The Seven Wonders are shown in Table 1. They are ranked in order from the factor with the greatest impact on yield (Weather, #1) to the factor with the smallest contribution to yield (Growth Regulators, # 7). The values represent a range of responses based on our research. For example, the weather can influence yield by more than 70 bushels/acre.

There are two important aspects of the Seven Wonders:

1. The higher a factor is on the list, the more control it exerts over the factors below it.
   
   
2. The factors interact!

In a favorable year, weather and nitrogen combine to contribute more than 50% of total yield. On average, the maximum response to nitrogen over an unfertilized check plot treatment is approximately 70 bushels/acre (Table 1 and Figure 1). The weather, however, influences every aspect of nitrogen loss, availability, and utilization by the plant. In a drought year such as 2005, the weather limited the response to N application to approximately 20 bushels/acre (Figure 1).

The weather influences the success of all of our management efforts. Continued research and technology, however, may provide us with tools to better mitigate the detrimental impacts of drought, high temperature, and other weather-mediated stresses.

Corn hybrids can vary by as much as 50 bushels/acre when grown in the same field in our research trials (Table 1). This variation can be attributed to hybrid interactions with weather (e.g., differences in drought and heat tolerance), as well as interactions with nitrogen and plant population. The interaction of hybrid, N rate, and plant population is particularly important since these are three of the factors for which the farmer has the greatest control over. We are characterizing this interaction to better understand the management requirements of hybrids in our Management Yield Potential trial.

The hybrid factor of the Seven Wonders also encompasses biotechnology traits such as those for control of corn rootworm. Our data shows that corn rootworm traits have already benefited corn nitrogen use (Figure 2). In this example, protection from corn rootworm increased yield with no fertilizer applied as well as the response to fertilizer application.

Previous crop (continuous corn versus corn following soybean) is the fourth wonder of the corn yield world. Accumulation of residue in continuous corn influences the availability of key nutrients like N and P. In the example shown in Figure 3, additional N was required to overcome the continuous corn yield penalty associated with 3rd year corn in 2006. In 2005 (3rd year corn) as well as both years of 5th year corn, no amount of extra N fertility could completely overcome the continuous corn yield penalty.

In addition to influencing nutrient availability, the accumulation of corn residue might also lead to a build-up of autotoxic compounds that reduce corn growth and yield. We are investigating the cause of the continuous corn yield penalty as well as corn stover removal as a management practice in our sustainability study.

Plant population must increase as we strive to achieve higher yields. Increased plant population, however, results in increased inter-plant competition that must be managed. As an example, in 2009, plant populations beyond 38,000 plants/acre resulted in a linear decrease in grain yield (Figure 4). Response to plant population is heavily infuenced by the weather, nitrogen availability, and hybrid.

Not all hybrids are created alike in their ability to tolerate increased plant population. We are identifying ‘racehorse’ hybrids, or those can respond positively to extra N and population, using our Management Yield Potential trial. In 2011, this trial showed that current commercial hybrids ranged from a 25 bushel/acre decrease to a nearly 12 bushel/acre increase as plant population was increased from 32,000 to 45,000 plants/acre.

Tillage can vary widely in its mechanism, timing, and degree. There is no one-size-fits-all approach to tillage, and practices may vary according to geography and crop rotation. The effect of previous crop on soil tilth following a tillage activity can be stark (Figure 5). In this example, tillage was needed to bury the accumulated residue from continuous corn production; however, extra passes were required to prepare a suitable seedbed. In contrast, minimal tillage was required to prepare a light, even seedbed when soybean was the previous crop.

Strip-tillage is one approach that simultaneously allows for increased residue retention and nutrient placement. We are utilizing strip-tillage accompanied by P placement as a management practice in our sustainability study.

The seventh wonder of the corn yield world (Growth Regulators) encompasses a wide variety of products that can modulate corn growth and yield. These products include seed treatments, ethylene blockers (e.g., Invinsa), and strobilurin fungicides. The ‘greening’ or ‘plant health’ effect associated with application of strobilurin fungicides is a good example (Figure 6). We have documented this effect in the absence of yield-limiting disease, especially under intensive management systems where all other factors are optimized. Although it is difficult to experimentally separate physiological effects from disease control, we believe that strobilurin fungicides elicit a change in the ethylene-mediated senescence trajectory of source leaves.