Watching The Corn Crop

While there are precious few management requirements remaining after the corn crop has reached waste high, walking the fields may lead to some improvements down the road.

Light Interception

When your corn has reached the silking stage, leaf expansion is completed, and light interception will be at its maximum. High yields will require a canopy that is intercepting 95% of the sunlight. Light interception of 95% means that as you look down between corn rows at noon on a clear day the patches of ground receiving sunlight are very small and scattered. Much more sunlight than this hitting the ground means yield potential will be limited.

University of Illinois research outlines the importance of the crop canopy in determining yield. Figure 1 illustrates how yields climb consistently as light interception increases towards 95%.

Figure 1. Relationship between light interception during grainfill and corn yield. Data are from a plant population trial conducted at Urbana, Illinois, and are averaged over 3 years (1992-1994). (Illinois Agronomy Handbook, 1999-2000, University of Illinois at Urbana-Champaign)

Some consideration to narrowing corn rows is based upon intercepting more light. If there is some value in this it may come early in the development of the corn canopy, although this has been difficult to show consistently. Research has shown that good stands of corn in both 20 and 30 inch rows could reach the 95 % light interception goal.

When scouting fields low plant populations will undoubtedly be the biggest threat to good light interception. Some growers may be surprised at the low densities that exist in their fields when they actually do some plant stand counts. Was it all the fault of the weather? In your early-planted fields, could you have increased seed drop, been more attentive to planting depth or changed herbicide options to improve emergence and hence final stand and light interception?

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Late Season Plant Health

Getting a solid corn canopy is one thing, keeping it there is another. How well does the canopy stay green and active throughout the grain filling period? Scouting for plant health later in the season may help identify nutrient deficiencies and hybrids that have poorer leaf disease resistance.

Lower leaf firing can be a result of either nitrogen or potassium deficiencies. Firing at the tip and then yellowing down the leaf edges indicates a potash deficiency. Firing at the tip and then yellowing down the middle of the leaf indicates low nitrogen. Firing of the bottom leaves is fairly common. It is often associated with dry weather, other soil problems (compaction or working the ground wet), and with eventual maturation of the plant later in the season. An Iowa study indicated that if there was a check strip of relatively high N fertility in the field, then leaf firing was a reliable indication of N status. Without the check strip, leaf firing was difficult to use as a specific indication of nitrogen shortfall in the crop.

The leaves of the corn plant must keep up to the demand of the ear for starch in order to prevent the plant from cannibalizing itself during the grain filling period. How is your late season plant health? Can you improve hybrid selection (Bt vs non-Bt, stay green characteristics), should you pay more attention to leaf disease scores, or is there room to optimize nitrogen rates to improve plant health during the critical grain filling period?

Early Silking

Corn plants may seem very healthy, with green leavesand low disease pressure, and yet still not meet the demands of grain filling. Why? One reason may be that the grain filling period has slid into that part of the season when days are shorter, light intensities are dropping and temperatures are cooler, thus the photosynthetic capacity of the plant is also diminished. The importance of early planting, good early growth, or early silking hybrids is not all about getting to black layer before the first frost. It is also about getting a significant portion of the grain filling period to occur in days, which are longer, and with more sunlight than what we get in September.

If your corn silks this year on August 3 instead of July 25, it may be easy to blame it on the weather, delayed planting or delayed emergence. But are there any management changes to be made? Drainage may the biggest limitation to getting corn planted and off to a fast start. Also consider some changes in hybrid selection, planter size, or tillage operations if your corn consistently silks in August instead of July.

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Starter Fertilizer

Starter fertilizer research at the University of Wisconsin during examined starter placement on a large number of fields that tested relatively high for P and K. There was a starter (5-cm by 5-cm planter applied band) versus no starter comparison. The average rate of application for the starter plots across the entire study was 15(N) – 26(P₂O₅) – 32(K₂O) lbs per acre. The researchersfound that there was an economic yield response in 40 of the 100 field sites tested. Table 1 outlines the number of sites tested each year and corn yields for starter and no starter treatments.

L.G. Bundy and T.W. Andraski, Journal of Production Agriculture. 12:664-670 (1999)

The study concluded that there was a significant tendency for starter fertilizers to have more of a positive impact when the following three conditions were met:

1. Delayed planting dates
2. Planting longer season hybrids.
3. Soil tests for K less than 140 ppm.

This is in contrast to the belief that starters in early planted/cool soils may have more of an impact. More frequent responses to starter fertilizer with late planting dates and hybrids may be due to stimulation of vital early season growth rates by starter fertilizer resulting in realization of more of the crop’s yield potential by the end of the growing season.

When considering management techniques that will speed early growth and canopy development starter fertilizer may play as much or more of a role in later planted corn as they do in the early planted fields.

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