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Wheat, rice, and maize collectively provide the bulk of energy and protein to the human diet worldwide. Over the last seven decades, China increased 5.8-fold the aggregated production of the three cereals, but further gains are required to meet the predicted demand of 776 Mt. by 2030. Here, we report the rate of genetic gain in yield of 0.65% year-1 for wheat between 1937 and 2012, 0.69% year-1 for rice between 1934 and 2011, and 0.77% year-1 for maize between 1950 and 2014 in China. National farm yield in the same periods increased 1.38% year-1 for wheat, 1.15% year-1 for rice and 1.32% year-1 for maize. Hence, genetic improvement (G) accounted for 47%-60% of the increase in farm yield, with improved management (M) and G × M interaction accounting for the balance. The absolute rate of genetic gain (kg ha-1 year-1) was proportional to environmental potential. Genetic gain in nitrogen utilization efficiency (yield per unit nitrogen uptake) was significant for all three crops and was achieved at the expense in grain N concentration that decreased at 0.19% year-1 in rice and 0.47% year-1 in maize. To guarantee future food security and environmental sustainability, breeding and agronomic efforts need to increase the rate of gain in yield and revert the declining trends in grain protein by improving nitrogen uptake and nitrogen harvest index. [ABSTRACT FROM AUTHOR]

On historical timescales, enhanced availability of water and nitrogen has been a major driver of improved crop yield worldwide. The interaction between water and nitrogen has received attention, but theories and conceptual tools lag behind our fast-increasing capacity to monitor crop nitrogen and water status using remote sensing. Theory, mostly developed in ecology, predicts that plant growth is maximized when all resources are equally limiting. Therefore, for a given level of stress, growth is maximized under colimitation of resources. The concept of colimitation is less developed in agriculture, where the notion of sequentially, single limiting factors has been dominating since von Liebig law of minimum. During the last two decades, the concept of colimitation of resources has been used to explain yield gaps in wheat, barley, and canola in Mediterranean-type environments of Australia and Europe, and in maize in the Pampas of Argentina. These studies conform to theory: high yield is usually associated with high water and N colimitation. In view of the increasing capacity to measure crop water and N status of crops using remote sensing, we reviewed N-water colimitation as this concept provides a robust framework to integrate otherwise dispersed measurements. First, we revise the concept of colimitation in terms of timescale, level of organization, and thropic level; an agronomic definition of colimitation is advanced. Second, we outline the methods to quantify colimitation between two resources in a generalized crop model, and between water and nitrogen in particular. Third, we summarize studies providing experimental and modeling evidence of colimitation. A notion of crop yield at the biophysical "boundaries" of nitrogen and water use efficiency is presented within the colimitation context. Finally, we propose future directions to develop tools to quantify water-nitrogen colimitation using remote sensing. [ABSTRACT FROM AUTHOR]