- A New Vision for Iowa Food and Agriculture, Food, Energy, and Agriculture Commentary
- September 15, 2010
For every contribution of $20 or more, you'll receive a copy of Francis Thicke's new book.
Agriculture And Climate Change
by Francis Thicke[The following is an excerpt taken from my book, 'A New Vision for Iowa Food and Agriculture'.]
Chapter 8. Agriculture and Climate Change
Climatologists tell us that our climate is changing. One of the causes of climate change is the growing concentration of heat-trapping “greenhouse gases” in the air. The most prevalent greenhouse gas (GHG) is carbon dioxide. However, in agriculture two other greenhouse gases are of major concern: methane, which has more than 20 times the GHG potency of carbon dioxide, and nitrous oxide, which has about 300 times the potency of carbon dioxide. Agriculture contributes about 8 percent of total GHG emissions in the U.S.
The release of nitrous oxide from soils is by far the largest source of GHG emissions from agriculture in the United States. Nitrous oxide emissions from soils can come from soil organic matter, legumes, manure and other sources, but the largest source is from synthetic nitrogen fertilizer. About 2.5 percent of synthetic nitrogen fertilizer applied to crops is released into the atmosphere as nitrous oxide. If farmers were to diversify crop rotations by including more nitrogen-fixing crops in rotations, less synthetic nitrogen would be needed, reducing nitrous oxide emissions. That would also reduce the amount of fossil fuel required to synthesize nitrogen fertilizer, concomitantly reducing carbon dioxide emissions.
Methane is the second-largest source of GHG emissions from agriculture in the United States. More than 70 percent of agriculture’s methane emissions come from enteric fermentation, a fancy term for gas produced in the digestive tracts of animals, mostly cows and other ruminant animals. Because rumen fermentation is a natural process, enteric methane cannot be eliminated, although research is finding that adjusting feeding rations can help reduce enteric methane emissions.
In the United States, about a fourth of the methane from animal sources comes from manure after it leaves the animal. That source of methane can be greatly reduced with appropriate management. For example, when manure is deposited in pastures or is composted, very little methane is generated. However, when manure is stored in liquid pits, it becomes anaerobic (as will be explained below), creating ideal conditions for methane to be generated in the liquid manure pit.
Because hogs are not ruminants, they produce very little methane from enteric fermentation. Nearly 90 percent of methane emissions from hog production in the United States is generated from manure while it is stored in liquid pits. Raising hogs on pasture or in deep-bedded housing would substantially reduce methane from hog production.
One “solution” that has been proposed to solve the problem of methane emissions from stored liquid manure is to build methane digesters next to CAFOs to produce and capture methane from the manure. While it is true that methane digesters will reduce methane emissions and produce methane fuel, it is important to also consider some of the limitations of methane digesters. One limitation is that methane digesters require animals to be in confinement so that their manure can be collected to be put into the digester. Animals that are out grazing in pastures spread their manure on the landscape where the manure decomposes and returns the nutrients to the soil, and does not emit much methane during decomposition. Also, when animals are housed on deep-bedded manure packs that are composted, methane emissions are much less than when manure goes into liquid manure pits. In other words, methane digesters not only require an industrial-style livestock production system in order to operate, but also, they are only needed for prevention of methane emissions in an industrial-style livestock production system.
Also, methane digesters are very expensive to build and almost invariably require large subsidies to make building them feasible. While many types of energy systems are currently being subsidized, it should be noted that subsidies for CAFO methane digesters also indirectly serve as subsidies for industrial livestock production systems. CAFOs already often receive subsidies to build liquid manure storage systems. Subsidizing CAFO methane digesters will amount to a double subsidy for the growth of the industrial livestock industry.
When considering the benefits of methane digesters to reduce methane emissions from stored CAFO manure, it is important to consider that there are alternative livestock production systems – that will be discussed in more detail below – that circumvent the need for methane digesters because they do not use methane-emitting liquid manure storage systems.
Considered solely from the energy-production point of view, methane from manure is not a high-producing source of energy. That is because when animals eat feed they utilize most of the energy of the feed for their own metabolism, which means that their manure is much lower in energy than the plant materials they ate. For methane generators connected to CAFOs to produce a lot of energy, additional biomass will need to be added to the manure.
[Stay tuned to this blog: I will be posting all the chapters from my book, 'A New Vision For Iowa Food And Agriculture' to this blog during the final weeks before the election on November 2nd. I look forward to any comments or questions you have.]
No Comments