Can Cover Crops & No-Till Reduce Dairy Greenhouse Gas Emissions?

The short answer: Cover crops and no-till may improve soil health by increasing soil structure and reducing soil erosion, but their impact on reducing greenhouse gas (GHG) emissions is highly variable and context dependent.

Break it down: To truly reduce dairy GHG emissions, we need to consider emissions from the whole farm system, including livestock, fertilizer, equipment, and more. How animal feed is produced is an important component of the system. So, the impact of cover crops and no-till on the soil has received a lot of attention.

That’s because agricultural practices that build soil carbon might offset GHGs by storing carbon dioxide (CO₂) from the atmosphere in the ground. We build soil carbon when the amount of carbon entering a system is greater than the amount leaving a system over a certain amount of time.

• In theory, cover crops may increase carbon entering the soil and no-till may reduce the amount of carbon leaving, building soil carbon.
  • Cover crops, defined here as crops planted to cover otherwise bare ground between crop rotations that are not grazed or harvested, increase photosynthesis.
    • As plants photosynthesize, they take up CO₂ from the atmosphere to build their plant structures.
    • As plants die and microbes decompose them, a portion of the carbon contained in their tissues is added to the soil.
  • No-till reduces soil disturbance, which can limit microbial breakdown of organic matter in the soil, and therefore, reduce the release of CO₂ back into the atmosphere.
• But in practice, the impact on soil carbon is not straightforward.
  • In a recent analysis, soil carbon appeared to accumulate in the upper 30 cm of soil in just under one third of U.S. cover crop comparisons, indicating high variability.
  • In a tillage analysis, no-till had more carbon than moldboard plowing in the upper 15 cm of soil. But no-till can also decrease carbon additions deeper in the soil, so carbon gains in the soil surface may be offset by carbon losses below.
• Results vary because of differences in soils, climate, and management practices. However, how we measure soil carbon, and to which depths, matters too.

Back it up: Relative changes, such as comparing soil carbon between no-till and full tillage, can show different results than tracking soil carbon changes over time. Depth also matters. Analyzing the top few centimeters of soil might be helpful for looking at nutrients, but may hide soil carbon changes deeper in the soil, which can change the soil carbon balance. To claim an overall increase in soil carbon, it’s important to measure changes even below 30 cm when the soil extends deeper, and if possible, across the whole soil profile.

• More work is needed to understand if, and when, cover crops and no-till can increase soil carbon overall. In one cover crop analysis, researchers found most studies sample only the soil surface. Similarly, many no-till studies don’t include deeper soil sampling. (Note: Studies of surface soils are still useful—they just aren’t best for determining if soil carbon increased to offset GHGs.)
• Deep samples suggest current best management practices may not build carbon. In a study of six long-term cropping system treatments, including conventionally and organically managed grain and forage systems (one with no-till and one with cover crops), none gained carbon after 20 years based on 90 cm samples. Some even lost carbon.
• Why carbon losses with depth? At the beginning, we discussed how building soil carbon requires more carbon coming in than leaving the system. While soil carbon changes are complex, our annual cropping systems don’t match the level of carbon inputs into the soil from prairies that built up much of our agricultural soils, even with cover crops and no-till.

The big picture: No-till and cover crops can be a step towards improving soil health and are an important addition in row crop systems. However, their role in offsetting GHGs through building soil carbon may be limited.

Photo by Randy Jackson.