Effect of different varieties of rosemary (Rosmarinus officinalisi) plant extracts on in vitro methane production of Eragrostis curvula hay

Abstract

Methane emissions from livestock contribute substantially to global greenhouse gases, which affects climate change and energy losses in ruminants that negatively impact productivity. To address this issue, the use of natural feed additives rich in plant secondary metabolites has gained interest as a sustainable alternative to synthetic additives. This study evaluates the potential of various rosemary (Rosmarinus officinalis) varieties as natural supplements to reduce methane production during the fermentation of Eragrostis curvula hay. Rosemary varieties, including McConnell’s Blue, Tuscan Blue, Roseus, Mountain Mist, and Silver Fox, were harvested at two physiological stages—eight weeks post-transplanting and at 50% flowering. Metabolomics analysis revealed significant differences in secondary metabolite concentrations across varieties and harvest stages. The different rosemary varieties were evaluated for in vitro gas production and organic matter digestibility and methane inhibition using standard procedures. Three different varieties of rosemary leaves were harvested at 8 weeks after planting (Season 1) and six varieties were harvested at 50% flowering (Season 2). The leaves were freeze-dried and extracted with 85% methanol and 15% water. The plant extracts used as a feed additive in in vitro rumen fermentation experiments at a rate of 75mg/kg feed DM. Wild Rosemary was different to the other varieties in terms of the concentration and presence of most of the metabolites phytochemical profiles, whereas McConnell’s Blue was the only variety that showed seasonal difference. In season 1, McConnell’s Blue variety increased total gas production (TGP) and Tuscan Blue variety decreased methane (CH4) and in vitro organic matter digestibility (IVOMD). In season 2, all the rosemary varieties plant extracts reduced TGP, but no effect was observed for (CH4). The study further identified positive correlations between some metabolites, such as carnosic acid, carnosol, and TGP, (CH4) and IVOMD. These findings underscore the potential of rosemary extracts as sustainable feed additives that support methane reduction.