Comparative in vitro evaluation of native Indonesian macroalgae on rumen fermentation characteristics, digestibility, gas production kinetics, and enteric methane mitigation in ruminants

Background and Aim: Enteric methane emissions from ruminants contribute significantly to greenhouse gas production, prompting research into sustainable feed additives. Macroalgae, rich in bioactive compounds, show promise in modulating rumen fermentation, improving digestibility, and reducing methane output. Indonesia’s diverse native macroalgae remain largely unexplored for these purposes, necessitating comparative evaluation to identify promising species for ruminant nutrition. This study aimed to conduct the first comparative in vitro evaluation of rumen fermentation patterns, digestibility characteristics, gas production kinetics, and enteric methane emissions using 14 native Indonesian macroalgae species, including brown (Sargassum sp., Padina sp., Turbinaria ornata), green (Boergesenia forbesii, Caulerpa racemosa, Ulva lactuca), and red (Palmaria palmata, Gelidium sp., Halymenia durvillei, Gracilaria verrucosa, Eucheuma cottonii, Gracilaria gigas, Eucheuma spinosum, Gracilaria coronopifolia) algae, to identify candidates for sustainable ruminant feed additives. Materials and Methods: Macroalgae samples were collected from various Indonesian locations, dried, and analyzed for chemical composition (dry matter, ash, crude protein, crude fat, crude fiber, nitrogen-free extract). In vitro fermentation was performed using a completely randomized design with five replicates per species. Samples (0.5 g) were incubated at 39°C for 72 h in buffered rumen fluid from fistulated Ongole crossbreed cattle. Parameters measured included total gas production, methane emissions (estimated via volatile fatty acid [VFA] profiles), ammonia, total and partial VFAs (acetate, propionate, butyrate, valerate, iso-butyrate, iso-valerate), acetate-to-propionate ratio, in vitro dry matter digestibility (IVDMD), in vitro organic matter digestibility (IVOMD), partitioning factor, microbial protein synthesis, and gas production kinetics. Data were analyzed using a one way analysis of variance with significance at p < 0.05 or p < 0.01, followed by post-hoc tests. Results: Chemical composition varied widely; red algae like Palmaria palmata had high crude protein (22.39 % dry matter), while brown algae like Padina sp. were ash-rich (74.39 % dry matter). Total gas production was highest in B. forbesii (54.75 mL; p < 0.01) and lowest in T. ornata (10.94 mL). Methane emissions and methane per incubated dry matter were lowest in Sargassum sp. (1.87 mM and 3.75 mM/g dry matter; p < 0.01), with Sargassum sp. and C. racemosa reducing methane by 71.86 %. Ammonia levels were similar across species (p > 0.05). Total VFA and propionate were highest in H. durvillei and B. forbesii (p < 0.01), with reduced acetate-to-propionate ratios. IVDMD and IVOMD were highest in H. durvillei (81.72 % and 69.53 %; p < 0.01). Gas kinetics showed B. forbesii with the highest asymptote (201.97 mL; p < 0.01) but slowest rate (0.01 mL/h). Positive correlations existed between crude protein and VFA/ammonia, while crude fiber inversely correlated with gas production and digestibility. Conclusion: H. durvillei emerged as optimal for enhancing rumen fermentation and digestibility, while Sargassum sp. excelled in methane mitigation. These species hold promise as natural additives for reducing environmental impacts in ruminant production, warranting in vivo validation for optimal inclusion rates and long-term effects.