In two experiments with lactating cows the effect of supplemented small doses of essential oils (0.1 g extract/kg; experiment I), Yucca schidigera saponin (0.1 g extract/kg) or Castanea sativa wood tannin (4.9 g extract/kg) were compared with unsupplemented diets and a soyabean meal-supplemented diet (only experiment II). Performance (liveweight change, milk yield and composition) was not affected although, in order to provoke effects, diets were deficient in protein by ≤10%. An exception was the soyabean meal-supplemented diet which also showed only non-significant trends towards higher persistencies of milk and milk protein synthesis. Effects of the extracts rich in saponins and tannins occurred in nitrogen conversion in the animal (less urinary and milk urea nitrogen excretion) and in manure. Nitrogen emissions from 8-weeks stored manure were lower relative to the unsupplemented diet. The main field of application of the extracts rich in tannins and saponins seems to be the mitigation of ammonia emission.