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Issue No. 3, Article 10/April 14, 2006

When Corn Follows Corn

If the drop in corn acreage and increase in soybean acreage in Illinois take place as expected this year, there will be less corn following corn in Illinois than there was in 2005. Still, some producers have continuous corn on a major part of their acreage now, and as long as there are more corn acres than soybean acres, some corn will follow corn.

While there is considerable debate about the yield consequences of corn following corn compared to corn following soybean, most planned, direct comparisons of these two systems continue to show some yield penalty when corn follows corn. This is not very consistentthere is little or no penalty in some years and a large one in others. Agronomists have spent decades trying to explain and alleviate this penalty, but to date no one has found the magic answer. Over the past decade, all of my research shows that corn following corn has yielded about 10 percent less than corn following soybean. That "rule of thumb" yield penalty has remained about the same for decades, and though we know that this average covers a range from 0 to 100 bushels per acre, it still is the yield loss that a producer should expect on average when making the conversion from corn following soybean to corn following corn.

At the same time, many (probably most) producers and consultants are absolutely convinced that there is no such yield penalty and that researchers simply don't know how to manage to get the same or even higher yields when corn follows corn. Such a contention gets support from the fact that many corn-yield contest winners use the same field year after year, which suggests to many people that corn in such fields actually gets better the more years the field is in corn. In none of our studies have we seen a consistent improvement in yield of corn following corn as compared to corn following soybean, so if this actually happens, it is masked completely by year-to-year variation. Nor do we see this penalty strongly correlated to yield level, meaning that a high yield of corn following soybean does not automatically mean less yield penalty for corn following corn.

How do we explain what seems to be a serious gap in understanding of such a common system of corn production? We probably can't do so yet, but one reason for this is that most producers who raise corn following corn do so first on their more productive fields, so yield expectations are already somewhat higher. The common idea that corn following corn improves with the number of years of continuous corn is to some extent reinforced by the fact that our expectations for yield change with time, and after a few years we have no real basis for comparison to corn following soybean, so there is little reason to imagine what a yield penalty might have been, especially when yields are high. But the best approach is, I think, still summed up by the producer who told me several years ago that he makes more money from corn following corn even if it does yield 10 percent less than corn following soybean.

If we are planting corn following corn in 2006, what should we do differently than if corn follows soybean? In general, not very much, but the following are some points to consider:

  • Hybrid selection for corn following corn remains a rather inexact science. In the past 2 years, we have had hybrid trials of corn following corn at three locations, and in only one of the six trials did we find good correlation between yields of the same set of hybrids following corn and following soybean. Most seed companies will suggest certain of their hybrids for corn following corn, but this set of hybrids tends to overlap with the set suggested for corn following soybean, and it tends to include the best hybrids offered by the company. Given that there tends to be somewhat more stress on corn following corn (perhaps related to root growth), choosing "defensive" hybrids for corn following corn might make sense. But most producers are reluctant to "play it safe" this way if they feel that the more defensive hybrids will yield less under good conditions. Characteristics that lead to high yield are important no matter what the preceding crop was.
  • If corn yields were much reduced by dry weather in 2005, corn following corn in 2006 has a few advantages. First, there is less crop residue, so there should be less underground "interference" for this year's crop. Remember to give credit for unused nitrogen from last year: We suggest taking the difference between application rate in 2005 (include N from DAP or MAP applied in the fall of 2004) and the actual yield in 2005, then reducing the N rate this year by half this difference. If last year's crop received 200 lb of N and yielded 100 bushels, then the credit this year would be half the difference, or 50 lb of N. It's likely that part of this credit is due to less tie-up by the reduced crop residue.
  • Because most fields with corn following corn were tilled last fall, working and planting them this spring will be slightly different from planting into less-tilled soybean stubble. Be sure to avoid working these fields when it's too wet. We think that roots of corn following corn typically find more barriers to growth than when corn follows soybean, and some of this might be related to working deep-tilled fields sooner than they should be.
  • Related to the previous point, corn following corn tends to respond more to management practices, such as protection from rootworm, that protect the roots and provide conditions for good root growth. Strong-rooted hybrids might be another way to help corn that follows corn, but this should not be the only basis for selection.
  • While our new N rate guidelines are formulated using the results of research done in corn following corn, they still show higher N rates for corn following corn, especially in northern Illinois. The fact that we still see slightly more N needed when yields of corn following corn are high suggests that we use N rates in the upper part of the suggested range when we are producing corn following corn on more productive fields.
  • Plant populations should not be different for corn following corn and corn following soybean. Most producers should be aiming to establish 28,000 to 32,000 plants per acre on average to above-average fields. Uniformity of seed placement is important regardless of the previous crop, though what it takes to get that might differ if the previous crop was corn. In particular, root balls or clumps of residue can be problems for seed placement. Uniformity of plant spacing down the row is not of much concern if stands are good.
  • There may be reason to watch fields more closely for fungal diseases when corn follows corn, especially if there is significant residue left on the surface after planting. The severity of diseases like gray leaf spot may be affected as much by tillage method and air movement (for example, fields bordered by trees) as by previous crop, though.

Finally, the data we have collected over the past 2 years indicate some possible promise of the corn-corn-soybean rotation. We do not find that second-year corn yields less than continuous corn, and in some cases it has yielded more. For producers without highly productive fields to convert to continuous corn, using the corn-corn-soybean rotations might be a good option. Stand by for more information on this study.--Emerson Nafziger

Emerson Nafziger

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