Effects of Drought on Corn Plants

The chance of drought somewhere across the U.S. exists every year. Understanding drought severity, duration and stage of crop growth while under drought can be helpful in setting proper yield expectations, as well as determining if harvesting early as silage will provide more value than previously intended grain. Monitoring conditions and yield potential of the crop throughout the season can help assess options for harvest.

Impact of drought timing on corn plants

Drought's biggest impact on corn yield loss potential depends on the crop stage at the time of drought and how long the drought persists. Peak water use occurs just prior to pollination and the weeks following, making it the most sensitive time for drought to occur.

Early-season drought often shortens plant height and limits the number of ovules (potential kernels) in developing ear shoots, but typically has minimal impact on yield if precipitation is received prior to pollination. The impact of drought on yield is much greater in the weeks just prior to and after pollination. Drought occurring throughout reproductive stages can cause 10%-50% yield losses depending upon what specific reproductive stage the crop is in when stress occurs (Table 1). Drought stress just prior to silking can delay silk exertion from husks resulting in poor pollination. Asynchronization between pollen shed and silking is one of the main reasons peak yield reductions (40%-50%) occur from drought stress at this timing. Drought stress occurring closer to the end of pollination commonly results in ovules not being pollinated, causing barren ear tips or aborting kernels after pollination.

Drought stress occurring after a successful pollination can still cause premature death of leaf tissue and kernels, resulting in a shortened grain fill period and lighter kernel weights. It can also cause ear shanks to prematurely collapse resulting in drooping ears and discontinuing movement of sugars to the ear before physiological maturity (black layer) is reached. Additional drought stress after black layer will have no impact on grain yield. Let’s take a closer look at the impact of drought during corn pollination.

Corn pollination during drought

According to Purdue University, stress during silk emergence and pollen shed can cause more yield loss than during almost any other growth and development stage.  At this stage, inadequate plant water potential can hinder pollination of the kernel ovule.  During the silking and pollen shed period, severe drought may reduce yield from 3% to 8% per day. During the 2 weeks following silking, extreme stress may further reduce yield up to 6% per day.

Corn silk emergence during drought

Severe drought stress in corn, as indicated by wilting of the plant, affects pollination primarily by its impact on silk elongation. Silks begin elongating from the ovules or potential kernels of the ear shoot about 7 days before silks are visible outside the husk tips. The silks from the butt of the ear elongate first, followed by those in the center and then the tip. Water stress around flowering and pollination delays silking, reduces silk elongation and inhibits embryo development after pollination. Moisture combined with heat stress interferes with the synchronization of silk emergence and pollen shed. 

Drought stress may desiccate silks leaving them unreceptive to pollen and the chance to fertilize the ovule. The tip silks are typically the last to emerge from the husk leaves; and if ears are longer than usual, the final silks from the tip of the ear may emerge after pollen shed. If severe drought stress continues into early kernel development, abortion of fertilized ovules can also result in an incomplete kernel set. Aborted kernels are shrunken, mostly white and different from unfertilized ovules in that development had already started.

Corn pollen shed during drought

In typical field conditions, an individual pollen grain will remain viable for 1 to 2 hours, but high temperatures with low relative humidity can rapidly degrade pollen. Temperatures at 100^o or above may literally kill pollen. Fortunately, pollen shed typically occurs during early to mid-morning hours before temperatures climb to such dangerous heights. “Fresh” pollen occurs over a few days until maturation is complete. If there is enough soil moisture to meet the plant’s needs, pollination can still be successful during lengthy periods of high temperatures. 

The University of Wisconsin Extension suggests 2 techniques to assess the success or failure of pollination. One popular method is the "shake test." Carefully unwrap the ear husk leaves and gently shake the ear. The silks from fertilized ovules will drop off. The proportion of silks dropping off the ear indicates the proportion of future kernels on an ear. Randomly sample several ears in a field to estimate the success of pollination. The second technique is to wait 10 days after fertilization of the ovules. Healthy developing kernels should appear as watery blisters during the R2 development stage.

Impact of drought on corn plant leaves

Drought will induce rolling of plant leaves, reducing transpiration and conserving plant moisture as a defense mechanism. Leaf rolling occurs when turgor (water) pressure is lost in the leaf’s cells due to a lack of water (as shown in Figure 1). Leaf rolling conserves water by decreasing the surface area of the leaf exposed to sunlight and reducing transpiration. However, it also reduces photosynthesis, which can decrease plant growth and development, and as a result, can limit yield potential. Photosynthesis reduction limits the ability of the plant to produce sugars and starches for developing grain. Leaf rolling during the heat of the day for a few hours will likely not cause yield loss; but if rolling persists 12+ hours a day, some grain yield loss is likely. Hybrids more prone to leaf rolling could be a potential positive trait since it is considered a defensive mechanism.

Impact of drought on corn root growth

Soil moisture is essential for proper root growth. Early-season drought can cause root tips to dry out and stop growing. Dry soils also cause brace roots to grow along the surface rather than penetrate the soils, which leads to standability issues later in the season (as shown in Figure 2). The decrease in root growth limits the surface area available to collect nutrients and water from the soil. Reduced secondary root development from early drought may reduce the ability of the plant to mine the soil for moisture and nutrients that are dependent on diffusion and root interception. If moisture availability doesn’t improve, overall plant growth can also be compromised.

Impact of drought on nutrient availability and uptake

In addition to drought limiting the biological processes that require water, it can also limit nutrient availability from the soil. Uptake of all nutrients can be lessened due to reduced root development under severe drought. However, of the main 3 nutrients, potassium (K) is the most likely to become deficient.

Positively charged K cations in the soil solution become tightly bound to negatively charged soil particles. The lack of an actively growing root system and less diffusion of K into soil solution can often cause K deficiency symptoms in soils with medium to high soil K test results. K is vital to several plant functions, including water and nutrient uptake and stalk health. Dry soils can exacerbate the plant’s inability to uptake K due to reduced physical mobility and root interception of K.

K deficiency symptoms start on the plant’s older leaves and can be identified by yellowing or firing on the leaf margins (as shown in Figure 3). Generally, drought will have less of an impact where K availability is adequate in the soil. Adequate K levels within the plant will also help to increase drought tolerance by supporting water uptake.

Impact of drought on corn disease and insects

In addition to nutrient deficiency, corn plants can be more susceptible to a different set of insects and disease from moisture stress. Spider mites and grasshoppers thrive in hot, dry weather causing a need to monitor crops closely for pest development in these conditions. Disease complexes are most often associated with wet conditions that are not present in a drought year. However, there are pathogens such as charcoal rot, Fusarium root rots and rusts that favor droughty conditions and can cause significant damage. Drought conditions can also promote certain types of ear mold development, which can potentially lead to the presence of specific mycotoxins in grain. Aspergillus and Fusarium molds are more likely to occur in hot, dry years. Aspergillus is responsible for the production of aflatoxins, and Fusarium molds have the potential to develop into fumonisins. Risk of grain infection will increase if grain was previously damaged by insect feeding. Mycotoxins can impact livestock milk production, reproduction and immunity and should be tested for if suspected prior to feeding.

Impact of drought on corn yield

Extended early-season drought can limit yield potential because of its impact on the plant’s development processes. The number of kernel rows on the ear is determined around the V6 growth stage, while the potential number of kernels is determined from approximately the V7 growth stage up until one week before silk emergence. As a result, extended periods of early drought can reduce the maximum number of potential kernel formation leading to possible yield reduction. Corn leaves that are rolled up for a couple of days likely won’t see significant yield loss, but corn that’s rolled up for the majority of a two-week period may see yield losses up to 20%. Extended early-season drought under extremely dry conditions can even lead to plant death.Yield reduction varies greatly depending on the severity and duration of the stress.

Harvest decisions for corn affected by drought

If pollination is good despite the heat, keep your plan to harvest as usual. When pollination is poor but some kernels are developing, the plant can continue to gain dry matter, making harvesting of your crops  still possible. In some areas, you can harvest poorly pollinated fields for silage use. During extreme situations of no pollination, the best quality forage will be found as close to flowering as possible. You could also leave the crop as a living cover crop until the fall before mowing or chopping.

Harvest management decisions depend on the remaining yield potential in the field. It is important to observe the estimated yield of each individual drought stressed field since each has unique conditions that impact yield. Also, be mindful of the development of stalk rots that may influence the standability of the field if choosing to harvest as grain.

To help make the most of available moisture next season, consider planting Golden Harvest® corn hybrids with Artesian™ technology. Artesian corn hybrids contain multiple genes for season-long drought protection, responding to water stress with multiple modes of action to maximize yield when it rains and increase yield when it doesn’t.

Contact your Golden Harvest Seed Advisor with questions or for additional agronomic insights.