This meta-analysis aimed to identify the optimal inclusion and threshold levels for black wattle, quebracho, and chestnut tannin extracts (TE) in sheep and cattle, in terms of their effects on dry matter intake (DMI), digestibility, weight gain, and nitrogen balance. Data were compiled from 44 scientific publications, representing 580 sheep and 742 cattle. The percentage change approach was used to quantify responses to TE inclusion. Mixed model analysis was performed to evaluate piecewise and linear regressions, with scientific publications treated as random effects, and TE level, TE source, animal species, supplementation period, dietary fibre level, dietary protein level, and animal age as fixed effects. The piecewise model provided the best fit for all responses. The optimal inclusion level was established at a 0% response, while the threshold was established at a 5% reduction in performance. With no differences between TE sources, the optimal inclusion level for DMI was 1.5 g TE/100 g dry matter (DM) and threshold was 3 g TE/100 g DM in both sheep and cattle. Sheep were slightly more able to digest crude protein than cattle (3.89 ± 1.91%) at the same TE inclusion level, but no other differences were observed between the two species. Black wattle TE reduced organic matter digestibility (−4.95 ± 1.82%), neutral detergent fibre digestibility (−8.62% ± 4.07), and acid detergent fibre digestibility (−16.83 ± 6.06) responses, relative to chestnut TE. Moreover, TE inclusion (particularly black wattle TE) raised faecal nitrogen while lowering urinary nitrogen. These differences between TE sources did not significantly influence the weight gain response, with an optimal inclusion level of 1.5 g TE/100 g DM, and a threshold of 2.3 g TE/100 g DM.