Purine excretion and estimated microbial protein yield in sheep fed diets differing in protein degradability

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• 作者：Webster ; A.J.F. ; Kaya ; Sule ; Djouvinov ; D.S. ; Kitcherside ; M.A. ; Glen ; E.F.
• 关键词：BEN ; basal endogenous nitrogen loss ; DOMD ; digestibility of organic matter in dry matter ; DUN ; digestible ; undegradable nitrogen ; ERDN ; effective rumen degradable nitrogen (=0.8QDN+1.0 SDN) ; FME ; fermentable metabolizable energy ; ME ; metabolizable
• 刊名：Animal Feed Science and Technology
• 出版年：2003
• 出版时间：March 31, 2003
• 年：2003
• 卷：105
• 期：1-4
• 页码：123-134
• 全文大小：90.7 K

文摘

The greatest sources of uncertainty concerning the application of the metabolizable protein (MP) system for ruminant feeding concern assumptions as to the extent and efficiency of synthesis of microbial protein (MiN) both from quickly, slowly and undegraded forms of dietary nitrogen (QDN, SDN and DUN, respectively) and from recycled endogenous nitrogen. In order to test these assumptions 12 castrated male lambs were allocated to three experimental semi-synthetic diets, QNF1.3, SNF1.3 and UNF0.8, formulated to provide different ratios of total dietary N to fermentable energy (FME) and to vary the proportions of total N provided by QDN, SDN and DUN. Thus, QNF1.3 and SNF1.3 were formulated to provide the same nominal ratio of 1.3g degradable N/MJ FME. UNF0.8 was formulated to provide a similar total N concentration to QNF1.3 but with the majority of N in the form of DUN thus providing only 0.8g degradable N/MJ FME. The diets were fed at two levels (maintenance and twice maintenance) according to a balanced changeover design. The two principal sources of dietary N were dark corn grains-plus-solubles (high QDN) and corn gluten meal (high DUN). MiN yield was estimated from urinary excretion of total purines. The “net” efficiency of incorporation of dietary N into MiN was estimated from increments of purine excretion between the two diet levels. Mean values for purine excretion from sheep were similar when the three diets were fed at the low, maintenance level. The net efficiencies of utilization of increments of ‘effective rumen degradable nitrogen’ (ERDN=0.8QDN+SDN) between maintenance and twice maintenance were QNF1.3=0.50, SNF1.3=0.94 and UNF0.8=0.59. The value of 0.94 for diet SNF1.3 provides strong support for the assumption that the efficiency of capture of SDN can approach 1.0. The low (0.50) net efficiency of capture of increments of ERDN from diet QNF1.3 suggests the MP system is overoptimistic in assuming that QDN can be captured with an efficiency of 0.8. When diet UNF0.8 was fed at the high level, 6/8 sheep failed to sustain food intake presumably because they were unable to sustain the microbial population of the rumen. This suggests that the capacity of sheep to recycle endogenous N into MiN may not exceed the maximum amount permitted by the assumption of the MP system even when DUN is fed in excess of requirement for growth.