The biofuel industry has been rapidly growing around the world in recent years. Several papers have used general equilibrium models and addressed the economy-wide and environmental consequences of producing biofuels at a large scale. They mainly argue that since biofuels are mostly produced from agricultural sources, their effects are largely felt in agricultural markets with major land use and environmental consequences. In this paper, we argue that virtually all of these studies have overstated the impact of liquid biofuels on agricultural markets due to the fact that they have ignored the role of by-products resulting from the production of biofuels. Feed by-products of the biofuel industry, such as Dried Distillers Grains with Solubles (DDGS) and biodiesel by-products (BDBP) such as soy and rapeseed meals, can be used in the livestock industry as substitutes for grains and oilseed meals used in this industry. Hence, their presence mitigates the price impacts of biofuel production on the livestock and food industries. The importance of incorporating by-products of biofuel production in economic models is well recognized by some partial equilibrium analyses of biofuel production. However, to date, this issue has not been tackled by those conducting CGE analysis of biofuels programs. Accordingly, this paper explicitly introduces DDGS and BDBP, the major by-products of grain based ethanol and biodiesel production processes, into a worldwide CGE model and analyzes the economic and environmental impacts of regional and international mandate policies designed to stimulate bioenergy production and use. We first explicitly introduce by-products of biofuel production into the GTAP-BIO database, originally developed by Taheripour et al. (2007). Then we explicitly bring in DDGS and BDBP into the Energy-Environmental version of the Global Trade Analysis Project (GTAP-E) model, originally developed by Burniaux and Truong (2002), and recently modified by McDougall and Golub (2007) and Birur, Hertel, and Tyner (2008). The structure of the GTAP-E model is redesigned to handle the production and consumption of biofuels and their by-products, in particular DDGS, across the world. Unlike many CGE models which are characterized by single product sectors, here grain based ethanol and DDGS jointly are produced by an industry, named EthanolC. The biodiesel industry also produces two products of biodiesel and BDBP jointly. This paper divides the world economy into 22 commodities, 20 industries, and 18 regions and then examines global impacts of the US Energy Independence and Security Act of 2007 and the European Union mandates for promoting biofuel production in the presence of by-products. We show that models with and without by-products demonstrate different portraits from the economic impacts of international biofuel mandates for the world economy in 2015. While both models demonstrate significant changes in the agricultural production pattern across the world, the model with by-products shows smaller changes in the production of cereal grains and larger changes for oilseeds products in the US and EU, and the reverse for Brazil. For example, the US production of cereal grains increases by 10.8% and 16.4% with and without by-products, respectively. The difference between these two numbers corresponds to 646 million bushels of corn. In the presence of by-products, prices change less due to the mandate policies. For example, the model with no by-products predicts that the price of cereal grains grows 22.7% in the US during the time period of 2006 to 2015. The corresponding number for the model with by-products is 14%. The model with no by-products predicts that the price of oilseeds increases by 62.5% in the EU during 2006-2015. In the presence of by-products, this price grows 56.4%. Finally, we show that incorporating DDGS into the model significantly changes the land use consequences of the biofuel mandate polices.