]

表 8 公式法测定家畜采食量 Table 8 Determination of livestock feed intake by formula method
3 小结与展望

放牧在陆地生态系统管理具有重要地位。放牧不合理化对草原生态环境造成破坏,采取正确的轮牧管理措施保护草原生态迫在眉睫。放牧家畜作为草原生态系统调控的关键性因子,采食量是基础参数。目前放牧家畜的选择性采食和采食植物部位的测定方法具有一定的局限性(表 9)。如测定结果误差较大、试验设计复杂、耗时费力、昂贵等。在测定放牧家畜采食量时,应充分考虑各种限制性因素,如不同放牧家畜的生理特性、年龄、品种、性别、草地生物量、不同物候期牧草家畜所采食牧草,以及主观性因素存在的人为干扰等。根据这些影响因素,了解放牧家畜的生活习性,采取多种方法结合来测定放牧家畜的采食量,确定载畜量,制定出合理的轮牧制度,对草原可持续利用具有现实意义。

表 9 采食量测定方法的比较 Table 9 Comparison of feed intake determination methods

放牧家畜采食量的测定首先取决于对家畜放牧行为和放牧生态系统的系统性认识。近年来,3S技术和传感器技术普遍应用于家畜放牧行为研究,与无人机、人工智能相结合成为主要的发展方向之一。植物蜡皮指示剂法以及NIRS技术也有广阔的前景。总之,降低试验成本、提高工作效率和结果的准确性仍然是放牧家畜采食量研究的首要目标。

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