Current high temperatures, coupled with drought in some growing areas, raise concerns about the potential for US production this season. Both corn and soybean crops are in or entering into critical growth stages, but the impact of weather conditions will vary by crop and development stage. Below are details about the impact of extreme heat and lack of moisture on corn and soybeans at different stages during the growing season.
Corn: For corn, the greatest water needs occur during the later vegetative growth stages, but early-season drought can have negative impacts that are felt throughout the entire growing season due to the effects on plant growth and nutrient uptake. Research has shown a negative response of corn yield to the accumulation of temperatures above 86 °F. The greatest impact of extreme heat stress on corn likely comes through intensification of water stress rather than the direct effect of heat itself. Higher temperatures cause the transpiration rate of plants to increase, placing a greater demand on soil water supply and potentially accelerating the onset of drought stress. Corn plants respond to water stress by closing their stomata, which helps preserve water but also reduces the rate at which plants are able to take in CO2 needed for photosynthesis. High nighttime temperatures (in the 70s or 80s) can result in wasteful respiration and a lower net amount of dry matter accumulation in plants, resulting in lower yields.
Growth stages of corn are divided into vegetative stages (V) and reproductive stages (R). Subdivisions of the V stages start with emergence( VE) then are designated numerically as V1, V2, V3, etc. through the last leaf stage. The number of leaves will fluctuate with hybrid and environment differences. The tassel is near full size but not yet visible at V15. The final vegetative state is tasseling (VT). Reproductive stages progress as follows: silking (R1), blister (R2), milk, (R3), dough (R4), dent (R5), maturity (R6).
Vegetative stage corn (VE – V1+) under heat stress: Early season development, up to V8, determines the size of the overall plant and the size of each leaf. Drought stress at this critical time period will reduce plant and leaf size. A small reduction in leaf size will not have a significant impact on yield, but the more the leaf size is reduced the less photosynthetic area will be available to contribute to yield. Drought stress that occurs between V6 to V8 can impact the number of kernel rows. While this trait is genetically controlled, it can be modified by the environment. If the corn product is genetically predisposed to have 18 kernel rows but ends up having less than that, it is most likely due to some stress that occurred between the V6 to V8 growth stages.
Beginning around 9 to 10 weeks after emergence, corn enters a critical period where successful pollination is required to convert potential kernels into viable, developing kernels. At the VT stage, tassels are fully visible and silks will emerge in 2 to 3 days. Silks are mainly composed of water, so drought can reduce the growth rate and emergence from the ear tip. Under drought conditions, the silks may be delayed in emerging and miss the pollen or desiccate to the point where they are not receptive to pollination resulting in completely barren ears. Additionally, drought stress can accelerate pollen shed, leading to increased potential for a lack of synchronization. Pollen production and viability can be reduced with daytime temperatures above 95˚ F. Heat stress alone will not usually negatively impact pollination if soil moisture levels are adequate, although prolonged temperatures over 100 °F can kill pollen.Reproductive stage corn (R1-R6) under heat stress: Kernel abortion can occur when successful pollination is followed by drought or heat stress and is usually more frequent at the ear tip. Drought or heat stress during the first 2 weeks after pollination is the most critical in determining if abortion will occur. The ideal temperature range for corn is between 86° F for daytime and 50° F for nighttime temperatures. When daytime temperatures are high, photosynthetic capacity is reduced, so fewer sugars are produced. Coupled with high nighttime temperatures the respiration rate is increased using the reduced amount of sugars produced impacting the number of kernels set and filled. Corn that is water stressed will usually curl the leaves, thus reducing photosynthesis. Reduction in photosynthesis lowers the number of nutrients provided to the developing kernels. Corn can be vulnerable to reductions in kernel weight through full maturity (R6 growth stage). Stalk weakness or lodging can result from severe heat or drought stress as the plant moves resources from the stalk to developing kernels. Pre-mature death can occur under severe drought or heat stress.
Soybeans: The largest negative influence on soybean yields is driven by high temperature and low soil moisture during the summer reproductive period. The ideal daytime temperature for soybean growth is 85℉. When temperatures exceed this threshold, especially for several subsequent days, heat stress can occur regardless of growth stage. Soybeans will be particularly stressed when excessive heat and drought conditions occur simultaneously and drive up soil temperatures leading to reduced moisture and nutrient uptake. High temperatures are somewhat less problematic during wet conditions. Soybeans are less sensitive to high nighttime temperatures than corn, however, yield impact can still be seen if nighttime temperatures exceed 85℉. Heat can also accelerate soybean maturity, as photoperiod and temperature interact to control flowering in soybeans.
Soybean growth is divided into vegetative (V) and reproductive (R) stages, similar to corn. The vegetative stages start with emergence (VE) and unrolled unifoliate leaves (VC). Stages are numbered from there according to how many fully-developed trifoliate leaves are present (V1 – V4 and higher). The reproductive (R) stages progress as follows: beginning flowering (R1), full flowering (R2), beginning pod (R3), full pod (R4), beginning seed (R5), full seed (R6), beginning maturity (R7), and full maturity (R8). The stages can overlap.
Vegetative stage soybeans (VE, VC, V1-V4+) under heat stress: High temperatures can result in slowed or stopped photosynthesis due to the plant closing its stomata in an effort to conserve moisture. Soybean root growth increases during drought conditions because plant carbohydrates are shifted to root growth. When adequate rainfall or soil moisture returns, vegetative growth will resume until the mid-seeding filling stage (R5.5). Temperatures above 86℉ can also reduce nodulation, and ultimately N-fixation in soybeans, and the effects could extend into the reproductive growth stages.
Reproductive stage soybeans (R1-R8) under heat stress: High temperatures in reproductive stage soybeans can result in aborted flowers, aborted small pods, aborted seeds in larger pods, and production of smaller seeds. Leaf loss can even occur under severe stress. Soybeans flower for several weeks so they have the opportunity to withstand short-term periods of stress, although moisture and drought stress may lead to shorter flowering times. At the R3 stage – marked when the plant has a pod on at least one of the upper four nodes of 3/16-inch long or longer – heat or moisture stress can reduce pod numbers, seed number per pod, or seed size, which may reduce yield potential. The ability for soybean plants to recover from temporary stress decreases from R1 to R5.
Generally, drought effects on soybean are not as severe as corn. This is a result of overlapping of development stages. When short-term drought stress results in flower or pod abortion, new flowers and pods will set when conditions improve. During prolonged drought stress, or when the stress occurs during pod set and seed filling stages, the compensatory ability is not as likely to occur. Drought can reduce pod number by up to 20 percent as a result of flower and pod abortion. (Sources: Iowa State, Dekalb, University of Nebraska, Purdue University)