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Fluid Journal : Summer 2016
14 The Fluid Journal Summer 2016 simulated the flowering dates for three different maturity classes at different CO2 and temperature scenarios for 2030 and 2050. These changes in climate (i.e., warmer winters) decreased the wheat growing season by up to 6 weeks. This shortened growth cycle decreased resource capture (growth- defining due to solar radiation capture and growth-limiting due to water and nutrient utilization), leading to a potential yield loss. Rezaei et al. (2015) proposed that warmer temperatures would advance phenological development, and increasing the rate would lead to a reduction of exposure to stress occurring later during the growing season. Lehmann et al. (2013) simulated different climate change scenarios with a range of management practices for winter wheat and maize in Switzerland, using a bio-economic model and the CropSys crop growth model. Their results showed climate change scenarios with reduced summer precipitation would increase the amount of irrigation required for economically viable yields. For winter wheat and maize, climate change reduced the optimal N fertilizer rate and there was greater uncertainty of profitability. For all climate scenarios, there was a decrease in grain yield for both crops between 20 and 40%, even under optimal management practices. During a recent study in Central Rift Valley of Ethiopia, Kassie et al. (2014) observed yield gaps of over 70% and that closure of the yield gaps would only be possible through improved crop and management practices. Water was the primary limitation in this environment to maize yield, and thus improved water management would be required to increase yields. There are no single or simple solutions to improving yields and no solid trajectories of crop yield by 2050, which leads to the conclusion that achieving yields necessary to feed future global populations will require innovative research and widespread deployment of agronomic practices. One approach for evaluating yield gaps is to evaluate the fraction of attainable yield. When this is expressed as a frequency distribution, a typical response is that 20% of the yield gap occurs over 85% of the time (Figure 3), as illustrated for Story County in Iowa. This response is typical of maize and soybean production across the Midwest and wheat production in the Great Plains. If we are going to use yield gaps as a method of analyzing where improvements can be made, then the approach will offer insights into factors limiting yield and the magnitude and frequency of the yield gap we are trying to close. If we assume the theoretical upper limit of yield is a function of the amount of light available and effectively utilized by the crop, then the primary production (Pn) can be described as the amount of light absorbed by the crop canopy and the conversion efficiency of light to photosynthesis (light-use efficiency) expressed as: Pn = Steiec/k where St is the annual integral of incident solar radiation (MJm-2), e1 is the efficiency that solar radiation is intercepted by the crop, ec is the efficiency at which intercepted solar radiation is converted to biomass, and k is the energy content of the biomass (MJg-1). The conversion of Pn into grain yield is expressed as: YC=nPn where YC represents the grain yield of a crop (kgha-1) and is the harvest index (the efficiency at which biomass is partitioned into the harvested product, e.g. grain). Maximum values for e1 are near 0.9, n values are near 0.6 and maximum value of ec for C3 crops are 0.024 and for C4 crops are 0.032 (Long et al., 2006). For C3 crops, the highest short-term ec values are near 0.035 and for C4 crops are near 0.043 (Beadle and Long, 1985; Piedade et al.; 1991; Beadle and Long, 1995). Relating light capture by the crop canopy with productivity can be traced back to Wilson (1967) with refinement by Monteith (1977). There has be extensive use of radiation use efficiency (ec; RUE) in the agronomic literature to describe the relationship between light capture by the crop canopy and productivity (Kiniry et al. 1989, Fletcher et al., 2013); Hatfield, 2014). Observations of maximum RUE are assumed to represent conditions when plant growth is not limited by water or nutrient stress. The concept of RUE is similar to water use efficiency (WUE) such that the maximum WUE describes the relationship between water use and plant productivity under non-limiting conditions. Linking WUE with RUE provides an opportunity for improved quantification of plant response to the environment. The value of closing the yield gap for increased food production is evident. The challenge remains on how to close the yield gap because of interacting complex factors due to water availability, nutrient supply, and the genetic diversity (Hogy et al., 2013; El-Sharkawy, 2014). A synthesis document on the effects of climate change on agriculture recommends a balance of research on genetics and management practices to enable adaptation to the effects of climate change (Walthall et al. 2012). Observations of traditional crop varieties, outperforming new drought-resistant varieties because of differing soil management practices during recent severe drought in Iowa provide anecdotal evidence supporting this approach. For this review we present an analysis of the potential role of understanding the interaction of genetics x environment x management (G x E x M) in meeting the production needs for 2050 and beyond. GxExM If the path to closing the yield gap is increasing YF via increasing the capacity of each unit of land to support higher yields (i.e., land productivity), then factors currently limiting yield and factors projected to limit yields warrant examination. Equation  demonstrates the value of greater radiation capture by the crop, increasing efficiency by which intercepted solar radiation is converted to biomass, and changing the energy content of the plant mass (Long). Long et al. (2006) expanded on the ec term and suggested altering canopy architecture to improve the distribution of solar radiation interception to prevent leaves from being light-saturated, increasing photo- protection to increase the efficiency of photosynthesis of leaves, increasing the catalytic rates of Rubisco (ribulose bisphosphate carboxylase oxygenase), and increasing the capacity for Rubisco regeneration. They also stated that these changes do not appear feasible during the next 10 to 20 years, thus suggesting that efforts to achieve crop production increases must focus on other means. If we accept the conclusion of Duvick (2005) for maize, that yield increases of the past 50