Treatment of corn ethanol co-products
Reduced GHG emissions from enteric fermentation
The GREET default distillers grains with solubles (DGS) co-product credit is based on displacement ratios of corn, soy and urea. Soybean meal is high protein cattle feed used to induce weight gain. Due to its high protein content and ease of digestibility, DGS displaces some soy on the animal feedlot. The overall protein content consumed by cattle is arguably the same from DGS as with other feeds, but the DGS feed results in shorter animal residence time on the feedlot.
For enteric fermentation emissions (i.e. methane produced from ruminant digestion), a significant factor is the time required to gain weight. A reduced DGS feed residence time therefore results in lower enteric fermentation emissions. Since the global warming potential (GWP) of methane is ~ 25 times that of CO2, a reduction in methane implies a GHG emission offset that should be included in the full life cycle analysis of corn ethanol.