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A Good Day in the Field

July 17, 2003

Today, July 16, 2003, is one of those "best" days that crops sometimes experience in the middle of the growing season. There are exceptions across Illinois, but in most places the corn and soybean crops are in good shape, there is adequate soil water from rains during the past week, and there are relatively few diseases, though insects are posing a threat to leaves and silks in some places.

What makes today such a good day? First is the temperature range: low 60s this morning, with a high expected to be about 85°F. These are nearly ideal temperatures for corn. The low was perhaps a bit low for soybean, but that crop is still not into its most critical time when temperatures will affect yield. For corn, night temperatures in the 60s prevent excessive loss of sugars through respiration. The low humidity that accompanies low night temperatures is also useful, in that it helps reduce disease development and means more rapid opening of stomata in the day (see the following).

The second factor that plants will respond positively to today is sunlight; it is supposed to be bright all day long. So today the crop will receive light energy in the visible range (plants use light in about the same range of wavelengths [colors] as we see with our eyes) at a rate of about 50 watts per square foot. (At 50% efficiency, that's a 100-watt bulb for every square foot.) Over the 11 hours or so of bright sunlight today, the visible light energy that will fall on the field is equivalent to about 450 kilocalories, or about the energy in two regular-sized Hershey chocolate bars, per square foot. If all of this energy were converted to sugar through photosynthesis at 100% efficiency, that would amount to about 1/4 pound of sugar per square foot, or some 10,000 pounds of sugar per acre. Because of various factors, most of which cannot be changed, field efficiency of photosynthesis tops out at less than 10%, even for the most photosynthetically efficient plants like corn, sugarcane, and grain sorghum. Even 8% efficiency, though, would mean about 800 pounds of sugars produced per acre on a good day like today.

If we produced 800 pounds of sugars per day for the 50 or so days of grain-filling in corn, that would mean about the dry-weight equivalent of 16 bushels per day, or 800 bushels per acre for the crop. Why don't we get closer to this number than we do? One of the less changeable factors is that corn kernels contain oil and protein, which take more energy to form than starch, which is just sugar molecules strung together. So just the conversion to corn kernels would use up perhaps 15% of the sugars produced. The plant also has to maintain itself, taking up nutrients and maintaining root, stalk, and leaf tissue. This may take another 15% of the sugars produced, bringing us down to about 11 bushels per acre for our "best" days and to about 560 bushels per acre for the crop if every day were a really good one.

Why don't we usually produce more than 30% or 35% of this "best possible" yield, even with our good soils and careful management? The main reason is that "best" days--days like today--are so rare. Most days are "stressful" for the crop, at least to some degree. I define stress as anything that reduces the amount of photosynthesis. In the field, sources of stress are numerous. Temperatures are usually not ideal, especially when night temperatures are high (above the low 70s), or, more rarely, when day temperatures are too low, below the mid-80s. Disease and insects can reduce the rate of photosynthesis through damage to the leaves. Cloudy weather can substantially reduce the amount of sunlight energy that falls in a day. Finally, lack of adequate soil water over time can reduce plant leaf area, and on a daily basis can cause leaf curling and reduced light interception, or can simply reduce photosynthetic rates when the leaf cannot get enough water to maintain transpiration.

Transpiration is the loss of water vapor through the leaves, and the rate at which it happens is directly proportional to the rate of photosynthesis; we cannot have a productive field without losing a lot of water from the soil to the air. Rapid transpiration means that the stomata (tiny openings) in the leaf surface are open to allow carbon dioxide gas in for photosynthesis, and it also means that the rapid evaporation of water from cell surfaces inside the leaf is helping to cool the leaf. This is all good, but it means that the crop uses up soil water fast, and so soil water depletes quickly if there isn't frequent rain to replenish it. Today, the crop will lose perhaps 0.2 inch of water, which is about 5,500 gallons per acre or about 1-1/2 pints per plant. If temperatures and windspeed were higher, this rate of loss would go up, even to the point where the roots might no longer be able to supply water fast enough, and stomata would close partway, reducing photosynthetic rate and raising leaf temperature. If soil water supplies decrease, this reduction in transpiration and photosynthetic rates happens even on otherwise good days. It's no mystery, then, why water supply is so critical.

As the result of a lot of good days so far this season in most areas, the crop is in very good condition, with more than 75% rated as good or excellent. Whether or not it stays there will have little to do with anything other than the number of "good days" we get for the crop during the next 50 days. Let's hope there are a lot of them.--Emerson D. Nafziger

Author: Emerson Nafziger


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