4. Corn productivity in response to climate

Corn productivity relative to climate is a function of both temperature and precipitation. Effects of increased temperatures have shown a large degree of variation with projections of reduced production by less than 5% with temperature increases of 1C [3] to over 50% with 4C increases [22]. Productivity of corn is affected by temperatures exceeding 35 C during pollination due to dehydration of the pollen [3]. Controlled environment studies have confirmed the effect of high temperatures on corn with temperatures greater than or equal to 3C above normal temperatures showing maize yield reductions of over 50% in grain yield [20, 21]. They observed an increased rate of phenology with increased temperatures; however, the largest effect on productivity was attributed to the increase in minimum temperatures during the grain-filling period. Field studies on corn have shown under field conditions yield reductions from 13 to 88% due to increased temperature 6C above normal temperatures [23]. The negative effects of high temperatures during the grain-filling period were attributed to pollen survivability and the efficiency of the grain-filling process. Increasing temperatures likely to be experienced under climate change demonstrate several negative effects plant growth and phenology. Lizaso et al. [24] recorded a reduction of corn yield under field and controlled conditions owing to reduced pollen viability as impacted by increased temperatures. A critical knowledge gap under future climate scenarios will be to evaluate the interaction of high temperature and increased humidity on pollen survivability and the efficiency of the pollination process. Lobell and Field [25] found maize yields decreased 8.3% per 1C rise without any additional effect due to water stress which was confirmed by Mishra and Cherkauer [26] for Midwest corn grain yields. Challinor et al. [27] compiled a meta-analysis of over 1700 published simulations for wheat (Triticum aestivum L.), rice (Oryza sativa L.), and corn. They found that without implementing adaptation strategies there would be a loss in yield in both temperate and tropical regions with only 2C of warming. They also found that adaptation practices could increase simulated yields by 7–15% with this same temperature increase; however, the practices were more effective in wheat and rice than for corn. There was consensus among the simulation models that yield decreases were be greater in the second half of the century with the greater declines in the tropical areas compared to the temperate regions. They estimated that corn yields would decrease by nearly 15% in temperate regions with a 4C increase and no adaptation but showed no decrease with adaptation practices [27].

Temperature and precipitation interact to affect corn productivity. Short-term water deficits and drought reduce growth and grain yield and are often the largest cause of crop losses. In the United States, drought was related to 41% of crop losses, while excess water was attributed to 16% of the yield loss [28]. Drought stress during the early and middle reproductive stages affected grain yields and these phenological stages were found to be the most sensitive to water stress [29]. Increases in spring precipitation can cause yield reductions due to aeration stress caused by flooded soils; however, drought stress remained the primary factor linked with reduced grain production [29]. In rainfed environments where corn is primarily grown, temperature and precipitation changes under climate change will negatively impact grain production and these interactions need to be more fully understood. In an analysis of wheat production in Europe, Semenov et al. [30], stated that understanding of the effects of higher temperatures and drought stresses during the booting and flowering periods would potentially lead to adaptation practices with the potential to reduce losses in grain numbers and grain weight. With both short-term water stress and drought as major factors affecting grain yield, improved water availability through more extensive root system and changes in root architecture would benefit yield stability [31]. The excess soil moisture in the root zone will require improved soil structure to facilitate gas exchange between the root system and the atmosphere [32]. The impact of precipitation is a combination of the precipitation amount and the soil water holding capacity. This was illustrated in an analysis by Egli and Hatfield [33] where they found average county level corn yields were a function of the soils ability to supply water.

development with no effect on the size of the corn plant at the end of the vegetative stage. There was a large difference in grain yield between temperature regimes with a faster rate of

Figure 4. Differences in total leaf collars, cumulative leaf area, and grain yield of three corn hybrids grown under normal

Corn productivity relative to climate is a function of both temperature and precipitation. Effects of increased temperatures have shown a large degree of variation with projections of reduced production by less than 5% with temperature increases of 1C [3] to over 50% with 4C increases [22]. Productivity of corn is affected by temperatures exceeding 35 C during pollination due to dehydration of the pollen [3]. Controlled environment studies have confirmed the effect of high temperatures on corn with temperatures greater than or equal to 3C above normal temperatures showing maize yield reductions of over 50% in grain yield [20, 21]. They observed an increased rate of phenology with increased temperatures; however, the largest effect on productivity was attributed to the increase in minimum temperatures during the grain-filling period. Field studies on corn have shown under field conditions yield reductions from 13 to 88% due to increased temperature 6C above normal temperatures [23]. The negative effects of high temperatures during the grain-filling period were attributed to pollen survivability and the efficiency of the grain-filling process. Increasing temperatures likely to be experienced under climate change demonstrate several negative effects plant growth and phenology. Lizaso et al. [24] recorded a reduction of corn yield under field and controlled

maturity with a subsequent reduction in grain production.

Ames, Iowa temperatures and normal +4C temperatures. (data redrawn from [20]).

4. Corn productivity in response to climate

100 Corn - Production and Human Health in Changing Climate

Evaluation of corn yield response to climate is complex because of the interactions of the impacts of temperature and precipitation. To provide a more robust framework for evaluating yield response the utilization of the yield gap as the difference between potential yield and actual yield has been utilized ([34]; van Bussel et al. [35]). This concept has been discussed and utilized for several decades but recently has been extended to create a yield gap atlas for the world. The yield gap approach allows for a quantitative assessment of the ability of the crop to achieve its potential yield and the inability of closing the yield gap can often be ascribed to climatic stress. Potential yield has been defined as "the yield of a cultivar when grown in environments to which it is adapted; with nutrients and water not limiting; and with pests, diseases, weeds, and other stresses effectively controlled" [36]. Potential yield (YP) is an expression of the ability of a crop canopy to convert solar radiation into dry matter with no stress during the growth cycle and radiation use efficiency can be used as a measure of this efficiency [37]. The goal of agronomic science is the evaluate practices and increasing the farmer yield (YF) may prove to be more fruitful than increasing potential yield (YP) ([38]; Lobell et al. [39]. Utilizing the yield gap approach provides a framework for evaluating the factors which affect crop yields and the phenological stage which these factors are having the most significant impact during the growing season. These studies are not simple analyses, because of the interactions of multiple factors affecting yield, and Sinclair and Ruffy [40] argue that nitrogen and water limit crop yield more than plant genetics and should be considered as the primary factors limiting yield. Understanding the yield gap requires being able to quantify both potential and actual yield and comparison among studies is often limited by the lack of consistent data and to advance our understanding of yield gaps will require standardized method for yield comparisons [41]. Fischer et al. [41] introduced attainable yield (YA) as a metric between YF and YP defined as the yield achieved by a producer under near optimum weather and management inputs. Hatfield et al. [42] utilized this approach on county level corn yields in the Midwest United States and defined the attainable yield as the years with the highest yield in the long-term record as illustrated in Figure 5. The values for attainable yield are derived by statistically fitting a line through the frontier of the yield observations and then computing the yield gap as the difference between the attainable and actual observed yield for each year. In this analysis, data from 1950 through the present are used because this represents the period of time with corn hybrids and enhanced production technology. This approach has

Climate Change Impacts on Corn Phenology and Productivity

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Hatfield et al. [42] utilized the yield gap approach for the Midwestern US to quantify the effects of climate variability on corn production and found three dominant climatic factors related to the yield gap. These were July maximum temperatures, August minimum temperatures, and July–August total precipitation. Yield gaps increased when July maximum temperatures exceeded 32C, August minimum temperatures exceeded 20C, and July–August precipitation totals decreased below 150 mm. The physiological reasons for these variables are related to the disruption of pollination (July temperatures), increased rate of senescence and reduced efficiency of grain-fill (August minimum temperatures), and water deficits during a period of the year with high crop water requirements (July–August precipitation). These relationships were observed for each county in the Midwest and utilized to project the impact of future climate change on the yield gap on corn production. They found that with the trends in temperature for the summer in the Midwest US that yield gaps would exceed 50% by the year 2075 in the southern portion of the Corn Belt. There were some counties in the Midwest in which excess moisture in the spring was related to the yield gap but these relationships were not robust enough for use in projections of future climates. The yield gap framework provides a robust method for assessing the impact of climate on yield variation over time and when combined with efforts similar to those used by Challinor et al. [27] could be used to quantify

been used for different crops and regions of the world to obtain yield gaps.

5. Agroclimatic indices to define corn production regions

Corn is produced around the world and within these areas there may be shifts in production areas due to the changing climate. Green et al. [43] have quantified the changes in the US Corn Belt and provided a geographic analysis to depict these shifts in distribution. Development and utilization of agroclimatic indices has value in being able to assess these shifts because they are related to temperature and precipitation. Neild and Richman [18] were among the first to use the GDD concept to define potential differences among corn hybrids. Development of tools to define where crops can be produced is critical to understand crop distribution and productivity [44]. Estimation of crop distribution within arable areas is necessary to determine whether a species can thrive in an agroclimatic zone and will become more critical with the projected increases in temperature. Zomer et al. [45] extended this concept to demonstrate how climate zones could be used to evaluate technologies that would enhance the ability of management practices to offset the impacts of climate change on crop production. There have continued to be advances in the development of agroclimatic indices to evaluate the suitability of a location for a particular crop since Neild and Richman [18]. Siddons et al. [46] cautioned that development of robust agroclimatic indices requires observations collected over long time periods and extensive observations from experimental locations. There has been an evolution in agroclimatic indices to include more factors affecting plant growth and development to derive values that characterize the environment and the potential for crop production. Typical factors are: average daily minimum temperatures below 0C; daily mean temperature to

the impact of adaptation practices.

Figure 5. Yield gap analysis for Story County, Iowa, USA using attainable yields derived from annual production values. (data obtained from the National Agricultural Statistics Service, www.nass.usda.gov, accessed March 8, 2018).

the period of time with corn hybrids and enhanced production technology. This approach has been used for different crops and regions of the world to obtain yield gaps.

climatic stress. Potential yield has been defined as "the yield of a cultivar when grown in environments to which it is adapted; with nutrients and water not limiting; and with pests, diseases, weeds, and other stresses effectively controlled" [36]. Potential yield (YP) is an expression of the ability of a crop canopy to convert solar radiation into dry matter with no stress during the growth cycle and radiation use efficiency can be used as a measure of this efficiency [37]. The goal of agronomic science is the evaluate practices and increasing the farmer yield (YF) may prove to be more fruitful than increasing potential yield (YP) ([38]; Lobell et al. [39]. Utilizing the yield gap approach provides a framework for evaluating the factors which affect crop yields and the phenological stage which these factors are having the most significant impact during the growing season. These studies are not simple analyses, because of the interactions of multiple factors affecting yield, and Sinclair and Ruffy [40] argue that nitrogen and water limit crop yield more than plant genetics and should be considered as the primary factors limiting yield. Understanding the yield gap requires being able to quantify both potential and actual yield and comparison among studies is often limited by the lack of consistent data and to advance our understanding of yield gaps will require standardized method for yield comparisons [41]. Fischer et al. [41] introduced attainable yield (YA) as a metric between YF and YP defined as the yield achieved by a producer under near optimum weather and management inputs. Hatfield et al. [42] utilized this approach on county level corn yields in the Midwest United States and defined the attainable yield as the years with the highest yield in the long-term record as illustrated in Figure 5. The values for attainable yield are derived by statistically fitting a line through the frontier of the yield observations and then computing the yield gap as the difference between the attainable and actual observed yield for each year. In this analysis, data from 1950 through the present are used because this represents

102 Corn - Production and Human Health in Changing Climate

Figure 5. Yield gap analysis for Story County, Iowa, USA using attainable yields derived from annual production values.

(data obtained from the National Agricultural Statistics Service, www.nass.usda.gov, accessed March 8, 2018).

Hatfield et al. [42] utilized the yield gap approach for the Midwestern US to quantify the effects of climate variability on corn production and found three dominant climatic factors related to the yield gap. These were July maximum temperatures, August minimum temperatures, and July–August total precipitation. Yield gaps increased when July maximum temperatures exceeded 32C, August minimum temperatures exceeded 20C, and July–August precipitation totals decreased below 150 mm. The physiological reasons for these variables are related to the disruption of pollination (July temperatures), increased rate of senescence and reduced efficiency of grain-fill (August minimum temperatures), and water deficits during a period of the year with high crop water requirements (July–August precipitation). These relationships were observed for each county in the Midwest and utilized to project the impact of future climate change on the yield gap on corn production. They found that with the trends in temperature for the summer in the Midwest US that yield gaps would exceed 50% by the year 2075 in the southern portion of the Corn Belt. There were some counties in the Midwest in which excess moisture in the spring was related to the yield gap but these relationships were not robust enough for use in projections of future climates. The yield gap framework provides a robust method for assessing the impact of climate on yield variation over time and when combined with efforts similar to those used by Challinor et al. [27] could be used to quantify the impact of adaptation practices.
