From Plows, Horses, and Harnesses to Precision Technologies in the North American Great Plains
Summary and Keywords
Since the discovery that food security could be improved by pushing seeds into the soil and later harvesting a desirable crop, agriculture and agronomy have gone through cycles of discovery, implementation, and innovation. Discoveries have produced predicted and unpredicted impacts on the production and consumption of locally produced foods. Changes in technology, such as the development of the self-cleaning steel plow in the 18th century, provided a critical tool needed to cultivate and seed annual crops in the Great Plains of North America. However, plowing the Great Plains would not have been possible without the domestication of plants and animals and the discovery of the yoke and harness. Associated with plowing the prairies were extensive soil nutrient mining, a rapid loss of soil carbon, and increased wind and water erosion. More recently, the development of genetically modified organisms (GMOs) and no-tillage planters has contributed to increased adoption of conservation tillage, which is less damaging to the soil. In the future, the ultimate impact of climate change on agronomic practices in the North American Great Plains is unknown. However, projected increasing temperatures and decreased rainfall in the southern Great Plains (SGP) will likely reduce agricultural productivity. Different results are likely in the northern Great Plains (NGP) where higher temperatures can lead to increased agricultural intensification, the conversion of grassland to cropland, increased wildlife fragmentation, and increased soil erosion. Precision farming, conservation, cover crops, and the creation of plants better designed to their local environment can help mitigate these effects. However, changing practices require that farmers and their advisers understand the limitations of the soils, plants, and environment, and their production systems. Failure to implement appropriate management practices can result in a rapid decline in soil productivity, diminished water quality, and reduced wildlife habitat.
Worldwide Agriculture Discovery and Change
Prior to the domestication of plants and animals, human societies obtained food by hunting, fishing, and gathering fruits, berries, nuts, grains, and vegetables. This food collection system resulted in periods of feast and famine. Over time, these societies learned that food security could be improved by planting seeds and harvesting the resulting crops. Food surpluses were then saved for use during times of shortage. It is likely that this process of food production led to plant domestication. For example, in the Neolithic Middle East, people harvested and stored grains that resisted shattering (Tanno & Willcox, 2006). A portion of this grain was consumed, and another portion was spread over the soil the following spring. The earliest grains were wild-growing cereals with heads that shattered (i.e., grain fell off the seedhead) at maturity (Tanno & Willcox, 2006). As grains were domesticated, selection was made for nonshattering heads (i.e., heads needed to be threshed to have grain separated from the ear), enabling greater grain collection.
This genetic selection of plants most likely started by chance (e.g., picking the largest seed, the most robust plants, etc.), but it was followed by “institutionalized” selection, a systematic method of choosing seed that would produce plants with the most desirable traits for the localized region. For example, the Moray archaeological site in Peru is thought to be the equivalent of an agricultural experiment station built by the South American Incas to test and create new cultivars that were acclimated to different climate conditions to improve food production throughout the many climatic zones of their empire (Mamani-Pati, Clay, & Smeltekop, 2014).
Each area of the world domesticated different plants. Asian, African, and European communities domesticated and grew wheat (Triticum spp.), barley (Hordeum vulgare), oats (Avena sativa), and lentils (Lens culinaris), whereas Central and South American communities domesticated and grew potatoes (Solanum tuberosum), corn (Zea mays), squash (Cucurbita moschata), and peanuts (Arachis hypogaea) (Mamani-Pati et al., 2014). In China, rice (Oryza sativa), soybeans (Glycine max), and pearl millet (Pennisetum glaucum) were domesticated.
Native American Agriculture in the North American Great Plains
Prior to the settlement of the northern Great Plains (NGP) by European settlers, Native American peoples hunted, gathered wild plants, and planted crops such as corn, beans (Phaseolus vulgaris), and squash. These crops were first domesticated in Central and South America over 8,000 years ago (Piperno, Ranere, Holst, Iriarte, & Dickau, 2009), and it is thought that they were slowly brought north through trade. Evidence suggests that about 800 years ago, NGP women started seeding corn, beans, and squash seeds into relatively small gardens (less than 0.24 hectares) (Gibbon & Ames, 1998; Schroeder, 1999; Munson-Scullin & Scullin, 2005; Drass, 2008). These gardens were generally located in sandy soils located near streams. This system was very effective, with squash providing mulch that helped control weeds, whereas legumes (beans) fixed atmospheric nitrogen, which in turn was eventually transferred to corn and squash (Smeltekop, Clay, & Clay, 2002). Corn yields then ranged from 627 to 1,250 kg/ha, whereas today’s corn yields can be more than 12,000 kg/ha.
Prior to European settlement, the primary agricultural tools were stone hatchets, pointed sticks, and wood and bone shovels and hoes, and Native Americans did not have plows or draft animals. Fire was an important component of the historical Native American management system; it combusted the aboveground residue remaining on the soil and helped control undesired plants, and caused short-term positive impacts on plant growth (Smart et al., 2013, 2016). However, if used too frequently, fire resulted in the loss of soil organic carbon and nitrogen. It is unclear how frequently intentional fires were used, or if most fires were from unintentional events (e.g., lightning strikes), but black carbon, which remains in soil after fires, indicates that fire was an important component of the prairie grasslands.
Great Plains Native American agriculture was changed by the introduction of the horse (Equus caballus) by the Spaniards. Horses allowed some tribes (such as the Lakota, originally inhabiting prairie areas from Minnesota to Montana and North Dakota to Nebraska) to become nearly full time bison hunters, whereas other tribes (such as the Mandan, originally inhabiting western North Dakota) specialized in planting and harvesting crops (Weltfish, 1965; Samson & Knopf, 1994; Goldewijk, 2001).
The Native American agricultural system is fundamentally different from the systems used by the settlers. Native Americans planted a diversity of corn, beans, and squash at a single location, whereas the settlers planted monocultures of annual crops which were then rotated from one location to another. Crop rotations are the foundation European agriculture. The rotations used by the settlers were based on the “Norfolk Rotation” (Barley-Clover/ryegrass-Wheat-Turnips) which nearly tripled England’s agriculture output in the 1700s. Since the 1930s, soils increasingly are fertilized and seeded with a single plant species. Plant diversity, which was mixed together in traditional agronomic sites, is now provided by following a two-, three-, or four-year rotation of crop species, and annual crops have become more resilient against extreme events. For example, during the 1930s drought, South Dakota corn yields ranged from 1,500 to 2,500 kg/ha, which increased to over 6,000 kg/ha during the drought of 2012 (Clay et al., 2014).
Development of Tools Used to Cultivate the Great Plains
A technology that changed North American Great Plains agriculture was the plow (Fig. 1), which was introduced by European settlers. This plow was not designed for human power, and it was built on multiple discoveries and technological changes from Asia, Africa, and Europe that occurred over a time period of 14,000 years. Technologies that were forerunners of the plow included the domestication of plants, sheep (Ovis aries), goats (Capra aegagru hircus), cattle (Bos taurus), and pigs (Sus scrofa domesticus or Sus domesticus), and the discovery of the yoke and early plows (Fig. 2) such as the ard.
Simple ards resemble hoes pulled by draft animals, and they basically are plows without the moldboard. An ard consists of a draft pole and yoke, a draft-beam, a stilt, and a share (Montgomery, 2007). The stilt is used for steering, and the share was pulled through the soil. The typical ard does not turn over the soil; rather, it creates a shallow furrow. This technology was best suited for thin coarse-textured soils. When the ard was attached to oxen by a yoke, the seeds were covered by soil. Compared with poking holes in the soil, the ard improved agricultural efficiency. However, not all ards were the same, and different designs were created for various situations. The use of the ard in thin upland soils had the potential to produce high erosion rates (Runnels, 1995; Kirch, 1997; Beach, Dunning, Luzzadder-Beach, Cook, & Lohse, 2006), which placed small communities at the tipping point of sustainability. However, not all communities used tillage implements in highly erodible environments. For example, the Inca of South America built terraces that reduced erosion (Mamani-Pati et al., 2014). In areas where domestic livestock were not available, such as Mesoamerica, plows and ards were never developed.
With time, the ard was modified into a plow that could cut and turn over the soil. The plow represented a significant technological improvement, and it was used to bury weed seeds and move subsurface nutrients to the soil surface. Many early plows consisted of a wooden moldboard and a metal coulter and share. The coulter sliced the soil, and the plowshare was the leading edge of the moldboard. About 2,500 years ago, plows with iron shares were developed in China (Greenberger, 2006).
The efficient use of the plow required an ability to connect the plow to the animal. Early yokes just fit over the animal’s head, which resulted in the front legs doing most of the heavy work (Fig. 2). Over time, the simple yoke was replaced with more complex systems that consisted of a throat-girth harness, breast-collar harness, and horse collar (Fig. 3). The breast-collar harness was developed in China between 481 and 221 bc, and it was introduced in Europe during the 8th century. The breastplate kept the surcingle (the strap that fastens around the animal’s girth) from slipping backward, which resulted in the animal pulling the load. The primary disadvantage of the breast-collar harness was that the pulling action came from the shoulders. When used on a horse, the pressure from this harness would limit the air supply by pinching the trachea. Therefore, for this harness, the ox was preferred over the horse.
The horse collar was a technology developed in China during the 5th century. The collar allowed the horse to push forward into the collar (rather than just a pulling motion), which more fully utilized the horse’s rear legs. In addition, it lowered the point of attachment between the plow and the horse, it did not push on the windpipe of the working animal, and it increased the rate and amount of land that could be cultivated. This advancement increased the use of horses in land cultivation. However, because these early technologies have long since disappeared, the energy efficiencies of these technologies are difficult to prove.
Along with the development of the horse collar was the modification of the plow. During plowing, a coulter makes a vertical cut, the share makes a horizontal cut, and the moldboard turns over the soil strip. The area between where the soil was lifted and where it was deposited is the furrow. In early moldboard plows, the cutting depth was adjusted by manually lifting the plow. This problem was overcome by installing wheels, which in turn allowed farmers to control the plowing depth and increase the plow’s weight and strength (White, 1962; Halberstadt & Halberstadt, 1997). The wheeled heavy plow was first used to cultivate soil in northern Europe between 900 and 1300 AD (White, 1962). The development of the plow is linked to increased food production, which in turn provided an opportunity for industrialization (White, 1962). Early plows turned the soil over in only one direction, and it relied on oxen or horses to pull it through the soil (Fig. 1). To simplify management and efficiency, field dimensions were dictated by the distance that horses or oxen could travel without rest. Early on, fields were plowed in rectangles, with the dimensions of 201 m (660 ft, or about a furlong) by 20 m (66 ft, or a chain). This area represented 1 acre (43,560 square feet).
The development of the steel plow in 1837 by John Deere and other blacksmiths of the period (Broehl, 1984) made plowing the Great Plains possible. This plow consisted of a beam, three-point hitch, height regulator, coulter, chisel, share, and moldboard (www.deere.com). The replacement of cast iron with steel and the concave contours also produced many advantages, including the fact that it was self-cleaning (Broehl, 1984), which improved efficiency and speed. Many of the cultural practices used by North American settlers arrived with the immigrants (USDA, 2004), and the practices initially adopted were directly related to their ethnic backgrounds (Shryock, 1939, Lemon, 1966). For example, German farmers preferred oxen over horses, and farmers from Scotland and Ireland combined crop production with livestock production. English settlers in the Plymouth Colony were taught by the Native Americans to use small fish to fertilize corn (Crawford, 1920).
Plowing the Great Plains
During the settlement of the Great Plains, native grassland sods were broken using animal-drawn plows and then seeded with annual crops such as wheat. The rapid conversion of the Great Plains grassland to cropland was facilitated by (a) high wheat prices, (b) favorable climatic conditions, and (b) the U.S. Homestead Act of 1862, which required that a settler claiming unsettled government land had to farm it for five years before title would be transferred. The land was cultivated on a biennial schedule, with one crop-growing year and a following year where the soil was allowed to rest and accumulate water. The year without crop cultivation was defined as a “fallow” year. Some shallow tillage was required in the fallow year to control weeds, typically in a mechanical way. During the 1920s and 1930s, the shallow tillage was accomplished with a one-way plow that pulverized the soil and increased the risk of erosion (Hansen & Libecap, 2004). The Dust Bowl of the 1930s resulted from a combination of factors, including (a) extremely low rainfall that was 15% to 25% below the normal rainfall of about 51 cm per year (20 inches per year); (b) deep plowing, which removed deep-rooted perennial grasses that provided soil structure; (d) the length of the drought, which persisted for almost 10 years; and (d) the small farm sizes and poor economic conditions, which resulted in lack of investment in erosion-control techniques.
Drought conditions during the 1930s were widespread, reducing crop yields in 27 states. The high winds, often exceeding 100 kilometers per hour, rapidly removed unprotected surface soil from plowed fields. The environmental impacts, such as loss of topsoil, dust-filled air, little plant growth, and the economic impacts of the Dust Bowl on the people living in the Great Plains, were staggering. In response to this problem, federal programs such as the Federal Emergency Relief Administration, the Federal Surplus Relief Corporation, the Civilian Conservation Corps, the Works Progress Administration, and the Drought Relief Service were created. In addition, the Soil Conservation Service, now called the Natural Resources Conservation Service (NRCS), was created by the U.S. Congress in 1935 after a dust cloud filled the sky over Washington, DC. The importance of sustainability was noted in Public Law 74-46, passed by the U.S. congress in 1935, which stated that “the wastage of soil and moisture resources on farm, grazing, and forest lands … is a menace to the national welfare” (USDA-NRCS, 2016).
Climate Change Impact on Salinization and Sodification in the NGP
Climate change in the northern Great Plains (NGP) is projected to increase both temperatures and rainfall in some areas, whereas other areas are expected to experience high temperatures with decreased rainfall (Schrag, 2011; Clay et al., 2014; Cook, Ault, & Smerdon, 2015; Reitsma et al., 2015). Climate change has the potential to affect productivity, soil carbon storage, cropping intensity, and land use (Burke et al., 1989; Miller, Amunderson, Burke, & Yanker, 2004; Bradford, Lauenroth, Burke, & Paruelo, 2006). These impacts are location dependent. The NGP is an area that encompasses about 72 million hectares (178 million acres) and includes portions of Wyoming, Montana, North Dakota, South Dakota, Nebraska, Alberta, and Saskatchewan. Today, a wide variety of crops are grown in the NGP, including corn, soybeans, wheat, canola (Brassica rapa), sunflowers (Helianthus annuus), and flax (Linum usitatissiumum). In 2015, 60% of the land in North Dakota, South Dakota, Montana, Wyoming, and Nebraska was grassland, alfalfa (Medicago sativa), hay (nonalfalfa) and shrubland (Han, Yang, Di, & Muller, 2012). The conversion of the region’s grassland to crops has the potential to reduce the amount of habitat for native species. Across the region, there were more shrublands in Wyoming (15.5 million hectares, or 38.3 million acres) than North Dakota (0.2 million hectares, or 0.5 million acres). The region’s grasslands are utilized by livestock and wildlife, such as the white-tailed deer (Odocoileus virginianus), pronghorn (Antilocarpra americana), black-tailed prairie dog (Cynomys ludovicianus), and grassland birds including the sharp-tailed grouse (Tympanuchus phasianellus) and several species of prairie chicken (T. cupido and T. pallidicinctus).
Land-use changes can also negatively affect soil sustainability. In many areas, land use, combined with climate change, has placed many soils at the sustainability tipping point. A major problem in the region is a growing salinity (calcium and magnesium salts) and sodicity (sodium salts). Plants will not grow in soil with high salt concentrations. Being in the middle of a continent with no ocean boundaries may make this an unexpected problem to solve. However, much of the NGP is underlaid with ancient marine sediments from vast, inland, shallow seas, which covered much of the landscape during the Mesozoic area. Periods of glaciation in the last 100,000 years of the Pleistocene mixed the salty marine sediments with material from the Canadian Shield and deposited these as glacial till in North and South Dakota. This geological activity results in areas of soils with high calcium, magnesium, and sodium salts deposited at random through the region. Rainfall has increased in this region in the past 25 years, causing the water table to rise. Native full-season perennial grass plants utilized more water than annual row crops planted in the region now. Therefore, the salinity and sodicity risks are increasing due to increased rainfall and grassland conversion to cropland, which reduces total transpiration. The net result is an increase in percolating water, which is turn raises the water table and provides an opportunity for water to transport salts already present in the glacial till from ancient marine sediments to the soil surface. Regionally and internationally, salt-affected soils are a serious and growing problem. High salt concentrations are estimated reduce productivity on over 10 million hectares (24.7 million acres) of land in the NGP and over 930 million hectares (2,298 million acre) worldwide (Szabolcs, 1989; Schrag, 2011; Cook et al., 2015).
Worldwide saline- and Na+-affected soils are separated into at least three groups: saline (high total salts), saline/sodic (high total salts and Na+), and sodic (high Na+). The classification of a salt-affected soil into one of these groups is based on the soil’s electrical conductivity (EC) and the amount of Na+ on the cation exchange sites [i.e., the exchangeable sodium percentage (ESP)] or within the soil’s solution phase [sodium adsorption ratio (SAR)]. Sodic soils traditionally have characterized as having SAR >13, whereas in the NGP, soils are at risk when SAR > 5 (He et al., 2015). The traditional approach to remediate a saline/sodic soil in the arid, irrigated regions of the southwestern United States is to apply water with a low electrical conductivity (EC), add a source of calcium (gypsum, lime), and allow for adequate drainage, which is most commonly done by installing tile drainage (Seelig, 2000; Carlson, Clay, Reistma, & Gelderman, 2013; Hopkins et al., 2012; Owen et al., 2014; Kharel et al., 2014; DeSutter et al., 2015; He et al., 2015). However, in semiarid, nonirrigated systems, such as those observed in the NGP and Australia, these remediation steps may actually worsen the problem (Northcote & Skene, 1972; McIntyre, 1979). The failure of traditional salt-affected best management practices (BMPs) in dryland systems is attributed to the failure to account for differences in the EC values of water leaching through the soil, differences in soil texture, and the failure to consider the water cycling across the topographic relief (Sumner, Rengasamy, & Naidu, 1998).
Climate Change Impact on Agriculture in the SGP
The southern Great Plains (SGP) includes portions of the Central Great Plains, High Plains, and Southwestern Tablelands ecoregions in Nebraska, Colorado, Kansas, New Mexico, Oklahoma, and Texas. In the SGP, temperatures are projected to increase and rainfall in many areas is projected to decrease (Cook et al., 2015). This area encompasses over 68.8 million hectares (170 million acres) and contains irrigation and crop production, grasslands, and shrublands. In 2015, approximately 58.8% of Nebraska, Kansas, Colorado, Oklahoma, and New Mexico was in grassland, alfalfa, hay (nonalfalfa), and shrubland (Han et al., 2012). Across this region, shrublands are greater in area in New Mexico (17.4 million hectares, or 43 million acres) than Nebraska (0.006 million hectares, or 0.015 million acres). Livestock production, as well as irrigated and dryland agricultural systems producing corn, soybean, wheat, cotton (Gossypium hirstutum), and sorghum (Sorghum bicolor), are important.
In the SGP, irrigation of sandier soil types is common. For example, in 2013, about 0.96 hectares (2.3 million acres), 1.15 hectares (2.8 million acres), 3.36 hectares (8.3 million acres), 0.172 hectares (0.43 million acres), and 0.281 hectares (0.69 million acres) were irrigated in Colorado, Kansas, Nebraska, Oklahoma, and New Mexico, respectively (USDA Census of Agriculture, 2014). The sources of irrigation water are the Colorado River and the High Plains (Ogallala) Aquifer. Critical issues in the region include, but are not limited to, the increasing temperatures, wildlife fragmentation, and decreasing depth of the High Plains Aquifer.
In the future, higher temperatures and lower rainfall combined with shrinking aquifers will likely limit food production (Shafer et al., 2014). The aquifer supporting SGP-irrigated agriculture is one of the most important in the world; currently, it provides irrigation water to approximately 30% of the irrigated lands in the United States (McGuire, 2013). Steward and Allen (2016) reported that the High Plains Aquifer depletion rate in more severe in the southern than the central Great Plains; for this reason, water conservation will become critical.
Agriculture in the North American Great Plains
North American agricultural products are shipped around the world. Associated with these market drivers is increasing agricultural intensification. For example, in South Dakota, approximately 0.728 million hectares (1.8 million acres) of land was converted from grassland to croplands between 2006 and 2012 (Reitsma et al., 2015, 2016). People are concerned about grassland to cropland conversion because it affects wildlife habitat fragmentation, water quality, and greenhouse gas emissions. Reitsma et al. (2015) and Reitsma, Clay, Clay, Dunn, and Reese (2016) were not able to identify a single cause of land-use changes; however, they did identify several important factors, including (a) a desire to increase financial returns, (b) increased absentee land ownership, (c) an aging workforce, (d) increasing temperatures and rainfall, (e) technology improvements, and (f) governmental policies.
Reitsma et al. (2015, 2016) point out that land-use change was not uniform; it occurred at locations where there was an economic opportunity. It is important to note that land-use changes that are not sustainable can result in a gradual or rapid decline in the soil resource. Worldwide, changes to unsustainable practices are a serious problem. For example, in Ethiopia, soil losses of 290 Mg (ha × year)−1 [260,000 lbs (acre×year)−1] were reported following the conversion of grassland to cropland (Fowler & Rockstram, 2001). Similar results have been reported for Turkey (Evrendilek, Celik, & Kilic, 2004).
The adoption of past technology improvements often has produced unexpected and unintended impacts on soils’ long-term productivity and cropping systems. For example, in the Great Plains of the United States, the development of the one-way plow improved weed control but contributed to the Dust Bowl during the 1930s, which is considered to be the greatest human-induced environmental disaster on record. Similarly, on another continent (Australia) at another time (the 1990s), the removal of shrubs allowed fence-row–to–fence-row wheat production that contributed to a rising water table and the salinization and sodification of footslope soils.
In many areas, the cultivation of genetically modified (GM) plants has been linked to the adoption of conservation tillage (Derpsch, Friedrich, Kassam, & Li, 2010; Clay et al., 2014; Lee, Clay, & Clay, 2014). For example, Argentina increased the no-tillage area from 300,000 hectares (741,000 acres) to over 9 million hectares (22.2 million acres) between 1990 and 2000 (Trigo, Chudnovsky, & López, 2003), which corresponded to the release of genetically modified organisms (GMOs). Similarly, Givens et al. (2009) reported that the use of glyphosate-tolerant crops in Illinois, Indiana, Iowa, Mississippi, Nebraska, and North Carolina was linked to the adoption of reduced tillage systems. Roberts, English, Gao, and Larson (2006) reported that the probability of a cotton producer to adopt conservation tillage increased if GM cultivars were planted. In Europe, where GM crop utilization was low, no-tillage adoption was also low (Brookes & Barfoot, 2010, 2011; Derpsch et al., 2010).
Benefits of the combined adoption of conservation tillage and GM crops include decreased erosion and improved soil resilience. McCarthy, Phost, and Currence (1993) reported that conservation tillage reduced erosion by 50%. Similar results were reported in Argentina, where Penna and Lema (2003) reported that converting from tillage to no-tillage reduced soil losses by up to 75%. The adoption of conservation tillage also reduces carbon dioxide emissions from Great Plains soil (Clay et al., 2012, 2015). Brookes and Barfoot (2011) found similar results, reporting that from 1996 to 2009, the linked adoption of GM traits and no-tillage/reduced tillage systems may be responsible for 115 billion kilograms (254 billion pounds) of CO2 being sequestered in soil. Due to higher temperatures, these gains may not be observed in southern environments.
The adoption of the no-tillage approach slows the rate that soil organic C (SOC) is mineralized and increases C sequestration in northern soil (Alvarez & Steinbach, 2009; Ismail, Blevins, & Frye, 1994; Karathanasis & Wells, 1989; Reeves, 1997; Rhoton, 2000; Clay et al., 2015). Clay et al. (2012) reported that increasing corn yields combined with conservation tillage adoption in South Dakota resulted in a 24% increase in SOC, which in turn increased soil resilience (Clay et al., 2015). However, these gains will not be seen in all climates. A negative relationship between temperature and SOC storage reported by Clay et al. (2010) suggests that increasing carbon sequestration in tropical environments will be much more difficult. For example, Causarano, Franzluebbers, Reeves, and Shaw (2006) and Sisti et al. (2004) reported that adopting no-tillage had a minimal impact on SOC in southeastern United States and Brazil. However, increasing agricultural intensification may also lead to wildlife fragmentation, reduced biodiversity, stress to pollinator populations, and increased soil erosion and salinization. Minimizing the long-term risks of land-use change requires careful assessment of the potential impacts on a growing human population and long-term sustainability.
Farmers are willing to adopt a new technology if it increases profits, fits within their production system, is easy to implement, and reduces labor requirements (Asmus, Clay, & Ren, 2013). GMOs can have direct and indirect impacts on most of these factors (Asmus et al., 2013; Chang, Clay, Hansen, Clay, & Schumacher, 2014). For example, prior to the release of glyphosate-tolerant crops, many different types of herbicides were routinely applied to control weeds (Sims & Guethie, 1992). After the release of glyphosate [N-(phosphonomethyl)glycine]–tolerant crops, multiple applications of herbicides were replaced with one or two applications of glyphosate. This change reduced the concentrations of herbicides found in groundwater, changed the types of weeds found in production fields, and increased the use of no-till planting. However, with the increase of herbicide-resistant weeds, the outlook for increased herbicide use with multiple modes of action and greater tillage intensity is changing the agricultural landscape once again.
A secondary consequence of the broad-use of GMO plants has been the development of herbicide-resistant weeds. In a general sense, resistance increases with frequency of use (Duke & Powles, 2008; Shaw et al., 2009). Herbicide resistance was first reported for triazine herbicides in the 1960s. Currently, 470 weed biotypes contain some type of resistance (Heap, 2016). Glyphosate resistance was first reported in Australia in 1998 (Powles, Lorraine-Colwill, Dellow, & Preston, 1998). Resistance has increased rapidly, and in 2016, there were 16 confirmed weed biotypes in the United States that were resistant to glyphosate (Heap, 2016). Best management practices that include the use of multiple control tactics, the application of the approved chemical rates, cleaning equipment, rotating mechanisms of action, and scouting fields can be used to reduce the spread and control of resistant weeds (Asmus et al., 2013; Moss, 2007). In summary, the adoption of reduced tillage system and GMO crops reduced erosion and improved resiliency and yields (Clay et al., 2012).
Potential Impacts of Climate and Land-Use Changes on Wildlife
Climate change will place many semiarid grasslands in the North American Great Plains, sub-Saharan Africa, Australia, and large portions of eastern and southern Africa, India, and Asia at the tipping points of sustainability. In the past, decreases in productivity combined with population growth have contributed to accelerated land degradation. For example, due to population increases, the rest time between fires in tropical slash-and-burn agriculture has decreased (Lininger, 2011). Similar changes have occurred in eastern Africa, where natural vegetation has been converted to farmlands, grazing lands, and urban centers (Maitima et al., 2009). Maitima et al. (2009) reported that associated with the land-use changes has been a decrease in indigenous plants and animals, decreased soil moisture, and increased erosion.
In the semiarid grasslands in North America, the same issues exist. The region’s grasslands provide habitats for birds, insects, livestock, and other animals. Concerns about grassland loss in the Great Plains focus on ecosystem fragmentation, reduction in biodiversity, increased pollution, climate alterations, the introduction of invasive species, and extinction of native species. In the 19th and 20th centuries in North America, the conversion of forests and prairies to farmland was thought to benefit wildlife by opening up the land and providing an additional food source (Brady, 2007). However, as farming progressed and became less diverse, the available habitat became less diverse. Generally, the conversion of grassland to croplands reduces available wildlife habitat (Miranowski & Bender, 1982).
Selected endangered species in the North American Great Plains include the burrowing owl (Athene cunicularia), greater prairie chicken loggerhead strike (Lanius ludovicianus), mountain plover (Charadrius montanus), piping plover (Charadrius melodus), sage thrasher (Oreoscoptes montanus), pallid sturgeon (Scaphirhynchus albus), burying beetle (Nicrophorus), black-footed ferret (Mustela nigripes), swift fox (Vulpes velox), peregrine falcon (Falco peregrinus), and least tern (Sternula antillarum). As agriculture intensifies, the number of isolated ecological niches is likely to increase.
Future of Agriculture in the North American Great Plains
The long-term sustainability of Great Plains agricultural activities must be conserved and preserved, as the region is truly the breadbasket of the world. One producer feeds about 155 people. With expanding populations, agriculture must continue to become even more efficient. Agriculture in the Great Plains is affected by complex interactions between land-use changes, extreme climatic conditions, government policies, and the farmer’s willingness to adopt sustainable practices. Depending on local conditions, climate change in the region may positively affect some areas and negatively affect other areas. Converting the region’s grasslands to cropland often leads to wildlife fragmentation. The need to maintain the region’s grasslands is confounded by a shrinking land base for producing food. Nationally, agricultural lands are converted to a variety of uses, including roads, parking lots, and residential areas. For example, according to the USDA NASS, 30 million hectares (74 million acres) of U.S. farmland were taken out of production between 1990 and 2012. Reduction in farmland intensifies the demands on all current farmland just to replace products from those areas, let alone boost outputs to nourish an increasing global population.
Providing food for a rapidly growing world population can be accomplished only by increasing the amount of cultivated land, reducing food waste, increasing genetic potential, and improving management. Due to the physical and biochemical constraints associated with increasing the amount of cultivated land or increasing the genetic yield potential, it is unlikely that either of these approaches can double food production by 2050 (Clay et al., 2014). For example, between 1950 and 2010, U.S. corn yields increased at a rate of approximately 93.5 kg (ha x year)−1 [1.5 bu (acre x year) −1]. These yield gains were attributed to many factors, including improved genetics, increased use of fertilizers, and better soil management.
When the yield gain rate is extrapolated from 2010 to 2050, this gain would produce a corn grain yield enhancement of 3.8 Mg ha−1 (60 bu/a) or further increase corn yields by 43%. This predicted yield gain falls far short of projected needs. Clearly, to protect the region’s soil resources, farmers and ranchers need to adopt precision farming and conservation techniques that will minimize the adverse impact of agriculture on the environment. Precision farming has been defined as the use of state-of-the-art technologies and decision support systems that optimize inputs while reducing waste. Precision conservation is the targeting of conservation practices to portions of the landscape, with a disproportionate impact on long-term sustainability and environmental quality.
Across the Great Plains, the following developments have occurred:
• Producers are increasing their reliance on information age technologies, and agricultural industries are making strategic investments to take advantage of new economic opportunities.
• Many farmers are purchasing precision technologies but are challenged with fully implementing variable rate treatments.
A 2014 survey of 86 South Dakota farmers showed that 58% were using differential corrected global positioning system–controlled autosteer. Erickson and Widmar (2015) had similar results in a survey of 171 agricultural retailers located in 35 states. The autosteer technology is used to reduce overlaps and skips, reduce the inward drift of implements, increases the amount of work that can be done in a day, and improves input placement (Shockley, Dillon, & Stombaugh, 2011). Even though autosteer could be used to apply variable rate treatments, this technology generally is not being used for this purpose. We believe that with time, precision technologies can help remediate problems and reduce production costs.
Climatic changes, financial concerns, public policies, and an aging workforce are concerns interacting to influence grassland and cropland land use in the Great Plains. In South Dakota, higher spring rainfall and temperatures have reduced the risk of growing crops. Associated with the reduced risk of crop production was over 700,000 hectares (1.8 million acres) of grasslands that were converted to cropland in South Dakota between 2006 and 2012. Environmental concerns of land-use change include the facts that wildlife habitat niches have become more isolated and fragmented, and populations have been reduced to unsustainable numbers. The adoption of precision technologies that allow farmers to match cultural activities with agronomic requirements may help mitigate these impacts. Precision farming is built on four core technologies: molecular biology, rapid communication, computer mineralization, and the development of improved climate prediction. Precision farming attempts to integrate these technologies into recommendations that will reduce inputs while maximizing outputs. For example, the use of molecular biology techniques allows scientists to quantify gene expression in response to abiotic and biotic stress. This work showed that in response to water stress, corn downregulated its nutrient capacity uptake and its ability to manage insect and disease problems (Hansen et al., 2013). Such information changes the way that the problem is visualized.
The adoption of sustainable systems requires that farmers and their advisers understand the limitations of the soils, the environment, and production systems. The same system will succeed in all environments. Understanding the limitations requires targeted research and the development of site-specific models. It is likely, that the use of GMO crops will continue in the future. Genetic modification provides the opportunity to better match plants to the environment while simultaneously improving the plants’ ability to withstand adverse conditions. The use of GMO crops has reduced the use of soil-based herbicides and increased the use of postemergence herbicides. However, it also has increased the risk of developing pest resistance. Associated with the use of GMO crops are increased no-tillage adoption, increasing amounts of organic C contained in soils, improved soil resilience, and an enhanced ability of plants to withstand adverse environmental conditions. The combined adoption of conservation tillage and GMO technologies has reduced the agricultural impact on the environment, and increased soil and water quality. Soil quality improvements are associated with reduced tillage and increased C sequestration. To minimize the development of pest resistance, the control mechanism must be rotated. Failure to implement appropriate management practices in the Great Plains can result in rapid declines in soil productivity and diminished water quality.
Alvarez, R., & Steinbach, H. S. (2009). A review of the effects of tillage systems on some soil physical properties, water content, nitrate availability and crops yield in the Argentine Pampas. Soil Tillage Research, 104, 1–15.Find this resource:
Asmus, A., Clay, S. A., & Ren, C. (2013). Summary of certified crop advisors’ response to a weed resistance survey. Agronomy Journal, 105, 1160–1166.Find this resource:
Beach, T., Dunning, N., Luzzadder-Beach, S., Cook, D. E., & Lohse, J. (2006). Impacts of the ancient Maya on soils and soil erosion in the central Maya lowlands. Catena, 65, 166–178.Find this resource:
Bich, A. D., Reese, C. L., Kennedy, A. C.Clay, D. E., & Clay. S. A. (2014). Corn yield is not reduced by in-season cover crop seeded after the weed free period. Crop Management13, 1–8.Find this resource:
Bradford, J. B., Lauenroth, W. K., Burke, I. C., & Paruelo, J. M. (2006). The influence of climate, soils, weather, and land use on primary production and biomass seasonality in the US Great Plains. Ecosystems 9, 934–950.Find this resource:
Brady, S. J. (2007). Effects of Cropland Conservation Practices on Fish and Wildlife Habitat. Fish and Wildlife Response to Farm Bill Conservation Practices, 07–1, 9–23. Retrieved from https://www.fsa.usda.gov/Internet/FSA_File/chap_1.pdf.Find this resource:
Broehl, W. G., Jr. (1984). John Deere’s Company: A history of Deere & Company and its Times. New York: Doubleday.Find this resource:
Brookes, G., & Barfoot, P. (2010). Global impact of biotech crops: Environmental effects, 1996–2008. AgBioForum, 13, 76–94.Find this resource:
Brookes, G., & Barfoot, P. (2011). GM crops: Global socio-economic and environmental impacts 1996–2009. April 14. Available at http://www.pgeconomics.co.uk.
Burke, I. C., Yonker, C. M., Pardon, W. J., Cole, C. V., Flach, K., & Schimel, D. S. (1989). Texture, climate, and cultivation effects on soil organic matter contents in U.S. grassland soils. Soil Science Society of America Journal,53, 800–805.Find this resource:
Carlson, C. G., Clay, D. E., Reistma, K., & Gelderman, R. (2013). Soybeans, salinity, and sodicity. In D. E. Clay, C. G. Carlson, S. A. Clay, L. Wagner, D. Deneke, & C. Hay (Eds.), iGrow Soybean: Best Management Practices (pp. 413–422). Brookings: South Dakota State University.Find this resource:
Causarano, H. J., Franzluebbers, A. J., Reeves, D. W., & Shaw, J. N. (2006). Soil organic carbon sequestration in cotton production systems of southeastern United States: A review. Journal of Environmental Quality, 35, 1374–1383.Find this resource:
Chang, J., Clay, D. E., Hansen, S. A., Clay, S. A., & Schumacher, T. E. (2014). Water stress impacts on transgenic corn in the northern Great Plains. Agronomy Journal, 106, 125–130.Find this resource:
Clay, D. E., Clay, S. A, Reitsma, K. D., Dunn, B. H., Smart, A. J., Carlson, C. G., Horvath, D., & Stone, J. J. (2014). Does the conversion of grasslands to row crop production in semi-arid areas threaten global food security?Global Food Security3, 22–30.Find this resource:
Clay, D. E., Carlson, C. G., Schumacher, T. E., Owens, V., & Mamani-Pati, F. (2010). Biomass estimation approach impacts on calculated SOC maintenance requirements and associated mineralization rate constants. Journal of Environmental Quality, 39, 784–790.Find this resource:
Clay, D. E., Chang, J., Clay, S. A., Stone, J., Gelderman, R. H., Carlson, C. G., Reitsma, K., Jones, M., Janssen, L., & Schumacher, T. (2012). Corn yields and no-tillage affects carbon sequestration and carbon footprints. Agronomy Journal, 104, 763–770.Find this resource:
Clay, D. E., Reicks, G., Carlson, C. G., Moriles-Miller, J., Stone, J. J., & Clay, S. A. (2015). Tillage and corn residue harvesting impacts surface and subsurface carbon sequestration. Journal of Environmental Quality, 44, 803–809.Find this resource:
Cook, B. I., Ault, T. R., & Smerdon, J. E. (2015). Unprecedented 21st century drought risks in the American Southwest and Central Plains. Science Advances. 1, e1400082.Find this resource:
Crawford, M. C. (1920), In the days of the Pilgrim fathers. Boston: Little, Brown, Norwood.Find this resource:
Derpsch, R., Friedrich, T., Kassam, A., & Li, H. (2010). Current status of adoption of no-tillage farming in the world and some of its main benefits. International Journal of Agricultural and Biological Engineering, 3(1), 1–25.Find this resource:
DeSutter, T., Franzen, D., He, Y., Wick, A., Lee, J., Deutsch, B., & Clay, D. E. (2015). Relating sodium adsorption ratio to its utility in the Northern Great Plains. Soil Science Society of America Journal, 79, 1261–1264.Find this resource:
Drass, R. R. (2008). Corn, beans, and bison: Cultivated plants and changing economies of the late prehistoric villagers on the plains of Oklahoma and Northwest Texas. Plains Anthropologist, Advances in Ethnobotany, 53, 7–31.Find this resource:
Duke, S. O., & Powles, S. B. (2008). Glyphosate-resistant weeds and crops. Pesticide Management Science, 64, 317–496.Find this resource:
Erickson, B., & Widmar, D. (2015). Precision agricultural services: Dealership survey results. The Center for Food and Agricultural Business, Dept. of Agricultural Economics and the Department of Agronomy, Purdue University, West Lafayette, IN.Find this resource:
Evrendilek, F., Celik, I., & Kilic, S. (2004). Changes in soil organic carbon and other physical soil properties along adjacent Mediterranean forest, grassland, and crop ecosystems in Turkey. Journal of the Arid Environment, 59, 743–752.Find this resource:
Fowler, R., & Rockstram, J. (2001). Conservation tillage for sustainable agriculture: An agrarian revolution gathers momentum in Africa. Soil Tillage Research, 61, 93–108.Find this resource:
Gibbon, G. E., & Ames, K. M. (1998). Archaeology of prehistoric Native America: An encyclopedia. New York: Taylor & Francis. See p. 24.Find this resource:
Givens, W. A., Shaw. D. R., Kruger, G. R., Johnson, W. G., Weller, S. C., Young, B. G., …, & Jordan, D. (2009). Survey of tillage practices in glyphosate resistant crops. Weed Technology, 23, 150–155.Find this resource:
Goldewijk, K. K. (2001). Estimating global land use change over the past 300 years: The HYDE Database. Global Biogeochemical Cycles, 15, 417–433.Find this resource:
Greenberger, R. (2006). Agriculture and food production. In The technology of ancient China (pp. 6–13). New York: Rosen Publishing Group.Find this resource:
Halberstadt, A., & Halberstadt, H. (1997). Plows & planting implements. Osceola, WI: Motorbooks International Publishers & Wholesalers.Find this resource:
Han, W., Yang, Z., Di, L., & Muller, R. (2012). CropScape: A webserver based application for exploring and disseminating US continuous geospatial cropland data products for decision support. Computers and Electronics in Agriculture, 84, 111–123.Find this resource:
Hansen, S., Clay, S. A., Clay, D. E., Carlson, C. G., Reicks, G.Jarachi, J., & Horvath, D. (2013). Landscape features impacts on soil available water, corn biomass, and gene expression during the late vegetative growth stage. The Plant Genome, 6, 1–9.Find this resource:
Hansen, Z. K., & Libecap, G. D. (2004). Small farms, externalities, and the Dust Bowl of the 1930s. Journal of Political Economy, 112, 665–694.Find this resource:
He, Y., DeSutter, T., Casey, F., Clay, D. E., Franzen, D., & Steele, D. (2015). Field capacity water as influenced by Na and EC: Implications for subsurface drainage. Geoderma, 245–246, 83–88.Find this resource:
Heap, I. (2016). International survey of herbicide resistant weeds. Weed Science Society.Find this resource:
Hopkins, D., Chambers, K., Fraase, A., He, Y., Larson, K., Malum, L., …, & Utter, R. (2012). Evaluating salinity and sodium levels on soils prior to drain tile installation: A case study. Soil Horizons, 53, 24–29.Find this resource:
Ismail, I., Blevins, R. L., & Frye, W. W. (1994). Long-term no-tillage effects on soil properties and continuous corn yields. Soil Science Society America Journal, 58, 193–198.Find this resource:
Karathanasis, A. D., & Wells, K. L. (1989). A comparison of mineral weathering trends between two management systems on a catena of loess-derived soils. Soil Science Society of America Journal, 53, 582–588.Find this resource:
Kharel, T., Clay, D. E., DeSutter, T. M., Malo, D. D., Carlson, C. G., & Reese, C. L. (2014). Chemical amendments for sodium leaching in saline-sodic soils. Proceedings of American Society of Agronomy Meeting, November 2–5, Long Beach, CA.Find this resource:
Kirch, P. V. (1997). Microcosmic histories: Island perspectives on “global” change. American Anthropologist, 99, 30–42.Find this resource:
Lee, S., Clay, D. E. & Clay, S. A. (2014). Impact of herbicide tolerant crops on soil health and sustainable agriculture crop production. In D. D. Songstad, J. L. Hatfield, & D. T. Tomes (Eds.), Convergence of food security, energy security, and sustainable agriculture (pp. 211–238). Berlin: Springer-Verlag.Find this resource:
Lemon, J. T. (1966). The agricultural practices of national groups in eighteenth-century southeastern Pennsylvania. Geographical Review, 56, 467–496.Find this resource:
Lininger, K. (2011). Small-scale farming and shifting cultivation. In D. Boucher, P. Elias, K. Lininger, C. May-Tobin, S. Roquemore, & E. Saxon (Eds.), The root of the problem. What’s driving tropical deforestation today? (pp. 1–8). Cambridge, MA: Union of Concerned Scientists.Find this resource:
Maitima, J. M., Mugatha, S. M., Reid, R. S., Gachimbi, L. N., Majule, A., Lyaruu, H., …, & Mugisha, S. (2009). The linkages between land use change, land degradation and biodiversity across East Africa. African Journal of Environmental Science and Technology, 3, 310–325.Find this resource:
Mamani-Pati, F., Clay, D. E, & Smeltekop, H. (2014). Modern landscape management in the Andean Highlands using technology developed by the Inca Empire. In G. J. Churcham & E. R. Landa (Eds.), The soil underfoot: Infinite possibilities for a finite resource (pp. 247–254). Boca Raton, FL: CRC Press.Find this resource:
McCarthy, J. R., Phost, D. L., & Currence, H. D. (1993). Conservation tillage and residue management to reduce soil erosion. G1650. Columbia, MO: University of Missouri Extension. Available at http://www.aphis.usda.gov/brs/aphisdocs/06_17801p.pdf.Find this resource:
McGuire, V. L. (2013). Water-level and storage changes in the High Plains aquifer, predevelopment to 2011 and 2009–11. U.S. Geological Survey Scientific Investigations Report 2012–5291.Find this resource:
McIntyre, D. S. (1979). Exchangeable sodium, subplasticity and hydraulic conductivity of some Australian soils. Australian Journal of Soil Research, 17, 115–120.Find this resource:
Miller, A. J., Amundson, R., Burke, I. C., & Yonker, C. (2004). The effect of climate and cultivation on soil organic C and N. Biogeochem, 67, 57–72.Find this resource:
Miranowski, J. A., & Bender, R. L. (1982). Impact of erosion control policies on wildlife habitat on private lands. Journal of Soil and Water Conservation, 37, 288–291.Find this resource:
Montgomery, D. R. (2007). Is agriculture eroding civilization’s foundation?GSA Today, 17, 4–9.Find this resource:
Moss, S. R. (2007). Herbicide-resistant weeds. In R. E. L. Naylor (Ed.), Weed management handbook (9th ed., pp. 225–253). Oxford: Blackwell.Find this resource:
Munson-Scullin, W., & Scullin, M. (2005). Potential productivity of midwestern Native American gardens. Plains Anthropologist, 50, 9–21.Find this resource:
Northcote, K. H., & Skene, J. K. M. (1972). Australian soils with saline and sodic properties. Soil Publication 27. Melbourne, Australia: CSIRO Publishing.Find this resource:
Owen, R. K., DeSutter, T. M., Reese, C. L., Malo, D. D., Kharel, T. P., Birru, G., …, & Carlson, C. G. (2014). Spatial variability of saline and sodic soils in the northern Great Plains. American Society of Agronomy Proceedings, Nov 2–5, 2014, Long Beach CA.Find this resource:
Penna, J. A., & Lema, D. (2003). Adoption of herbicide tolerant soybeans in Argentina: An economic analysis. In N. Kalaitzandonakes (Ed.), Economic and environmental impacts of agrotechology (pp 203–220). New York: Kluwer-Plenum.Find this resource:
Piperno, D. R., Ranere, A. J.Holst, I, Iriarte, J., & Dickau, R. (2009). Starch grain and phytolith evidence for early ninth millennium B.P. maize from the Central Balsas River Valley, Mexico. Proceeding of the National Academy of Science of the United States of America, 106, 5019–5024.Find this resource:
Powles, S. B., Lorraine-Colwill, D. F., Dellow, J. J., & Preston, C. (1998). Evolved resistance to glyphosate in rigid ryegrass (Lolium rigidum) in Australia. Weed Science, 46, 604–607.Find this resource:
Reese, C., Clay, D. E., Clay, S. A., Bich, A., Kennedy, A. C., Hansen, S. A., & Miller, J. (2014). Winter cover crops impact on corn production in semi-arid regions. Agronomy Journal, 106, 1479–1488.Find this resource:
Reeves, D. W. (1997). The role of soil organic matter in maintaining soil quality in continuous cropping systems. Soil Tillage Research, 43, 131–167.Find this resource:
Reitsma, K. D., Clay, D. E., Clay, S. A., Dunn, B. H., & Reese, C. L. (2016). Does the U.S. cropland data layer provide an accurate benchmark for land-use change estimates? Agronomy Journal, 108, 266–272.Find this resource:
Reitsma, K. D., Dunn, B. H., Mishra, U., Clay, S. A., DeSutter, T., & Clay, D. E. (2015). Land-use change impact on soil sustainability in a climate and vegetation transition zone. Agronomy Journal, 107, 2362–2372.Find this resource:
Rhoton, F. E. (2000). Influence of time on soil response to no-till practices. Soil Science Society of America Journal, 64, 700–709.Find this resource:
Roberts, R. K., English, B. C., Gao, Q., & Larson, J. A. (2006). Simultaneous adoption of herbicide-resistance and conservation-tillage cotton technologies. Journal of Agricultural and Applied Economics, 38, 629–643.Find this resource:
Runnels, C. N. (1995). Environmental degradation in Ancient Greece. Scientific American, 272, 96–99.Find this resource:
Samson, F., & Knopf, F. (1994). Prairie conservation in North America. BioScience, 44, 418–421.Find this resource:
Schrag, A. M. (2011). Addendum: Climate change impacts and adaptation strategies. In S. C. Forrest, H. Stand, W. H. Haskins, C. Freese, J. Proctor, & E. Dinerstein (Eds.), Ocean of grass: A conservation assessment for the Northern Plains conservation network and Northern Great Plains ecoregion. Bozeman, MT: World Wildlife Fund-US.Find this resource:
Schroeder, S. (1999). Maize productivity in the eastern woodlands and Great Plains of North America. American Antiquity, 64, 499–516.Find this resource:
Seelig, B. D. (2000). Salinity and sodicity in North Dakota soils. NDSU Extension. Fargo, ND: North Dakota State University.Find this resource:
Shafer, M., Ojima, D., Antle, J. M., Kluck, D., McPherson, R. A., Petersen, S., …, & Sherman, K. (2014). Great Plains: Climate change impacts in the United States. In M. Melillo, Terese (T. C.) Richmond, & G. W. Yohe (Eds.), The third national climate assessment (pp. 441–461). Washington DC: U.S. Global Change Research Program.Find this resource:
Shaw, D. R., Givens, W. A., Farno, L. A., Gerard. P. D., Jordan, D., Johnson, W. G., …, & Owen, M. D. K. (2009). Using a grower survey to assess the benefits and challenges of glyphosate-resistant cropping systems for weed management in US corn, cotton, and soybean. Weed Technology, 23, 134–149.Find this resource:
Shockley, J. M., Dillon, C. R., & Stombaugh, T. S. (2011). Whole farm analysis of the influence of auto-steer navigation on net returns, risk, and production practices. Journal of Agricultural and Applied Economics, 431, 57–75.Find this resource:
Shryock, R. H. (1939). British versus German traditions in colonial agriculture. Mississippi Valley Historical Review,26, 39–54.Find this resource:
Sims, B. D., & Guethle, D. R. (1992). Herbicide programs in no-tillage and conventional-tillage soybeans (Glycine max) double cropped after wheat (Triticum aestivum). Weed Science, 40, 255–263.Find this resource:
Sisti, C. P. J., dos Santos, H. P., Kohhann, R., Alves, B. J. R., Urquiaga, S., & Boddey, R. M. (2004). Change in carbon and nitrogen stocks in soil under 13 years of conventional or zero tillage in southern Brazil. Soil Tillage Research, 76, 39–58.Find this resource:
Smart, A. J., Perkins, L. B., Schramm, T. N., Nelson, M. J., Bauman, P. J., Clay, S. A., & Clay, D. E. (2016). The effects of patch-burn grazing on vegetation structural heterogeneity in the northern tallgrass prairie of South Dakota. Great Plains Research, 26, 57–70.Find this resource:
Smart, A. J., Scott, T. K., Clay, S. A, Clay, D. E., Ohrtman, M., & Mousel, E. M. (2013). Spring clipping, fire, and simulated increased atmospheric nitrogen deposition effects on Tallgrass Prairie vegetation. Rangeland Ecology and Management, 66, 680–687.Find this resource:
Smeltekop, H., Clay, D. E. & Clay, S. A. (2002). The impact of intercropping sava snail medic cover crop on corn production. Agronomy Journal, 94, 917–924.Find this resource:
Steward, D. R., & Allen, A. J. (2016). Peak groundwater depletion in the High Plains Aquifer, projections from 1930 to 2110. Agricultural Water Management, 170, 36–48.Find this resource:
Sumner, M. E., Rengasamy, P., & Naidu, R. (1998). Sodic soils: A reappraisal. In M. E. Sumner & R. Naidu (Eds.), Sodic soils: Distribution, properties, management, and environmental Consequences. New York: Oxford University Press.Find this resource:
Szabolcs, I. (1989). Salt-affected soils. Boca Raton, FL: CRC Press.Find this resource:
Tanno, K., & Willcox, G. (2006). How fast was wild wheat domesticated?Science, 311, 1886.Find this resource:
Trigo, E., Chudnovsky, D., & López, A. (2003). The adoption of transgenic crops in Argentina an open-ended story. Winnipeg: International Centre for Trade and Sustainable Development.Find this resource:
U.S. Department of Agriculture (USDA). (2004). Climate and man. Reprint ed. Honolulu, HI: University Press of the Pacific.Find this resource:
U.S. Department of Agriculture (USDA) Census of Agriculture. (2014). Farm and ranch irrigation survey (2013). Vol. 3, Special studies, part 1, United State Department of Agriculture, Washington DC.Find this resource:
U.S. Department of Agriculture–Natural Resources Conservation Service (USDA-NRCS). (2016). A brief history of NRCS. Available at http://www.nrcs.usda.gov/wps/portal/nrcs/main/national/about/history/.
Weltfish, G. (1965). The planting of the crop. The lost universe: Pawnee life and culture (pp. 95–123). Lincoln: University of Nebraska Press.Find this resource:
White, L., Jr. (1962). Medieval technology and social change. Oxford: Oxford University Press.Find this resource: