*Article* **Effect of Sainfoin (***Onobrychis viciifolia***) Pellets on Rumen Microbiome and Histopathology in Lambs Exposed to Gastrointestinal Nematodes**

**Daniel Petriˇc 1,2, Michaela Komáromyová 2,3, Dominika Batt'ányi 1, Martyna Kozłowska 4,5, Weronika Filipiak 4, Anna Łukomska 6, Sylwester Slusarczyk ´ 7, Malgorzata Szumacher-Strabel 4, Adam Cie´slak 4, Marián Várady 3, Svetlana Kišidayová 1 and Zora Váradyová 1,\***


**Abstract:** Our study analyzed the ruminal fermentation and microbiome, hematological profile, and abomasal histopathology of lambs experimentally infected with a gastrointestinal nematode (GIN) and fed sainfoin pellets (SFPs; 600 g DM/d/animal) for 14 d. Twenty-four lambs infected with *Haemonchus contortus* were divided into two separated groups: animals fed meadow hay (control) and animals fed SFPs. The ruminal contents, fermentation parameters, and microbiome in vitro and in vivo were determined using molecular and microscopic techniques. Ruminal contents in the SFP group indicated smaller populations of *Archaea* (*p* < 0.001), *Methanomicrobiales* (*p* = 0.009), and lower methane concentrations in vitro (*p* = 0.046) and in vivo (*p* = 0.030) than the control group. The relative abundance of *Butyrivibrio fibrisolvens* quantified by real-time PCR was higher in the lambs with the SFP diet (*p* = 0.05). Haemonchosis affected the number of red blood cells of the lambs (*p* < 0.001). The lambs in the SFP group had a higher percentage of damaged abomasa glands than did the control group (*p* = 0.004). The consumption of SFPs by GIN-infected lambs may affect ruminal methanogens and subsequently decrease methane emission without undesirable changes in the ruminal microbiome or the health of the animals.

**Keywords:** bacteria; flavonoids; hematological profiles; methane concentration; plant bioactive components; ruminal fermentation; sheep
