Strategic use of tannins as means to limit methane emission from ruminant livestock

In two experiments, the effects of supplementing tannins via a tannin-rich legume or a tannin extract on methane emission were determined. In Experiment 1, applying the rumen simulation technique, the tannin-rich legume Calliandra calothyrsus and the low-tannin legume Cratylia argentea were supplemented alone or in combination with each other to a tropical grass diet. In Experiment 2, six growing lambs, receiving either temperate grass or grass–legume diets, were supplemented with 0 or 25 g of Acacia mearnsii tannins/kg dietary dry matter. In Experiment 1, different from supplementation with C. argentea to the grass, methane emission did not increase when C. calothyrsus was added to the grass-alone diet and decreased linearly when C. argentea was successively exchanged by C. calothyrsus in the legume supplement. However, this decrease was associated with a corresponding reduction in the feeding value of the mixed diets. By contrast, in Experiment 2, the supplementation of the tannin extract decreased methane release (kJ/MJ gross energy intake) by 13% on average without affecting body energy retention. These results suggest that particularly extracted tannins can be useful in mitigating methane emission without major losses in feeding value of the diet while very tannin-rich shrub legumes such as C. calothyrsus, despite being also effective in limiting methanogenesis, are restricted in their application due to the simultaneous depression of the feeding value of the diet.