Climate Change Consequences for Agriculture in Iowa

AgMRC Renewable Energy &Climate Change Newsletter
September 2011

Natalia P. Rogovska, Post Doctoral Research Associate
Richard M. Cruse Professor, Department of Agronomy Iowa State University

Many recent climate trends such as increases in the number of frost-free days, annual and springtime precipitation, the frequency of intense precipitation events, and dew-point temperature, as well as decreased fall precipitation, have relevance to Iowa agriculture. Some of the changes are favorable to agriculture, but others are not.

Farmers have responded to these changes by planting corn and soybean earlier to take advantage of the longer growing season, installing more subsurface tiles to drain excess soil water faster, and purchasing larger combine heads to facilitate harvest in the fewer hours without dew. Increased soil erosion rates are requiring many farmers to adopt additional conservation practices aimed at improving soil and water quality. Higher monthly rainfall and increased transpiration from crops coupled with reduced winds have created favorable conditions for survival and spread of many unwanted pests and pathogens. Increased use of pesticides and other chemicals, used in response to these pests and pathogens, is likely to raise the level of chemicals and contaminants in our water and food.

*   Reprinted with permission from Climate Change Impacts on Iowa

Crop Production and Yield

Corn and soybean yields have been rising steadily since the 1940s (Figure 1), with the average yearly increase in the last decade being 2.5 and 0.6 bushels per acre, respectively (USDA NASS 2010). Improved management and genetics, higher fertility, and reduced drought stress have all been partially credited for this yield increase. Despite great improvements in yield potential, crop production remains highly dependent on climate in conjunction with other variables. The overall effect of climate change on crop productivity in Iowa remains unclear, as positive climatic events could be overridden by the impacts of poor management or genetics, or favorable management and genetics could override negative climate events. Regardless of these interactions, it is certain that climate changes will affect future crop production.

Greenhouse and growth chamber studies suggest increases in atmospheric carbon dioxide (CO2) will generally have a substantial positive effect on crop yields by increasing plant photosynthesis and biomass accumulation. Field research, however, indicates conclusions from such controlled conditions overestimate the “fertilization” effect of CO2 on crop yields (Schimel 2006). Despite earlier reports of substantial effects, field experiments found no increase in corn yield and an estimate of only 14 percent increase in soybean yield (Long et al. 2006). Increasing CO2 concentrations change the competitive advantage of different plant types.

Greater precipitation during the growing season, as we have been experiencing in Iowa (Takle 2011), has been associated with increased yields; however, excessive precipitation early in the growing season adversely affects crop productivity. Waterlogged soil conditions during early plant growth often result in shallower root systems that are more prone to diseases, nutrient deficiencies, and drought stress later in the season (Stolzy and Sojka 1984). An Iowa study indicated that waterlogged conditions are responsible for an average 32 percent loss in crop yields, and 100 percent crop production loss is expected in four out of 10 years on poorly drained areas (Kanwar et al. 1984). The maximum crop damage is observed when flooding occurs at the early stages of growth (Bhan 1977, Chaudhary et al. 1975). These data reinforce the common understanding that a dry spring followed by a wet summer is much better for yields than a wet spring followed by a dry summer.

Frequent or excessive early season rainfall also can delay planting beyond the optimum for crop production, as happened in 2008 (Pope 2008). Shortened growing seasons pose a substantial risk of yield reduction.

Figure 1. Average yield of corn and soybeans in the US.  Corn yields have increased since 1940 on average 2.5 bushels per year, and soybeans have increased on average 0.6 bushels per year. These increases are traced partially to favorable climate. Variability about this trend also is caused by climatic or weather phenomena. (USDA NASS 2010)


According to De Bruin and Pedersen (2008), delaying soybean planting in Iowa from late April to late May or June results in a 12 to 41 percent yield reduction. Switching to early maturity varieties does not negate yield loss and in many cases might not be an option, because seeds are often prepaid well in advance of planting.

An increase in temperature, especially during nighttime, reduces corn yield by shortening the time in which grain is accumulating dry matter (the grain fill period). According to Takle (2011), Iowa’s nighttime temperatures have been increasing more rapidly than daytime temperatures. In 2010, corn yield forecasts dropped from the previously projected 179 to 169 bushels per acre due to warm temperatures during the grain fill period (Elmore 2010).

Crop Management

Changes in precipitation patterns and an increase in temperatures are affecting nitrogen (N) management for crop production. Increased rainfall in early to late spring forces crop producers to delay in-season fertilization, which often results in a yield penalty (Balkcom et al. 2003). Moreover, excessively wet soils are prone to N losses via denitrification, with an estimated 4 to 5 percent loss of N per day when soils are saturated (Sawyer 2008). Application of N in the fall when soil temperatures fall below 50 degrees Fahrenheit is a common practice in Iowa. According to Iowa Environmental Mesonet observations (2010), soil temperatures in Ames, Iowa, historically are almost certain to drop below 50 degrees Fahrenheit by October 2. However, on October 24, 2010, most of Iowa still had soil temperatures into the mid to high 50s, which resulted in delaying N application that fall. The first fall day when the average soil temperatures are below 50 degrees Fahrenheit is occurring later and later in the season (Figure 2), a change that delays the time of fall N application.

Figure 2.  Iowa’s fall soil temperatures.  First day of the fall season when the average 4-inch soil temperature was below 50 degrees Fahrenheit. With current warming temperatures, this date is occurring later in the season, a change that is delaying fall nitrogen application. (Iowa Environmental Mesonet 2010

Excessive rainfall can force farmers to delay planting, as was mentioned in the previous section, or to replant damaged acres, a practice often recommended if flood damage occurred during the early vegetative stages (Pedersen 2008). In contrast, crops affected by floodwaters later in the season, when replanting is not an option, are often lost. In recent years, many crops along rivers and in potholes have been lost along with substantial amounts of applied nitrogen fertilizer.

The current changes in precipitation, temperature, wind speeds, solar radiation, dew-point temperatures, and cloud cover imply less ventilation of crops and longer dew periods. Soybean plants in particular readily absorb moisture, making harvest problematic. One adaptive approach to these conditions involves farmers purchasing larger harvesting equipment to speed harvest, compensating for the reduced daily time suitable for soybean harvest.


The recent extreme weather events involving greater intensity and amount of rainfall (Takle 2011) have increased the erosive power of Iowa’s precipitation, resulting in significant erosion of topsoil. According to the USDA’s National Resource Inventory report, Iowa’s average erosion rate is estimated to be 5 tons per acre per year. Best science indicates soil renewal rates are closer to 0.5 tons per acre per year (Montgomery 2007). Iowa Department of Agriculture and Land Stewardship showed that an estimated 2.3 million acres – about 10 percent of Iowa's cropland – had that year suffered severe erosion damage, which is defined as an annual erosion rate of 20 tons or more of soil per acre. In some areas, erosion rates exceeded 50 tons per acre per year (Figure 3) (Iowa Daily Erosion Project 2010). The impact of climate change on the erosive force of precipitation in the US is expected to increase by as much as 58 percent (Nearing 2001). Moreover, today’s soil erosion rates are expected to increase exponentially as precipitation continues to rise; consider for example that a 20 percent increase in precipitation has been shown to increase erosion rates by an estimated 37 percent (Lee et al. 1996).

Figure 3. Average soil loss in Iowa in 2008. The increased intensity and amount of rainfall during this flood year resulted in a soil loss of more than 50 tons per acre in some townships (orange). This contrasts with the “tolerable average” of 5 tons per acre per year (blue shades). The rise in the intensity and amount of rainfall has increased the erosive power of Iowa’s precipitation. (Iowa Daily Erosion Project 2010)

Crop Health and Use of Pesticides

Iowa’s current increases in temperature, soil wetness, and humidity can favor the development and establishment of plant diseases, leading to more severe disease epidemics. A prime example is the wet soil conditions of the 2010 growing season, which are often cited as the cause for the widespread epidemic of soybean sudden death syndrome that year. This syndrome has the potential of reducing yields by up to 100 percent. 2010 was the worst year for soybean sudden death syndrome since it was discovered here in 1994 (Yang 2010). Soybean Asian rust is present in the southern US but has not yet been observed in Iowa. However, warmer winters and wetter summers, with extended periods of summer warmth and leaf wetness, raise concern among plant pathologists about possible spread of this disease to Iowa (NCSRP 2005). Overall, the increasing risks of plant disease resulting from increased precipitation have led to more frequent use of foliar fungicides in the past several years in Iowa (Yang et al. 2008).

With increasing temperatures and soil moisture, weeds are posing more problems and are proving more difficult to control. In 2008, weed pressure was exacerbated due to heavy rains and saturated soils that reduced the efficacy of pre-emergent herbicides and reduced development of crop canopy, allowing weeds to thrive (Hartzler 2008). Weeds are more genetically diverse than crops, and therefore in the face of climate changes are more likely than crops to show enhanced growth and reproductive stability. The southern US, with its higher average annual temperatures and greater precipitation, has much greater estimated crop loss due to weeds (when herbicides are not used) than more northerly regions (Bridges 1992).

With warming temperatures and elevated CO2 concentrations, many weeds are migrating northward (Backlund et al. 2008). The efficacy of herbicides is expected to further decrease with rising CO2 levels (Ziska and Goins 2006). The growing weed problem cannot be solved by using standard methods of weed control alone, such as tillage or repeated in-season cultivation, because these may not be as effective as herbicides and will lead to greater rates of soil erosion. Current rainfall patterns also make mechanical cultivation more difficult. With increasing temperatures, harmful invasive species might pose new agricultural challenges, since many invasive weeds and plant pathogens can now overwinter in regions that were previously too cold for them.

Water Quality

The increased rainfall frequency and intensity now experienced in the Midwest (Karl et al. 2009) produce more pollution and sediment due to increased surface water runoff and subsurface drainage (IPCC 2007). Currently, nitrate loss via tile drains is one of the biggest concerns for water quality. Long-term monitoring of agriculturally-related drainage in Gilmore City, IA, showed that nitrate loss is greatest in years with higher precipitation and hence greater tile flow (Figure 4). Depending on the rainfall, the annual nitrate loss varied from 1 to 67 lb of nitrate-N per acre (Lawlor et al. 2008). Due to increases in the amount of precipitation, the number of acres that are artificially drained is likely to increase, exacerbating problems of nutrient-related water-quality issues.

Figure 4. Comparison of annual precipitation and nitrate-N loss via tile drainage. Nitrate loss via tile drainage (bottom line) is greatest in years with higher precipitation (top line), and Iowa’s precipitation and rainfall intensity have been increasing. (Adapted from Lawlor et al. 2008)

Runoff of nutrients such as phosphorous and nitrogen, pesticides and herbicides, and various manure-derived pathogens and antibiotics poses a threat to water quality in Iowa and downstream. Following the 1993 Midwestern floods, the Gulf of Mexico’s dead zone doubled in size because of increased nutrient runoff that year (Epstein 1998).

Animal Production

Despite the fact that Iowa ranks first in hog and fifth in cattle production nationwide (USDA NASS 2008), there is a lack of information about the effects of climate change on animal production in Iowa. Nevertheless, our general knowledge and principles pertaining to livestock and extreme weather events are applicable to Iowa’s changing climate conditions.

High temperatures have been shown to reduce summer milk production, impair immunological and digestive functions of animals, and increase mortality rates among dairy cattle (Klinedinst et al. 1993, Nienaber and Hahn 2007, Mader 2003). On days when the ambient temperature exceeds 90 degrees Fahrenheit, the risk of sow mortality doubles (Carlton 2004). In 1992, 1995, 1997, 1999, 2005, and 2006, the loss of cattle during extended heat episodes exceeded 100 head for some Midwestern farms (Backlund et al. 2008). In 1995, livestock-related economic losses due to heat stress were estimated to be $31 million in Iowa alone (Hahn et al. 2001).

In general, domestic livestock can adapt to gradual changes in environmental conditions; however, extended periods of exposure to extreme conditions greatly reduce productivity and are potentially life threatening. During adverse heat events, management alternatives, such as the use of bedding in winter or sprinklers in summer, need to be considered (Mader at al. 2009).

Research Gaps

There are many uncertainties associated with predictions and assumptions about the effect of climate change on agriculture. These uncertainties beg to be addressed by research. Most studies to date have only considered the average climate change rather than extreme climate events. However, extremes, not averages, will delineate successes and failures. Currently, scientists have insufficient understanding about CO2 fertilization effects on crops. Attempts to study these effects are often hindered by failure to consider other interactive factors such as pests, diseases, weed pressure, adaptation measures, and technological improvements. The same is true when studying other factors.

There is a need to develop strategies to help crops cope with climate variability through plant breeding, and through selection for increased tolerance to water stress and improved nutrient use efficiency, and through tolerance to temperature extremes during grain fill periods. More advanced soil and water conservation technologies must be developed.


Recent weather events and climatic trends are stressing agriculturally related resources. Increased rainfall and frequency of much heavier-than-normal rainfall events result in disproportionately negative impacts on soil and water resources and on crop production. Increasing dew-point temperatures and reduced wind flow have potential near-term crop disease impacts. Subsurface drainage is increasingly necessary to maintain acceptable crop yields. Elevated precipitation and early season rainfall increasingly delay planting, increase nitrate nitrogen losses, and affect nitrogen fertilizer application timing. Climate extremes, not the averages, frequently control productivity of crops and livestock.

References Cited

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