**3. The role of management**

While improved genetics have contributed greatly to increasing corn productivity over the last 70 years, other factors also played a role. Duvick [5] has noted that corn productivity per plant has not changed much over the last few decades, suggesting that, under favorable conditions, the efficiency of photosynthesis for corn (as a C4 plan) has not improved. If so, what is the source of corn productivity growth? Duvick [5] argued that most of the historical increases in US corn yields are due to increases in plant density. Thus, corn productivity gains have come from the interactions between the plant and its environment, along with improvements in farm management and cultural practices. Over the years, new corn hybrids have been selected to be more resistant to lodging and more tolerant of biotic stress (pest damage, weed competition, disease) and abiotic stress (adverse weather, poor soil conditions). These genetic changes have interacted with improved management practices, including fertilizer use, irrigation, tillage system, weed control, pest management and crop rotation. Fertilizer applications remedy soil nutrient scarcity, as corn yield is very responsive to nitrogen [5]. When available, irrigation alleviates soil water scarcity and drought. Pest and weed populations can be (at least partially) controlled and suppressed by tillage, crop rotations and by the use of pesticides (insecticides and herbicides). Crop rotation had been used by farmers for centuries to reduce pest and weed infestation and to restore soil fertility [19–21].

The hypothesis that management and genetic biotechnology interacted in generating recent corn productivity gains have been investigated by Chavas and Shi [22] and Chavas et al. [23]. They found evidence of the important role of management and of interaction effects between technology and management. First, they documented how biotechnology has been a major driver of improved corn productivity over the last decade. They also explored how the benefit of GE traits can vary with agro-climatic conditions. Second, they showed how GE hybrids provide enhanced control of pest damages, thus reducing exposure to both risk and downside risk (the provability of facing low yields). Reducing risk exposure is a major part of the benefits of GE technology [24]. Importantly these GE benefits can go beyond the farm if the suppression of pest population is regional [25]. Third, Chavas and Shi [22] and Chavas et al. [23] showed how crop rotation and GE technology provide alternative ways to control pest populations, indicating that they behave as substitutes in the corn production process. Fourth, they reported the presence of synergy between biotechnology and plant density as they affect corn productivity. By improving pest control, GE hybrids make it possible to obtain greater productivity from higher plant density, evidence that the observed growth in corn productivity has been the outcome of important synergies between genetics and improved management.
