The increasing antimicrobial resistance among mastitis pathogens necessitates alternative therapeutic approaches in dairy cattle. This study evaluated the antimicrobial efficacy of selected plant extracts against both reference and production strains of mastitis-associated microorganisms in vitro, and assessed the in vivo effects of an extruded feed additive (the wormwood-poplar extract additive) containing Artemisia absinthium and Populus balsamifera extracts, on udder health and milk quality in cows with subclinical mastitis. In vitro assays demonstrated significant antimicrobial activity of citrus peel (Citrus spp.) and spruce (Picea abies) extracts against Candida spp., Escherichia coli, and Staphylococcus aureus, including resistant field isolates. The 15-day in vivo trial involving 60 Holstein cows revealed a 68% reduction in somatic cell counts (p = 0.017) following supplementation, achieving compliance with top-grade milk standards without compromising milk composition or sensory attributes. These findings underscore the potential of phytogenic compounds as effective, natural alternatives for mastitis control, promoting sustainable dairy production and reduced antibiotic reliance.

Mastitis in dairy cows, caused by pathogens such as Staphylococcus aureus, Escherichia coli, and Candida albicans, leads to substantial economic losses and contributes to antimicrobial resistance, emphasizing the need for natural alternatives. This study assessed the phytochemical composition, antioxidant activity, and in vitro antimicrobial effects of ethanol extracts from ten medicinal plants, including Populus balsamifera buds, Syzygium aromaticum, and Humulus lupulus, as well as two multi-component plant mixtures and commercial products against reference strains and field isolates of mastitis pathogens. Extracts exhibited total phenolic contents ranging from 2.5 to 43.0 mg GE/g, with Populus balsamifera and Syzygium aromaticum demonstrating the strongest DPPH radical scavenging activity (IC₅₀ 1.89–2.9 mg/g extract). Disc diffusion assays demonstrated broad-spectrum inhibition, particularly for Populus balsamifera (15.1–19.1 mm inhibition zones) and Humulus lupulus (9.0–18.4 mm) against key pathogens; phenolic and flavonoid contents positively correlated with antimicrobial activity (r = 0.63–0.99, p < 0.001). Multi-component mixtures provided consistent broad-spectrum effects (12.6–17.2 mm). These phenolic-rich plant extracts represent promising alternatives to reduce antibiotic use in dairy mastitis management.