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Use of Tillage and Conservation Cropping in the US

Tillage is used to control weeds, incorporate crop residue and fertilizers, and prepare land for planting. However, minimizing soil disturbance and maintaining soil cover are critical production practices needed for improving or maintaining soil health.

Soybeans in Mississippi are grown on a variety of soil series, from sandy loam to clay. Each of these soils has unique properties and/or conditions that may benefit from some sort of tillage. There is no set rule for if, when, and how much of a particular type of tillage should be used to grow soybeans or any other crop on any of these soils. However, the preponderance of evidence indicates that tillage per se is not required for optimum soybean production on any of these soils.

A Sept. 2018 report from USDA-ERS titled “Tillage Intensity and Convervation Cropping in the United States–EIB-197” by Claassen, Bowman, McFadden, Smith, and Wallander summarizes results from a national-level survey of farming operations that was conducted to identify/estimate tillage and conservation practices’ adoption and use in the production of corn (2005, 2010, and 2016), soybeans (2002, 2006, and 2012), cotton (2003, 2007, and 2015), and wheat (2004, 2009, 2017) in the US.

To ensure that the survey results are interpreted correctly, the below glossary of terms used in the report is shown.

•   Conservation Tillage: Tillage management practices that result in a STIR (see below) of less than or equal to 80, and do not use a moldboard plow. Conservation tillage practices can include mulch-till, no-till, and strip-till.

•   Conservation Crop Rotation: A sequence of crops in the same field grown for the purpose of supporting soil health. Specifically, it includes at least one high-residue crop (e.g., corn), at least one low-nitrogen crop (e.g., grass or legume), and attains a threshold level of >1.5 average annual residue (values range from 0.25-4.0). The emphasis is on rotations with high-residue crops than can improve soil health when used in conjunction with conservation tillage., particularly strip-till or no-till. Example values are 2.0 for corn and wheat, and 1.0 for soybeans and cotton. Thus, a corn-soybean biennial rotation is not considered a conservation crop rotation system since its average annual value is 1.5.

•   Conventional Tillage: A combination of tillage management practices that result in a STIR >80.

•   Crop Residue: Plant material from a crop, such as cornstalks or soybean stubble, that remains after harvest.

•   Cover Cropping: Planting a crop for seasonal cover, as when cereal rye is planted during the winter months between cash crops.

•   Mulch-Till: A type of conservation tillage where soil is tilled (for example with a chisel plow or disk) but soil disturbance is low (STIR <80).

•   No-Till: The practice of refraining from tilling the soil from harvest of the previous crop to harvest of the current crop.

•   Residue Harvest: The practice of harvesting the residue of a cash crop for a secondary use.

•   Soil Health: The continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans. Soil health benefits from conservation tillage may be fully realized only when practices such as no-till or strip-till that minimize soil disturbance are used consistently over time.

•   Soil Tillage Intensity Rating (STIR): A numerical index that represents the type and severity of soil disturbance caused by tillage operations. The STIR value incorporates the type, speed, depth, and degree of disturbance caused by tillage.... The STIR is the sum of STIR values of individual tillage operations.

•   Strip-Till: A tillage system used to minimize soil disturbance in row crops by tilling only in narrow strips where seeds are planted. In fields where strip-till is used, the STIR is typically <80.

Major findings from the survey that pertain to tillage and conservation systems used in US soybean production follow. Remember these are results through 2012–previous surveys of soybean practices were conducted in 2002 and 2006. Also, using STIR rather than methods based on soil residue cover (previously used to estimate conservation tillage adoption) focused the estimates on soil disturbance rather than a combination of soil disturbance and residue cover from a previous crop (see p. 15-17 of the above publication for a description of the differences in the two methods).

•   Conservation tillage (no-till and mulch-till) was used on about 70% of soybean acres in 2012.

•   The share of no-till in soybean conservation tillage systems was 56%, or 40% of the total soybean acres in 2012.

•   In 2002, 2006, and 2012, no-till soybean acres were 35%, 45%, and 40% of total US soybean acres. Thus, there was a slight decline in no-till soybean acres between 2006 and 2012.

•   Continuous tillage was used on about 45% of US soybean acres in 2012, whereas alternating no/strip-till and continuous no/strip-till acres comprised 34% and 21% of soybean acres, respectively.

•   Over 60% of soybean acreage in the South used conservation tillage systems. However, since the “South” in the survey included the Southern Seaboard, Mississippi Portal, and Eastern Uplands (see map on p. 7 of above publication), it is not possible to determine the breakdown for the lower Mississippi River Delta region.

•   Only 12% of US soybean acres were grown in a conservation crop rotation. ~23% of soybeans grown in alternating no-till/strip-till systems used conservation crop rotations, whereas <6% of continuous till and continuous no-till/strip-till soybean acres were devoted to conservation crop rotation systems.

•   Survey results suggest that residue-increasing practices were more common when soybeans were grown in continuous or alternating no-till/strip-till systems than when grown with continuous tillage.

To increase sustainability in US soybean cropping, conservation programs will increasingly focus on reducing tillage and adopting cropping practices to improve soil health. The above survey results can be used to determine the present use of these practices, as well as where their adoption can or should be increased.

Composed by Larry G. Heatherly, Nov. 2018,