LAUSR

The present study was carried out to determine the effect of Acacia nilotica, a tropical plant rich in hydrolysable tannins (HT), on rumen fermentation and methane (CH4) production in vitro. We used leaves and pods from Acacia nilotica alone and combined. The combination of HT from Acacia nilotica leaves and pods and condensed tannins (CT) from Calliandracalothyrsus and Leucaenaleucocephala were also evaluated to assess potential differences in biological activity between HT and CT. Four series of 24-h incubations were performed using rumen contents of 4 sheep fed a tropical grass (natural grassland based on Dichanthiumspp). A first experiment tested different levels of replacement of this tropical forage (control (CTL) without tannins) by Acacia nilotica leaves or pods: 0:100, 25:75, 50:50, 75:25 and 100:0. A second experiment tested the mixture of Acacia nilotica leaves and pods in different proportions: 100:0, 75:25, 50:50, 25:75, and 0:100. A third experiment tested the 50:50 combination of Acacia nilotica leaves or pods with Calliandracalothyrsus and Leucaenaleucocephala. Acacia nilotica pods and leaves had a high content of HT (350 and 178 g/kg DM, respectively), whereas Calliandracalothyrsus and Leucaenaleucocephala had a high content of CT (361 and 180 g/kg DM, respectively). The inclusion of HT from Acacia nilotica leaves and pods decreased CH4 production dose-dependently (P < 0.01). Total replacement of the CTL by Acacia nilotica decreased CH4 production by 64 and 55% with leaves and pods, respectively. Pods were richer in HT than leaves, but their anti-methanogenic effect did not differ (P > 0.05). Although Acacia nilotica leaves and pods inhibited fermentation, as indicated by the lower gas production and VFA production (P < 0.01), this effect was less pronounced than for CH4. Volatile fatty acid production decreased by 12% in leaves and by 30% in pods as compared to the CTL alone. Positive associative effect was reported for VFA, when HT-rich sources and CT-rich sources were mixed. Combining the two sources of HT did not show associative effects on fermentation or CH4 production (P > 0.05). Hydrolysable tannin-rich sources were more effective in suppressing methanogenesis than CT-rich sources. Our results show that HT-rich Acacia nilotica leaves and pods have the potential to reduce ruminal CH4 production.