Effects of tannins on ruminal degradation and excretory pattern of N and implications for the potential N emission from the manure

Two experiments were carried out to assess the effects of supplementing tannin-rich legumes or extracted tannins on ruminal degradation of crude protein and the excretory pattern of nitrogen (N). In Experiment 1, using the in vitro gas-transducer technique, various tannin-rich forage legumes were supplemented alone or in combination with the tannin-free legume Vigna unguiculata to a tropical grass diet. In Experiment 2, six growing lambs fed temperate grass or grass–legume diets were supplemented with 0 or 25 g of Acacia mearnsii tannins/kg dietary dry matter. In Experiment 1, supplementation of the grass with tannin-rich legumes (i.e. Calliandra calothyrsus or Flemingia macrophylla) alone did not increase the supply with degradable protein compared to the pure grass diet, and suppressed apparent ruminal protein degradability. By contrast, V. unguiculata clearly increased amount and proportion of rumen-degradable protein in the diet. When combined with V. unguiculata, the tannin-rich legumes could be used in proportions of up to 10% of the complete diet without affecting protein degradation rate. In Experiment 2, tannin supplementation decreased ruminal ammonia concentration and resulted in a shift in N excretion from urine to faeces without affecting body N retention. This shift in N excretory pattern is of practical relevance because urinary N is prone to ammonia emission during manure storage and application. These results confirm that tannin-rich legumes suppress ruminal protein degradability and suggest that dietary tannin supplementation can be useful in mitigating potential gaseous N emissions from animal excreta.