Innovations for Improving Productivity and Nutrient Use Efficiency - Maize Systems of North America.
Daniel Walters1, Achim Dobermann1, Tony Vyn2, and Sylvie Brouder3. (1) Department of Agronomy and Horticulture, University of Nebraska-Lincoln, 261 Plant Science Bldg, Lincoln, NE 68583-0915, (2) Purdue University, 3450-B Lily Hall, West Lafayette, IN 47907-1150, (3) Purdue Unversity, Department of Agronomy, West Lafayette, IN 47907-2054
The predominant rainfed and irrigated maize (Zea mays L.) cropping systems in North America are ones in which maize is grown in rotation soybean (Glycine max L.) or as a continuous monocrop. Maize production in North America is a resource-intensive enterprise and relies heavily on the annual input of fertilizers and is responsible for nearly 40% of global maize supply. Average maize grain yields in this region have increased linearly at a rate of 110 kg/ha/yr over the past 35 years with a yield average of approximately 8.8 Mg/ha. This gain in productivity has been realized through the continued adaptation of improved crop management technologies and genetic improvement in maize hybrids that compliment these technologies. The application rate of fertilizer nitrogen (N), phosphorus (P) and potassium (K) fertilizers to maize rose quite quickly during the 1960's and 1970s but since 1980 N rates have leveled off and P and K use has declined steadily. This dynamic has led to a dramatic increase in partial factor productivity (kg grain per kg nutrient applied) of these essential macronutrients. Technologies that have resulted in these improved nutrient use efficiencies include increased stress tolerance of improved maize hybrids, improved production practices such as conservation tillage, planted population control and weed control options as well as improved nutrient management with regard to form, placement and timing of fertilizer materials. The trends in nutrient use efficiency gain, yield and demand for grain (especially with the recent onset of maize-grain ethanol production) lead one to question the long-term sustainability in maize yield gain to meet an ever increasing global food and energy demand without a renewed increase in fertilizer consumption. An analysis of maize yield contest winners in the United States and simulation model results indicate that farm yields are presently only 40 – 60% of the attainable yield potential and so an appreciable exploitable yield gap remains in the production potential of the region. To maintain and even increase the efficiency of nutrient use, more precise and diverse nutrient management strategies are needed. In order to exploit the existing yield gap, close attention will be needed to maintaining adequate nutrient supply as the margin of error between nutrient excess and deficiency decreases rapidly as yield potential is approached. At present, fertilizer rate recommendations are primarily based upon soil testing which result in somewhat stagnant management strategies as fertilization decisions are made on the basis of rather empirical relationships that are insufficient to meet the need for greater input use efficiency at higher yield levels. Also, with the widespread adaptation of conservation tillage, nutrient stratification in soil has resulted in a need to reassess soil testing and fertilizer application strategies. The intensification of nutrient management to exploit existing yield potential will require technologies that are based on a quantitative understanding of the relationship between yield and nutrient uptake as well as the critical timing of nutrient supply in relation to crop demand. It is imperative that nutrient use efficiencies are maintained and/or improved to avoid adverse impact on the global environment and the economy of agricultural production systems. At the same time, intensification strategies must be designed that improve soil quality as this contributes to nutrient and energy use efficiency and thus reduces the impact of nutrient use on emissions of greenhouse gases and any adverse effects on water quality. This will require a more interdisciplinary research approach in order to address the entire spectrum of agricultural, ecological and environmental functions of our agricultural systems.