Beside management practices, corn production depends on environmental conditions. The main environmental factors influencing corn growth are temperature, moisture, and solar radiation. Especially important are temperature and moisture.
The optimal average temperatures for corn growth range between 68 and 73°F. However, the optimum temperature varies over the corn growing season and between daytime and nighttime.
Corn can survive short exposure to low and high temperatures of 32 and 112°F, respectively. Cooler temperatures slow down the growth of plants. Growth decreases once temperature drops to about 41°F. Temperatures between 32 and 28°F have very little effect on corn. Extremely low temperatures cause freeze damage, the severity of which will depend on the temperature, duration, and corn growth stage. Extended low temperatures at seedling stage that reduce the soil temperatures to below freezing two inches below the surface may kill corn.
Later in the season, a long exposure of corn to temperatures below 28°F can damage corn by damaging the “growing point”. The growing point for corn is located in the center of the stem and below the soil surface until the V5 - V6 growth stage (5 - 6 corn leaves with collars). At the V6 growth stage, corn would be approximately 12 inches tall. It is important to remember that although corn can germinate and grow slowly at about 50°F, the planting should start when the average soil temperature reaches 55°F at the top two inches.
Poor germination and stand usually are the result of low soil temperatures.
Corn yield may also be reduced due to high air temperatures (95°F and higher) during pollination. High temperatures during this time can cause damage to pollination if plants are under drought stress. During moisture stress, especially at low relative humidity, high temperatures can desiccate silks and damage or kill pollen. Pollination will not be affected by high temperatures if there is adequate moisture in the soil, because pollen shed usually occurs during morning hours.
Corn productivity is also correlated with the length of growing season, which is characterized by the Growing Degree Day (GDD) accumulations (referred to as “heat units”). The GDD is accumulated from the day after planting until physiological maturity. The GDD calculation method most commonly used for corn in the United States is the 86/50 method.
Growing degree days are calculated as the average daily temperature minus 50 using the following formula:
GDD = ((Tmax + Tmin) / 2) – 50
where Tmax is the maximum temperature and Tmin is the minimum temperature. If the maximum temperature is above 86°F, 86 is used, and if minimum temperature is lower than 50°F, 50 is used in the calculations of the average temperature. Corn hybrids require a certain number of accumulated GDDs to reach maturity. Long-season hybrids require more GDDs to reach physiological maturity than short-season hybrids. This GDD method is more accurate in corn production than traditional method of days from planting to maturity.
Highest corn yields can only be obtained under optimum moisture conditions during the growing season. Moisture stress at any of the growth stages will result in potential yield reduction. Corn has generally high water requirements and can use about 0.25 inch of water per day during rapid growth. However, water usage may increase up to 0.35 inch per day during pollination.
Corn yields are reduced when evapotranspiration exceeds water supply from the soil at any time during corn growth. Evapotranspiration is the sum of the water loss from the soil through evaporation and water used by plant during transpiration. Major water loss in early corn growth is from evaporation. During moisture stress, the availability and uptake of nutrients are also reduced and therefore stress weakened plants are also more susceptible to disease and insect damage.
The yield loss due to water stress will depend on the growth stage of corn during the drought stress as well as the length and severity of the drought. Corn is most sensitive (highest potential yield loss) to water stress during pollination, followed by grain-filling, and vegetative growth stages.
Water stress during vegetative growth stages will reduce stem and leaf cell expansion as well as dry matter accumulation due to lower water and CO2 intake. This results in reduced plant height and leaf area, and lower yield potential. Severe moisture stress is indicated by leaf wilting. Corn plants in the initial phases of drought wilt in the afternoon. During longer periods of drought, wilting occurs earlier in the day. Eventually, leaves remain wilted all day. A 10 to 20% yield reduction may occur if drought stress occurs during vegetative stages of corn.
Moisture stress during pollination is the most critical for reducing yield potential of corn. Severe moisture stress will result in delayed silking and reduced pollination due to lack of viable pollen and reduced synchronization between silking and pollination. Under severe stress, some plants will not form any silks, or silks will emerge after pollen production has ended; therefore, resulting in poorly developed ears. The moisture stress during this stage may cause up to 50% yield reduction. Kernels, especially near the tip of the ear, are most susceptible to abortion during the first two weeks following pollination. Tip kernels are generally fertilized last and are less vigorous.
During the kernel dough stage, yield losses are mainly due to reductions in kernel dry weight accumulation. Stress during dough and dent stages will decrease kernel weights and often causes premature black layer formation. Once grain has reached the physiological maturity, moisture stress will have no further physiological effect on final yield. High plant populations in moisture-limiting environments increase moisture stress and silking problems, which lead to reduced kernel number.