By Roy A. Ulrich, Technical Agronomist, Dekalb & Asgrow, Southern Ohio
“So, what do you think this corn and bean crop will yield?”
This is the most commonly asked question of agronomists at field days during the month of August. Usually, the most common response from an agronomist is, “Well, it depends.” Then this is quickly followed by a synopsis of the growing season, either slanted towards a positive yield outcome or a less than favorable one, but with the caveat and easy out of “there is a lot of yield still to be built in the month of August.” So, with that thought in mind, let’s examine one of the main drivers in corn yield production in the month of August that none of us have control over: the weather. Of specific concern at the time are temperatures — not the daytime high temperatures of the mid 90s we saw in the middle of the month, but the high nighttime temperatures.
So why is temperature important to corn production and so vitally important during grain fill? Since corn growth and development is directly linked to the accumulation of Growing Degree Units (GDU) which is calculated by the formula: Daily Maximum Temperature + Daily Minimum Temperature)/2-50 with the maximum high temperature of 86degrees F and the minimum low temperature of 50 degrees F. This daily weather information paired with a corn products data on GDUs from mid-pollination to black layer will give us the number of calendar days for a given product to produce photosynthates and turn them into dry matter in the form of kernels.
Let’s look at how this works out. For example, a 113-day corn product reaches mid-pollination at 1,330 accumulated GDUs since planting and black layers or reach physiological maturity at 2,825 accumulated GDUs since planting. Simple arithmetic gives the grain fill period of that product a total of 1,495 GDUs for that product during grain fill. During the grain fill period if the daytime high temperature averages 86 degrees and the nighttime low temperature averages 60 degrees this results in an average GDU accumulation of 23 GDUs per day. In that case, that product would have the opportunity to fill grain for 65 calendar days. Let’s see what happens if the night-time temperatures were increased to 73 degrees over that same period. How important is that 13-degree increase? If the daytime high temperature remained the same at 86 degrees, it would result in a GDU accumulation of 29.5 GDUs per day. In that case the same product would only be in grain fill for 50 calendar days. This would result in a lost opportunity of 15 days to make energy and convert it into grain, ultimately lowering overall yield regardless of any other factors.
There is another negative effect on corn when examining increased night-time temperatures. Not only does increased night-time temperature reduce the number of days for corn to fill grain, but it also increases the amount of maintenance respiration required by a plant to repair plant cells. This maintenance respiration results in sugars being used to repair the plant and as a result they are not available for grain fill. Remember that 13-degree increase in night-time temperatures from above and how it drastically reduced the days of grain fill? A 13-degree increase in temperature also doubles the rate of maintenance respiration resulting in twice the amount of energy lost to keep the plant functioning resulting in them not contributing to grain fill. If thinking about a corn plant as a power plant, then higher night-time temps result in fewer days of production and more of that energy being consumed to fix the plant with a net overall result of less energy output or less yield.
While increased night-time temps are not favorable to a corn crop, they are favorable to fungal pathogens such as gray leaf spot which reduces the available leaf area to capture sunlight and produce energy. Controlling foliar leaf diseases becomes even more important when overall energy production will be limited due to a shortened grain fill period and higher amounts of energy used for maintenance respiration.