No. 3 Article 8/April 10, 2009

Corn Planting Date Revisited

March 2009 was marked by swings in both temperature and precipitation in different parts of Illinois, with rainfall ranging from 2 to 3 inches above normal in the northern half of the state to 1 to 2 inches below normal in the southeastern part. Temperatures were above normal over most of the state, with departures ranging from 3 degrees or more in southern areas to less than 1 degree in northern areas. The weather changed rather quickly in the latter part of March, and the first week of April has produced temperatures of 5 to 7 degrees below normal and rainfall ranging from less than normal in western Illinois to an inch or more above normal in the eastern part of the state. So "on average" the spring weather so far hasn't been too bad, but in most areas it has been less than ideal.

Some people took advantage of the warm, dry weather to plant in mid-March. Reports of planting were most numerous from parts of southwestern Illinois, including the "south of Springfield" area that often is among the earliest to report a start to planting. Some of this corn had emerged by the first of April, and since then it has suffered from the low temperatures, including temperatures below freezing on April 6 through 8 and daytime highs early this week in the 40s. Some light snow has helped frost the cake for some of these fields. Night temperatures in the mid- to upper 20s are not very friendly to emerged corn plants, of course, and it's possible that some will be frozen to near seed depth. Very small corn plants may not survive this. Even those that do survive to grow back from the seed will have used up so much of the seed reserve (what has not in the meantime been lost to microbial action in cool, wet soils) that they may not come back with much vigor. As we learned in 2005, freeze damage that results in plants growing back slowly and without their normal vigor may well result in yield loss, even if stands end up okay. At this point, it would be prudent to have seed lined up to replant most fields that were planted in March.

Most of eastern Illinois has wet soils at present, though other parts of the state are starting to dry out. The first date of planting in our corn planting trials this year was made on Friday, April 3, at Monmouth, and the date at Perry was April 7. The cool temperatures will slow drying, however, and it's unlikely that planting of large acreages will get underway unusually early this year. Especially when soil temperatures are well below 50 degrees like they are in most areas now, it is unwise to plant into soils before they are dry enough to work and plant well. Remember that soil at field capacity is at its most "compactable," and even flotation tires can do some damage when they carry heavy equipment across soils that have not warmed up enough to dry out to below field capacity.

The 2008 growing season in Illinois was a very unusual one, with cool, wet conditions through May, adequate to excessive rainfall through July in many places, dry weather in much of August, and rainfall and favorable temperatures in September and October that allowed crops to fill well as they matured late. Yields were high. This affected the planting date work that we did in 2008, with flat or even increasing yields as corn was planted later in central and southern Illinois. At Perry, averaged over populations ranging from 30,000 to 40,000, corn planted on April 7 yielded 205 bushels per acre and that planted on May 30 yielded 215. The extreme example was at Brownstown, where the April 17 planting yielded 169 bushels per acre and the June 6 planting yielded 231.

These results are unusual enough to skew the response averaged over the four years of the study, even including data from other locations. As a compromise, I included the 2008 data from Perry and excluded that from Brownstown. But this serves as a reminder that all planting date responses ever assembled reflect the large amount of variability that occurs over years, and so they are not nearly as precise at predicting planting date responses as the numbers might seem to indicate. Averages over years and locations are the best we can do to make predictions, but the chances that these predictions will hold exactly in any given year are very small.

Figure 1 gives the planting date response curves for northern Illinois (Monmouth and DeKalb), central Illinois (Urbana and Perry), and southern Illinois (Brownstown and Dixon Springs), updated--with the exception noted above--with the 2008 data. We used the same hybrid for the northern locations and Urbana, and a later-maturing one in the southern locations. The latest-planted corn matured with no difficulty in all years and locations, though some of the latest plantings might have had more insect or disease pressure than earlier plantings.

Figure 1. Planting date responses averaged over two sites and four years (2005-2008) for each of three regions in Illinois.

The responses shown in Figure 1 are converted to numbers in Table 2, with average yields and the average loss in yield per day of delay during each 10-day period from early April to early June. According to this, yield losses reach 1 bushel per day by early May in northern and southern Illinois, while the loss is more gradual in central Illinois, reaching 1 bushel per day of delay only in the last third of May. Is central Illinois really this different? The answer is probably not, but including the data from Perry in 2008 effectively decreased the penalty predicted from delayed planting. Unless we know that a year like 2008 can never happen again, there's no good reason to exclude the data.

Given the similarity of the responses to planting delay in southern and northern Illinois, it would not be unreasonable to use the yield losses from those regions to predict response to planting date anywhere in Illinois. That would mean losses of 1, 1.5, and 2 bushels per acre for each day of delay during the first, middle, and last third of May, respectively. Due to higher yields in northern Illinois, putting these losses on a percentage basis means greater rates of decline in southern Illinois than in northern Illinois. It's an old debate about which method is better, but because "bushels pay the bills," I tend to favor the use of bushels and not percentages.

Planting on May 1 is expected to yield only 7 or 8 bushels less than planting on the optimum date in the southern and northern parts of Illinois and less than that--only about a bushel less than planting on the optimum date (April 20)--in central Illinois. So delays in planting until past the end of April, though they cost some yield, do not automatically mean large yield losses. Planting even two or three weeks after the optimum date might well produce higher yields than planting into cool, wet, compacted soils closer to, or before, the optimum date.

Negative yield "losses" for the periods in early April mean that yields actually increase as planting is delayed during that period, because the "best" planting date is later than early April. Such yield penalties for planting very early are typically not large, but they are not uncommon--of 20 site-years (not counting sites with unusual circumstances such as delayed earliest plantings), the earliest planting yielded more than the second planting 11 times, the second planting yielded more than the first planting 8 times, and at one site there was no difference.

When the weather is consistently warm after the first planting emerges, early planting tends to do better relative to later planting. But if warm temperatures early are followed by cool or cold temperatures after corn has passed the 2- or 3-leaf stage (this would most commonly be the largest, earliest-planted corn), we think there can be a temperature-related decrease in yield potential. In 2006 at Urbana, where the first planting yielded almost 40 bushels per acre less than the second planting, there was a period in mid-May during which high temperatures averaged less than 50 degrees. In contrast, April and May 2008 were both cool months, and even though corn emerged and grew slowly from the first two or three planting dates, we did not see lower yields from the earliest planting dates at most locations. We do not know much about this phenomenon, but for now we can only hope that May (as well as the last half of April) turns out to be consistently warm. With the exception of the unusual year in 2008 (when May was like a typical April and June was somewhat like a typical May), two of the three highest corn yields in Illinois have come in years when May temperatures were above normal.--Emerson Nafziger

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