Tannins are secondary plant compounds which have been extensively studied in order to improve the nitrogen use efficiency (NUE) of ruminants. A meta‐analysis was performed of 58 in vivo experiments comparing milk yield, composition and nitrogen metabolism of lactating dairy cows fed diets with or without tannins. The meta‐analysis shows that tannins have no impact on corrected milk yield, fat and protein content or NUE (p > .05). However, tannins reduce ruminal ammoniacal nitrogen (N) production by 16% (from 10.95 to 8.47 mg/dl on average), milk urea by 9% (from 15.82 to 14.03 mg/dl) and urinary N excretion (−11%; p < .05). This is compensated for by a lower apparent N digestibility (61.51% with dietary tannins compared to 66.17% without). The effect of tannin on N metabolism parameters increases with tannin dose (p < .05). The shift from urinary to faecal N may be beneficial for environment preservation, as urinary N induces more harmful emissions than faecal N. From a farmer's perspective, tannins seem unable to increase fat‐ and protein‐corrected milk yield or reduce feed protein requirements and thus have no direct economic benefit. Potentially less costly than tannin extracts, forage or by‐products naturally rich in tannins could still be useful to reduce the environmental impact of ruminant protein feeding.

中文翻译：

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日粮单宁对泌乳奶牛产奶量和组成，氮分配和氮利用效率的影响：荟萃分析。

单宁是植物的次要化合物，已被广泛研究以提高反刍动物的氮利用效率（NUE）。对58项体内实验进行了荟萃分析，比较了饲喂含或不含单宁的日粮泌乳奶牛的产奶量，组成和氮代谢。荟萃分析显示，单宁酸对校正后的牛奶产量，脂肪和蛋白质含量或NUE均无影响（p  > .05）。但是，单宁将瘤胃中的氨态氮含量降低了16％（平均从10.95降低到8.47 mg / dl），牛奶尿素降低了9％（从15.82降低到14.03 mg / dl），尿中N的排泄量（−11％；p <.05）。这可以通过较低的表观氮消化率来补偿（使用日粮单宁的消化率为61.51％，而使用单宁的为66.17％）。单宁对氮代谢参数的影响随单宁剂量的增加而增加（p  <.05）。从尿液向粪便N的转变可能有利于环境保护，因为尿N比粪便N引起更多的有害排放。从农民的角度来看，单宁似乎无法增加经脂肪和蛋白质校正的牛奶产量或降低饲料蛋白质需求，因此没有直接的经济利益。天然单宁丰富的草料或副产品的成本可能比单宁提取物便宜，但仍可用于减少反刍动物饲喂蛋白质对环境的影响。