Influence of partial replacement of Heteropogon contortus hay with Leucaena leucocephala cv. Taramba and physiological stage on dry matter utilization, volatile fatty acids, and ruminal microbiota in Bali cattle

The objective of this study was to evaluate the effects of partial replacement of Heteropogon contortus hay (HCH) with Leucaena leucocephala cv. Taramba (LLT) and physiological stage (PS) on dry matter (DM) intake and digestibility (DMI and DMD), volatile fatty acids (VFA), and ruminal microbial communities in Bali cattle ( Bos javanicus domesticus ), using a completely randomized design with a 3 × 2 factorial arrangement [PS × dietary treatment (DT)], with three replicates per treatment. Eighteen male Bali cattle were used, assigned to three PS: weaned (<10 months; 92.1 ± 16.3 kg), growing (1–2 years; 157.5 ± 14.4 kg), and adult (>2 years; 177.5 ± 20.7 kg), and within each PS two DT were evaluated: a control group fed exclusively with HCH and a group with partial replacement (40% of DM) of HCH with LLT. DMI and DMD were affected by DT, PS, and their interaction ( P ≤ 0.008), increasing with the partial replacement of HCH with LLT and showing a greater response in growing animals, followed by adults and weaned animals. This replacement increased total VFA concentration (mM), as well as acetate ( P = 0.007), iso-butyrate ( P = 0.001), butyrate ( P < 0.001), and valerate ( P = 0.002) concentrations, whereas propionate was influenced ( P = 0.029) mainly by PS. Likewise, the number of OTUs, microbial richness, and diversity increased with LLT inclusion, and PCoA and ANOSIM analyses evidenced a clear microbial separation according to DT ( P < 0.05). At the taxonomic level, Bacteroidota and Bacillota dominated in all groups (>79%), with a higher abundance of Bacteroidota in animals receiving LLT. Additionally, LLT enriched taxa such as Prevotellaceae , Xylanibacter , Christensenellaceae R-7 group , and Succinivibrionaceae , whereas HCH favored the Rikenellaceae RC9 group and Spirochaetaceae . LEfSe analysis identified Prevotellaceae (LDA = 5.21) and Xylanibacter (LDA = 5.15) as biomarkers of the LLT treatment. In conclusion, partial replacement of HCH with LLT modified DMI and DMD, increased total and individual VFA concentrations, and altered ruminal microbial composition in Bali cattle, with more pronounced effects in growing animals, suggesting its potential as a feeding strategy in tropical forage-based systems.