No. 4 Article 7/April 20, 2007

Dealing with the Delayed Start to Corn Planting

The fact that less than 1% of Illinois corn was planted by April 15 means that the start of corn planting is considerably later than it's been in recent years. With so much on the line with high corn prices this year, there is growing concern that we are starting to lose some yield potential for each day that passes with little planting progress. Corn that we planted here at Urbana on April 2 was just starting to germinate on April 17, with roots and shoots less than a quarter of an inch long. The seed was still very sound, reflecting its past two weeks in the refrigerator that the soil has been. There has thus been little agronomic advantage to planting in early April this year.

Illinois is a large state, so characterizing effects of planting date on yield is not easy or very precise. If we use data for the whole state, we see that corn yield is not very well correlated with an early start to planting, as indicated by the percentage of corn planted by April 10 (Figure 1). But the percentage of corn planted by the end of April does show some correlation with yield (Figure 2). Though it's not a perfect predictor of yield, this indicates that each percentage point increase in corn planted by April 30 means another half bushel in statewide yield. The three highest percentages (all above 72%) planted by the end of April during the past 14 years were in 2004, 2005, and 2006, long enough for some people to develop this as an expectation.

Figure 1. Percent of corn planted by April 10 and corn yield in Illinois, 1993 through 2006. Source: National Agricultural Statistics Service.

Figure 2. Percent of corn planted by April 30 and corn yield in Illinois, 1993 through 2006. Source: National Agricultural Statistics Service.

With typically rapid planting after April 30 regardless of how the year turns out, this correlation tends to decrease at later dates. But the message is that we get most of the crop planted during April in the better years and that failure to do so tends to result in low yields. The adage that "it matters when you finish planting, not when you start" appears to hold true. While there are plenty of anecdotes about individual fields that were planted late and still yielded well, years that allow timely planting tend to be better corn years.

We are not yet to the end of April, so there's little reason to start serious worrying about when we'll finish planting and how much the "delay" will cost. With today's equipment, we can plant faster than ever before. We have planted at more than 5% per day over some 10-day periods in several recent years, and if every field in Illinois were fit to plant at the same time, we likely could plant at least 75% of the corn crop in a week. We estimate that the median number of days Illinois producers need to plant their entire corn crop is about 5. This might be higher by half a day or so this year because of increased corn acreage.

Even with the ability to plant fast, wet fields and some wet weather ahead will probably delay corn planting past the ideal time. The "standard" data used to predict planting delay penalties is some that we generated 15 years ago. Tables are in the Illinois Agronomy Handbook, as well as a replant calculator based on these data. It shows that planting in late April produced the highest yields, and that yield declined by 3% from May 1 to May 10, by 6% from May 10 to May 20, and by 10% from May 20 to May 30. This is about 1/2 bushel per day of delay for the first 10 days of May, 1 bushel per day during the second third of May, and 1-1/2 bushels per day for the last third.

More recent work showed similar responses to planting date, but a little more penalty to late planting. Averaged over 7 trials in the northern half of Illinois in 2005 and 2006, we found the following yield response, averaged over 30,000 and 35,000 populations.

The data in this table start in early April and include a couple of cases where planting in early April produced lower yields than planting in late April. Like the earlier data, the highest yield was produced in the last week of April--April 24, to be precise. Yield penalties for planting delays were about twice as great, on a bushel basis, than in the earlier work. Corn lost yield at about 1 bushel per day from May 1 to 10, 2 bushels per day from May 10 to 20, and almost 3 bushels per day during the last third of May. This is on a higher base yield than was the earlier work, so percentage reductions are not as different, but it's reasonable to conclude, at least as a preliminary finding, that the yield penalty from late planting has not decreased with newer hybrids.

In the southern part of the state, yields declined much more rapidly after mid-April in 2006, with yield loss close to 50% by early June at Dixon Springs. Growing conditions were very favorable at Dixon Springs in 2006 (the highest yield in the mid-April planting was more than 270 bushels per acre), but at Brownstown, yields were lower and the planting date response was much less. We are continuing these trials, but at this point I can say that delaying planting past April is likely to cause as much yield loss in southern Illinois as in the rest of the state. Of course, with higher rainfall in southern Illinois, planting early is often not possible.

It's been a tough start to April, but there's hope that we will catch up quickly with a return to better temperatures and less rainfall. One lingering effect of the cold temperatures is slow drying of the surface soil. But some excess water has drained away, so planting will get under way in some areas as soon as soil can warm up (signaling that drying is more complete) and there are a few days without rain.

Related to the planting date question is the question of whether anhydrous ammonia should be put on before planting, how many days there should be between NH3 application and planting for the crop to be safe, and whether it might be better to wait until after planting to apply NH3. Our normal recommendation has been to wait at least 3 to 5 days between application and planting. But ammonia applied to cool, wet soils does not spread very far from the point of release, and the application slot tends to remain intact. If the soil dries out after application in such conditions, then ammonia can be released from the application band and can move up the application slot, causing damage to any plant tissue that is in the vicinity. In such cases, even waiting two weeks after application to plant may not prevent injury. If the soil does not dry out, there is little danger of injury, since ammonia will stay put in wet soils.

Ammonia often goes on when soil is considered a little too wet to plant, which contributes to the problem. In fact, soils that are in good condition for ammonia application are also in good condition to plant. If it gets to be late April before either operation can be done, it might make more sense to plant first and apply N later. With RTK and autosteer, it might be possible to avoid any problem with ammonia by applying it between where the rows will be planted. It is also possible to apply ammonia immediately after planting rather than wait until the crop has emerged.--Emerson Nafziger

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