Corn response to drought varies by the timing, duration, and severity of the drought. Extreme heat amplifies the effects of drought on a corn crop. Generally speaking, the worst time for a major drought to occur is from about 2 weeks before flowering (pollen shed / silk emergence) to 2 weeks after flowering. However, that does not mean that severe drought has no effect on corn earlier or later in the season. Licht and Archontoulis (2017) provide estimates of potential yield loss due to stress for different corn development stages.
One of the early symptoms of drought stress is leaf rolling by the plants which occurs when leaf transpiration demand is no longer matched by soil moisture uptake. The rolling of the leaf reduces the leaf area exposed to sunlight, which effectively reduces transpiration demand but also decreases photosynthetic activity.
Conventional agronomic dogma says that leaf rolling that begins late morning or midday but recovers under cloud shadow or evening hours is not serious stress, although there is probably a limit to how many days of such leaf rolling the plant can endure without incurring loss. Leaf rolling that begins nearly at sunup and does not recover until well after dark is certainly serious stress. At some point, severe drought stress will result in the slow death of leaf chlorophyll and the leaves begin to turn from normal green to a dark green-gray, then to simply gray, and finally the straw color of complete death.
Visually defining "severe" stress during the grain fill period is more challenging because leaf rolling does not occur as dramatically as earlier in the season. Short of continuing death of leaves or plants, there are few other indicators of "severe" stress during the last 30-45 days before maturity. One such indicator, though, is premature ear declination (Nielsen, 2020b).
The following table references "direct" and "indirect" effects of stress on yield. "Direct" yield effects refer to direct impacts on the yield components (ears per land area, kernels per ear, and weight per kernel). "Indirect" yield effects are primarily those which impact the uniformity and size of the photosynthetic "factory", i.e.., the overall size and health of the plant. Crop responses to drought early in the season often impact yield later in the season primarily by virtue of their impact on kernel weight during grain fill. Thus, plants that are stunted prior to silking impact yield later during grain filling.
The table also refers to "ovules". Remember that the ear size determination period prior to silking is all about developing the number of potential kernels, i.e., the ovules (Nielsen, 2023). Actual kernel number is determined by the success of pollen fertilizing the ovules during pollination and the initial survival of those newly developing kernels (Nielsen, 2020a).
In the midst of all this doom and gloom, let me remind you that the corn plant has an amazing resilience to drought stress and can recover surprisingly well once soil moisture has been restored. The overall stress tolerance of today's hybrids is far superior to that of previous generations. If you find yourself stressed out over drought stress, remember the infamous words of Yogi Berra... "It ain't over until it's over."
| Timing | Possible crop response | Direct yield effect | Indirect yield effect |
|---|---|---|---|
| Germination & Emergence | No germination Variable germination Failed germination Delayed germination |
Stand loss | Variable emergence |
| V1 - V5 | Death of lower leaves Desiccation/death of nodal roots "Floppy" corn (Nielsen, 2022) Whole plant death Stunted plant growth Leaf rolling Reduced Ps rate |
Stand loss | Smaller Ps factory Inefficient Ps factory Variable development |
| V6 - V14 | Death of lower leaves Desiccation/death of nodal roots "Floppy" corn (Nielsen, 2022) Stunted plant growth Leaf rolling Inhibition of brace roots Whole plant death Reduced Ps rate Reduced nutrient uptake Reduced # of ovule rows Reduced # of ovules per row |
Stand loss |
Smaller Ps factory Inefficient Ps factory Variable development |
| V15 - VT | Stunted plant growth Inhibition of brace roots Leaf rolling Leaf death Whole plant death Reduced Ps rate Reduced nutrient uptake |
Stand loss | Smaller Ps factory Inefficient Ps factory Variable development |
| Silking (R1) | Whole plant death Leaf rolling Leaf death Reduced Ps rate Reduced nutrient uptake Hastened tassel development Delayed silk development Poor pollen+silk "nick" Reduced silk viability Reduced pollen viability Pollination failure |
Stand loss Reduced kernel # |
Smaller Ps factory Inefficient Ps factory Variable development |
| Blister (R2) Milk (R3) |
Leaf death Plant death Reduced Ps rate Kernel abortion |
Stand loss Reduced kernel # |
Smaller Ps factory Inefficient Ps factory Variable development |
| Dough (R4) Dent (R5) Phys. maturity (R6) |
Leaf death Plant death Reduced Ps rate Premature ear declination Premature plant senescence Premature black layer Premature stalk rot development Aflatoxin (Aspergillus ear rot) |
Stand loss Reduced kernel weight |
Smaller Ps factory Inefficient Ps factory Variable development |
| Post-maturity | Hastened whole plant death Poor stalk integrity Stalk rots Stalk lodging Ear droppage Aflatoxin (Aspergillus ear rot) |
Pre-harvest kernel loss At-harvest kernel loss |
Harvesting difficulties |
Licht, Mark and Sotirios Archontoulis. 2017. Influence of Drought on Corn and Soybean. Iowa State Univ. Integrated Crop Management. https://crops.extension.iastate.edu/cropnews/2017/07/influence-drought-corn-and-soybean [URL accessed June 2023]
Nielsen, RL (Bob). 2018. Effects of Severe Stress During Grain Filling in Corn. Corny News Network, Purdue Univ. Agronomy Extension. http://www.kingcorn.org/news/timeless/GrainFillStress.html [URL accessed June 2023]
Nielsen, RL (Bob). 2020a. Kernel Set Scuttlebutt. Corny News Network, Purdue Univ. Agronomy Extension. http://www.kingcorn.org/news/timeless/KernelSet.html [URL accessed June 2023]
Nielsen, RL (Bob). 2020b. Premature Ear Declination in Corn. Corny News Network, Purdue Univ. Agronomy Extension. http://www.kingcorn.org/news/timeless/Droopy.html [URL accessed June 2023]
Nielsen, RL (Bob). 2022. "Rootless" or "Floppy" Corn Syndrome. Corny News Network, Purdue Univ. Agronomy Extension. http://www.kingcorn/news/timeless/FloppyCorn.html [URL accessed June 2023]
Nielsen, RL (Bob). 2023. Ear Size Determination in Corn. Corny News Network, Purdue Univ. Agronomy Extension. http://www.kingcorn.org/news/timeless/EarSize.html [URL accessed June 2023].