Plant secondary metabolites (PSM|) are used as livestock feed additives instead of chemicals for their multiple effects on rumen microbiota, ammonia N production and fermentation end products. In this study we investigated the effect of two kinds of PSM used as feed additives and compare their effect to feeding whole alfalfa. Fifteen small tail Han rams were randomly allocated into five groups (n= 3/ group). CONT group (basal diet), HT group (basal diet supplemented with 2 g of Hydrolizable tannin), PF group (basal diet supplemented with 2g of Pueraria flavone) G3 group (basal diet +200g of Gannon 3 alfalfa) and G9 group (basal diet +200g of Gannon 9 alfalfa). For 30 days. At the end of the experiment rumen and serum samples were collected for SOD analysis, rumen pH, ammonia N concentration, total and proportional VFA and PCR quantification of total bacteria, F. succinogenase, methanogens and protozoa population. An in situ ruminal digestion kinetics experiment for soybean NDF was applied. Results showed that serum Superoxide dismutase enzyme (SOD), ruminal pH and ruminal ammonia N were not influenced by any of the treatments, but total volatile fatty acids (TVFA) and molar proportion of acetic acid were significantly reduced in HT group. Protozoa and methanogens were significantly increased in G9 and PF groups. Tannins and flavonoids of different of different sources and in different concentrations affect rumen ecosystem differently but, did not affect the serum SOD.