Corn Production Guide (continued)

A-1130, May 1997

Irrigation Management

Corn needs between 18 to 22 inches of soil moisture during most growing seasons to achieve maximum yield potential. In North Dakota, irrigation is used to supplement rain to maintain optimum soil moisture for growth. Under these conditions, corn is capable of producing 8 to 14 bushels of grain corn and 1.25 to 1.75 tons of silage for each inch of additional applied water. Corn variety maturity length will affect seasonal water use. For example, water use during a particular growing season will be greater for 90 day corn than for 80 day corn.

The water that evaporates from the soil near a corn plant plus the soil water used by the corn is called evapotranspiration (ET) or simply water use. The frequency and amount of irrigation depends on the growth stage of the corn (which determines the daily water use), the water-holding capacity of the soil in the root zone and the prevailing weather conditions.

Corn Rooting Depth and Water Use

Corn is a relatively deep rooted crop. Typically, in deep soils, roots grow laterally 12 to 18 inches from the stalk and downward to a depth of 4 feet or more. About 90 percent of the roots will be found in the top 3 feet, which is considered the effective rooting depth for irrigation purposes. Over the course of a growing season, about 40% of the water used by corn will come from the first foot of soil, 30% from the second foot and 20% from the third foot. Less than 10 percent will be obtained from the soil below 3 feet.

Average corn water use will increase from about 0.03 inches per day soon after emergence to over 0.27 inches per day during ear formation (Figure 1). However, during July and August, hot, windy days can push water use to over 0.35 inches per day. The water use is given as a depth measurement because it is assumed that corn removes soil water from under every square foot of soil surface in the field.

Figure 1. Corn water use and soil moisture management criteria. (8KB b&w chart)

Water Holding Capacities of Soil

The depth and water holding capacity of soil has a great influence over when and how often irrigations are required. Soil texture determines the amount of available water it will hold (Table 1). Note that the greater the water holding capacity of the soil in the root zone, the less frequent the irrigations should be. It is important to know the soil texture and water holding capacity of the dominant soil type in a corn field and use that information for making irrigation decisions.

Irrigation Water Management

It is desirable to have a soil profile that is near field capacity at planting. Most years this will happen naturally with normal winter snow and spring rainfall. Less than a full soil moisture profile to a depth of at least 3 feet at planting could hinder root development later in the season. Also, stored soil moisture in the root zone serves as a supplement during high water use periods.

From emergence to the onset of tassels (about 40 days), corn is relatively drought tolerant. It can withstand up to 60 percent soil water depletion in the root zone without a significant impact on yields (Figure 1). However, from the onset of tasseling to the blister kernel stage (40 to 80 days after emergence), soil moisture levels in the root zone should not be depleted more than 50 percent to achieve maximum yields. After blister kernel development, corn can again withstand 60 percent soil water depletion without much impact on yields.

The period of greatest water stress sensitivity coincides with the time of highest water use demands (July and August). Corn water use will average around 7 to 8 inches in July and 6 to 7 inches in August. With temperatures in the 80s, corn will use about 1.75 inches per week (net). Temperatures in the 90s will increase the water demand to around 2.1 inches per week (net).

Most center pivots are set to apply from 0.5 to 1 inch of water per revolution. For a center pivot system covering 128 acres with 800 gallons per minute (gpm) of capacity, it will take about three days to put on 1 inch (net) of irrigation water. Therefore, when the corn begins to tassel it is critical that the soil moisture profile be monitored frequently, or it may be difficult to keep up with corn water use during periods of high temperatures and wind. Scheduling of irrigations during these periods is extremely important.

Corn planted on relatively deep soil where the full 3-foot root zone can develop should receive at least 1.0 inches (net) of water each irrigation during the period of highest water use.

Corn planted on shallow soils (12 to 24 inches of top soil) underlain by coarse sand and gravel can pose irrigation management problems. The roots will be concentrated only where there is top soil, thus this becomes the management root zone. A shallow root zone means there is less available water. For this situation, applying less water (0.5 to 0.7 inches) more frequently would be produce better results than applying a larger amount less frequently.

For corn grain, the last irrigation of the season is determined by the maturity of the corn kernels. Corn should be irrigated until sufficient soil moisture is available to ensure the milk layer in the kernel moves down to the tip of the kernel or black layer formation. This generally occurs about 55 days after 75% of the plants have visible silks on the ears. Yellow dent corn is usually well dented at maturity.

Irrigation Scheduling

Determining when to start and stop an irrigation system is a very important part of irrigation water management. Since irrigation is used to supplement rain, it is extremely important to have at least two rain gages for each irrigated field. They should be located on opposite sides of the field to provide an accurate estimate of the amount received over the entire field. They should be located so that they measure only rain, not applied irrigation water.

Soil in the root zone is the reservoir that stores the water for use by corn. Soil moisture levels in the root zone determine the criteria for when to start and stop irrigations. There are several soil moisture monitoring tools available to determine the soil moisture level at a particular time and place.

Direct soil moisture measurement can be done several ways. The "soil feel" method is the most widely used. It involves using a soil probe to obtain a soil sample from a certain depth in the root zone, then determining the amount of soil moisture by squeezing the soil in the palm of your hand. For corn, soil samples should be checked at 1 and 2 feet below the soil surface. To be accurate, using the soil feel method requires considerable experience with a variety of soil textures.

Soil moisture can also be measured with mechanical devices such as tensiometers and soil moisture blocks. When these are used, one or more of these devices are buried at different levels in the root zone and at several locations in the field. For corn, the root zone soil moisture should be monitored at 1 and 2 feet below soil surface. The amount of soil moisture is determined by either reading a gage or using a portable meter. These devices only indicate the soil moisture status at that particular location. Electronic methods which measure soil moisture levels based on the changes in measurable electronic properties of the soil are also available.

Using just soil moisture measurement for irrigation scheduling can create more work during the growing season for the irrigation manager. Soil moisture measurements must be made two or three times during the week and at several locations in the field. It is important to sample the most common soil types in the field. Consulting the county soil survey will show where these soils are located in the field.

Another form of irrigation scheduling is to use estimated corn water use values. This method, sometimes called the "crop water use replacement method," is based on obtaining daily estimates of corn water use and accurately measuring the amount of rain received on the field. Irrigations are scheduled to replace the amount of soil moisture used by the corn minus the amount of rain received since the last irrigation. Estimations of water use for corn based on maximum daily temperature are shown in Table 2.

The best choice of tools for irrigation scheduling is a combination of in-field soil moisture measurement and a recorded daily soil water accounting procedure. This method, called the "checkbook" method, has also been used successfully for many years in Minnesota and North Dakota. The checkbook method is a soil moisture accounting method which uses daily corn water use values and the soil water-holding capacity to predict the time and amount of water needed to replenish what has been removed from the root zone since the last irrigation or rain. A circular on irrigating using the checkbook method is available from any county extension office in Minnesota and North Dakota.

Table 2. Average corn water use based on maximum daily air temperature, week after emergence and growth stage (inches/day). (30KB b&w table)

Estimation of Pre-harvest Corn Yields

There are several techniques for estimating corn grain yield prior to harvest. This version was developed by ag. engineering at the University of Illinois and is the one most commonly used. A numerical constant for kernel weight is figured into the equation in order to calculate grain yield. Since weight per kernel will vary depending on hybrid and environment, the yield equation should only be used to estimate relative grain yield. For example, yield will be overestimated in a year with poor grain fill conditions, while it will be underestimated in a year with good grain fill conditions.

Step 1. Count the number of harvestable ears per 1/1000th acre (Table 4).

Step 2. Count the number of kernel rows per ear on every fifth ear. Calculate the average.

Step 3. Count the number of kernels per row on each of the same ears, but do not count kernels on either the butt or tip that are less than half size. Calculate the average.

Step 4. Yield (bushels per acre) equals:

(ear #) X (avg. row #) X (kernel #)
90

Green Snap Damage

Green snap or "brittle corn" are terms often used to describe the breakage of corn stalks caused by high winds primarily during the elongation (rapid growth) period of vegetative growth.

Green snap can occur at most any vegetative stage after the growing point reaches the soil surface (8 inches tall), but corn usually is most susceptible from the 14-leaf to tassel stages. Rapidly growing plants adequately supplied with water and nutrients are predisposed to green snap. Cells at each node site are rapidly dividing. The new cells push-up and elongate to form internodes, thus increasing plant height and leaf exposure. These rapidly growing cells will be thin-walled at first with little strength tissue or fiber. Therefore they become quite vulnerable to breakage by wind, cultivation, anhydrous application or any physical activity that bends the stalk. At night they are extremely vulnerable since the plants are turgid (full of water). During mid-day they are less susceptible as the cells are less full of water and stalks are more flexible.

Corn being grown under the best conditions of plentiful moisture, high N rates, warm temperatures and optimum plant populations are vulnerable to green snap. Usually breakage occurs just below the primary ear node site. Some hybrids appear to have a longer time period when the stalks are susceptible to breakage. Most corn seed companies test/screen using artificial green snap tests to eliminate those corn lines that tend to have weaker stalks.

Hail Damage

Prior to and for some time after emergence, the corn plant is relatively immune to hail damage. At emergence, the plant's growing point is below the soil surface and remains there for about three weeks, until five or six leaves have fully emerged. Because the growing point is in the leaf whorl and below ground level, plant damage due to hail at these early stages rarely results in any significant yield loss.

Approximately three weeks after emergence, all nodes and internodes are developed, and the growing point is elevated above the soil surface.

For the next four to five weeks, the plant grows rapidly and becomes more and more susceptible to hail damage up through tasseling — the most critical period. Once past tasseling, hail has progressively less effect on yield loss.

Estimating total yield loss

Total corn yield loss from hail damage is estimated by adding the expected yield loss caused by stand reduction, the expected loss caused by defoliation, and the expected loss caused by direct ear damage. Remember, however, that this is only an estimate of the percent yield loss. As with undamaged corn, extremely favorable weather during the rest of the growing season can cause actual yields to be higher than expected. Similarly, unfavorable weather can cause greater-than-anticipated reductions.

Frost Damage

Early Season - When early frost kills corn leaf tissue, producers worry about whether or not corn plants will recover. The key to assessing corn seedling viability is to find and observe the "growing point." The growing point is where all new tissue originates and is protected below ground until the plants reach the V-5 stage. Removal or death of leaf tissue above the growing point has only a small effect on corn growth and yield at these early stages.

The growing point can be found by pulling the entire corn plant, including roots and splitting the entire plant lengthwise. If the growing point was below ground and white or creamy in appearance, then injury didn't occur. Observations of frost damage are best made by waiting at least two or three days after frost occurred. If the growing point appears healthy and is white to light yellow color several days after frost, full plant recovery is likely. Plants with extensive leaf tissue damage will likely recover if the growing point is not injured by early frost. New leaves should appear within three to four days if growing point is uninjured.

Late Season - If a killing frost occurs before grain fill is complete, yield potential and quality could be affected. A killing frost can occur when the temperature in the crop canopy drops from 32�F to 28�F for a short time (5-10 minutes) or if the canopy temperature stays at 32�F for four to five hours. This is adequate to kill the entire plant. A lighter frost of 30-32�F lasting an hour or two could kill leaves but not the stalk or ear shank. When only a portion of the leaves are killed, those not killed can continue to function and contribute to grain yield if good growing conditions follow frost. The effects of late season frost on killing leaves at various growth stages are shown below:

USDA market grades of shelled corn

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A-1130, May 1997