RESUMEN
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.