The objective of this work was to study nutritional strategies for decreasing methane production by ruminants fed tropical diets, combining in vitro and in vivo methods. The in vitro approach was used to evaluate the dose effect of condensed tannins (CT) contained in leaves of Gliricidia sepium, Leucaena leucocephala, and Manihot esculenta (39, 75, and 92 g CT/kg DM, respectively) on methane production and ruminal fermentation characteristics. Tannin-rich plants (TRP) were incubated for 24 h alone or mixed with a natural grassland hay based on Dichanthium spp. (control plant), so that proportions of TRP were 0, 0.25, 0.5, 0.75, and 1.0. Methane production, VFA concentration, and fermented OM decreased with increased proportions of TRP. Numerical differences on methane production and VFA concentration among TRP sources may be due to differences in their CT content, with greater effects for L. leucocephala and M. esculenta than for G. sepium. Independently of TRP, the response to increasing doses of CT was linear for methane production but quadratic for VFA concentration. As a result, at moderate tannin dose, methane decreased more than VFA. The in vivo trial was conducted to investigate the effect of TRP on different ruminal microbial populations. To this end, 8 rumen-cannulated sheep from 2 breeds (Texel and Blackbelly) were used in two 4 × 4 Latin square designs. Diets were fed ad libitum and were composed of the same feeds used for the in vitro trial: control plant alone or combined with pellets made from TRP leaves at 44% of the diet DM. Compared to TRP, concentration of Ruminococcus flavefaciens was greater for the control diet and concentration of Ruminococcus albus was least for the control diet. The methanogen population was greater for Texel than for Blackbelly. By contrast, TRP-containing diets did not affect protozoa or Fibrobacter succinogenes numbers. Hence, TRP showed potential for mitigating methane production by ruminants. These findings suggest that TRP fed as pellets could be used to decrease methane production.

An in vivo trial was conducted in sheep to investigate the effect of three tropical tannin-rich plants (TRP) on methane emission, intake and digestibility. The TRP used were leaves of Glyricidia sepium, Leucaena leucocephala and Manihot esculenta that contained, respectively, 39, 75 and 92 g condensed tannins/kg DM. Methane was determined with the sulphur hexafluoride tracer technique. Eight rumen-cannulated sheep of two breeds (four Texel, four Blackbelly) were used in two 4 × 4 Latin square designs. Four experimental diets were tested. They consisted in a tropical natural grassland hay based on Dichanthium spp. fed alone (C) or in association with G. sepium (G), L. leucocephala (L) or M. esculenta (M) given as pellets at 44% of the daily ration. Daily organic matter intake was higher in TRP diets (686, 984, 1054 and 1186 g/day for C, G, L and M respectively; p < 0.05) while apparent organic matter total tract digestibility was not affected (69.9%, 62.8%, 65.3% and 64.7% for C, G, L and M respectively; p > 0.05). Methane emission was 47.1, 44.9, 33.3 and 33.5 g/kg digestible organic matter intake for C, G, L and M, respectively, and was significantly lower (p < 0.05) for L and M than for G and C. Our results confirm the potential of some TRP to reduce methane production. The strong decrease in methane and the increase in intake with TRPs may be due to their presentation as pellets.

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The present study was carried out to determine the effect of Acacia nilotica, a tropical plant rich in hydrolyzable tannins (HT), on rumen fermentation and methane (CH4) production in vitro. We used leaves and pods from A. nilotica alone and combined. The combination of HT from A. nilotica leaves and pods and condensed tannins (CT) from Calliandra calothyrsus and Leucaena leucocephala 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 Dichanthium spp.). A first experiment tested different levels of replacement of this tropical forage (control [CTL] without tannins) by A. nilotica leaves or pods: 0:100, 25:75, 50:50, 75:25 and 100:0. A second experiment tested the mixture of A. 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 A. nilotica leaves or pods with C. calothyrsus and L. leucocephala. Acacia nilotica pods and leaves had a high content of HT (350 and 178 g/kg DM, respectively), whereas C. calothyrsus and L. leucocephala had a high content of CT (361 and 180 g/kg DM, respectively). The inclusion of HT from A. nilotica leaves and pods decreased CH4 production dose-dependently (P < 0.01). Total replacement of the CTL by A. nilotica decreased CH4 production by 64 and 55% with leaves and pods, respectively. Pods were richer in HT than leaves, but their antimethanogenic effect did not differ (P > 0.05). Although A. 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 when compared with the CTL alone. Positive associative effect was reported for VFA, when HT-rich sources and CT-rich sources were mixed. Combining the 2 sources of HT did not show associative effects on fermentation or CH4 production (P > 0.05). Hydrolyzable tannin-rich sources were more effective in suppressing methanogenesis than CT-rich sources. Our results show that HT-rich A. nilotica leaves and pods have the potential to reduce ruminal CH4 production.