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Fluid Journal : Summer 2016
19 The Fluid Journal Summer 2016 wheat yields under no-till during years with limited rainfall during conventional vs. no-tillage systems comparisons for southern Italy. No-till systems had an advantage over conventional tillage because of reduced soil water evaporation coupled with the enhanced soil water availability induced by better water holding capacity (Hulugalle and Entwistle, 1997). Greater attention is warranted to the relationship between soil organic matter and water holding capacity as described by Hudson (1994), along with soil management practices required to increase soil organic matter and root exploration into the soil profile. Unger et al. (1991) found conservation practices that maintained crop residue on the soil surface had a positive impact on water conservation and translated into increased water availability for the crop in semi-arid regions, leading to greater yield potential. Manipulation of the soil and adoption of conservation practices can have a positive impact on WUE; therefore, adopting these practices can provide one avenue for crop yield enhancement to meet future food needs (Hatfield et al., 2001). The amount of yield increase will vary depending on the atmospheric conditions during the growing season; however, a more stable soil water supply fosters less variation of crop yield. Genetic improvement Genetic improvement of crops has proven to advance yields over the past 50 years and as Duvick (2005) pointed out, yield increases in maize can be attributed equally between genetic improvement and management improvements. Yield trends in crops have shown a positive advance and yet Grassini et al. (2013) found these increases vary among rice, wheat, and maize. They conducted a thorough analysis of rice yield in China, India, Indonesia, Republic of Korea, Vietnam, California and the south central United States; for wheat in Australia, China, France, India, the Netherlands, and the United Kingdom and for maize in Brazil, Central Africa, China, India, Italy, the eastern U.S. Corn Belt, the western rain-fed U.S. Corn belt, and the western U.S. irrigated Corn Belt, using several different statistical models fit the annual grain yields. They found that there was evidence of a yield plateau in different countries (e.g. rice in eastern Asia and wheat in northwest Europe), whereas in others there was a linear trend in grain yields. They suggested that attention be directed to yield trajectories for more effective strategic planning about future production estimates. The assumption that yield improvement trends will be maintained at the current rates may not be true. George (2014) points out that in spite of advances in germplasm and agronomic advances there, more attention is warranted to enhancing agronomic practices to increase actual yield if yields in developing countries are expected to increase. Mueller et al. (2012) suggested that closing the yield gap is possible through a combination of intensive nutrient and water management; however, these authors acknowledged the presence of a changing climate and did not consider the potential disruptions to crop production due to changing climate as a barrier to achieving potential yields. Neumann et al. (2010) conducted a global scale analysis using frontier yields, yield gaps and efficiencies for maize, wheat, and rice production. They defined the frontier yields as the highest observed yield for a combination of conditions and used the definitions of van Ittersum et al. (2003) to quantify the variables for their frontier analysis. van Ittersum characterized these variables as growth-defining (potential crop yield under a given physical environment where conditions cannot be managed), growth-limiting (water and nutrient limitations preventing the crop from achieving potential yield), and growth- reducing (pests, diseases or pollutants and require some agronomic management to reduce these yield impacts). Actual crop yield represents the interaction among these three factors (van Ittersum et al. 2003). Neumann et al. (2010) defined the process of closing the yield gap as intensification and would require a country-specific process to close the yield gap because the yield constraints are not uniform across countries: e.g., in developing countries the lack of capital investment, access to technology, and infrastructure may be the liming factors, whereas in developed countries, improved genetics and management may be viable options. Feeding 9 billion people presents a major challenge because of a combination of factors. Increased production is not possible without new lands and increased yield and cropping intensity (Gregory and George, 2011). Smith et al. (2010) estimated that per capita land area will decrease and actually declined from 0.415 ha in 1961 to 0.214 ha during 2007, and they estimated average cereal yield will need to increase by 25% from the 3.23 Mg ha-1 of 2005-2007 to 4.34 Mgha-1 during 2030. To meet future production requirements, Gregory and George (2011) summarized that only 20% would come from new land and 80% from intensification (increased yields and greater cropping intensity). These conclusions would suggest that increased emphasis on improved management for nutrients and pests would increase the capacity for food production. West et al. (2010) argue that clearing land for agricultural production will lead to increased carbon losses, and for the tropics, the efforts must be directed toward increasing crop yields rather than clearing more land. Increasing productivity will require increased management and improved agronomic techniques. Unfortunately, since the land resource will become a premium, this option will remain as the most viable solution. Challenges There are numerous constraints to crop yield, and the constraints must be overcome to produce the quantity of crops necessary for the projected population of 2050. However, there are additional aspects of the production puzzle that need further discussion. One is the quality of grain or produce to achieve nutritional security and supply the calories to sustain the population. To achieve this goal will require an emphasis on nutrient management, forms, and timing of nutrients to ensure adequate nutrient availability for optimum crop production. To achieve this goal will require we fully embrace the linkage of ecology with agriculture to ensure we achieve both production and environmental goals at the same time. An emerging challenge for agronomists is the need to evaluate the quality of the grain with as much vigor as the evaluation of grain yield. There have been limited studies on