Proc. of the 4 th Animal Wealth Research Conf. in the Middle East & North Africa Foreign Agricultural Relations (FAR), Egypt, 3 – 5 October (2011) pp. 01 - 11 CARVACROL AND EUGENOL AS MODIFIERS OF RUMEN MICROBIAL FERMENTATION, AND METHANE PRODUCTION IN VITRO Yosra Ahmed Soltan ab* , Amr Salah Morsy bc , Rafael Canonenco Araujo, Hani Mohamed Elzaiat ab , Sobhy Mohamed Abdalla Sallam a , Helder Louvandini b and Adibe Luiz Abdalla b a Animal Production Department, Faculty of Agriculture, University of Alexandria, Egypt. b Laboratory of Animal Nutrition, Centre for Nuclear Energy in Agriculture, University of Sao Paulo, Brazil. c Agricultural Research Centre, Animal Production Research Institute, Dokki, Egypt ABSTRACT Carvacrol (CAR) and eugenol (EUG) were used as natural alternatives modifiers of rumen microbial fermentation, and methane production in vitro using semi-automatic gas production system for 24 h incubation. The substrates, 500 mg of Tifton hay plus concentrate (50:50 w/w) were incubated with rumen inoculum (25 mL) and 50 mL buffer with no additives (control) or the treatments monensin (MON) (3 μM/75 ml of culture fluid) as positive control, CAR and EUG at [(5, 10 and 20) and (10, 20 and 30) μl /75 ml of culture fluid)], respectively. The results indicated that both CAR and EUG at 10 and 20 μl/75 ml of culture fluid respectively presented similarity in rumen total gas production (GP), CH 4 and the truly degraded organic matter (TDOM) when compared to MON. Monensin showed the highest (P< 0.0001) molar proportion of propionate and the lowest (P< 0.0001) acetate: propionate ratio compared with all the treatments whereas all the tested essential oils increased (P<0.0001) the molar proportion of butyrate. Although the highest dose of CAR (20 μl) presented the lowest (P< 0.0001) CH 4 production, this reduction was associated with reduction of acetate, propionate, butyrate and total short chain fatty acids (SCFA) production, which reflected general fermentation inhibition. All treatments decrease (P< 0.0001) protozoa count compared with the control and there were found no significant differences (P>0.05) in pH and ammonia (NH 3 ) concentration. These results suggested that both of CAR and EUG in moderate levels could be also used as natural modifiers of rumen fermentation to decrease CH 4 emission, but in a different mode of action than monensin. Key words: gas production, essential oils, monensin, short chain fatty acids.