Journal of Animal and Feed Sciences, 29, 2020, 206–214 https://doi.org/10.22358/jafs/127694/2020 The Kielanowski Institute of Animal Physiology and Nutrition, Polish Academy of Sciences, Jabłonna The use of yeast-fermented cassava roots as a sole source of protein in beef cows C. Promkot and P. Pornanek 1 Rajamangala University of Technology Isan, Faculty of Natural Resources, Department of Animal Science SkonNakhon Campus, Post Box 47160, Thailand KEY WORDS: beef cattle, cassava, root, yeast, Saccharomyces cerevisiae, protein Introduction Farming ruminants in tropical conditions is often challenged by the shortage of roughage (both of its quantity and quality), and especially a lack of protein sources during the drought season (Wanapat and Devendra, 1992), resulting in reduced livestock yield (Leng, 1990). One of many ways to solve the shortage of good quality roughage during the dry season is feeding livestock a concentrate feed. However, such type of feed is expensive as it is produced from protein sources such as soybean meal (SBM) a locally rare animal feed ingredient that must be imported. In efect farmers do not supplement their livestock with concentrate feed or use only limited concentrate feed, which leads to a decreased livestock production. Finding a cheaper protein source that can be procured locally to replace SBM in concentrate feed will be a solution. Cassava is considered as an important economic crop in tropical countries and it is extensively cul- tivated. Therefore, cassava as inexpensive protein source is widely used among livestock produc- ers. Cassava can be fermented with yeast to pro- duce yeast-fermented cassava products with a high level of protein and has the potential to be an al- ternative source of protein replacing SBM. This yeast-fermented cassava product is also cheaper than SBM (yeast-fermented cassava product costs approximately 1015 THB per kg, while soybean meal – 1525 THB per kg; 1 USD = 31.78 THB). The use of yeast-fermented cassava products as a source of protein in ruminant feed has been widely studied (Boonnop et al., 2009, 2010; Wanapat et al., 2011; ABSTRACT. Four ruminally-cannulated non-pregnant Brahman beef cows (body weight (BW) = 430 kg) were used to study the effects of yeast-fermented cassava roots (YEFECAR) as the sole source of protein on dry matter (DM) intake and nutrient digestibility. Animals were allotted to groups fed YEFECAR at levels: 0, 20, 30, 35% DM, according to a 4 × 4 Latin square design. Cows were fed rice straws (as a roughage) ad libitum; concentrate treatments (1.5% BW per day) were offered twice daily. It was indicated that the use of YEFECAR in a concentrate diet reduced feed intake (P < 0.05) but had no effects on nutrient digestibility, rumen ecology (pH, NH 3 -N), rumen volatile fatty acid concentration, rumen bacterial population and microbial protein. Accordingly, it can be concluded that the use of YEFECAR as the primary source of protein at 20% DM or higher in concentrate feed can lower the feed intake, which may subsequently affect livestock yield. To ensure optimal utilization of YEFECAR as the main source of protein in ruminant feed further studies on increasing the sulphur content in yeast medium prior to the production of YEFECAR are needed. Received: 16 December 2019 Revised: 2 June 2020 Accepted: 7 August 2020 1 Corresponding author: e-mail: ppitukpol@hotmail.co.th