By: Roy Ulrich, Technical Agronomist for Dekalb/Asgrow, previously published on Ohio Ag Net
The growing season has been quite variable across the region this year so far. For some regions of Ohio, the start to the growing season may have been slightly delayed, but once it was fit the crop went in relatively fast and stress free. For other regions, the growing season was extremely late to get started and each management step has been a struggle to accomplish between all the rains. So, whether your crop started out stress free or it has been under stress since the beginning, the state’s corn crop has transitioned from the vegetative stages into the ever-critical reproductive stages.
The first of those stresses to show up in fields this year was foliar diseases. The warm, humid weather conditions of late June were the perfect environment to develop foliar diseases in corn. The development and progression of these foliar diseases leads to a direct loss in the photosynthetic ability of a plant as leaf surface area is compromised and lost for photosynthesis.
The second stress is water stress due to the lack of rainfall this summer. Some areas are already several inches below the 10-year average since planting. The plant needs that water to cool itself through evapotranspiration. Moisture stress can be magnified by conditions that limit root development including sidewall compaction, tillage compaction or insect damage.
Heat stress on a corn crop can negatively affect corn yield from two separate fronts. The first is the number of days that a plant is in the grain fill period. Corn development is based on growing degree units (GDUs), which are calculated by the day max temp + day minimum temp/2 – 50 with the highest maximum temperature being 86 degrees and the lowest minimum being 50 degrees. High nighttime temperatures and high daytime temperatures together increase the daily gain of GDUs. For example, a day with a high of 86 and a low of 74 results in 30 GDUs accumulated. Contrasted with a cooler day with a high of 78 and a low of 52 results in 15 GDUs accumulated.
Each corn product has a set number of GDUs that it takes to complete grain fill or to move from R1 silking to R6 blacklayer at physiological maturity. So, if a product requires 1,550 GDUs to move from R1 to R6, the more GDUs per day that are accumulated the fewer days that the plants can produce photosynthate and accumulate dry matter in the form of grain. The other way that heat can negatively impact corn yield is when nighttime temperatures are high, dark respiration rates are high. This means that plants must burn more sugar to keep the plant cool and for plant maintenance. Photosynthate that is burned during this dark respiration is not available for grain fill.
Peter Thomison, Ohio State University Extension corn specialist, reports that for every 13-degree increase in temperature, respiration rates may double. So, with high nighttime temperatures, a corn plant is burning more energy for maintenance and to stay cool, which results in less energy left to utilize for grain fill. The high temperatures result in a double hit to yield potential as it takes the plant fewer days to reach the required GDUs for blacklayer, which results in less total photosynthate produced for grain fill. In addition, plants are burning more of that energy for maintenance at night. Simply put, there is less total energy produced and more of that energy lost to maintenance in a growing season with high day and nighttime temperature compared to a growing season with cooler day and nighttime temperatures.
Stress during grain fill may have a direct impact on corn yield. At this point in the growing season these are some of the main stresses on the corn crop. As I am writing this, the forecast is for the trend of above average temperatures and below average rainfall to continue so these stresses are more than likely to continue through grain fill, unless the weather pattern begins to change.