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Corn Planting Delays

May 3, 2002
The official estimates have the Illinois corn crop 25% planted by April 28, compared to a 5-year average of 32% and the 2001 number of 46% planted by this date. The United States had about 25% of the corn crop planted at the end of last week, with a wide range among states; Missouri had about 62% planted and Iowa 33%, but Indiana had only 4% of its corn crop planted. While these numbers are not record setting, this planting season is shaping up as one of the more difficult ones of recent years, at least in Illinois and states east of here.

Besides the slow progress to date, many fields are quite wet; drying rates have been slow due to cool temperatures; and the planting progress has been very uneven, with some areas mostly planted and others yet to start. There is clearly a lot of stress building, as producers watch fronts continue to march across the state, and with the "problem" date--that after which yields will decline with each day's delay in planting--rapidly approaching. Data that we use to predict effects of planting delays on yield in fact show a slightly slower rate of yield loss with planting date delays than most people think, but recent springs have allowed timely planting, and many of us start to worry by the time the calendar turns over to May, if not before. We certainly worry about when we will finish, even with a modest delay in when we start.

The data we use to predict planting delays are from research we did in the northern half of the state in the late 1980s. Such studies are very difficult, given that we can rarely plant whenever we want to in April and May. They can also be compromised by planting when the soil's not fit to plant, thus poorly representing what producers will do. But the work we did went fairly well due to dry springs (1988 and 1989, which were followed by very dry summers also), and we think the numbers are reasonably predictive over much of Illinois. I am not aware of similar data from studies in southern Illinois, but I think it's reasonable to expect similar responses there. Avoiding dry weather later in the season may be more important in those soils, but the growing degree-day accumulation is faster in May and June, and the frost date is later, which helps to compensate for soil effects. It's only an anecdote, but some of the highest yields we have had at Brownstown were in 1993, when planting was in early June, but it rained all summer long.

The results of the planting date study, including information needed to make replant decisions, are in the Illinois Agronomy Handbook. With a full stand, we found the following yields as planting was delayed:
Planting dateYield, % of maximum
April 30 100
May 4 99
May 9 97
May 14 95
May 19 91
May 24 86
May 29 81

Although yield loss accelerates with planting delays, to about 1% (1 1/2 bushels) per day of delay by the end of May, it starts more gradually, with a total loss of only about 5% (7 to 8 bushels) by mid-May. It is important to recognize that, while these numbers may represent average losses we can expect, they will almost never represent losses that might be experienced within a given field in a given year.

Some of the variability in response to planting date is due to continuing weather factors (for example, the recent weather has been cool with very slow-growing degree-day accumulation, meaning that the development of corn whose planting is delayed is not falling very quickly behind that of planted corn, as it would be if the weather were warm and corn planted in mid-April were up and growing). In contrast, April 2001 was very warm, and late-planted corn (which in 2001 meant corn planted later than early May) never caught up; yields suffered as a result, especially in areas that had low rainfall later in the summer. In contrast, we accumulated only about 247 GDD in April this year at Urbana, and more than 60% of that was accumulated during the warm stretch from April 11 to 19, by which time very little corn had been planted. Rainfall in July and August will, as always, be the most important factor affecting yield of corn in Illinois. Once it warms up in May (providing we get the crop mostly planted by then), differences in development between corn planted in mid-April compared to that planted in mid-May will not be large.

Management decisions we make can and will affect how the crop responds to delayed planting. During the rather gradual declines in yield potential during the first half of May, we can in most fields do more damage by working and planting when soils are too wet than we can even gain by planting 3 or 4 days earlier. Of course, there is no guarantee that the weather will turn warm and dry any time soon. But the time to take desperate measures--such as planting as soon as "I can get through without getting stuck"--is certainly not here yet. Compaction before and during planting, which makes root growth more difficult, has the same practical effect as late planting--it makes good yields more dependent on good weather later in the season than we would like the case to be.

In most cases, we probably want planting to be the first operation done as soon as soil conditions allow, with nitrogen and herbicide application done afterward so that planting is not further delayed. But we also need to be reasonable in our approach to difficult weather conditions, recognizing that very good yields are possible even when corn is planted in May.--Emerson D. Nafziger

Author: Emerson Nafziger

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