Iowa State University Extension corn agronomist Roger Elmore has been getting two often-asked questions from farmers this past week.

1. Is the corn planting window closing?

2. What about corn that is already planted?

Is Iowa's Corn Planting Window Closing?

As of Sunday April 29 USDA-NASS reported half of Iowa's corn lay in seed beds, the other half in seed bags. That rate puts Iowa 18% ahead of the five-year average. "Note that 41% of Iowa's anticipated 2012 corn crop was planted in one week, with only 4.3 days suitable for fieldwork. If we can use that number for days corn was planted, that's 5.9 million acres in 4.3 days or 1.37 million acres per day. It proves farmers can plant a lot of corn quickly in Iowa when conditions are right. Unfortunately, weather conditions the week of April 30-May 5 weren't as favorable. How should we think about this?"

* Corn already planted: By April 22 farmers had 9% of Iowa's corn planted. Four percent of that was planted between April 16-22, and 4% between April 9-15, and 1% prior to April 8. "Depending on your specific location, all of the corn planted prior to April 15 should be emerged or very close to emerging now based on average heat unit accumulations across the state," Elmore noted on May 2. "However, heat unit accumulation (growing degree days or GDD) has been a bit less in the northern third of the state, so corn in those areas planted before April 15 may be close to emergence but not quite there yet. It takes about 90 to 120 GDD's from planting to emergence."

Is Iowa's Corn Planting Window Closing?

* Check corn emergence: Corn planted after the April 15 date, including that planted the week of April 23 to 29, should be well along in the germination process but not yet emerged as of May 2. For more information on this, read the article Elwynn Taylor and Roger Elmore wrote about corn's germination process.

"Once emergence occurs, you need to evaluate plant stands carefully – whether you expect good emergence and seedling survival or not. Poor stands and plant-to-plant variability lower yield potential," says Elmore. "However, depending on the potential date of replant, keeping the surviving stand may be the best option – even with variable plant heights and development."

There are two situations that may cause you to consider replanting:

1) If corn plants emerged non-uniformly, resulting in different plant developmental stages within a row but plant populations are reasonable, replanting will not likely be of benefit, says Elmore. Although the smaller plants compete with their larger neighbors for resources, only extreme conditions warrant replanting. If half the plants are two-leaves behind the rest of the plants within a row, yields can be reduced by 5% to 10%. You can estimate yield loss in fields exhibiting non-uniform development by using a tool on uneven emergence posted at our website.

2) If corn populations are significantly lower than desired, then replanting may be of benefit, he adds. Elmore suggests you consider several things and make comparisons when determining if a specific field fits this category:

* Estimate stands. Measure the existing plant population in several random areas in the field. Use the 'Replant Checklist' for steps to evaluate an existing stand in a problem field.

* Estimate yields. The most important factor in deciding whether or not to replant is to calculate expected yield with the current stand versus what you could potentially have if you replanted. The accompanying table provides guidelines for this decision. The data shows relative yield potential for numerous planting dates and plant populations based on recent yield data, planting date trends and modern ranges in plant populations.

"The replant decision rarely comes easy," notes Elmore. "Numerous factors determine a field's yield potential. Consider data like that presented in the accompanying table as a tool to use in approximating what may result — based on our best available research data. Please realize though that actual yield losses may be greater or less than what is shown."

Corn not yet planted: is the window closing?

Optimum Iowa corn planting dates range from mid-April to the end of April in north central and northeast Iowa and to the first or second week in May in other parts of Iowa. "The table in my March 27 article, Best planting dates for Iowa, summarizes the recommendations for various regions of Iowa," says Elmore. "So, what is there to say the first week of May when only half of Iowa's corn is planted?" Here it is in a nutshell:

Be patient if corn is not yet planted, advises Elmore. It is far better to wait for good soil conditions than to 'mud in' corn.

He adds, "There is little question based on our research trials that, on average, yield potentials begin to drop after May 2 in north central and northeast Iowa; after May 13 in southwest, south central and southwest Iowa, and after May 18 in northwest, west central, central and east central Iowa. Those are average responses and we don't know what 2012 will offer. If 2012 provides 'average' growing conditions, the accompanying table contains some ideas on potential yield reductions associated with delayed planting. For example, if you target 35,000 plants per acre and you are able to plant between May 5 and May 15, you may experience 96% of the original yield potential."

Certainly it is time to plant if soil conditions allow, he adds. 'Mudding in' corn could cause trouble the entire growing season. Soil temperatures are back to normal for this time of year across most of the state; normals are about 50 F. "So soil temperatures are pointing in our favor for planting corn," he says. "The calendar clearly shows we should not think too hard about planting, but the call for more rain this past week did – and should – slow planting progress." When soil conditions are favorable, it is time to plant corn, he adds. Is the window for planting corn closing? Not yet!