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Fluid Journal : Summer 2016
12 The Fluid Journal Summer 2016 ▼ DOWNLOAD needed to increase production. One of those technological advances is how we address nutrient and pest management in agricultural systems. Adding to the demand for increased production is the call to redirect agriculture toward a more sustainable path for food security (Godfrey et al. 2010) because of growing competition from nonagricultural sectors for land, water, and energy. These assessments illustrate the complexities of producing food and feed to satisfy global food demands of the future. Clearly, a challenge to the agricultural community exists, there are no simple solutions, and there is an urgent need for revolutionary (vs. evolutionary) innovations. GxExM Here we explore the current state of agricultural production and where opportunities and challenges exist to increase yields. A suggested framework for approaching the challenges is built on an effort to understand the interactions among genetics, environment, and management, referred to as genetics x environment x management or simply: G x E x M. Yield gap A path to increased yields begins with a look at where current farmer yields are relative to potential yield. 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” (Evans and Fischer, 1999). Potential yield (Yp) is a measure of the capacity of a crop to convert solar radiation into dry matter with no stress during the growth cycle. Cassman et al. (2003) and Lobell et al. (2009) present a case for closing the yield gap, the difference between potential yield (Yp) and farmer yield (YF), rather than seeking to increase Yp. A yield gap approach addresses all factors affecting crop yields and also when these factors affect yield during the growing season. Sinclair and Ruffy (2012) suggest that N and water limit crop yield more than plant genetics and thus N and water should be considered the primary factors limiting yield. This would argue for a renewed emphasis on nutrient management, especially nitrogen management to increase crop productivity (Spiertz, 2009). Fischer et al. (2014) suggest that closing the yield gap requires a more standardized method for yield comparisons and propose that yield gap be expressed as a percentage of the YF because increased production will come from greater YF rather than Yp . Attainable yield was defined as the yield achieved by a producer under near optimum weather and management inputs. Hatfield (2010) and Egli and Hatfield (2014a, 2014b) looked at county level YF for Iowa, Kentucky, and Nebraska and found the frontier of the upper bound of YF achieved an optimal, which could be treated as YA for comparison with YF from any year. The results indicated that differences between YF and YA provide insights to the limitations of crop yield for any year. This is illustrated in Figure 1 with county level yield data for Iowa to show the difference between Ya and Yf and the variation of yield gap values each year. These data are typical of county- level yields and show yield gaps as a fraction of Ya have decreased since 1950 because yields have increased. Egli and Hatfield (2014) found that soil quality as defined by the National Crop Commodity Productivity Index (NCCPI) was positively related to soybean yields across Iowa and Kentucky; however, when irrigation was present this relationship was no longer valid as found in Nebraska county yields with extensive irrigation. Environmental conditions will have a major impact on our ability to close yield gaps, and increased attention on the factors limiting yield will provide insights into future increases of productivity. Lobell et al. (2009) summarized a comparison of YF with YP data of maize, rice, and wheat. Using a combination of simulation models and experimental observations to estimate Yp they found that the yield gap for maize ranged from 44 to 84%, for wheat 11 to 60%, and for rice 16 to 70%. Their observations suggest that much can be achieved to meet production needs by closing the yield gap for these crops. For rain-fed crops, the average yield gap was close to 50% of the Yp. They were not as optimistic for irrigated crops as observed yields, where YF was nearly 80% of Yp . And they noted that increasing YF of irrigated crops is necessary for further yield gains (Lobell et al. 2009). The assumption that YF will continue to increase at the same rate as the past 50 years is challenged by other complicating factors. Brisson et al. (2010) analyzed trends of European wheat yields and showed that the lack of genetic improvements was not the Figure 1. Attainaable yield, actual maize yields, and yield gaps from 1950 through 2012. (data from USDA-NASS).