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Feeding ruminants herbage-based diets offers numerous advantages, but there are also some disadvantages. These diets are often imbalanced and, due to a high crude protein (CP) content critical regarding nitrogen (N) use efficiency. One potential solution is dietary supplementation with condensed tannins (CT), which are found in certain forage legumes and plant extracts. The CT bind to dietary CP and reduce its ruminal degradation rate, which might have positive effects on performance but also on the environment shifting the N excretion pathway from urine to feces and decreasing the ammonia (NH3) volatilization potential of the excreta. The effectiveness of CT is influenced by their dietary content and structure, both of which can be affected by various factors, making it challenging to assess their impact comprehensively. In our studies using sainfoin as a sole feed or as part of a forage-based diet for sheep and dairy cows, we consistently observed a reduction in N excretion in urine. In dairy cows, this was associated with a decrease in NH3 emissions, but also resulted in lower milk yields, depending on the comparison diets used. Therefore, it is not possible to make universally valid conclusions about the effects of CT in herbage-based feeding systems. However, when applied optimally, CT can provide ecological benefits without adversely affecting animal performance.