ABSTRACT.- This systematic review-meta-analysis (MA) estimates the impact of including tannins in beef and dairy cattle diets on short-chain fatty acids (SCFA), ammonia nitrogen (N-NH3), dry matter digestibility (DMD), and methane (CH4) production. The effects are described according to the sources (natural or extract), type of tannin (condensed, CT, or hydrolyzed HT or blend), total tannin concentration in the source, and doses. The bibliographic search was performed in five electronic databases: CAB-Abstracts, PubMed, Scopus, Science Direct, and Web of Science. Predefined protocols were used at all stages of the systematic review process. An MA for random effects was performed for each indicator separately with the mean difference between the control (without tannin) and treated (with tannin) groups. Twenty-seven articles were included for this study, of which 20 were for SCFA, 20 for N-NH3, 14 for DMD, and 10 for CH4. The inclusion of HT reduced the total SCFA concentration (p = 0.005). Within a dose range of 10-40 g/kg of a CT extract, the total SCFA concentration was reduced by 0.804 mM in the rumen fluid (p