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Make Corn Replant Decisions Carefully

Make Corn Replant Decisions Carefully

Farmers with corn emerging should be evaluating stands and scouting for black cutworm, and those who are considering whether or not to replant corn should get advice from ISU Extension specialists. Tune in to this week's "Crop Minute" interview for more information.

Roger Elmore, Iowa State University Extension corn specialist, discusses the things to consider when making the decision whether or not to replant corn. And Erin Hodgson, ISU Extension entomologist, encourages farmers who have corn emerging to scout those fields for signs of black cutworm. Those are the topics discussed in this week's ISU Extension "Crop Minute" interview (which is actually two interviews this week). So, it's more than a minute's worth of good, practical, useful information for farmers.

Elmore refers to the table on page 12 in the ISU Corn Field Guide for current relative yield potential of corn, according to the planting date and population. Look for the weekly "Crop Minute" on the right side of the ISU Extension ICM News homepage, under More Resources. Or click here to go directly to the May 9, 2011 interviews (mp3) to hear what Elmore and Hodgson have to say.

Last week was a fantastic opportunity for planting corn in Iowa

A lot of corn went into the ground during the May 1 to May 11 period, before rains generally fell over most of the state to halt fieldwork. Thus, the last week or so was a fantastic week for planting corn in Iowa. USDA's weekly weather and crop survey showed that 61% of the state's corn acreage was planted in one week, during the seven days ending May 8. "The state of Iowa is in pretty good shape now, as far as corn planting is concerned. We caught up on planting progress thanks to favorable weather during the first week of May," notes Elmore.

"We're up to the 5-year average now, which is 69% planted as of May 8," he adds. "That's good. You have to remember that the 5-year average is influenced by last year's almost record-pace for planting. But the bottom line is, after a cold and wet April 2011, when hardly any corn was planted in the state, Iowa's planting progress is finally doing pretty well now, as we've had much better planting weather in May."

Look at your cornfields now for emergence, seedling health issues

Farmers in some areas of Iowa are concerned about early planted corn which went into the ground when soil was a little too cold and wet during the first half of April. "The corn planted in early to mid-April probably will be emerging from the ground this week, so this is a good time to get out into your fields and look to see how your corn is emerging," says Elmore. "Are there gaps in the stand? If so, dig and see if the seedlings are actually coming up or not. Assess the corn stand. People are talking about poor stands in some of those early-planted fields."

To assess corn stands in fields, look at the uniformity of emergence and take a stand count before you decide to replant. "Depending on corn population levels that are now present and the calendar date when you could replant, it may be a good idea to replant corn in some fields or areas of fields where the crop emergence or growth of the corn is poor," says Elmore.

Elmore has information on the ISU Extension corn website that can help farmers make replant decisions. It has a checklist you can use. The website is Only about 3% of Iowa's 2011 corn acreage was planted before mid-April.

Continue to scout fields as corn emerges, and evaluate the stand

Scout your fields as the corn emerges and if you have gaps in the stand, dig and see what's going on with seedlings in the soil, advises Elmore. Learn from the mistakes made this spring. Some of the corn planted in early to mid-April when soils were cold and wet is showing sidewall compaction and related issues.

"Corn planted before mid-April is starting to spike through the soil now," observes Paul Kassel, ISU Extension field crop specialist at Spencer in northwest Iowa. "The long cold spell in April has caused some loss of stand in some fields. Corn stands of 30,000 or more plants per acre generally result in maximum yields. If stands are reduced to 25,000 you should expect to get about 95% of maximum yield. This assumes that the remaining stand is fairly uniform."

The cost of replanting and the yield loss from late planting need to be compared to any yield loss you can expect to see from stand reductions, for you to make a well-informed, good decision about whether or not to replant, he says. The table found on page 12 in the ISU Extension publication CSI 0001, Corn Field Guide, is very helpful:

Influence of planting date and plant population on corn grain yields in Iowa

              ---------------- Corn Yields (% of maximum) ----------------
Stand      April 20 -     May 5 -     May 15 -     May 25 -     June 5 -
X 1,000   May 5          May 15     May 25        June 5         June 15
   35         100               96              87               70                54
   30           99               95              86               69                53
   25           95               91              83               67                51
   20           89               85              77               63                48

   15           81               78              71               57                44

   10           71               68              62               50                38

Remember, numerous gaps of up to 4 to 6 feet long in the corn stand can reduce yields by an additional 5% to 6%, says Kassel. The usual method to check corn populations is to measure off 1/1,000th of an acre in a row. That is 26 ft. 2 in. in 20-inch row width, 17 ft. 5 in. in 30-inch rows, 14 ft. 6 in. in 36-inch rows and 13 ft. 9 in. in 38-inch rows. For more information on checking corn stands, visit the ISU Extension corn website which is listed previously in this article.

What about killing corn to replant corn? What are the herbicide considerations for replanting? If you need to replant April-planted corn, here is an article from the University of Illinois on killing emerged corn for replanting corn in that same field:

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