Corn replant decisions in 2004

What a weekend! Who could have predicted that it could go so wrong? Only four days ago I talked to a reporter and shared the good news from Iowa that we got most of our corn and soybean planted during the optimum-planting window and gave our crop the best start for many years. Yes, I know that the weather has been on the cool side and we have seen a little bit of frost injury but no significant replanting has been done in Iowa. This weekend definitely changed that.

Drowning corn

On Friday and Saturday we received, on average, 6 to 10 inches of rainfall along with significant hail injuries and a few "minor" tornadoes. Many reports this morning described severe wind injury with wind gusts of 100 miles an hour. The hardest hit area is the northern two-thirds of the state, where most of our corn and soybean acres are grown. It is still too early to say how many acres need to be replanted. Another front is moving in this afternoon and we may receive another half-inch rain this evening.

Corn that currently is not underwater is stressed and yellow because of the cool temperatures and the wet, saturated soils. Stand evaluations and assessments probably need to be made in each field. Based on my travels around the state yesterday, most farms had a field or part of a field under water, significant erosion, and large gulleys. Regardless of the cause of the damage, the key to survival and regrowth is the health of the growing point. Flooding at any time when the growing point is below the water level can kill the corn plant in a few days. If recovery from the whorl is not visible within four or five days of improved soil conditions, then counts of healthy plants should be made and replanting decisions considered. The cool temperatures that we saw yesterday and today, after the damage, slowed both the recovery and deterioration of the corn, which may force us to wait a few extra days before making an accurate assessment of the field.

Flooding

When you evaluate your corn stands, first you need to determine the plant population in the field (stand counts in a row length equal to 1000th of an acre and count the plants to determine population), then you need to evaluate plant health three or four days later, assess the unevenness of stands, compare the yield of a reduced stand to that of a replanted stand, calculate replanting costs, and evaluate the risk factors of replanting. Optimum populations do vary across locations, but these variations are relatively small.

If you are in a situation where corn needs to be replanted because of last weekend's weather, one of the first questions that pops in my mind is "How late we can plant full-season corn hybrids in Iowa?" Although delayed planting shortens the growing season, corn hybrids adjust well to this delay. Corn yields do not begin to decline rapidly until planting is delayed beyond mid-May. Based on previous Iowa State University research on corn yield response to planting date, corn yield will be 68 percent of the relative yield potential compared to corn planted at the optimum window (April 20 to May 5). A general rule is that if planting is delayed until May 25, you should select a hybrid that matures five days earlier than an adapted full-season hybrid for that area. If planting is delayed another seven days, select a hybrid that matures another five days earlier than the previous one. In general, the date to switch maturities is later in southern Iowa. Generally speaking, if corn planting is delayed until June 10 to 15, very early maturing corn hybrids should be used; after June 10 to 15, corn planting is risky unless the corn can be used for silage. If you decide to switch to another crop, such as soybean, please remember that the corn herbicide program that you used earlier may not allow you to plant soybean.

For more information on corn planting in Iowa, refer to PM 1885: Corn Planting Guide.

Influence of planting date and plant population on corn grain yield (adapted from http://www.extension.iastate.edu/Publications/PM1885.pdf).

Planting Date
April 20-May 5 May 13-19 May 26-June 1 June 10-16 June 24-28
Final Stand* Relative yield potential (percent)
28,000-32,000 100 99 90 68 52
24,000 94 93 85 64 49
20,000 81 80 73 55 42
16,000 74 73 67 50 38
12,000 68 67 61 46 35

* Assumes a uniform plant spacing

This article originally appeared on pages 50-51 of the IC-492 (9) -- May 31, 2004 issue.

Updated 05/30/2004 - 1:00pm