Natural kaolin minerals with varying iron contents reduced cattle's enteric methane generation in vitro

Abstract

Kaolin, a dominant mineral group in geophagic soil, often contains iron impurities that incur purification costs. Valorizing kaolin for livestock applications like ruminant methane (CH₄) suppression without purification is promising but not well studied. This study used two kaolin clay samples collected from a Western Australia deposit, and tested their inclusion in a basal diet for CH₄ inhibition in an in vitro animal model using viable cattle rumen fluid and paunch. Physicochemical characterization showed that one clay sample (Kao) had kaolinite as the dominant mineral species and a trace of halloysite, while another one had additional minerals and impurities such as iron oxides and illite/mica (KaoFe). Regardless of the CH₄ produced by the basal diet, an approximately 5 % inclusion of clays significantly reduced enteric CH_4, with reductions of 23 % (Kao) and 37 % (KaoFe) compared to the control. On average, the in vitro digestibility (IVD), coefficient of fermentation efficiency (CFE), microbial crude protein (mCP), and ammonia nitrogen (NH₃–N) were 64 %, 77 %, 144 g/kg DM, and 21 g/kg DM, respectively. These results showed no significant differences between clay-treated and control groups, which indicates the clay's non-toxic nature in the rumen. Biochemical and physicochemical parameters and their correlations suggest that clay inclusion in the basal diet increased hydrogen sinkers, such as propionic acid and CH₄ inhibitory ions, such as Al³⁺, Si⁴⁺, Fe²⁺/Fe³⁺, and K⁺. And the inclusion of the KaoFe sample contributed to more of these. Given that the cattle naturally ingest soil minerals through geophagy with good palatability, this study emphasizes the potential valorization of natural kaolin clays as feed supplements. It advances a cost-effective and biocompatible feed additive for rumen CH₄ abatement without adversely affecting the digestive system.