A composite feed additive approach for methane mitigation and improved rumen fermentation: toward sustainable livestock production.

Sustainable strategies to mitigate methane (CH₄) while maintaining rumen efficiency are important for climate-smart livestock production. This study evaluated a composite feed additive combining tamarind seed husk tannins with sodium nitrate, magnesium sulphate, calcium propionate and sodium bicarbonate, alongside each component group tested separately. Three independent in vitro experiments were conducted, one per additive, each with five inclusion levels (0, 1, 2, 3 and 4% of substrate dry matter) and three replicate syringes per level. Rumen liquor from a fistulated buffalo bullock was incubated for 24 h with 200 mg of a total mixed ration substrate. Total gas and CH₄ production, in vitro true digestibility, volatile fatty acids, ammonia nitrogen, partitioning factor and microbial biomass production were measured. Methane yield fell linearly with increasing inclusion of every additive (p < 0.001). Tamarind seed husk reduced CH₄ from 33.50 to 26.31 mL/g DM, the composite additive from 32.00 to 20.15 mL/g DM, and the chemical additive reduced CH₄ from 32.49 to 17.94 mL/g DM at 4% inclusion. In vitro true dry matter digestibility was unchanged by tamarind seed husk (p = 0.682) and increased with both the chemical additive (59.65 to 61.63%, p = 0.012) and the composite additive (60.58 to 62.91%, p = 0.008). Microbial biomass production rose by 55% with the composite additive (21.99 to 34.14 mg per 200 mg substrate, p < 0.001), and the acetate-to-propionate ratio fell from 3.13 to 2.16 (p < 0.001), driven principally by a decline in acetate rather than a rise in propionate. At 4% of substrate dry matter the chemical additive gave the greatest reduction in CH₄, while the composite additive combined a reduction of similar magnitude with the highest digestibility, partitioning factor and microbial biomass production of the three treatments. Both warrant further evaluation. Because each additive was incubated as a separate run, additive type is confounded with run and a synergistic interaction between components could not be established; a single simultaneous incubation is required to test this directly.