Biofuel co-product credits? The answer is blowing in the wind

The success of biofuel policies depends on delivering GHG reductions. Life cycle analysis (LCA) is the primary tool used in biofuel policies to estimate well-to-wheel GHG emissions and ensure that the intended GHG reductions targets are achieved. Therefore, the effectiveness of these policies depends on the quality of the LCA.

There are many important sources of GHG emissions in the life cycle of biofuels. One major source is the indirect GHG emissions changes caused by the market-mediated effect of large-scale biofuel production. Most of the debate around indirect effects is focused on indirect land use change (ILUC), but land use change is not the only potential source of emissions credits and debits. At a March 2014 workshop [.pdf] the California Air Resources Board (ARB) indicated that ILUC GHG emissions of corn ethanol obtained from the Global Trade Analysis Project (GTAP) model would be lower if GHG credits associated with reduced “enteric methane” emissions due to use of dried distillers grains with solubles (DDGS) in cattle feed were included. Since methane is a potent GHG gas with a 100-year global warming potential (GWP-100) 25 times higher than that of carbon dioxide, a lower production of methane implies a potential for appreciable GHG emission offset.

DDGS can be a good substitute for animal feed ingredients such as corn, soybean meal, barley, and wheat, and has been used by the animal feed industry in the U.S. and elsewhere. Reduced demand for the feed ingredients displaced by DDGS avoids GHG emissions and provides an emissions credit. These offsets offered by DDGS are routinely accounted for in estimating carbon intensities of corn ethanol in regulations, including the California Low Carbon Fuel Standard (LCFS) and the U.S. Renewable Fuel Standard (RFS2).

Cattle feed is one of the principle markets for DDGS, which are a co-product of corn ethanol production. Diets rich in DDGS can have a higher lipid (fat) content than alternative diets, and studies have associated this with a decrease in enteric methane production. Enteric methane—that is, methane from the intestine—is produced in the rumen of ruminants such as cattle and goats, where bacteria carry out fermentation to assist in food digestion. Most of the methane produced in the rumen is released as belching. In addition, a small amount of enteric methane is also produced in the large intestine of ruminants. This enteric methane is released as flatulence. Specifically, DDGS diets reduce methane formation by exerting toxic effects on and impacting the activities of cellulolytic bacteria, methanogen bacteria, and protozoa, thereby reducing the amount of organic matter fermented in a rumen.

As it happens, EPA has already taken into account GHG credits from enteric methane reductions in estimating the carbon intensity of corn ethanol when it finalized the Renewable Fuel Standard (RFS2) in 2010. For this, EPA used the Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) model default credit of 3.2 g CO₂e/MJ ethanol. This credit is in addition to the land-use credits realized from the displacement of ingredients such as corn and soybean meal from animal feed rations by DDGS.

The credit of 3.2 g CO₂e/MJ is by no means an insignificant result. By EPA’s assessment corn ethanol only just qualifies as a renewable fuel under the RFS2, having average GHG reductions of just over the minimum 20% required. Hence, the 3.2 g CO₂e/MJ credit played an important role in ensuring that corn ethanol qualified as renewable fuel. Without the credit, the GHG reductions would be only 18%.

This begs the question of how confident we are that DDGS use indeed results in reduced enteric methane emissions. Is there any possibility of GHG burden shifting?

A recent study by Benchaar et al. using sophisticated experiments reports a strong linear relationship between the amount of corn DDGS in animal feed and the amount of enteric methane. As DDGS incorporation rates in cattle diets were increased from 0% to 10%, 20%, and 30%, there were corresponding decreases in enteric methane emissions by 2%, 4%, and 8%, respectively. These reductions are equivalent to 248 g CO₂e/day/cow, 495 g CO₂e/day/cow, and 990 g CO₂e g/day/cow, respectively (Table 1).

However, benefits of DDGS in terms of methane reductions are likely to be at least partially offset by an increased release of nitrous oxide (N₂O) from urine and feces, which is 298 times more powerful as a greenhouse gas than carbon dioxide. The same study by Benchaar et al. showed that cows fed with DDGS diets produced urine and feces with more nitrogen. This is because the nitrogen content of DDGS is higher. For example, cows fed with 10%, 20%, and 30% DDGS diets excreted 11g, 17 g, and 32 g more nitrogen per day compared to the cow fed with 0% DDGS diets. About 0.25%–2.25% of nitrogen present in the excreta (urine and feces) would be eventually converted to N₂O by anaerobic bacteria and/or aerobic bacteria. Assuming that 1.5% N would be released as N₂O, N₂O emissions would offset somewhere between 20% and 30% of CO₂e savings resulting from methane reductions (Table 2).

An even stronger result was reported in a life cycle study by Hunerberg et al., who found that when beef cattle were fed wheat and corn DDGS diets containing higher levels of nitrogen, N₂O produced from manure offsets all the savings offered by reduced enteric methane emissions.

Potentially even more important, a team of scientists in Canada also found an increase in manure and urine production when cattle were fed DDGS diets. More manure implies more methane emissions if the manure is stockpiled or stored as slurries in storage tanks that are not equipped with methane recovery. For example, the same team found that feces obtained from pigs fed with wheat DDGS released about 40% more methane than pigs fed with control diets (no DDGS), simply because pigs fed with DDGS produce more feces and hence more organic matter.

According to the American Biogas Council [.pdf], only 241 farms in the US have anaerobic digesters to capture methane. The American Biomass Council also estimates that there are about 12,000 additional dairy and pig farms where anaerobic digester installations could be potentially installed. This suggests that at most a couple of percent of manure has been used for anaerobic digestion, with the rest most likely applied to agricultural land after storage. Even though methane generation from open storage is not going to be as high as that from dedicated anaerobic digesters, there will still be more methane emissions from manure from DDGS-fed cows compared to the manure from cows not fed with DDGS.

This implies that DDGS diets may simply shift some of the methane emissions from the digestive system to manure handling (assuming methane produced from manure is not captured and utilized). And that is an example of GHG burden shifting. Therefore, in the absence of good manure management practices, the benefits apparently offered by a reduction in enteric methane emission cannot be automatically assumed.

Overall, based on these studies it is likely that an increase in methane emissions from manure coupled with an increase in N₂O emissions for DDGS-fed cattle can offset 40% to more than 100% of the savings provided by reduced enteric methane emissions, depending on nitrogen levels in DDGS diets.

The implication of this for policy is that credits for avoiding methane emissions should be limited to cases where it can be demonstrated that methane from manure is captured or reduced through good manure management practices. Otherwise, we run the risk of overestimating the appropriate emissions credits, due to methane burden shifting. There is also a case for establishing a guideline on the allowable nitrogen level in diets containing DDGS, in order to avoid increased N₂O emissions.

On a final note, as mentioned above, the expected decrease in enteric methane production is due to the higher lipid content (fat) of DDGS. However, this means that if corn oil is extracted from DDGS (an increasingly common practice, with the oil being used to produce corn biodiesel) the resulting reduced-oil DDGS should potentially be ineligible for any credit from reduced enteric fermentation.