While there are precious few management options remaining after the corn crop has reached waist height, 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 intercepts 95% of the sunlight. As you look down between corn rows at noon on a clear day the patches of ground receiving sunlight should be very small and scattered. Much more sunlight than this hitting the ground means yield potential will be limited.

Research work from the University of Illinois outlines the importance of the crop canopy in determining yield. Figure 1 shows how yields climb consistently as light interception increases towards 95%.
Some consideration has been given to narrowing corn rows in order to intercept more light. If there is some value in this practice, it may come in the form of increased light interception early in the development of the corn canopy, although even this has been difficult to show consistently. Research has shown that good stands of corn in both 20-inch and 30-inch rows could reach the 95% light interception goal.

The greatest threat to good light interception this year will undoubtedly be low plant populations. How low are your stands? Some growers may be surprised at the low densities that exist in their fields (even the ones that they thought were pretty good) when they actually get out and do some plant stand counts. Was it all the fault of the weather? Could you have increased seed drop in your early planted fields, planted a bit shallower or changed herbicide options to improve emergence and hence final stand and light interception?

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)

Late Season Plant Health
Of course getting a solid corn canopy is one thing, keeping it there is another. Here your scouting really focuses on how well the canopy stays green and active throughout the grain filling period. Scouting for plant health later into the season may allow you to sort out some hybrids that have poorer leaf disease resistance and/or allow you to look for nutrient deficiencies. From the nutrient perspective, 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, and is often associated with dry weather, other soil problems such as compaction or working the ground wet, and of course 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? From a management perspective, 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
Your corn plants may seem very healthy, leaves green, low disease pressure, etc., etc., and yet are still not meeting the demands of grain filling as effectively as they might. 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 and/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, but how about considering some changes in hybrid selection, planter size, or tillage operations if your corn consistently silks in August instead of July?

Along these lines, it may be worth re-mentioning the starter fertilizer research done in Wisconsin during the years 1995-1997. This study examined starter placement on a large number of fields that tested relatively high for P and K. The starter fertilizers employed in this study were not always identical at all of the test sites, but there was always 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 researchers, L. Bundy and T. Andraski, University of Wisconsin, found that on these soils there was an economically profitable corn response in 40 of the 100 field sites tested. Table 1 outlines the number of sites tested each year and corn yields recorded for starter and no starter treatments.

The study revealed a significant tendency for starter fertilizers to have a greater positive impact when the following three conditions were met:

• delayed planting dates
• planting longer season hybrids
• 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 on these types of fields. More frequent responses to starter fertilizer with late planting dates and longer season hybrids may result from 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 it comes to 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 it does in the early planted fields.

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