Our ten day forecast looks discouraging. Highs above 95 degrees every day for the next nine days, with no real chance of rain in that time period. This has been raising a fair number of questions by producers. What impact will this week have on the growing season? What can the plants do to compensate? How resilient is the crop right now? So we did some digging for the facts. The nature of this discussion tends to bear a more negative, frustrating tone. We do not want this article to be discouraging, but rather to help you learn the science and mechanisms behind plant reactions to heat and drought stress. Stick with us and we hope to provide you some useful information on what’s going on in the field.
The Current Situation
Historical climate data shows us somewhere around 3-5 inches below average precipitation from May 1 until now. This substantial decrease in available precipitation as well as low residual soil moisture coming off of a dry growing season and winter last year has resulted in the crop living rainfall to rainfall.
At the current growth stage, VT- R2, a corn plant is at its maximum water use requirement. We calculate the water use as ET or evapotranspiration, which is the combination of water a plant is using during transpiration as well as soil water evaporation. ET at these stages range from 0.35-0.5” per day, with values even higher in hot, low humidity conditions. If we assume the lower end of that range, a corn plant is using 2.45 inches of water per week. With such limited subsoil moisture as well as infrequent rains, it is possible that some locations do not have 2.45 inches of water available.
Stresses During Pollination
The two weeks prior to pollination and two weeks after are critical for yield potential in a corn plant. Yield is primarily correlated with the number of kernels per acre and secondarily the size of the kernels. Since the sheer number of kernels is most important for yield, preserving and retaining the max number of kernels is critical. Hot weather preceding pollination can result in early pollen drop and delayed silking, resulting in poor synchrony for the pollination process. Even if timing of silks and tassels matches, hot temperatures can result in non viable pollen and desiccation of silks. It appears we have come through the majority of pollination well, with many ears fully pollinated. We have a lot of viable kernels, which means the potential for yield is there. However, if your field is still pollinating, it could be at risk for more variable pollination with the forecasted hot temperatures. However, a large portion of pollen shed occurs early in the morning, coinciding with cooler temperatures, so the potential risk for poor or spotty pollination is reduced.
The uniformity and amount of pollination completed can be tested through the shake test. Carefully shuck an ear of corn, being careful to not disturb the silks. Once the last shuck is removed, shake the ear of corn. Every kernel that is pollinated will no longer have a silk attached. Kernels that are not pollinated will still have a silk attached. We are able to see the uniformity of pollination and amount yet to be pollinated with this method. Another method is to wait until late blister/beginning milk to see which kernels are increasing in size.
Stresses After Pollination
Since heat and drought stress during the pollination window was not a concern for many fully pollinated fields, what concern do we have heading into blister and milk stages of corn? In the first two weeks after pollination, kernels may begin to abort at the ear tip and progressively move farther down the ear if droughty conditions continue. If this continues on into the milk stage, larger portions of ears will abort kernels and the remaining kernels will be smaller and lighter. During the blister and milk stages, drought stress can result in yield reductions as high as 6% per day. This comes as both a reduction in possible kernels per acre and lighter remaining kernels. Assuming a yield goal of 200 bushels per acres, this could result in a loss of up to 12 bushels per acres per day. Keep in mind, this severe of an impact is found in the driest conditions and we will likely not see an impacts that severe initially.
The Plants Response
So what can a plant do to compensate? It is the plants natural response to preserve yield as long as possible. A plant does this through various defense mechanisms. While these mechanisms ultimately result in some form of yield loss, it does so at a lesser rate or impact than if the plant didn’t use any defense mechanisms at all. As we all know, one of the first defense mechanisms is leaf rolling. Plants roll their leaves as a way to reduce the leaf area index and consequently reduce surface area for transpiration, or moisture loss. This mechanism helps keep the moisture in the plant. The degree and speed to which a plant rolls is largely determined by genetics. Leaf rolling is very common and we even see it early in growing season during the heat of the day. These instances do not affect yield. However, when we start seeing leaf rolling lasting larger portions of the day, or even more than 12 hours, yield will be compromised, particularly if this takes place around pollination. If leaves stay rolled through most of the night, the impact on yield will be most severe.
Another method for compensating is leaf loss. It is normal for a plant to lose some of the lowest leaves and devote resources to leaves conducting a large portion of photosynthesis. However, as drought stress continues, more and more leaves will be cannibalized from the lower canopy in order to preserve the rest of the canopy. The very bottom three to four leaves really do not contribute to a large part of photosynthesis, so their loss is not consequential. However, as leaves are lost farther up the canopy, a reduction in potential photosynthesis occurs, leading to a reduction in the available resources for grain fill. While initial leaf loss will occur from the bottom up, if the drought is prolonged, top leaf death or dieback may occur. As this death of leaves moves both from the top down and bottom up, a middle portion of green leaves will be retained as long as possible to conduct photosynthesis.
Ultimately, if drought stress is not mitigated, it will result in premature plant death. This of course shortens the window for grain fill and results in smaller ears with lighter kernels.
The nutrient status of the plant is also affected by increasing drought stress. Without uptake of water, there is also no uptake of nutrients. In particular, we will see deficiencies of our non-mobile nutrients like phosphorus and potassium. As the soil surface dries out, the pore space increases. The P and K ions must diffuse a larger distance to reach to reach the root system. At some point, the ions may not be able to move the distance needed. This, in essence, creates a superficial deficiency. We often see K deficiencies over P deficiencies at this point, due to the fact the plant needs nearly two times the amount of potassium as phosphorus for growth and development. This results in deficiencies for fundamental processes for plant life. The plant will attempt to deal with this shortage by pulling nutrients from the lower leaves and stem. This will ultimately result in poor stalk strength and possibility of stalk rots and other diseases. You can check for this cannibalization by cutting open a stalk and looking for light and hollow stalks, often with pink discoloration. We are seeing the beginning of this shortage of nutrients on the lower leaves; many are exhibiting potassium deficiencies.
The Plants Ultimate Goal
While all this discussion can feel doom and gloom, it is important for us to know the physiological processes the plants will go through as this hot and dry spell continues. Remember, each corn plant is genetically programmed to retain as much yield as it can. It does this at the expense of transpiration, photosynthetic rates, and its own plant structure. Keep this in mind however when you see a field full of rolled plants. They are doing their best to preserve yield for as long as possible.