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Issue No. 9, Article 10/May 22, 2009

Replanting Corn

Corn planting in Illinois continues its slow progress, with only 20% of the crop planted by May 17. While drying weather is finally here this week and planters are starting to move in places, several of the short periods during which we made some planting progress were ended by heavy rainfall events. This means that some stands are not very good, and some of those that haven't been in the ground long enough to emerge will struggle. As much as we'd rather not have to think about this, especially with the poor start to the planting season, some of the fields in Illinois that were planted in recent weeks will need to be replanted.

One question that is increasingly heard as planting is delayed or when replanting is certain is whether to go to an earlier-maturing hybrid. If the first-choice hybrid is midseason to slightly later than midseason--about 111-, 113- and 115-day RM in northern, central, and southern Illinois, respectively--there is little reason to switch to an earlier hybrid if planting can be done by the end of May in northern Illinois or by the end of first week of June in central and southern Illinois. The cushion--the difference between expected GDD accumulation and the GDD requirement for a given corn hybrid--may be zero or even negative when planting such hybrids very late. But late-planted corn accumulates fewer than the normally required number of GDDs, which means that the cushion may not need to be as large. This drop in GDD requirement is not very consistent across years, and it is often proportional to yield loss from late planting, perhaps because hot, dry weather late in the season both speeds development of late-planted corn and decreases yield. But early-maturing corn planted late is likely to be hurt even more by weather-related problems than mid-maturity hybrids would be, and both will be damaged by early frost. This suggests that going to earlier hybrids, especially if they are not adapted, may provide little or no benefit when planting is late.

We had a lot of experience in 2008 replanting drowned-out spots in fields, and there's not much that needs to be said on this topic. If the spots are large enough, simply replant them once soils have dried enough. Waiting long enough for soils in low spots to be in good shape can be a test of patience, but such areas will likely need to be harvested later than the rest of the field anyway, so there's little to be gained by pushing to plant while soils are still wet. When soil at the depth of planting forms a ball that deforms instead of crumbling when pressed with your thumb, it is still too wet. The first planting is usually completely dead in such areas, so there's no concern about competition from older plants.

In those relatively rare cases when stand loss is more or less uniform across a field, the replant decision can be assisted by data on the expected yield increase from replanting. Over the past four years we have accumulated data on both planting date and plant population from six locations around Illinois in order to create new numbers to aid in the replanting decision. Table 1 has the results of this work from the two northern Illinois sites.

The data in the table are given as percent of maximum yield to take into account the different yield potentials in different fields. While these data are from northern Illinois, the planting date response in southern Illinois was similar. Plant population response differed slightly, with highest yields in southern Illinois produced at about 33,000 plants. The change to lower-than-optimum populations, however, was similar across years at most locations, so the data can be used at any Illinois location.

To determine the advisability of replanting, first find the expected yield (percent) at the existing combination of the first planting date and the population remaining in the damaged stand. If the date and populations are between two lines or columns on the table, simply estimate. Then estimate the yield for a full stand planted on the replant date.

Here is an example: A stand planted on April 25 is now at 22,500 healthy plants per acre, uniformly distributed. Estimating halfway between 20 and 25 (thousand) and halfway between April 20 and 30 gives an estimated 83% of maximum yield. If I replant on May 25 at the population of 35,000 to give maximum yield (this was dropped slightly to take seed cost into account), I can expect a yield of about 81% of maximum. In this case the yield expected from replanting is actually less than would be expected from keeping the existing stand.

If there were only 20,000 plants from an April 25 planting, then replanting on May 25 would be expected to produce about 2 percentage points more than leaving the reduced stand. Multiply the expected yield from this field by .02 to see how many bushels this would amount to. If yield from the field is expected to be 200, then replanting would be expected to increase yield by 4 bushels per acre. If this would be expected to cover replant costs (possible only if replant seed were free), replanting would be indicated.

In cases where the numbers show that replanting would likely result in little yield benefit, it is advisable to use some common sense and experience before finalizing the decision. Factors in favor of replanting in such cases include non-uniformity in plant size; poor distribution of plants, with a lot of row sections where two or more plants are missing together; signs of disease or other damage on surviving plants; and a gut feeling that replanting in this particular field is likely to do well. It is important to make the replant decision based on estimated effects on net income, not on an emotional basis or a need to "make the field look better." At the same time, listening to one's gut isn't always a bad thing, especially when an objective process shows that the outcome of replanting is likely to be a wash.

Factors that work against replanting in cases of little expected yield increase include high cost of replant seed; droughty soil where late-planted corn has typically come under a great deal of stress in past years; fields where foliar diseases are likely to be a problem with later-planted corn (for example, those with trees on two or three sides); soils that are still not really dry enough at the time of replanting, where damage from replanting is likely to decrease yield; and high costs of drying the wet grain that typically results from late planting.

Stand problems that vary widely across the whole field are a serious challenge when making a replant decision. If less than a fourth or so of the field has very low stands while the rest of the stand is good, it might be feasible to "patch-plant" those sections affected. The patch-planting operation is as much art as science, especially with a 16- or 24-row planter, where the whole width is used even if only three or four rows need replanting. Some newer planters allow individual units or sections to be shut off, but it would be a tedious operation to do this by sight, and many people would decide instead to simply plant the whole width wherever some replanting is needed.

In most cases of patch-planting, the dropped population should be set to no more than the number of seeds needed to restore the most thinned place in the field to its full stand. If it is possible to change planting rates from the tractor seat, that would be helpful, but of course it would get tedious having to change this frequently after estimating on the go how many plants are missing. There is also the issue of planting right over the old row to try to damage it or between the old rows in order to keep both stands.

I saw one patch-planted field in 2008 that had double stands in perhaps a fourth of the field. That may not have been a huge problem under the great conditions we had in 2008, but it would be a big problem under average weather. Leaving the option to cultivate out, or perhaps to direct a herbicide to kill, the less desirable stand (old or new) in different parts of the field makes sense to a few people, but the inability to guess whether to leave or destroy the old stand at planting often carries over to an inability to make that choice when both stands are present.--Emerson Nafziger

Emerson Nafziger

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