Introduction
Recently, plants and plant extracts have been suggested as alternative additives to be used
in animal feeding [1]. Plants provide a foremost source of biologically active molecules [2]
that may modify ruminal fermentation enhancing the efficiency of utilization of feed energy
and protein and decreasing methane and nitrogenous emissions [3,4]. Roots of Rheum
palmatum and bark of Frangula alnus, when used as additives, have been reported to
modify ruminal fermentation, decreasing methane production in in vitro batch cultures [5].