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:

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

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