It’s not a question of IF there will be more corn planted, but rather one of “HOW MUCH.”  Some reports would seem to indicate that the wife’s flower garden and kids’ sandbox will both be occupied by Zea maize in 2007.  Further, it’s clear that there will be more corn planted in fields that were corn last year, and maybe even the year before.  Planting corn on corn is generally considered a situation to be avoided because of increased pest management challenges and the inherent reduction in yield potential compared to first year corn.  To some degree these risks are manageable and that will be the focus of our discussion here. 

Losing the rotation effect

        The rotation effect is real.  Figure 1 shows the average yield loss of corn planted at the Arlington Research Farm from 1987 – 2005 for first year corn compared to varying lengths of continuous corn.

Figure 1.  Corn yield response in a corn-soybean rotation and following five years of soybean (Arlington, 1985-2005)

        From this long term rotation study, we see that one year out of corn is all that is needed to maximize rotation effect for corn.  An 11 percent yield reduction occurred for 2^nd-year corn (176 bu/a compared to 159 bu/a) and a 16 percent yield reduction was measured in the 3^rd through 5^th years (152 bu/a).  Finally, corn grown continuously throughout the trial averaged 148 bu/a, a 19 percent yield reduction from 1^st year corn.  These results are similar to those found in other rotation studies. 

        Yield reductions with continuous corn are highly correlated with tillage.  That is, the more aggressive the tillage, the less yield reduction.  Yield reductions are greatest with no-till corn on corn, a bit better with strip till, and better yet with chisel plowing.  Moldboard plowing is no longer in the discussion because of its negative environmental consequences.  Even though tillage has an impact on expected yield, no system can come close to negating the inherent rotation effect. 

Managing pests

        Pest control is a major consideration in corn on corn systems.  The most obvious is the control of rootworms, which must be dealt with through the use of a soil applied insecticide or by planting a rootworm resistant hybrid.  When using the latter strategy, remember that a 20 percent refuge of non-Bt corn must also be planted.  Three options exist for rootworm control on the refuge corn:  1) take your chances that there won’t be a rootworm problem and do nothing (odds are actually pretty good), 2) use a high rate formulation of seed applied insecticide, or 3) apply a standard rate of soil applied insecticide.  

        In addition to corn rootworm, soil and plant residue borne plant diseases can also be more of an issue with corn after corn.  Hybrid selection for resistance, seed fungicides, and foliar fungicides are options for a higher level of disease control.  Many diseases that afflict corn are environment dependent; hence, weather in 2007 will be the main driver of whether or not we see increased disease pressure because of more corn on corn acres. 

Soil fertility issues

        The need for phosphorus and potassium are driven by soil test, yield level, and whether the corn will be harvested as grain or silage.  The rules don’t change as a result of the preceding crop.  That’s not the case for nitrogen (N).  There is typically a need for more N when corn is planted after corn compared to 1^st-year corn.  The exact rate is determined by the soil yield potential and the N:corn price ratio.  Most area soils have a high yield potential and so N:corn price ratio will be our primary determining factor for maximizing return to N.  I used the Nitrogen $ Rate of Return Calculator spreadsheet to derive the following table: 

         Depending on whether you booked N early and how optimistic you will be that corn price will hold, the optimum rate of N ranges from 120 to 145 pounds per acre.  One reason for bumping these rates slightly higher is a situation where high plant residue cover exists.  Conversely, don’t forget to take appropriate legume and manure credits.  Often overlooked are 2^nd-year N credits for alfalfa.  A 50 lb/acre N credit can be taken for 2^nd year corn following a good to fair stand of alfalfa.      

        Finally, corn after corn will perform best on your most fertile, well-drained soils. Keep plant densities above 30,000 plants per acre.