Warm Nights & High Yield: Oil & Water?

R.L. (Bob) Nielsen
Agronomy Dept., Purdue Univ.
West Lafayette, IN 47907-2054
Email address: rnielsen at purdue.edu

he “dog days of August” are upon us with warm and uncomfortably muggy days accompanied by warm and uncomfortably muggy nights. Invariably, conversations down at the local cafe over coffee or the neighborhood tavern over a few beers turn to the inevitable belief that “…these warm nights simply cannot be good for the corn crop.”

One of the concerns often expressed relative to warm nights during the grain fill period is that excessively warm nighttime temperatures result in excessively high rates of maintenance respiration by plants. That physiological process oxidizes photosynthetic sugars and provides energy for the maintenance and repair of plant cell tissue, which helps the photosynthetic “factory” continue to operate efficiently.

Excessive rates of maintenance respiration, in response to excessively warm nighttime temperatures, are thought to wastefully “use up” photosynthetic sugars that would otherwise be available as photoassimilate for use by the developing kernels. While the relationship between temperature and the rate of maintenance respiration is well documented, the practical significance of excessive maintenance respiration rates to a full-size corn crop during the grain fill period is not well documented in the research literature.

Perhaps more importantly, warmer temperatures from pollination to kernel maturity increase the calendar rate of crop development (how fast the crop moves through the grain filling stages). Consequently, the number of days in the grain fill period decreases and kernel maturity occurs earlier with warmer temperatures than with cooler temperatures. It is also true that warmer temperatures increase the photosynthetic rate (amount of photosynthetic sugars produced per day), but it appears that shorter grain fill duration trumps faster photosynthetic rate and so yield tends to decrease with warmer grain fill periods.

Another consideration with grain fill periods that are warmer than usual AND muggy is that such weather conditions are conducive for the development of several important foliar corn diseases, including gray leaf spot (Cercospora zeae-maydis). Severe outbreaks of such diseases early in the grain filling period can destroy significant amounts of the “solar capture components” (i.e., leaves) of the photosynthetic “factory”, resulting in significantly less production of photosynthetic sugars, which translates into yield loss.

Having said all that, does history support the belief that “…these warm nights simply cannot be good for the corn crop?” Considering that July and August by and large represent the bulk of the grain fill period for corn in Indiana year in and year out, that time period merits a look to see whether there is a strong relationship between statewide temperature deviations and grain yield.

Figure 1 illustrates that relationship for statewide departures in July & August temperature and statewide grain yields for the period 1994 through 2015. Five of those years represented record-setting or near record grain yields and three represented very poor corn years with statewide yields more than 10% below trend, including the 2012 drought year when statewide corn yield was nearly 38% below trend. All the other years represent statewide corn yields with single-digit departures from trend.

There is a reasonable relationship between these departures in temperature and departures in yield. In general, cooler July and August grain fill periods tend to result in above average statewide corn yields and vice versa for warmer than normal July & August grain fill periods. However, temperature departures from normal only account for about 38% of the annual variability in statewide grain yields.

Obviously, rainfall during the grain fill period is also important in determining grain yield. Figure 2 illustrates the relationship for statewide departures in July & August rainfall and statewide grain yields for the period 1994 through 2015. While the nature of the relationship is what one would expect (rain makes grain), it is not a very strong one. Rainfall departures from normal only account for about 13% of the annual variability in statewide grain yields.

Figure 3 illustrates the relationship between statewide grain yield and a simple index that combines the temperature and rainfall departures. The graph basically reinforces what we expect: Cooler and wetter July & August grain fill periods tend to have above average statewide corn yields and vice versa for warmer and drier July & August grain fill periods.

So, where does this discussion leave us relative to the 2016 Indiana corn crop? After all, statewide corn crop condition ratings throughout the season have been consistently strong, averaging more than 70% “good” to “excellent” in USDA-NASS parlance (USDA-NASS, 2016b).

Statewide temperatures for July were slightly cooler than average and rainfall slightly above average (MRCC, 2016), both of which are positive for yield prospects. The exceptions to the positive rainfall data are those areas of northeast Indiana that were drier than average.

August to date has been 1 to 3 degrees F warmer than normal across the state and, more importantly, zero to 50% of normal rainfall has occurred for the northern two-thirds of the state (MRCC, 2016). Neither climate departures are favorable for finishing a corn crop strongly.

The corn crop statewide is certainly poised to finish with statewide yields above trend. The question lies mostly with “how much” above trend. The answer lies mainly with what happens weather-wise the remainder of the grain filling period.