Tillage, the act of cultivating soil in order to improve crop growing conditions, is closely associated with the emergence of settled societies. Critical for food security, incentivized by crop sector profitability, and implicated in environmental degradation events such as the US Dust Bowl era, the activity has long been matter for public policy. Our intent here is to highlight recent developments in supply and demand for the activity as well as public interest in overseeing whether and how soil cultivation occurs.
Tillage, a suite of practices adapted to the intended crop, a location’s soils and climate, and available technology, include any or all of cutting, hoeing, loosening, turning, and elevating the soil. Power sources have shifted from tool-assisted human hand labor through animal-drawn wooden plows to the combustible engine. Commencing with eighteenth century innovations, specialized tillage equipment has become a prominent purchased input category.
While the activity requires substantial energy, labor, and mechanical inputs, farmers derive many benefits. Tillage long before planting can bury preceding-crop residues as well as the insects and plant pathogens they harbor while allowing time for nature to further loosen the soil. Tillage activities just prior to planting can dry and warm the soil for early planting, provide a seedbed that fosters quick emergence, bury or mangle weeds that would otherwise compete for nutrients and sunshine, and make available soil-stored nutrients. Tillage between crop plants and rows can control weeds during the crop season.
Tillage operations generate external costs and benefits to users of land, air, and water resources (i.e., the public at large). The external effect that may resonate most clearly with the public is soil erosion arising from intensive or unwise cultivation. Fertile soil is mostly privately-owned, and optimal use for private goals may conceivably lead to severely degraded fertility. Eroded soil may end up in waterways, impeding service flows including transport, recreation, and potable water. Tillage can also limit land capacity to filter nutrient runoff into waterways and to mitigate floods by delaying or dispersing rainfall. Disturbed soil may give rise to dust storms or fine particulate matter that impair human health and can also foster carbon release, resulting in higher greenhouse gas (GHG) emissions.
In what follows, we discuss tillage as an input into agricultural production, pointing out policy issues along the way and emphasizing the nexus between tillage and seed technologies. We also discuss climate external effects. We conclude with some speculations on the future for tillage and related activities.
Tillage as an input
The role of tillage in a cropping system depends on many context-specific factors. Corn can be tillage-intensive, and especially so when planted after corn with residue that the farmer would like to bury. Available tillage system choices include: conventional tillage that combines traditional moldboard plowing to completely invert the top soil layer with disk or harrow to break clods and make a smooth seedbed; reduced or conservation tillage using a chisel plow, which opens the topsoil while leaving much surface residue intact; and no-till where the soil is not deeply disturbed but seed and any agrichemicals are placed into a slit while maintaining soil structure. Conservation practices, such as strip till or mulch till, mix the previous elements to preserve residue and soil organic matter while creating a suitable, weed-free seedbed. While more intensive tillage can positively affect that planting’s yield, the effect is generally not large and cost considerations or environmental and related constraints primarily determine the tillage choices.
Each tillage type involves specialized equipment that can be owned or custom-hired, where conventional tillage is more demanding in both labor and machinery requirements. Higher tillage intensity also requires comparatively more energy but less herbicide. Perry, Moschini, and Hennessy (2016) show that from 1998 to 2011 adoption of low-tillage-intensity systems for the US soybean crop tended to increase with fuel price but decrease with both herbicide price and soybean price. Larger operations preferred low intensity systems, likely due to scale economies and the propensity for larger operations to rent land distant from a tillage operation base.
Figure 1 shows GDP-deflated energy prices paid by US farmers and also farm-level corn price deflated energy prices during 1990–2021. When compared with either inflation or corn prices, energy prices have been higher in the period’s second half, an interval over which lower intensity tillage practices have generally expanded for both corn and soybeans crops.
Soil type and landscape topography can be important determinants of the choice made. Erosion-prone soils and/or a significantly sloped field may tilt the choice toward lower intensity tillage practices in order to keep soil in place. Eligibility considerations for certain government payments may also affect the choice. Weather too can be a determinant because fall and early spring conditions may not allow for field operations. In addition, as intensive tillage acts to dry out the soil, it will be favored when seeking earlier planting and disfavored when soil water availability is low.
Seeds and weeds
Since the 1950s, herbicides have largely superseded tillage as a primary weed control input in US agriculture. However, the suite of pre-emergence herbicides popular through the 1980s required application to tilled soil for effectiveness, and so tillage remained important. In the early 1990s, the advent of Roundup ReadyTM crops, including both corn and soybean, fundamentally changed the need for tillage. These crops can tolerate glyphosate, a broad-spectrum herbicide that kills any plant that is not genetically resistant, allowing regular application post-emergence and reducing need for tillage associated with pre-emergent formulas.
The rise of glyphosate resistant crops enabled farmers to control weeds without tillage. With weed control assured through chemicals alone, no-till farming has offered both profitability and environmental advantages. After an initial investment in no-till planting equipment, a farmer can cut costs by reducing the number of costly field passes (Perry, Moschini, and Hennessy 2016) while retaining crop residues, reducing soil erosion, and building organic matter. Glyphosate resistant crop uptake boosted low-tillage system adoption rates, while also increasing use of glyphosate and reducing use of other herbicides (Perry, Moschini, and Hennessy 2016; Van Deynze, Swinton, and Hennessy 2021).
However, tillage declined, only to rise again. As reliance on glyphosate and related herbicides grew, many weed species evolved genetic resistance to those herbicides. Since 2007, US farmers have been increasing their use of tillage (as well as non-glyphosate herbicides) to control resistant weeds. The rate at which farmers have returned to tillage is driven in part by the number of herbicide-resistant weed species they face (Van Deynze, Swinton, and Hennessy 2021). Figure 2 depicts percent adoption of conservation tillage (solid blue) and no-till (dashed blue) in US soybeans, as well as the growth in the maximum and mean number of glyphosate-resistant weeds identified at state level. Low-intensity tillage has declined from a peak adoption of ~70% as glyphosate has become less effective, falling to adoption levels not seen since the early days of Roundup Ready™ adoption in the early 2000s.
Tillage and climate
Agricultural soils have been identified as an important source of biogenic GHGs, such as carbon dioxide (CO2) and nitrous oxide (N2O), as some farming practices also add excess nitrogen to soils or stimulate soil organic matter decomposition, leading to more GHG emissions. Global human-induced N2O emissions have increased by 30% since 1980—nearly 90% of this increase is attributed to enhanced direct and indirect emissions from agricultural soils (Tian et al. 2020). The share of agricultural N2O emissions is similarly important in the United States, with corn and soybean production accounting for over 90% of its increase (Lu et al. 2021). Consequently, mitigating GHG emissions from the agricultural sector while maintaining productivity and economic viability has become a major challenge for policymakers. Tillage practice plays a critical role in affecting agricultural soil GHG emissions because it stirs crop residues into soils while altering soil temperature, moisture, and aeration conditions. Further complexities arise when tillage interacts with fertilization choices and other management practices.
Using data from a nationwide farmer survey to drive a process-based land ecosystem model, Yu et al. (2020) estimate that historical tillage practices in the US corn-soybean cropping system have led to a soil carbon loss of 10.3–15.2 MMT C yr-1 (MMT-million metric tons, or 1012 g) from 1998 to 2016, much larger than the carbon sequestered annually in Conservation Reserve Program land (~ 7.78 MMT C yr-1). Furthermore, reduced tillage intensity has lowered soil CO2 and N2O emissions at a rate of 5.5 MMT CO2 e yr-1 during 1998–2008 (Lu et al. 2022), an annual rate close to the gross GHG emissions from all Iowa residential fossil fuel consumption in 2020 (IDNR 2020). However, under growing pressure from weed resistance, intensified tillage during 2009–2016 increased soil CO2 and N2O emissions by 13.8 MMT CO2 e yr-1, more than offsetting the GHG mitigation benefit gained through reduced tillage one decade earlier. Figure 3 shows how GHG fluxes have responded to the tillage intensity change (first declined and then increased) from 1998 to 2016. Compared with these changes in soil emissions, the agricultural machinery GHG emissions associated with tillage intensity change is trivial. These findings alert us to the need for developing sustainable weed management practices while using reduced tillage or no-till to mitigate GHG emissions.
Driven by profit, technical innovations, and some regulation, tillage choices have changed dramatically since World War I, when tillage was horse-powered and chemical herbicides usage very limited. Looking forward, technological change as well as fuel and commodity prices are likely to retain their importance in determining whether and how soil is cultivated. With precision conservation, for example, tillage operations avoid highly erodible field sub-plots and sensor-driven robotic sprayers treat weeds individually to avoid the need for extensive spraying or tillage as a substitute. Climate policy that either subsidizes GHG uptake, through carbon credits, or taxes GHG emissions is also likely to become an important factor in tillage choice. To the extent that fertilization practices affect cultivation, evolving approaches to nutrient management may also affect tillage choices.
While the totality of scientific evidence points to a public policy stance that would, in general, encourage low tillage intensity as affected through publicly supported research and outreach activities, more detail in policy prescriptions are as yet challenging. The science underlying tillage and its environmental implications is complex because soil physics, chemistry, and biology are inherently dynamic, place-specific, and as yet poorly understood. Matters for consideration when thinking about policy approaches to tillage include how to collect field-level information for research and for management, how to avoid incentive structures that might deliver short-run benefits but do long-run harm, and the optimal choice between carrot (e.g., carbon credits) or stick (e.g., cropping plans to be eligible for program payments) given budget limitations.
Iowa Department of Natural Resources (IDNR) 2020. "2019 Iowa Statewide Greenhouse Gas Emissions Report." Iowa Department of Natural Resources.
Lu, C., Z. Yu, D. Hennessy, H. Feng, H. Tian, and D. Hui. 2022. "Emerging Weed Resistance Increases Tillage Intensity and Greenhouse Gas Emissions in the U.S. Corn-Soybean Cropping System." Nature Food.
Lu, C., Z. Yu, J. Zhang, P. Cao, H. Tian, and C. Nevison. 2021. "Century-long Changes and Drivers of Soil Nitrous Oxide (N2O) Emissions across the Contiguous United States." Global Change Biology 28(7): 2505.
Perry, E., G. Moschini, and D.A. Hennessy. 2016. "Testing for Complementarity: Glyphosate Tolerant Soybeans and Conservation Tillage." American Journal of Agricultural Economics 98(3): 765–784.
Tian, H., R. Xu, J.G. Canadell, R.L. Thompson, W. Winiwarter, P. Suntharalingam, E.A. Davidson et al. 2020. "A Comprehensive Quantification of Global Nitrous Oxide Sources and Sinks." Nature 586(7828): 248–256.
Van Deynze, B., S.M. Swinton, and D.A. Hennessy. 2022. "Are Glyphosate-Resistant Weeds a Threat to Conservation Agriculture? Evidence from Tillage Practices in Soybeans." American Journal of Agricultural Economics 104(2): 645–672.
Yu, Z., C. Lu, D.A. Hennessy, H. Feng, and H. Tian. 2020. "Impacts of Tillage Practices on Soil Carbon Stocks in the US Corn-Soybean Cropping System during 1998 to 2016." Environmental Research Letters 15(1): 014008.
Hennessy, D., C. Lu, S. Swinton, and B.V. Deynze. 2022. "The Tillage Input: Technical Change, Markets, and Policy." Agricultural Policy Review, Fall 2022. Center for Agricultural and Rural Development, Iowa State University. Available at www.card.iastate.edu/ag_policy_review/article/?a=149.