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Issue No. 7, Article 11/May 12, 2006

Looking at Early Corn Development

While corn planting is more than 90% complete by now in Illinois (it was officially reported at 85% as of Sunday, May 7), I had a question this week about why later-planted corn isn't emerging as quickly as expected. Some such questions are coming from parts of Illinois where the crop was pounded by heavy rain after planting, and I'll address those at the end of this article. But it might help to review the time to emergence in corn and to look forward a few weeks as the corn crop starts to develop.

We have for a long time used as a rule of thumb that it takes about 110 to 120 growing degree-days (GDDs) from planting to emergence. This isn't an iron-clad rule, but when temperature and soil moisture conditions are within normal ranges, it works fairly well. Table 7 uses historical temperature data from Urbana to predict the number of days after different planting dates needed to accumulate about 115 GDDs and contrasts this with actual days needed to reach these GDD totals this year.

The table explains why people who were amazed to see the crop come up in about a week when planted in mid-April in some areas (it was warmer in some of the drier, more-planted areas than in Urbana) are wondering why later-planted fields are coming up no faster than that, and in some cases are taking longer to emerge. April 2006 had more than the normal GDD accumulation in most parts of Illinois, but just as remarkable is the consistency of temperatures over the whole month and the steady pace of GDD accumulation that resulted. Not only has this meant good, consistent emergence in most areas without the cold weather interruptions that we often get, but emergence has been very good in most fields, and consistency of stands is good in these fields.

The article "Staging Corn Plants and Implications Associated with Herbicide Applications" in this issue describes corn growth staging during early growth. We can use GDDs to predict stages as the crop grows, but we need to keep in mind that such predictions aren't perfect, and there is no substitute for going into fields to stage the crop on the day we need to know its stage. But GDDs will often get us close, especially when temperatures stay within normal ranges and we don't have setbacks caused by frost, hail, and the like.

Dr. Bob Nielsen at Purdue University suggests that it takes about 85 GDDs to add one leaf stage up to the 10-leaf stage (V10) using the leaf collar method of staging, then about 50 GDDs per leaf from V10 to tasseling. That's probably simplified to some extent, based on what we know about plant development. From emergence to about the V6 stage, the stem of the corn plant elongates very slowly underground, with the tip of the stem (growing point) reaching the soil surface only by about V6, at which point the entire stem is no more than 1 inch long. Stem elongation starts to accelerate after that but reaches "full speed" only about the time the 10th internode is elongating, when the plant is about waist high.

The slow growth of the stem early in the season means that the emergence of leaves and leaf collars depends mostly on leaf expansion and elongation. That's why leaf appearance is relatively slow early in the season. Corn planted at Urbana on March 30 is now at the 4-leaf stage, and about 400 GDDs have accumulated since planting. The crop emerged on April 15, at a GDD accumulation of about 120 (there were about 20 GDDs each day on April 14 and 15, so we'll use the total through April 14 here). Since then, an additional 280 GDDs have accumulated. We estimate that it takes only about 30 to 40 GDDs from emergence to V1, so about 240 GDDs have accumulated since V1, and three V-stages have been added. This means that about 80 GDDs have accumulated per leaf stage, very close to Nielsen's suggested requirement.

So why did I say above that the 85 GDDs per leaf stage up to V10 is "simplified"? We expect the appearance rate of leaves to accelerate as soon as stem elongation starts, which is at or a little before V6. In that case, it's likely that GDDs per stage start dropping by V7 or so. As the last several leaves appear before tasseling, stem elongation is very rapid, and GDDs per leaf usually drop below 50. But it doesn't make a lot of sense to try to finesse GDDs per leaf stage much more than using two rates, especially when these predictions are not that precise in any case.

As an example of how we can use GDDs to predict development, we can use 160 GDDs from planting to V1 and 85 GDDs per leaf stage up to V10 to predict the date plants will reach V6 (585 GDDs) from different planting dates, based on average and 2006 GDDs. Table 8 uses 2006 data at Urbana up to May 9, then average GDDs after that.

So we see that expected dates of reaching V6 are moved up by 5 or 6 days compared to average for early-planted corn this year, as a result of the relatively warm temperatures during the first half of April. As the days get warmer and growth accelerates during June, these differences will become less obvious, and if we get some rainfall and average temperatures during June, much of the Illinois crop should be on track to pollinate a few days ahead of normal in 2006.

While the corn crop has emerged or is emerging well in most of Illinois, we are aware that there are some emergence problems in areas that received heavy rainfall the last few days of April and first few days of May. These problems were generally in the central part of the state, where more than 3 inches of rain fell in some places. Corn planted after April 18 to 20 in these areas was not yet up when the rains came, and it was relatively cool after the rains, which reduced drying rates and meant that soils stayed wet longer. Soil crusts have formed as well, though cool and cloudy weather after heavy rain usually means that soil crusts are not as strong as when it's sunny and windy.

People in affected areas have been rotary hoeing fields to help emergence. If more than 130 or 140 GDDs have accumulated since planting and there's no sign of emergence, dig up seedlings to see if rotary hoeing will help. If seedlings show clear symptoms from being in saturated soils (soft rot of the kernel, discoloration of the seeding, rotting of tissue), then they might fail to grow vigorously even if hoeing frees them, and replanting might be necessary. If they show "compressed" growth that comes from trying to emerge against a physical barrier (short, thick coleoptile, leafing out), then hoeing might help, but it needs to be done as soon as possible. Rotary hoeing tends to do more damage to plants that are not very well rooted, so there is the potential to harm more than help if root growth has been restricted by the wet conditions. If it appears likely that the population of healthy seedlings will be less than 75% or so of planted seeds, then replanting might be in order. According to our replant model (see the calculator), the stand at which replanting on May 15 (with seed at no cost) would not increase net return is 21,000 plants per acre. That assumes that surviving plants are healthy and productive, however, and if some of them are not, then the existing population needed to produce as much income as a replanted stand increases.--Emerson Nafziger

Emerson Nafziger

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