The production of corn-based ethanol in the U.S. has been dramatically increasing in recent years, and consequently so has the quantity of co-products, especially distillers dried grains with solubles (DDGS), generated from this industrial sector. DDGS is sold as a commodity for animal feed, and like corn, its bulk physical and chemical properties are quality attributes that can be used in trading by merchandisers. Consistency in product quality has been a cause for concern in the industry, and the lack of standard methods for physical and chemical property determination potentially confounds the issue, especially when results of attributes measured by different methods have not been evaluated for method parity. Toward that end, this research examined the sources of DDGS bulk density variability using samples from six commercial ethanol plants (three from Indiana, and three from South Dakota) and conducted in two different laboratories. Variations due to sample handling, container filling method, filling height, and container size were also examined. It was ascertained that DDGS source accounted for most of the bulk density variability observed. It was also determined that filling height and measuring container size resulted in significant differences in the resulting bulk density values.