Effect of Leucaena leucocephala and corn oil on ruminal fermentation, methane production and fatty acid profile: An in vitro study

Aims: This in vitro study aimed to examine the effect of proportions of Leucaena (Leucaena leucocephala (Lam.) de Wit) to Napier grass (Pennisetum purpureum Schumach) or levels of corn oil (CO) and their interaction on ruminal fermentation, methane (CH4) production and fatty acid profile. Methods: The experiment was conducted as a 4 × 3 factorial arrangement following a completely randomised design with two factors. The treatments were according to the proportion of Leucaena and Napier grass (in g/kg DM, Treatment (T)1 = 0: 750 (control), T2 = 250: 500, T3 = 500: 250, T4 = 750: 0). Three levels of CO (in mg rumen fluid, CO1 = 0, CO2 = 10, CO3 = 20 respectively) were added to each of the diet, giving a total 12 dietary treatments. Key results: Replacing Napier grass with Leucaena at 500 g/kg (T3) and 750 g/kg (T4) levels increased the molar volatile fatty acid concentration, microbial protein synthesis (P < 0.001) and ammonia nitrogen concentration (P = 0.003), whereas ruminal protozoa concomitantly decreased (P < 0.05). The addition of CO at 10 mg also reduced the number of ruminal protozoa compared with the control (P < 0.001). A significant Leucaena × CO interaction was observed on the increase of ammonia nitrogen and microbial protein synthesis, and CH4 production was simultaneously suppressed (P < 0.001). There was also a significant Leucaena × CO interaction on increasing concentration of C18:1 cis-9, C18:2 cis-10 cis-12 and α-linolenic acid, which thus contributed to the increase of n-3 polyunsaturated fatty acids accumulation in the culture (P < 0.001). However, the concentration of C18:0 was not influenced by the treatments (P > 0.05). Conclusion: This study demonstrated that the inclusion of Leucaena into a Napier grass-based diet at 500 g/kg and 750 g/kg DM positively affected rumen fermentation, reduced CH4 formation and increased beneficial fatty acids in the rumen. Although CO had similar positive effects on CH4 production and targeted beneficial fatty acids, it reduced the microbial protein synthesis at inclusion of 20 mg/mL DM. Overall, there were synergistic interactions between Leucaena and CO in reducing CH4 production and improving the fatty acid profile in the rumen. Implications: It is possible to improve animal productivity while reducing the environmental impact of livestock production through inclusion of tannin-containing Leucaena and CO in ruminant diets in tropical regions where C4 grasses typically have low nutritive value. © 2021 CSIRO.