While I really don't think gas from manure is the grand solution to climate change like this article seems to state. It doesn't actually make a difference when it comes to fertilizer. You don't burn the manure itself, you burn the gas produced from the manure. Afterwards you have this black sludge that is actually an even better fertilizer than before because the digestion in the methane production process breaks down the organic matter in the manure and makes especially the nitrogen contained within shift from being tied up in proteins and organic matter to being pure ammonia and nitrate. I have myself witnessed how a local pig farmer increases his wheat yields significantly once he installed a biogas digester and started fertilizing with the pig manure digestate instead of raw pig manure.

If we are going to have animals, we might as well put all the manure into a biogas digester before using it as fertilizer. You can even run tractors off the stuff if they are properly concerted. A few farmers that have biogas digesters have totally stopped buying diesel.

However for the US specifically I think step 1 is starting to actually use the manure at all. You guys have these poop lagoons over there where the manure just rots in a pool and never gets used on the field. In the EU that's illegal.

This industry is happy to let manure runoff into local ecosystems and lead to massive waterway pollution and toxic algae blooms. Biogas is no better with the tanks used for holding manure for biogas regularly leaking out and/or overflow

This is not an industry that is super concerned about fertilizer shortages and usage. Animal agriculture makes fertilizer demand increase and worsens the problem, even if manure was used 100% optimally

Thus, shifting from animal to plant sources of protein can substantially reduce fertilizer requirements, even with maximal use of animal manure

https://www.sciencedirect.com/science/article/abs/pii/S0921344922006528

For an example of how large the fertilizer difference can be: Producing 1kg protein from beans uses 12x less fertilizer than doing the same from beef

To produce 1 kg of protein from kidney beans required approximately eighteen times less land, ten times less water, nine times less fuel, twelve times less fertilizer and ten times less pesticide in comparison to producing 1 kg of protein from beef

https://pubmed.ncbi.nlm.nih.gov/25374332/

Actually, you don't burn the manure. You get energy from composting manure, which produces gas, which can be burned in turbines to produce electricity. There are some local coops that do this.

The leftovers is fertilizer.