Last week in the Bulletin, I mentioned that it would be better for corn if the temperatures during the next 2 months averaged a few degrees below normal. Someone this week mentioned the "conventional wisdom" that while the weather was warm and humid "at least it's good weather for corn." It might be time for a little thinking about temperatures as the corn crop enters the period of rapid grainfilling. |
Although the belief that weather that's uncomfortably warm and humid is good for corn probably makes us feel a little better when we're having such weather, such conditions really are not ideal for corn growth and yield. In fact, weather that's comfortable for ushighs in the mid-80s and relatively low humidityis also ideal for corn. Humidity by itself is not so much a direct factor in corn growth conditions, but indirectly it is important because it is associated with night temperatures. We usually associate humidity with how comfortable it is in the afternoon, but it's often more useful to consider the dewpoint, which is temperature at which the air is saturated. The dewpoint doesn't change with temperature and so is a more direct measure of the concentration of water vapor in the air.
Night temperatures usually fall to near the dewpoint in our climate, so high dewpoints mean higher night temperatures. High night temperatures are associated with higher respiration rates in the plant. Respiration uses up sugars formed in photosynthesis, leaving less available to fill grain. When the dewpoint is high, dew (which forms because leaf temperatures fall below the dewpoint at night) dries from the corn leaves more slowly in the morning as the air temperature rises. When the leaves stay wet longer, leaf diseases may develop more extensively. The effects of such dampness on insects is mixed, because some diseases of insects may be favored in some cases.
High dewpoints are associated with uncomfortable weather because night temperatures stay relatively high and sweat evaporation, which is our cooling mechanism, is slow when relative humidity is high. The "90° and 90% humidity" that people talk about is usually an exaggeration, though: relative humidity falls to 90% when the temperature is only about 3° or 4° above the dewpoint. Dewpoints in the 70s are quite uncomfortable, and dewpoints in the 80s are downright tropical. Afternoon relative humidity values in the 70s are uncomfortable on warm days, and they need to get below 50% (which happens when the temperature is about 20° above the dewpoint) before it starts to feel comfortable to humans.
As the temperature rises above the dewpoint in the morning, the relative humidity (the percentage of water vapor in the air relative to what it would contain if saturated at that temperature) drops. Because water vapor content in the air changes relatively slowly over time (except when weather fronts pass through), the greater the rise in temperature from the morning low to the afternoon high, the lower the afternoon relative humidity. Low relative humidity slightly increases the rate of transpiration (evaporation of water from inside the leaves), but wind speed usually affects transpiration rates more than relative humidity.
We often think of high rates of transpiration as negative because they result in more rapid loss of water from the soil. When water supply is adequate, as it is in most areas this year, rapid water loss from leaves means more evaporative cooling of leaves, and also more rapid uptake of carbon dioxide, that is, higher photosynthetic rates. Rapid water loss is also associated with bright sunlight, which also means higher rates of photosynthesis. So we would normally consider a day of relatively rapid water loss to be a good day for the corn crop.
The ideal scenario of night temperatures in the low 60s, high temperatures in the mid- to upper 80s, low relative humidity, and bright sunlight would translate into very rapid grainfilling rates for this year's corn crop. We normally consider that it takes about 55 days from pollination to physiological maturity, which would mean a grainfilling rate between three and four bushels per day. During the middle of this period, about a month after silking, filling rates probably approach five to six bushels per acre per day. Cloudiness and loss of leaf area to insects, diseases, or nutrient deficiencies reduces these rates and results in lower yields. It is important that every day's sunlight be used to the maximum extent possible by the crop, and we need to do what we can to assure that the canopy is full and healthy right up to physiological maturity.--Emerson Nafziger