Department of Dairy Science, Virginia Tech, Blacksburg 24061.
Department of Animal and Poultry Sciences, Virginia Tech, Blacksburg 24061.
J Dairy Sci. 2021 Mar;104(3):2935-2955. doi: 10.3168/jds.2020-19447. Epub 2021 Jan 15.
The objective of this meta-analysis was to quantitatively summarize variations in urea kinetics related to ruminant species, diet composition, and ruminal fermentation. A database of 31 studies measuring urea recycling kinetics were used to derive 2 sets of linear mixed-effects regression models. Study was used as a random intercept and regressions were weighted by 1 divided by the standard error of the mean observation. Models were compared, when appropriated, using the concordance correlation coefficient, root estimated variance associated with study (σˆ) and error (σˆ) and corrected Akaike information criterion values. From a dietary standpoint, most response variables were affected by measures reflecting dietary crude protein [(CP; e.g., N-NH or rumen-degradable protein (RDP)] and by variables reflecting dietary energy content [e.g., total digestible nutrients (TDN), dietary starch, or ruminal pH]. Dietary CP, N-NH, and TDN typically had positive slopes on urea N entry rate (UER; g/d and g/kg), whereas starch and TDN/RDP had negative slopes on UER (g/kg). On the other hand, increasing TDN increased gastrointestinal entry rate (GER; g/kg), whereas an opposite effect was observed for RDP. Increasing diet RDP content reduced the urea N returned to ornithine cycle (ROC; g/kg) in most models. Ruminal variables also reflected the importance of N and energy supplies. Ruminal ammonia concentration significantly affected ROC (g/d and g/kg), used for anabolism (UUA; g/kg), ROC:GER, UUA:GER, and the incorporation of recycled urea N into microbial N relative to gastrointestinal entry rate of urea. Ruminal pH significantly affected GER:UER and ROC:GER ratios. Total digestible nutrients had a positive slope on UUA (g/kg). Increasing the ratio of energy to protein (TDN:RDP) increased the GER:UER ratio, decreased the ROC:GER ratio, and increased the UUA:GER ratio and the incorporation of recycled urea N into microbial N relative to gastrointestinal entry rate of urea N. Comparison among models revealed that species was an important explanatory variable affecting most response variables. However, whether these differences are related to the intrinsic N metabolism of each species or due to the diet variation remains unclear. Understanding these differences could lead to improvements in N use efficiency in ruminant diets by formulating more precise low-N diets considering the particularities for each species.
本荟萃分析的目的是定量总结与反刍动物种类、日粮组成和瘤胃发酵相关的尿素动力学变化。使用了 31 项测量尿素回收动力学的研究的数据库来推导出 2 套线性混合效应回归模型。研究被用作随机截距，回归通过 1 除以平均值观测的标准误差进行加权。当适当的时候，通过一致性相关系数、与研究相关的根估计方差 (σˆ) 和误差 (σˆ) 以及校正的赤池信息量准则值来比较模型。从日粮的角度来看，大多数反应变量受到反映日粮粗蛋白的措施 [(CP; 例如，N-NH 或瘤胃可降解蛋白 (RDP)] 和反映日粮能量含量的变量 [例如，总可消化养分 (TDN)、日粮淀粉或瘤胃 pH] 的影响。日粮 CP、N-NH 和 TDN 通常对尿素氮进入率 (UER; g/d 和 g/kg) 有正斜率，而淀粉和 TDN/RDP 对 UER (g/kg) 有负斜率。另一方面，增加 TDN 会增加胃肠道进入率 (GER; g/kg)，而对于 RDP 则观察到相反的效果。增加日粮 RDP 含量会降低大多数模型中尿素氮返回鸟氨酸循环的量 (ROC; g/kg)。瘤胃变量也反映了氮和能量供应的重要性。瘤胃氨浓度显著影响 ROC (g/d 和 g/kg)、用于合成代谢的尿素利用 (UUA; g/kg)、ROC:GER、UUA:GER 和回收尿素氮在微生物氮中的掺入相对于胃肠道尿素进入率。瘤胃 pH 显著影响 GER:UER 和 ROC:GER 比值。总可消化养分对 UUA (g/kg) 有正斜率。增加能量与蛋白质的比值 (TDN:RDP) 会增加 GER:UER 比值、降低 ROC:GER 比值、增加 UUA:GER 比值以及回收尿素氮在微生物氮中的掺入相对于胃肠道尿素氮进入率。模型间的比较表明，物种是影响大多数反应变量的重要解释变量。然而，这些差异是与每个物种的内在氮代谢有关，还是由于饮食的变化尚不清楚。了解这些差异可以通过制定更精确的低氮日粮来提高反刍动物日粮的氮利用效率，同时考虑到每个物种的特殊性。
