In vitro screening of various tropical foliages, seeds, fruits and medicinal plants for low methane and high ammonia generating potentials in the rumen

An extensive in vitro screening of various leaves, seeds and fruits of tropical multi-purpose shrubs and trees (MPT), leaves of medicinal plants and residues of leguminous food-feed crops was conducted to test their methane and ammonia generating potential. Plant materials were incubated with ruminal fluid/buffer mixture in four replicates for 24 h using the Hohenheim gas test method. In the group of the MPT leaves, different accessions of Acacia angustissima and Sesbania sesban were tested. Comparing the methanogenic potential, significant differences occurred both between and within the four plant groups tested. Generally, the methane-to-total gas ratio was lower for the leaves of the MPT and the medicinal plants than for the leguminous crop residues and the MPT seeds and fruits. Out of the latter group, Albizia rhizonse and Sapindus saponaria showed the lowest methanogenic potential. Within the MPT leaves the lowest methanogenic potential was found with Samanea saman and different Acacia and Sesbania species, plants known to contain plant secondary metabolites able to suppress methanogenesis. Differences between accessions of either Acacia angustissima or Sesbania sesban were large, illustrating the need to differentiate between accessions. The protozoal population was affected by the plant secondary metabolites to a smaller extent than expected, and leaves of Acacia angustissima and Sesbania sesban did not show any anti-protozoal effect. The plant group with the highest variation in their anti-protozoal effects was the MPT fruits and seeds with protozoal counts ranging between 0.6 and 5.7 × 104/ml incubation liquid. Ammonia concentrations largely differed between and within the four plant groups with values ranging from 7 to 16, 10 to 33, 9 to 20, and 15 to 23 mmol/ml incubation liquid for the MPT leaves, for the MPT fruits and seeds, for the medicinal plant leaves, and for the leguminous food-feed crop residues, respectively. The correlation coefficient between plant crude protein content and ammonia concentration was as high as 0.75, indicating that plant secondary compounds such as tannins did not excessively inhibit feed protein degradation. In conclusion, various plant materials, including accessions of Acacia angustissima, Sesbania sesban and Cajanus cajan, were found promising to approach the goal of an improved nutrition of small ruminants in the tropics at simultaneously limited methane emissions.