Lambs were divided into 3 groups of 6 animals each. For 35 days lambs were fed a diet including 2% rapeseed oil, 1% fi sh oil and 0.1% carnosic acid (the control group) or two experimental diets supplemented with 0.35 mg • kg -1 Se as selenized-yeast (SeY) (the SeY diet) or selenate (the selenate diet). Muscles (Musculus longissimus dorsi (MLD) and Musculus biceps femoris (MBF)), ruminal fl uids and microbiota were collected from each lamb. SeY supplementation most effectively stimulated the accumulation of straightchain volatile fatty acids (VFAs), iso-branched-chain VFAs, CO 2 and CH 4 in the ruminal fl uid. The contents of CO 2 , CH 4 and VFAs including straight-chain VFAs with the exception of iso-branched-chain VFAs were most effectively reduced by the selenite diet. The control diet most effi ciently increased the concentration sums of odd-saturated fatty acids (odd-SFAs) and iso-SFAs in microbiota. The SeY diet most effi ciently reduced acetic acid to propionic acid ratio in the ruminal fl uid. The selenate diet improved animal performance by reducing ruminal concentrations of CH 4 and CO 2 . The SeY diet and especially the selenate diet reduced the biohydrogenation to C18:0 when compared with the control diet. The selenate diet more effi ciently reduced the concentration sums of all SFAs (SFAs) and all fatty acids (FAs) in MLD and MBF than the SeY diet, which most effectively increased the concentrations of SFAs and FAs in MLD and MBF. The selenate diet most effectively increased the body mass gain of lambs.Key words: carnosic acid, fatty acids, methane, ovine rumen, seleno-compounds, volatile fatty acids INTRODUCTIONThe most common supplemental chemical forms of selenium (Se) for ruminants are selenite, selenate or selenized yeast (SeY) [1,2]. The chemical form of Se affects Unauthenticated Download Date | 5/11/18 9:28 AM Acta Veterinaria-Beograd 2016, 66 (3), 373-391 374 biosynthesis yield of Se-complexes and Se-proteins in ruminal microorganisms [3]. Se-methionine (Se-Met) is more effi ciently accumulated in the animal body than inorganic forms of Se. Organic chemical forms of Se undergo less alteration in the rumen leading to less of insoluble chemical forms of Se [1]. Ruminal micro-organisms are capable to metabolize organic and inorganic forms of Se into Se-amino acids (Se-AA). The poor bioaccumulation of inorganic Se has been linked to the ruminal environment whereby selenate or selenite is reduced by microorganisms to insoluble and unavailable elemental Se, which is excreted in the feces [1,3]; selenate has higher relative bioavailability value than selenite. Ruminal microorganisms can reduce excessive doses of organic or inorganic forms of Se to selenides or unabsorbable Se o . Interestingly, Serra et al. [4] showed that dietary Se-compounds change the ruminal production of volatile fatty acids (VFAs). In fact, numerous studies showed that Secompounds affect ruminal microorganism yield and activity of microbiota [1][2][3][4][5][6]. Thus, Se-compounds affect VFAs production and rumen micro...