Summer Heat and Corn Yields

by: T. Marc Schober

Questions about diminishing crop conditions have been raised in July, as the Corn Belt has been plagued with unseasonably high temperatures for most of the past two weeks. This prolonged period of hot, dry weather caused the USDA to lower crop condition estimates for both corn and soybeans in each of the past two weeks. According to a recent publication by UBS Investment Research, “The 2011 corn crop is off to a start that we believe would be consistent with below normal growing conditions.” Since 1992, below trendline crop conditions at this point in the growing season have resulted in a national average of less than 154.7 bushels/acre, said UBS.

There are a number of factors to determine how the levels of stress and potential yield damage caused by hot weather. To estimate potential losses, one needs to analyze the following factors collectively: soil moisture, plant maturity, duration of prolonged heat and temperatures, plant population, and genetic makeup of the plants.

Soil Moisture and Plant Maturity

Soil moisture can be both a yield enhancer and inhibitor, varying on timing within the plant's lifecycle and quantity of moisture within the soil. Excess moisture can cause delays in planting, which may affect yields if the plant does not experience a full growing season. Another issue with wet soils is the possibility of nitrogen leaching. Of the three major nutrients, nitrogen moves through the soil profile the fastest and producers run the risk of losing applied nitrogen with excess rainfall.

Producers can take tissue samples from plants during the growing season to monitor not only nitrogen, but all major and micronutrients. This management technique allows producers to apply nutrients more efficiently and allows them to tailor specific nutrient plans for each field.

During germination, excess moisture can have a lasting affect on a plant's development. Abnormally high amounts of water will cause plants to set a shallow root system in order to stay alive versus drowning. During warm, dry months a shallow root system may inhibit the plants ability to search deep into the soil for moisture and nutrients. Finally, damp growing conditions leave plants susceptible to diseases, which can lead to standability issues and make harvest difficult.

Without adequate moisture, plants become defensive and focus on survival. This means they do not collect the nutrients needed to fully mature, thus adding additional stress. Stress during pivotal growing stages such as pollination, silking, and grain fill may result in shorter ears, increased tip back, and possible abortion of kernels. All of which will contribute to yield reductions. Producers with irrigation equipment can assist plants through these crucial time periods by watering them daily, while others are at the mercy of the weather and their soil structure.

Unlike soybeans, once a corn plant reaches maturity, its kernels no longer continue to absorb moisture. Soybeans will continue to absorb moisture until they are harvested. This can cause harvest delays because soybeans are more susceptible to damage during drying versus corn. The only way soil moisture affects the corn harvest is if machines are unable to operate without getting stuck.

Temperature Stress

Corn plants produce energy (sugars) through the process of photosynthesis, which combines sunlight, water, and carbon dioxide. The plant then utilizes dark respiration, which uses the sugars produced during photosynthesis for growth and maintenance. However, unlike photosynthesis, respiration is a continuous process that does not require sunlight.

When night time temperatures remain above 70 to 80 degrees Fahrenheit, respiration rates are higher and dry matter accumulation is lower. Ohio State University has reported that for each 13 degree increase in temperature, respiration rates may double. During these periods the plant uses more sugars produced during photosynthesis for maintenance instead of growth. Essentially, the plant is working so hard to stay alive that it has less energy available for developing kernels, thereby lowering potential grain yield.

High night time temperatures result in faster growing degree unit (GDD) accumulation that can lead to earlier corn maturation. Where cool night time temperatures result in slower GDD accumulation that can lengthen the grain filling period and promote greater dry matter accumulation and grain yields. Research performed by the University of Illinois indicates that corn grown with night time temperatures in the mid-60s out yielded corn grown with temperatures in the mid-80s.

Plant Population

To maximize yields, farmers select planting populations based on soil characteristics, annual rainfall (natural or irrigated), and genetic makeup of their seeds. Corn populations can range from the low tens of thousands to over forty thousand plants per acre.

Producers will plant higher populations in soils composed of silt loams versus lesser quality soils containing more clay or sand. Depth of topsoil is also taken into consideration when determining plant population. Based on geography, topsoil levels can range from nonexistent to greater than six feet. Soils with more topsoil can hold and store water more efficiently, compared to those with a sand or gravel base. Compared to soils with minimal topsoil, those with multiple feet of topsoil will hold water further down in the profile, which allows growing plant roots to find additional sources of water during long dry periods.

Understanding annual rainfall is also important when planting higher populations. All crops need sufficient moisture throughout the growing season. To produce top end yields, moisture availability during tasseling, silking, and pollination is imperative. Water usage during these growth stages can increase to 0.35-0.50 inches per day.

Hybrid Genetic Selection

Selecting corn hybrids that are built to withstand the stresses of heat and drought-like conditions is important for growers to understand. Producers in arid climates or below average soil quality will naturally plant lower populations, but selecting hybrids known for the ability to handle these environments is just as important to maximizing profits.

As mentioned above, periods of high night time temperatures add stress to maturing corn plants. During ear fill, which occurs directly after pollination and fertilization, corn plants may respond to stress by remobilizing or moving stored sugars from the stalk and leaf tissue to developing ears. This is commonly referred to stalk cannibalization, which weakens the stalk and makes it more susceptible to diseases. Research from Purdue University shows, that even if stalk rot does not develop, loss of stalk integrity and structure from the remobilization can increase the risk of stalk breakage.

Stalk strength ratings vary between hybrids and are based on; stalk diameter, stalk rind thickness, stalk strength components, and disease tolerance. Weak stalks are more susceptible to stalk rots, which can lead to upwards of a 20% loss of yield potential. In addition to cannibalization, stalk weaknesses can be attributed to:

• A reduction in photosynthesis due to foliar diseases
• Damage to plant structure from hail or insect and nematode feeding
• Difficult growing environment from nutrient deficiency or imbalance, compaction, excessive plant
population, or moisture extremes

Producers can limit yield losses by selecting hybrids that fit their growing conditions best and by scouting fields to access stalk and plant health. Iowa State University recommends scouting before black layer or 40 to 50 days following pollination. Scouting fields weekly by walking in a zigzag pattern through the field and pinching at least 100 random plants. Pinch tests can be done by pinching the lower stalk internodes between your thumb and forefinger. If more than 15% of the stalks fail the pinch test (stalks easily collapse under minimal pressure), the field should be harvest as soon as possible to avoid harvesting delays that accompany downed corn.


Producers are in no means in complete control of their yields. However, developing and following proper management techniques on a farm by farm basis will allow producers to maximize their returns. The management techniques listed below will assist producers in developing their strategic plans and reaching their goals:

• Select hybrids with good stalk ratings and strong tolerance to foliar diseases
• Plant appropriate populations for each hybrid
• Maintain balanced levels of fertility, especially nitrogen and potassium
• Help to avoid or minimize stress to corn, especially during pollination and grain fill
o Control insects to help avoid root and stalk injury
o Manage weeds with different modes of action, including adding other herbicides to control
glyphosate (Round-Up) resistant weeds
o Rotate crops to reduce pathogen survival
o Provide adequate drainage to help avoid conditions conducive to rot or other diseases
o Provide timely applications of irrigation (where applicable) to help avoid moisture stress
• Harvest in a timely manner to help minimize the risk of harvest losses

Disclosure: I have no positions in any stocks mentioned, and no plans to initiate any positions within the next 72 hours.