In vitro effects of Tribulus terrestris seed powder as feed additive on methane emission and digestibility in local cattle breed of Sri Lanka

Abstract

Methane (CH₄) emissions from ruminant livestock contribute significantly to global greenhouse gas accumulation and represent a loss of dietary energy. The use of plant-based feed additives rich in secondary metabolites has gained attention as a sustainable strategy for methane mitigation. This study evaluated the in vitro effect of Tribulus terrestris seed powder, a saponin-rich natural additive, on methane production, protozoal population, and feed digestibility using rumen fluid collected from local cattle breeds of Sri Lanka. A basal substrate composed of rice bran and paddy straw was supplemented with T. terrestris seed powder at graded inclusion levels (10%, 30%, and 50%), alongside an unsupplemented control. In vitro fermentation was conducted under anaerobic conditions, and methane production was measured at 6-hour intervals for 24 hours, while in vitro dry matter digestibility (IVDMD) and protozoa counts were determined after incubation. Results demonstrated a significant (p < 0.05) reduction in methane production with increasing levels of T. terrestris supplementation compared to the control. The methane-reducing effect was accompanied by a marked decline in rumen protozoa populations, indicating a possible saponin mediated inhibition of protozoa- associated methanogenesis. Moderate inclusion levels of T. terrestris maintained acceptable digestibility, whereas higher levels tended to reduce IVDMD. Overall, the findings suggest that T. terrestris seed powder has strong potential as a natural methane-mitigating feed additive for climate-smart livestock systems, provided that optimal inclusion levels are used to balance methane reduction and nutrient utilization in local cattle production systems of Sri Lanka.