Carbon Footprint and the Frequency of Manure Removal

Carbon footprints have become important topics for sustainability. In pursuit of carbon neutrality, many technologies are promoted. Some require large capital investment or changes to a farm’s infrastructure. For example, adding aeration to existing manure storage. Or adding capturing biogas capture and cleaning to make renewable methane. But not all practices that reduce a farm’s greenhouse gas emissions must be so complicated. More frequent manure applications can also reduce greenhouse gas emissions.

 Manure methane production is proportional to methane potential and percent conversion during storage. The potential depends on the ration fed and completeness of digestion in the animal. The percent conversion is based on the storage length and temperature. Higher temperatures, or longer storage times, lead to greater conversion.

 In 2015 I wrote a paper exploring methane production from deep pit manure storage. I examined how temperature, volume, and properties influenced biological activity. Using this information, I put together a model to estimate how much methane a pig space produced per year. I use this model to explore how different application strategies impact methane emissions.

 Assume the base case will be a swine finishing operation that land applies manure once per year in the fall. Let the initial manure depth be 30.5 cm in November. Each pig has 0.9 m² of floor space and generates 4.9 L/headspace-day of manure at 9% solids. The methane production rate would be estimated at 0.0996 L CH₄/L-day. The annual methane emission is 19 kg CH₄/animal space-year.

 How would switching to twice a year manure application alter this estimate? Using the same assumption, adding manure removal in May would reduce the estimated methane emission to 10 kg CH₄/animal space-year. This is a 46% reduction.

 These calculations assume no change in nitrogen use efficiency. Making synthetic nitrogen is energy intensive. Thus, our ability to use manure as a fertilizer is important to our greenhouse gas footprint. Our work with spring manure has shown that it can be a method to increase nitrogen use efficiency.

 New technologies, like in-season manure application, may further reduce methane emissions. This model suggests that applying in spring, summer, and fall might result in a 70% reduction.

 So, what does this mean? California offers a carbon offset market at approximately $30 a ton per CO₂ equivalent. At this price, going from once a year to twice a year manure application would reduce emissions by 225 kg CO₂ per pig space. This is worth about $6 a pig space. Similarly, going to an in-season application would be worth about $10 per pig space.

 At 5,000 pig sites, around $50,000 a year in potential revenue. This revenue could help buy new equipment or pay labor to make such changes achievable.