Cover Crops’ Climate Hype

Cover cropping, the practice of planting secondary crops during the off-season to improve soil health, has roots stretching back to antiquity. Ancient Romans used crops like lupines to enhance soil fertility without the use of manure, and figures like George Washington recommended “crops grown to replenish the soil.” The U.S. Department of Agriculture (USDA) later promoted cover cropping to combat the severe soil erosion witnessed during the Dust Bowl.

Today, enthusiasm for cover crops has surged as the focus has shifted towards their potential role in mitigating climate change. High-profile advocates, including celebrities like Woody Harrelson and Rosario Dawson, assert that cover crops can significantly sequester carbon in the soil and reduce greenhouse gases. Even President Biden endorsed the practice of “farmers planting cover crops, so they can reduce carbon dioxide in the air.”

Now there’s a growing raft of government and private sector climate mitigation programs set to provide farmers with billions of dollars in the coming years to adopt cover crops. However, much of this money could have a bigger climate impact if spent on other agricultural practices. Some efforts to promote cover crops could even raise emissions. Proponents of climate-smart agriculture need to take a harder look at when and where cover crops cut emissions, and shift support to more cost-effective and fail-safe climate strategies.

Key Findings:

• Cover crops have received the largest share, 21%, of funds that the Inflation Reduction Act (IRA) provided to two major USDA conservation programs in order to mitigate climate change. USDA is on-track to spend nearly $2.5 billion in IRA funds through these programs on cover crops by 2026.

• USDA estimates many farming practices, such as improved fertilizer management and modifying livestock feed, reduce greenhouse gas more cost-effectively than cover crops.

• Cover crops’ net climate benefit is highly variable and substantially lower than typically claimed when accounting for emissions from growing cover crop seeds, planting and terminating cover crops, indirect land-use change resulting from lower yields, and other factors.

• USDA should 1) identify when and where cover crops have the biggest climate benefit per dollar spent, 2) stop directing climate mitigation dollars to cover crop scenarios that do not lead to net mitigation, and 3) increase support for other agricultural climate mitigation strategies such as improved fertilizer management, improved manure management, and administering methane-reducing cattle feed additives.

Climate-Focused Cover Crop Efforts on the Rise

USDA has awarded over $150 million in federal funds earmarked for climate mitigation to farmers to plant cover crops since 2023 and is on track to award nearly $2.5 billion in climate-related funding in the next three years for cover crop adoption if current trends continue. USDA research programs also award more funding to climate mitigation projects involving cover crops than to any other climate mitigation strategy.

The Environmental Quality Incentives Program (EQIP) is one of the largest sources of support for farmers to adopt cover crops. Administered by USDA’s Natural Resource Conservation Service (NRCS), EQIP provides a cost-share to farmers, ranchers, feedlot operators, and other agricultural producers to implement a wide range of conservation practices that improve soil health, reduce erosion, and enhance water quality. Out of the 150+ farm practices eligible for EQIP funding, cover crops receive the most support. Since 2014, NRCS obligated about $914 million (8.6%) in standard EQIP funding for cover crops, according to USDA data.

The Conservation Stewardship Program (CSP) is another major initiative administered by NRCS that supports farmers in adopting cover crops. CSP offers financial and technical assistance to help farmers manage and enhance their existing conservation efforts. According to USDA, about $20.2 million in CSP funds (3.1%) were obligated in 2023 for farmers to implement various forms of cover cropping.

In addition, the Inflation Reduction Act (IRA) of 2022 expanded funding for EQIP and CSP, providing $8.45 billion and $3.25 billion, respectively, from Fiscal Year 2023 to 2026 to support farmers in adopting farm practices that USDA determines “improve soil carbon, reduce nitrogen losses, or reduce, capture, avoid, or sequester carbon dioxide, methane, or nitrous oxide emissions.” Among the more than 50 such practices USDA included in its latest list, cover crops stand apart. Thus far, NRCS has obligated more than four times as much EQIP IRA funding to cover crops than to any other practice, $156 million, about 28% of all EQIP IRA spending to-date. An additional $8 million in IRA funding for CSP has been directed to cover crops, about 4% of CSP’s IRA spending to-date.

So far, USDA has spent nearly 7% of EQIP and CSP IRA funding, of which about 21% went to cover cropping. If this trend continues, USDA will spend nearly $2.5 billion in IRA EQIP and CSP funds on cover crops by 2026.

This is in addition to the substantial funding that USDA awarded to projects involving cover crops through its Partnerships for Climate-Smart Commodities. This $3.1 billion initiative, which aims to expand markets for low-carbon commodities, funded projects across the country involving a wide range of commodities. At least one project, led by the National Fish and Wildlife Foundation, is focused exclusively on increasing cover crop adoption. With up to $95 million in federal funding, the organizations involved aim to support implementation on more than 1 million acres. Though most other funded projects focus on a wide variety of practices, over ¾ of the $2.4 billion that has been approved to-date is for projects that involve cover cropping.

In short, there is an unprecedented level of federal support for cover crop adoption.

Limits to Climate Benefits Cropping Up

The climate benefits of cover crops are generally overstated and overestimated. In many cases, paying farmers to plant cover crops has little to no climate benefit. Understanding the reasons why can help maximize the climate benefits of cover crop efforts and illustrates the need for greater support for other climate mitigation practices.

First, while adding cover crops to a farm increases soil carbon sequestration on average, it often has little to no impact on carbon stocks. A 2022 review by one of the leading cover crop researchers found that in 71% of cases studied in the U.S., adding cover crops did not increase soil carbon sequestration. It found that when cover crops did increase carbon sequestration, there was wide variation in the impact, ranging more than four-fold from 0.08 to 0.37 metric tons (MT) of carbon per acre per year. Likewise, scientists at The Nature Conservancy compiled 32 studies of cover crops in the U.S. corn belt, finding that cover crops increased soil carbon by a mere 0.34% on average. Nearly half of the comparisons found a reduction or no change in soil organic carbon with the addition of cover crops.

To be fair, these findings are not necessarily nationally representative. They are simply based on reviews of existing studies. However, reviews of studies in other regions find similar, though sometimes more modest, limitations. For instance, one recent meta-analysis examined the impact of cover crops in temperate climates around the world. It too found that while the average impact on soil carbon was positive, in a substantial share of cases (21%), there was no increase.

Second, legume cover crops, which fix nitrogen from the air, often increase nitrous oxide (N₂O) emissions, offsetting some of the climate gains from carbon sequestration. Two meta-analyses examining this issue indicate that legume cover crops substantially increase direct N₂O emissions. Incorporating the residues from the crops into the soil, as is generally done on farms that till their soil, likely increases N₂O emissions.

Though the effect on N₂O emissions can be large, it doesn’t necessarily offset all the climate benefits from carbon sequestration. Some studies find that legumes have smaller effects on N₂O emissions. And when farmers plant legumes, the added nitrogen can enable them to reduce fertilizer application and its associated emissions. Moreover, most U.S. farmers plant non-legume cover crops, somewhat obviating the issue. Still, even non-legumes can substantially increase N₂O emissions, emphasizing how any full assessment of cover crops’ climate impacts must consider their impact on N₂O.

Third, planting cover crops requires running a tractor through the field one more time than is typically done, burning fuel that emits CO₂. In many cases, farmers also must run machinery to kill the cover crop, which is often done by applying an herbicide, tilling the soil, or either cutting or crimping the plant with special equipment. A 2023 USDA report estimated that emissions from burning fuel typically offset 7% of the average climate benefits from non-legume cover crops in the Corn Belt, but in some cases can offset over 15% in Iowa, the largest corn-producing state.

Accounting for these three factors, planting cover crops may have, on average, a modest climate benefit per dollar spent. USDA’s 2023 report estimated that planting cover crops reduces emissions at a cost of at least $30–$75 per MT CO₂e. For comparison, the same report estimated that a variety of farm practices—including improved fertilizer management, biochar application, alternate wetting-and-drying of rice, manure digesters, and modifying livestock feed—could reduce small-to-moderate amounts of emissions for less than $20 per MT CO₂e.

Marginal Abatement Cost of Implementing Cover Crops in the United States

However, even these modest figures for cover crops’ cost-effectiveness overestimate their climate benefits. Several other factors limit cover crops’ mitigation potential.

1. Deep soil carbon loss: A small body of emerging evidence suggests that when planting cover crops does increase soil carbon in the upper soil, the gains can be offset partly or entirely by losses deeper in the soil. A 2019 study, for example, found that planting winter cover crops in a conventional corn system resulted in greater soil carbon in the upper 30 cm of the soil than leaving the soil bare. However, there was an overall loss in soil carbon down to 2 meters. This may be because the additional biomass from the cover crops results in more microbial activity in the deeper soil that converts soil organic matter into CO₂. Yet, this dynamic is poorly understood. Few studies have measured carbon levels deeper than 30 cm, let alone to 2 meters. More research is needed to confirm whether and when this deep soil carbon loss occurs, why it happens, and how to minimize it.

2. Yield reductions: Cover crops can have varying effects on the yields of cash crops like corn and soy. A 2024 global synthesis of the literature found that while legume cover crops can increase yields, at least for corn and barley, non-legume crops have no significant impact, and that repeated cover cropping for more than four years significantly reduces yields. The study found that in the U.S. Midwest planting non-legume cover crops like rye, which is most commonly done, has a median effect of reducing yields by 0.5%. Several other analyses have also found reductions in yields from current cover crop strategies in the Corn Belt. A 2022 study that used satellite imagery to detect cover crop usage and estimate yields concluded that cover crop adoption reduced yields by about 5.5% for corn and 3.5% for soy. The Nature Conservancy’s review of cover crop studies in the Corn Belt also found modest yield declines.

Decreases in yields or overall productivity can raise commodity prices, spurring producers elsewhere in the world to expand their cropland, resulting in deforestation, other land-use change, and related emissions. When productivity rises, the opposite typically occurs: land sparing. A 2023 paper in Nature Sustainability estimated that a 5% reduction in U.S. crop productivity, which might be expected with cover crop adoption, would expand global cropland area and land-use change enough to offset at least 70% of the soil carbon benefits from cover crops.

There are additional impacts from reducing productivity. Many crops are produced in the U.S. have a lower carbon footprint than in other countries. For example, the United Nations Food and Agriculture Organization (FAO) reports that producing corn, wheat, and other cereal crops in the U.S. results in less on-farm emissions (e.g. from fertilizer) per bushel than the global average. Therefore, when productivity declines in the U.S., it not only causes land-use change globally but also can increase emissions from farm inputs like fertilizer.

Recent U.S. Cover Crop Adoption Reduces Maize Yields

3. Permanence and reversibility: As with many regenerative farming practices, cover cropping does not necessarily add carbon to the soil permanently. Tilling the soil, which most farmers do at least once every few years, typically releases some of the carbon that cover crops add. Likewise, if farmers stop planting cover crops and leave their fields fallow, a significant portion of the additional carbon that was stored will eventually be lost through erosion or conversion by microbes into CO₂. This is particularly troubling since many farmers do not consistently plant cover crops and will occasionally fallow their fields. A USDA survey found that about half of corn grain and soybean acres only had cover crops planted 1 out of every 4 years. Data from the USDA Census of Agriculture paints a similar picture, indicating that farmers either stop or alternate adoption on at least 15% of acres where cover crops are grown.

Soil Organic Carbon Gradually Falls after Cover Crop Disadoption

4. Cover crop life cycle: Growing the cover crop seeds that farmers plant involves land use, fertilizer, fuel, and other resources that contribute to agriculture’s carbon footprint. One study estimated that growing enough seed for all corn producers to plant rye, the most common cover crop, would require nearly 1.8 million hectares, equivalent to 4.4% of U.S. corn acres. The impact of this additional land use is generally not considered in any reviews, meta-analyses, or modeling of the net climate impacts of cover crops. Neither are the emissions from fertilizer, fuel, and other inputs used to produce cover crop seed.

5. Additionality: Whatever benefits cover crops do have cannot solely be attributed to government programs or private sector efforts aimed at increasing their use. Some farmers would have adopted cover crops even without the initiatives. That is, any climate benefits are not always additional. Generally, levels of additionality depend on the upfront cost and long-run economic benefit of any given practice, in addition to how incentive programs are designed. A 2021 study estimated that among Iowa farmers who received government payments to adopt cover crops, only 54% of acres would have been planted with cover crops were it not for the payments. Studies in other areas have found higher rates, but agree that some payments ultimately go to farmers who would have adopted cover crops anyway. Perhaps most concerning, a 2022 study found that while EQIP payments are relatively effective at increasing cover crop adoption in the Corn Belt, CSP payments have historically reduced cover crop use. This counterintuitive outcome is likely due to how CSP encourages and pays for maintenance of existing practices, not just adoption of new practices, and therefore can spur farmers to focus on practices that are easiest to adopt.

Accounting for just some of the above factors can dramatically reduce the estimated benefits of cover crops. For example, the widely used COMET-Farm model estimates that non-legume cover crops in the Corn Belt reduce emissions by an average of 0.39 MT CO₂e/acre, accounting for soil carbon sequestration and changes in nitrous oxide emissions from the soil and fertilizer application. Typical emissions from diesel use for planting and killing cover crops offset about 7% of the benefit. Nitrous oxide emissions from growing the cover crop seed offsets another 4%, putting aside any other emissions associated with seed production. If planting cover crops reduces productivity by 5%—whether by reducing yields, increasing costs, or requiring more cropland for growing seed—that would reduce the climate benefit by nearly 47% more. If payments for cover crop adoption are not all additional, the benefits are further eroded.

To be sure, some often overlooked factors could increase cover crops’ mitigation potential. One of the most important is albedo. Plants reflect more sunlight than bare soil so planting cover crops can often reduce temperatures. One analysis estimated that this “albedo” effect, under typical conditions in the US, has a climate benefit about 10–38% as large as the effect of cover crops on soil carbon sequestration.

Another important climate benefit that’s often not estimated is the avoided emissions from producing fertilizer. Cover crops, especially legumes, can increase soil nitrogen levels, thereby enabling farmers to apply less fertilizer. This not only avoids the nitrous oxide emitted when applying the fertilizer but also the emissions from manufacturing fertilizer. Yet when farmers plant non-legume cover crops, some apply more nitrogen fertilizer to the cash crops afterward. In the latest National Cover Crop Survey, over half of respondents who grew winter cereal cover crops like rye before corn reported that they applied additional fertilizer when planting the crop. This can help avoid yield declines. When cereal cover crops decompose, they can tie up nitrogen in the soil that the cash crops need. Little research has examined how this response by farmers affects emissions, but greater nitrogen fertilizer application generally increases emissions.

In short, the climate benefit of cover crop adoption in the U.S. is likely significantly smaller than typically estimated. But there are no models, tools, or datasets that incorporate all the important factors to estimate cover crops’ climate impact in different situations. USDA and others are on track to spend billions of dollars to cut agriculture’s emissions through cover crops based on uncertain estimates that miss a big piece of the climate picture. This must change.

Recommendations

USDA, and others funding cover crops for climate mitigation, should do three things: 1) develop the data and tools needed to identify when and where cover crops have the biggest bang for their buck, 2) stop directing climate mitigation dollars to cover crop scenarios that do not lead to a net reduction in greenhouse gas emissions, and 3) diversify their support for other agricultural climate mitigation strategies.

Develop Data and Tools to Optimize Cover Crop Spending

The impact of cover crops on soil carbon and nitrous oxide emissions depends on a variety of factors like soil type and climate, and theoretically, can be modeled. In practice, however, farm-level estimates from leading models like the DayCent Model used by USDA’s COMET-FARM tool are highly uncertain. The farm-level uncertainties in DayCent are reportedly too large to determine if a particular farming practice will have a positive or negative climate impact. Governments and companies interested in supporting cover crops as part of their climate mitigation efforts should therefore not only use such models to identify where cover crops will have the most and least benefit but also support their development. This includes funding and conducting studies of how different types of cover cropping affect long-term yields, soil carbon throughout the soil profile, nitrous oxide emissions, and albedo, among other factors.

In addition, USDA should update its greenhouse gas calculators to account for how cover crops—and other farming practices—affect yields and thereby land-use change. USDA is piloting a program where farmers who adopt cover crops, no-till, and more efficient fertilizers can sell corn ethanol and soy biodiesel into the growing market for Sustainable Aviation Fuel. The program relies on the GREET model to calculate the carbon footprint of ethanol and biodiesel, estimating how selling crops into the ethanol market can contribute to indirect land-use change. But USDA and GREET do not treat cover crops consistently, ignoring whether planting them affects land-use change. Likewise, the COMET-FARM tool, which is used to estimate the climate benefit of many USDA-funded climate mitigation projects, does not consider how farming practices alter yields or land-use change. Incorporating these factors would likely result in a very different set of farming practices being identified as most climate-friendly: those that raise yields and productivity.

Stop Some Climate-Smart Funding of Cover Crops

USDA should also establish more narrow definitions of when cover cropping is not “climate-smart” and therefore should not qualify for EQIP and CSP IRA funding. For instance, USDA should not use funds earmarked for climate mitigation to support growing non-legume cover crops in places without sufficient water and nutrients where they would be expected to reduce yields or result in greater fertilizer application. In some instances, USDA should consider only supporting legume cover crops. Legumes are more likely to raise yields and though they can increase soil N₂O emissions, they generally add more nitrogen to the soil therefore allowing a reduction in fertilizer application. However, it is important to ensure that farmers using this approach actually reduce their nitrogen fertilizer application.

Diversify the Portfolio of Climate Strategies

Out of all the agricultural climate mitigation strategies Breakthrough assessed in our From Lab to Farm analysis, USDA already provides the greatest amount of research funding for projects related to cover crops. Research programs, at USDA and elsewhere, should prioritize other mitigation strategies. Many emerging farming practices and technologies have a large potential to cut emissions but are still in an early stage of development and need far more R&D funding than they currently receive. These include developing cattle breeds, forages, feed additives or drugs, and vaccines that cut methane emissions from grazing animals without harming animal health or productivity. To be sure, research is still needed to assess and optimize cover crops’ impact, to breed crops that require less land for producing seed, and other related areas. But these efforts should be narrowly focused on addressing the key limitations facing cover crops today, ensuring that other promising mitigation strategies also receive adequate funding.

Likewise, USDA should shift more of its IRA “climate-smart” funding to increase farmer adoption of other practices and technologies with well-proven climate benefits. Without more accurate data on cover crops’ climate and yield impacts, we can’t comprehensively assess how cost-effectively they cut emissions compared to other practices. But existing comparisons suggest USDA funds would be better spent elsewhere. USDA itself ranks 17 practices higher in terms of their climate benefits than cover crops. These include improving livestock feed management, which can reduce their nitrous oxide or methane emissions; optimizing the application of nitrogen fertilizer and other nutrients; and seeding rangeland with grasses and other plants that increase productivity and soil carbon storage. USDA and others such as the Environmental Defense Fund also estimate that several practices—including improved fertilizer management, alternate wetting-and-drying of rice, improved manure management, and administering methane-reducing cattle feed additives like 3-NOP—can cut tens of millions of tons of emissions at less than $30/tCO₂, less than USDA’s minimum estimate for cover crops. Yet these practices all receive significantly less IRA-funded support from USDA as well as far less USDA research funding.

There is little evidence that paying farmers to plant cover crops meaningfully fights climate change. With billions of taxpayer dollars in its hands for climate mitigation, USDA needs to better prioritize efforts that are cost-effective, transparent, and science-based. The original intention of planting cover crops was to improve soil fertility, reduce erosion, and manage water. There is a clear need to continue supporting cover crops for these purposes. But much of the climate mitigation funding going into them would be better spent elsewhere.

Acknowledgments: Many thanks to Mike Badzmierowski for valuable comments and suggestions. All opinions expressed and any errors are solely the author’s.