How should one differentiate between “liquid/slurry without a natural crust” and “uncovered anaerobic lagoon”? Is it dependent on whether the solids are above or below 5%? Or is it dependent on some type of stabilization and treatment with water being deliberately added?

The 5% solids cut-off is a good rule of thumb for many situations. However, it is correct that the best distinction of an anaerobic lagoon is one where there is some treatment and deliberate water addition (ie. If we flush the manure out of the barns, if we add water to vacuum it, if we do SLS, etc.). In a case where the only water added is rainwater and maybe wash water (not flush water), the manure pit is likely best described as a slurry.

A farm that uses solid-liquid separation has multiple uses for the manure solids after in-vessel composting (windrow composting, solid off-farm, and bedding). Should all of that be taken into consideration or should it all be considered as going through the in-vessel composting?

We only have the option to specify 2 management strategies for the solid fraction after SLS, so there isn’t a perfect answer to match the farm’s situation. If we could enter more than 2 strategies, we could capture all 4 of the above and weight them by the amount of time (in-vessel composting, windrow composting, sold off-farm, and bedding) – but since we can only do 2, I would focus on those where the manure spends the most ‘time’ of the year, e.g. bedding and sold off farm.

How do you handle compost barns as manure management system?

We would recommend using ‘deep bedding with storage > 1 month’ as a proxy for a compost barn system. The FARM ES model does not list ‘compost barn’ as an option because there is currently insufficient research available on the GHG emissions from compost barn systems.

Why does daily spread have a lower footprint than an anaerobic lagoon?

The boundary of the ‘manure’ section of FARM ES is storage and handling. The emissions factor for manure management therefore does not cover emissions after the manure is applied. Because daily spread does not involve manure storage, it has a lower manure emissions factor. Emissions after land application (including daily spread) are incorporated in the feed production emissions category.

The feed production emissions category, which includes an estimate of emissions from land application, is based on LCA research using USDA and other datasets. It is not specific to the farm’s individual field practices.

For additional context, see the following straightforward explanation of the emissions tradeoffs between daily spread and other manure storage. There are many considerations that go into manure management decisions beyond GHGs (e.g. water quality, soil health, etc.). UW Extension has fact sheets on the topic of GHGs and manure.