ASI: It’s all about timing

Two words for time in ancient Greek are chronos and kairos. Chronos referred to sequential or chronological time. Kairos depicted “a time in between,” when something special happened, an opportune or supreme moment, a time of either crisis or opportunity. While kairos is qualitative; chronos is quantitative. The clock ticking on my desk marks chronos; the wedding this weekend is a kairos moment — at least for the newlyweds and their families!

With corn it’s all about timing, too, both chronos and kairos. A few seconds, minutes, hours or days here or there — “nicks” on the measuring stick of time — and stresses during the landmark events can make all the difference in life for plants.

Pollen shed and silking

Everything this year points to a wide window for both pollen shed and silking. I wrote about earlier silking and yield in this column in July 2011. No doubt earlier silking is in part associated with earlier planting, and in addition, early silking is loosely correlated with higher yields. However, many exceptions occur.

For example, although we had relatively early silking dates from 2005 to 2007 and again in 2010, silking nearly a week later in 2009 resulted in the highest corn yield in Iowa’s history. Our second-best yield occurred in 2004, which silked about the same time as those of 2005-07 and 2010.

As for the most critical kairos for corn, we know that stress during the critical pollination and silking period often reduces yield potential. Water stress rises to the top as the most notorious stress factor, although high temperatures, defoliation (from hail, insects, etc.) and extremely high plant populations, among others, reduce yield during this critical time, especially when coupled with drought stress.

During flowering, plants use more water (0.35 inch or more per day) than at any other time. This is in part because silks have the highest water content among all parts of the corn plant.

Anthesis-silk interval, or ASI, is an indicator of stress. It’s one of the best indicators of how plants respond to stress during flowering by measuring the time in days — chronos — between pollen shed and silk emergence. Along the same lines, we often talk about the “nick,” referring to the overlap of these two critical developmental stages.

The ASI for older hybrids in good condition may have been two to three days with a range up to a week or more. Corn breeders over the last five to six decades worked toward developing hybrids that shed pollen and silk at nearly the same time. And they have succeeded! With some modern hybrids, silks actually emerge before pollen shed even begins.

These reductions in ASI have helped stabilize modern corn yields in stressed environments. In situations where water is limited, silk emergence and elongation slows. Pollen shed remains constant or accelerates. In older hybrids, water stress often resulted in a loss of nick; thus when silks emerged, there was no pollen source. Barren plants or ears with fewer kernels per ear resulted.

By condensing the window of time between tassel and silk emergence, we are more assured of having good pollination with modern hybrids. This is true even if the silks are delayed a couple days or more due to stress from lack of moisture.

Pollen shed occurs over five to eight days, and silks are viable and receptive to pollen up to seven to 10 days. Smaller ASI values means a greater chance of successful seed set, increased kernel numbers and increased yield.

The wide planting window this year offers challenges as well as potentials. Taking note of ASI in your fields will indicate how the crop is faring at flowering. This is the supreme kairos for corn. Did pollen shed and silking coincide in near-perfect synchrony, or did stress delay silking and not pollen shed? In the long run, the impact of stress conditions during this kairos will spell out in yield.

Elmore is the Iowa State University Extension corn agronomist.

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Planting dates

Last month in this column we discussed the multiple 2012 planting dates, as well as some of the pluses and minuses associated with them. On average we can and often do say earlier planting results in greater yields and later planting results in lower yields. Of course, as with any average, these generalities aren’t always true. Sometimes later planting dates result in the best yields.

Since planting, the 2012 crop has endured much. Emergence problems plagued a good share of corn planted the last week of April (when we planted 41% of our corn) as temperatures turned cold in early May.

Seedling blights, imbibitional chilling, side-wall compaction and “rootless” corn reduced stands in various parts of Iowa to the extent that warranted replanting in some cases. Intense weed pressure, abnormally dry conditions and early rootworm larvae feeding all plagued us, and just now as I write, Goss’s wilt snuck its way in again!

This article published in the July, 2012 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.