Farms aren’t a major contributor to greenhouse gas emissions. However, even the less-than-2% contribution — 1.4% for dairies — can be further reduced by using a few strategies for manure handling.

With dairy cattle, managing the diet can help reduce emissions. In fact, 25% of dairy farms’ contributions to greenhouse gas emissions come from bovine belches. But manure — representing 52% of greenhouse gas emissions on dairies — is the bigger issue.

Pasture-based management and daily manure spreading can help reduce greenhouse gas per unit of farmland used, said Jason Oliver, Cornell Pro-Dairy senior Extension associate. But it also generates a greater greenhouse gas per unit of milk produced.

“It’s a great option for smaller herds, but less feasible for larger or consolidated herds” that typically have manure stored, he said at the recent North American Manure Expo.

“Manure’s effect is a human impact due to storage to protect water quality,” Oliver added. “You have the opportunity to put manure out when conditions are right. We can’t abandon storage. We can reduce how much we store and reduce solids content.”

The time of year also matters, especially when thinking of methane gas buildup. The warm summer months are when methane builds up, he said. But spring and fall are typically when farmers empty out storage.

“Let’s find ways to get manure out in the summer,” Oliver said.

One way to reduce methane is by killing or inhibiting methanogens by adding manure additives, such as those that cause acidification to reduce pH.

Another way is using a mechanical separator and coagulator. Separated solids at 30% dry matter can be reduced by extracting up to 60% “tea water.” “You also need to keep solids dry,” Oliver said.

Lauren Ray, an agricultural sustainability engineer with Cornell, said manure solid separators can be challenging to manage from an emissions standpoint.

Many farms recycle separated solids for bedding, reducing the amount of manure stored. But aerobic composts and digesters, even those that are well managed, can still produce greenhouse gas emissions and lose plenty of ammonia.

“Greenhouse gas can be reduced by managing aeration, or turning; moisture, through using a roof; and carbon-to-nitrogen ratios through using bulking materials,” Ray said, adding that anerobic digestion of manure can capture biogas that’s 60% methane.

“With digesters, we’ve seen 35% to 40% reduction of methane,” she said. “We’re degrading the most volatile solids. One tool is an optical gas camera to find leaks in the system.”

Leaks in digesters can occur unintentionally and even during normal operations. Venting is sometimes required, especially during a system disturbance.

Another treatment option is using a nitrogen manure enricher.

“It compresses ambient air and injects N₂ into manure to reduce ammonia loss and methane,” Ray said. “It works well for smaller farms. It uses electricity, so we’re studying financial viability.”

Ray has also looked at how using a cover on manure storage vessels can affect methane emissions. For example, a manure crust “may reduce methane by up to 40%, but only if a thick, dry crust is present and is less than 7% dry matter.”

“Other materials such as straw, biochar and synthetic crusts may be a solution,” she added.

Using a plastic cover can be helpful in that no water can enter the manure storage, and no gas will leak out. But the manure should be separated, Ray said.

Sergeant writes from central New York.