Cassava (Manihot esculenta, Crantz) is widely grown in the tropical and sub-tropical areas (Sommart et al., 2000;Wanapat, 2009). Its root is a good source of rumen fermentable carbohydrate and has been used with NPN especially urea in ruminants (Wanapat, 2009). 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 (8.0 g/100 g of protein) (Yamada and Sgarbieri, 2005). Yeast products are beneficial by enhancing dry matter (DM) intake and overall animal performance in ruminants Therefore, the objective of this research was to study the effect of YEFECAP in concentrate mixtures in lactating dairy cows fed on rice straw.