Gas (CO2 and CH4) and ammonia production in the rumen represent major sources of lost carbon and nitrogen, respectively. The essential oils of some plants have been shown to decrease gas and ammonia production by selectively inhibiting rumen microbes. Particularly, those of Zataria multiflora (ZEO; thymol 21%, carvacrol 32%) and Mentha spicata (SEO; carvone 55%) were evaluated in vitro as ruminant-feed additives. The experiments employed mixed rumen microbes and a hyper-ammonia-producing bacterium (HAP) isolated from the rumen of a Mehraban sheep. Both ZEO and SEO decreased in vitro fibre digestibility and also gas production by mixed rumen microbes that were fermenting a typical growing-lamb diet. ZEO decreased ammonia concentration in mixed culture of rumen microbes, but SEO exerted the opposite effect. A bacterial isolate (MT8) was obtained from the rumen of a Mehraban sheep, and the 16S rRNA gene sequence indicated that it was most closely related to Clostridium bifermentans. Isolate MT8 exhibited rapid ammonia production when peptides were the growth substrate, which indicated that MT8 was a HAP. Both oils inhibited the growth and ammonia production of isolate MT8. However, ZEO decreased ammonia production at lower doses, and to a greater degree, than did SEO. These results indicated that both essential oils could potentially be used to modulate rumen fermentation. The detrimental effects on fibre digestion could be problematic in high-forage diets, and this requires further investigation. Isolate MT8 is the first described HAP from the Mehraban sheep rumen. Results on ammonia production by isolate MT8 and mixed rumen microbes indicate differential mode of action of each oil on this parameter.
