本试验旨在探讨牦牛尿中嘌呤衍生物(PD)排出量对饲粮氮水平的响应规律,并基于此估测了瘤胃微生物氮(MN)产量,以期为高寒牧区牦牛的科学饲养提供参考.选取4头体重[(192±12)kg]相近、年龄(3岁)相同的去势公牦牛,采用4×4拉丁方试验设计将牦牛分为4组,各组饲粮氮水平分别是1.03%、1.95%、2.85%和3.76%,每组1头;试验分为4期,每期21 d,包含15 d的预试期和6 d的正试期.结果表明,牦牛尿中PD主要由尿囊素和尿酸组成,尿囊素/PD和尿酸/PD分别为0.69~0.76、0.23~0.30,黄嘌呤与次黄嘌呤的含量极少.当饲粮氮水平升高时,尿中PD、尿囊素、尿酸以及马尿酸排出量均线性增加(P<0.05),而尿酸/PD和嘌呤氮指数(PNI)均线性降低(P<0.05).瘤胃细菌嘌呤碱基(RNA当量)含量、瘤胃细菌氮含量以及瘤胃MN产量都随着饲粮氮水平升高而线性增加(P<0.05),但饲粮氮用于合成MN的效率[即瘤胃MN/食入氮(NI)]却线性降低(P<0.05).基于尿中PD排出量(mmol/d)和瘤胃MN产量(g/d)与NI(g/d)之间良好的线性关系,构建了如下数学模型:PD=0.58NI+18.28,MN=0.18NI+22.18.综合得出,当牦牛饲粮氮水平为2.85%时,牦牛瘤胃MN产量最大,为42.60 g/d,而PNI以及饲粮氮用于合成MN的效率却在低氮(1.03%)条件下达到最高,这一结果揭示了牦牛对低氮饲粮中氮素营养高效利用的特点,解释了牦牛对青藏高原饲料营养匮乏的适应性的营养机理.%This experiment was conducted to explore the regularity of urine purine derivatives ( PD) excretion respond to dietary nitrogen level, and based on this, ruminal microbial nitrogen ( MN) production was esti-mated, thus provided references for appropriate feeding of yaks in cold farming area. Four 3-year-old castrated male yaks [body weight was (192±12) kg] were divided into 4 groups with 1 yak per group in a 4×4 Latin square design. Dietary nitrogen levels of different groups were 1. 03%, 1. 95%, 2. 85% and 3. 76%, respectively. The experiment consisted of 4 periods, each lasted for 21 d, allowing a 15-d pre-test period and a 6-d test period. The results showed that urine PD were mainly consist of allantoin and uric acid, allantoin/PD and uric acid/PD were 0.69 to 0.76 and 0.23 to 0.30, respectively, and negligible contents of xanthine and hy-poxanthine were found. As dietary nitrogen level increasing, urine PD, allantoin, uric acid and hippuric acid extractions were increased linearly ( P<0.05) , however, uric acid/PD and purine nitrogen index ( PNI) were linearly decreased ( P<0.05) . With the increase of dietary nitrogen level, ruminal bacterial purine bases ( RNA equivalent) content, bacterial nitrogen ( BN) content and MN production were linearly increased ( P<0.05) , and efficiency of dietary nitrogen converted into MN [ ruminal MN/nitrogen intake ( NI ) ] was linearly de-creased ( P<0. 05 ) . There were a strong relationship between urine PD extraction ( mmol/d ) , ruminal MN ( g/d) and NI ( g/d) , the following mathematical equations were established as follows: PD=0. 58NI+18.28, MN=0.18NI+22.18. In summary, ruminal MN production is the highest (42.60 g/d) when dietary nitrogen level is 2.85%, whereas PNI and efficiency of NI converted into MN are the highest under the condi-tion of low dietary nitrogen level (1.03%). The results indicate the great efficiency of nitrogen utilization when low nitrogen provision in yaks, and their adaptation nutrition mechanism to the deficiencies in intake of nutrient of yaks in Tibetan plateau.
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