Cassava is widely grown in the tropical and sub-tropical areas Its root is a good source of rumen fermentable carbohydrate and has been used with NPN especially urea in ruminants While, dietary yeasts have been widely used as a ruminant feed especially with Saccharomyces cerevisiae since it contained useful nutrients for ruminants especially high lysine composition.the objective of this research was to study the effect of YEFECAP in concentrate mixtures in lactating dairy cows fed on rice straw.