Introduction

Ruminant livestock production systems face significant challenges, including reduced rumen fermentation efficiency, dietary energy loss as methane and environmental consequences of greenhouse gas emissions. Enteric methane represents a 2%–12% loss of metabolizable energy (ME) from feed and constitutes a major greenhouse gas with substantial global warming potential (Xie et al. 2025). The extensive use of chemical and antibiotic feed additives to improve rumen fermentation efficiency has encountered increasing limitations due to microbial resistance development and potential hazards to human and animal health (Salman et al. 2026). Consequently, attention towards natural additives, particularly plant‐derived compounds, has intensified as sustainable and safe alternatives for modulating rumen fermentation and mitigating methane production (Kazemi 2019; Kazemi and Eskandari Torbaghan 2019). Therefore, identifying natural feed additives capable of simultaneously improving rumen fermentation efficiency and reducing enteric methane emissions has become a major research priority for sustainable ruminant production systems.

Essential oils, as bioactive secondary metabolites of plants, have garnered considerable interest in small ruminant nutrition due to their capacity to modulate rumen fermentation, influence microbial populations and exert antioxidant and anti‐inflammatory effects (Idowu et al. 2025; Nasir et al. 2026). Evidence suggests that essential oil supplementation can simultaneously improve energy and nutrient utilization efficiency while effectively reducing greenhouse gas emissions, particularly methane (Caroprese et al. 2023). Recent studies have further demonstrated that essential oils can selectively suppress methanogenic archaea and ruminal protozoa while modulating bacterial populations involved in fibre degradation (Dorantes‐Iturbide et al. 2022; Muslykhah et al. 2026). However, the efficacy of essential oils varies considerably depending on their botanical origin, chemical composition, dosage and dietary conditions, emphasizing the importance of evaluating individual plant species before their practical application (Yohannis et al. 2026).

Perovskia abrotanoides Kar. (Lamiaceae) is a perennial plant widely distributed across arid and semi‐arid regions of Iran and Central Asia (Sajjadi et al. 2005). This species has traditionally been valued in Iranian medicine for its anti‐inflammatory, antiseptic and antimicrobial properties. Phytochemical investigations have demonstrated that P. abrotanoides represents a rich source of bioactive compounds, including monoterpenes and sesquiterpenes in the essential oil, alongside diverse phenolic and flavonoid constituents in various extracts (Sajjadi et al. 2005). The essential oil has been reported to contain 1,8‐cineole, camphor, α‐pinene and β‐pinene as major active components responsible for its antibacterial and antifungal activities (Mahboubi and Kazempour 2009; Ashraf et al. 2014). Furthermore, methanolic and ethanolic extracts from different plant organs, particularly flowers, exhibit high total phenolic content and notable antioxidant capacity, indicating substantial potential for inhibiting oxidative processes and microbial activities (Mazandarani et al. 2010; Ashraf et al. 2014). Despite these well‐documented pharmacological and antimicrobial properties, the potential application of P. abrotanoides essential oil as a natural rumen modifier has received little scientific attention, and its effects on rumen fermentation characteristics and methane mitigation remain largely unexplored.

Although an increasing number of studies have investigated plant‐derived essential oils as natural rumen modifiers (Nhara and Baloyi 2025; Nasir et al. 2026), the reported effects on fermentation, methane mitigation and nutrient utilization remain inconsistent across literature. Most previous studies have focused on commercial essential oils or a limited set of conventional medicinal plants (Nasir et al. 2025; Muslykhah et al. 2026). Information regarding the biological activity of P. abrotanoides essential oil in the rumen ecosystem is still scarce, and the relationship between its unique chemical composition (particularly its high concentration of oxygenated monoterpenes) and its influence on methane production, microbial populations, nutrient degradability and microbial efficiency has not been comprehensively evaluated under controlled in vitro conditions.

Key fermentation parameters, such as gas and methane production, volatile fatty acid (VFA) concentrations, NH3–N, protozoal populations and microbial biomass synthesis efficiency, serve as critical indicators for evaluating the effects of feed additives on rumen health and functionality (Patra and Saxena 2009). Comprehensive evaluation of these parameters provides valuable insights into the mechanisms through which bioactive plant compounds influence microbial metabolism, fermentation efficiency and energy utilization in the rumen.

Accordingly, the objectives of the present study were to determine the chemical composition of P. abrotanoides essential oil and to investigate the effects of different inclusion levels of whole‐plant essential oil on gas production kinetics, methane production, fermentation parameters, nutrient degradability, protozoal population and microbial efficiency indices in sheep rumen fluid under controlled in vitro conditions. We hypothesized that supplementation with P. abrotanoides essential oil would produce dose‐dependent modifications in rumen fermentation, resulting in reduced methane production and protozoal populations while improving microbial biomass synthesis and energy utilization efficiency. Furthermore, we hypothesized that an optimal supplementation level would mitigate methane emissions without adversely affecting overall nutrient degradability.