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Issue No. 19, Article 6/July 31, 2009

Do More Corn Plants Need More Nitrogen?

The Illinois corn crop continues to develop slowly, with just over 50% silking as of July 26. It's truly a unique experience to reach the end of July with as much as a third of the Illinois crop yet to reach the pollination stage. Moisture has been good and the crop canopy continues to improve, though the percentage of the crop rated good or excellent continues to languish in the low 60s. The fact that this rating hasn't been moving much reflects the unevenness in many fields, with ongoing water damage and compaction effects, and often with yellow and stunted corn. It doesn't help that temperatures remain below average, though as we said in 2008, cool days during the growing season can be okay as long as we get enough of them.

The range of yellow to dark green within and among corn fields is a reminder that nitrogen is one of the really difficult things to manage in corn production. We know that soils with higher organic matter can provide a great deal of N to the corn crop; each percentage point of organic matter represents about 1,000 pounds of organic N in the top 7 inches of soil, and between 1% and 3% of this N can become available to the crop in a year as a result of microbial action. Over the range of soils, weather, and crop conditions in Illinois fields, the amount of N provided by the soil in an average year ranges from less than 20 or 30 pounds to more than 150. A 200-bushel corn crop takes up about 240 pounds of N, and fertilizer N needs to make up the difference between crop need and soil supply. It's little wonder we see so much variation in response to fertilizer N.

One part of estimating how much fertilizer N the crop requires is trying to determine how much the crop will need. While we can't really guess at this without knowing the yield level, some management factors do often affect yield, and hence affect N uptake. One such factor is plant population. We have heard a consistent message that "higher" plant populations are needed to reach the yield potential of modern hybrids. If that's the case, wouldn't higher populations always need higher rates of N fertilizer?

The idea that higher corn plant populations must have more fertilizer N to avoid deficiency seems so obvious that many would wonder why we'd even ask the question. Articles in the popular farm press have explained how each additional plant in the field needs just so much more N. In calls I've received about planting errors that result in populations of 50,000 or more, the first impulse seems to be to put more N on so that yields won't suffer so much.

We conducted a study over 10 site-years where we varied N rate from 60 to 240 pounds per acre and plant populations from 20,000 to 40,000 per acre (final stand). Across the seven locations where there was a response to N rate (the study years included 2006, where N responses were reduced by carryover N following the dry 2005), the response to N was not consistently affected by population level (Figure 1). The lowest populations seemed to require less N, but the two higher populations (33,300 and 40,000 plants) needed about the same amount of N to maximize yield.

Figure 1. Nitrogen fertilizer rate response at different plant populations (shown in thousands), averaged over 7 Illinois site-years.

So why didn't 40,000 plants respond to a higher N rate more than 33,300 plants? Some might think it was just a fluke, or that we didn't choose the "right" hybrids, or that we just messed up. I don't think any of these is a good explanation. Instead, I think we simply see here that different things limit yields in different fields and years, and that we aren't very good at guessing, or controlling, what that will be. For example, water often limits yields, and when this happens fewer plants are needed, and higher N rates can actually reduce yields in some cases by increasing leaf area and photosynthetic (and transpiration) rates, thus accelerating the onset of drought. So when water is short, more plants and more N can both contribute to yield loss, and the strategy of boosting both can backfire.

So what do you do with N rate if you end up with high plant population, whether by accident or intent? Treat it exactly as you would with a more "normal" population. Understand that corn may well start to "fire" if it runs out of water, but that shortage of N is not the direct cause of such firing. In other words, understand what limits yields, and manage accordingly. From a common-sense standpoint, raising populations from, say, 32,000 to 35,000 would normally increase yields by only 5 to 10 bushels, and this would likely require less than 10 pounds of additional N to be taken up. That small amount of N can easily be "lost" given that higher populations mean slight changes in plant size, harvest index (proportion of grain weight to plant weight), root growth patterns, and so on.

For more dramatic evidence on this point, note that some of the low-lying areas in wetter parts of the state have both low populations and light green or yellow leaves, even though we would expect the N supply to be better per plant at low populations. Some of this yellowing is a result of N loss, but in many cases root growth is also compromised, and N uptake may be affected more by root damage than it is by loss of N.

This is the time of the season when looking at such things as N and population responses can help us to sharpen our management. Remember: Canopy counts--we will not harvest good yields if leaf cover is not very complete and if leaves don't have the good, green color that indicates high photosynthetic capacity.--Emerson Nafgizer

Emerson Nafziger

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