单宁酸调控肉牛排泄物氧化亚氮生成关键成分的规律与分子机制

Nitrous oxide (N2O) is a potent greenhouse gas. N2O is also the key factor which damages the ozonosphere of the earth. Beef farm is an important source of N2O. The N2O on beef farm is produced in nitrification and denitrification. The conversion efficiency of dietary nitrogen (N) of beef cattle is usually low and the N excretion is high; the ratio of urinary N excretion is 2-3 times of faecal N; while the efficiency of urinary N used for producing N2O is higher than that of faecal N. The percentage of urea N in total urinary N is 76.7-85.2%, the hydrolysing rate of urea is high and urea N is the main component for producing N2O. Hippuric acid is an aromatic compound. The percentage of hippuric acid N in urinary N is 4.1-5.1%, the hydrolyzing rate of hippuric acid is slow while it has the potential to inhibit N2O production. Decreasing urinary N especially urea N excretion and increase hippuric acid excretion favors decreasing N2O production. Tannic acid which is an aromatic compound has the effect to decrease ruminal degradability of dietary protein and NH3-N concentration and improve the utilization efficiency of dietary protein. The aromatic compounds including gallic acid, pyrogallol and resorcinol produced from hydrolysis of tannic acid could be used as the precursors of hippuric acid. It is hypothesized that tannic acid has the effects of lowering the ratio of urinary N/faecal N, urea N excretion, increasing hippuric acid excretion and decreasing N2O production. The project plans to verify the hypothesis and clarify the molecular mechanism.

氧化亚氮(N2O)是高强度温室气体,也是破坏大气臭氧层的关键因素。肉牛场是N2O的重要来源。肉牛场N2O是粪尿含氮(N)化合物经硝化和反硝化产生的。肉牛对日粮N转化率低、N排泄量大；肉牛的尿N排泄量一般是粪N的2-3倍；且尿N生成N2O的效率高于粪N。尿中的尿素N占尿N76.7-85.2%,尿素分解速度快,是生成N2O的主要原料。马尿酸是尿中的芳香族代谢物,马尿酸N占尿N 4.1-5.1%,分解速度慢,且具有抑制N2O生成的潜力。减少肉牛尿N特别是尿素排泄量、提高马尿酸排泄量有利于减少N2O生成。单宁酸能够降低日粮蛋白质瘤胃降解率和氨浓度、提高日粮N利用率。单宁酸在瘤胃中被分解产生的芳香族化合物没食子酸、焦棓酸和邻苯二酚可被用作合成马尿酸的前体。本项目推测日粮中添加单宁酸能够降低肉牛尿N/粪N比例和尿素排泄量、提高马尿酸排泄量,减少肉牛场N2O生成量。本项目拟验证该推测并阐明其分子机制。

本项目研究了日粮中单宁酸(tannic acid, TA)调控成年肉牛瘤胃发酵、营养物质消化、瘤胃微生物区系及氮(nitrogen, N)代谢规律，TA在肉牛体内的代谢特点，应用静态培养技术(static incubation technique)检测了日粮中添加TA减少肉牛尿液氧化亚氮(nitrous oxide, N2O)产量的效果，并比较了可水解单宁单体没食子酸(gallic acid, GA)与TA的效果。.结果显示，TA在肉牛瘤胃中可被水解为焦棓酸(pyrogallol)、GA和间苯二酚(resorcinol)。日粮中添加TA时，尿液中可检测到、而粪便中不能检测到焦棓酸和间苯二酚。日粮中添加13.0-26.0 g TA/kg 干物质(dry matter, DM)降低瘤胃液氨态氮(ammonia nitrogen, NH3-N)浓度和具有降低挥发性脂肪酸(volatile fatty acid, VFA)浓度的趋势，降低瘤胃原虫、产甲烷菌和Ruminococcus albus的相对丰度。添加16.9 g TA/kg DM不影响门水平上的主要细菌Firmicutes和Proteobacteria和属水平上的主要细菌Prevotella_1 和Christensenellaceae_R-7_group的相对丰度。添加13.0-26.0 g TA/kg DM降低尿N和尿素排泄量，提高马尿酸(hippuric acid)和粪N排泄量，降低日粮粗蛋白(crude protein, CP)消化率和尿N/粪N比值，不影响日粮N转化率。日粮中添加16.9 g TA/kg DM降低肉牛尿液N2O-N产量达40.7% (日粮CP 111 g/kg DM) 和45.8% (日粮CP 136 g/kg DM)。.GA在瘤胃中可被水解为焦棓酸和间苯二酚。日粮中添加GA时，肉牛尿液中可检测出、而粪便中不能检测出这两种产物。GA影响瘤胃发酵及微生物区系的作用弱于TA。日粮中添加15.2 g GA/kg DM不影响日粮主要营养物质消化率及N代谢，但降低尿液N2O-N产量达28.5% (日粮CP 114 g/kg DM)和30.9% (日粮CP 151 g/kg DM)。.饲喂成年肉牛低CP日粮，或在日粮中添加TA或GA可显著减少肉牛尿液N2O-N产量。GA减排N2O-N效果低于TA。