Irrigation and Drought Management

R. W. Heiniger, Crop Science Department

Irrigation Management for Corn

The use of irrigation for growing corn in North Carolina has increased steadily over the past 30 years. The major advantages of irrigation in corn production come from an increase in yield potential and more consistent yields over time. Comparisons of commercial fields over a seven year period found that irrigated corn fields yielded over 215 bushels per acre on average; while non-irrigated fields on the same farm over the same period averaged only 140 bushels per acre. Furthermore, the irrigated yields during the seven years ranged from 194 to 245 bushels per acre; while non-irrigated yields ranged from 13 to 204 bushels per acre.

The disadvantages of irrigating corn are the high initial cost of the equipment, the increase in management requirements, the cost of operation and maintenance, and the lack of good quality water resources. The lack of large quantities of good quality water is the primary limitation to the use of irrigation for corn production in North Carolina. Average net seasonal irrigation requirements for corn in North Carolina range from 5 inches on the organic soils in the tidewater region to nearly 16 inches on the sandy soils of the coastal plain region. The characteristic water use pattern of corn (Figure 4-1) shows that corn can use water at a rate of 0.35 inches per day just before and after silking. Single day peak use rates can approach 0.50 inches. Since many sandy soils contain less than 2 inches of plant available water, sandy soils in the coastal plain can sustain corn growth for only about a week during the silking period before more water is required. At a use rate of 0.35 inches per day and an irrigation system that is 85 percent efficient (modern center pivot) the gross water supply must be capable of producing 7.8 gallons per minute per acre. For center pivot system covering 130 acres, this translates into a system capacity of 1,000 gallons per minute. Because of the lack of high quality water, few, if any, center pivot systems in North Carolina have such a capacity. Under these conditions, the tendency is to try to spread too little water over too many acres. Southeastern growers tend to begin watering too late in the growing season and generally wait too long after rainfall events to resume irrigation. This leads to yield reductions and a failure to return a profit above the cost of the irrigation. The key to success in using irrigation is to keep the soil water storage reasonably full before the peak use period occurs and starting irrigation before crop stress is visible. Therefore, it is important to understand the water requirements of corn by growth stage and to use scheduling to determine when to irrigate and how much water to apply.

Irrigation Requirements by Growth Stage

Germination and Seedling Stage (0 to 45 days after planting). Only a very small amount of soil water is necessary to germinate the seed, but adequate water in the top 12 to 18 inches of soil is essential to produce strong seedlings. On medium to fine textured soils or on organic soils, this early season water requirement is normally supplied by rainfall. On light textured, sandy soils, irrigation may be required to germinate the seed and continue proper development. Sands hold little water, so a physiological drought may occur at any time where soil water storage is limited.

During the rapid growth stage (14 to 45 days after planting), the corn plant is reasonably tolerant of soil water stress. During this period, the water use rate is increasing rapidly and some wilting in the late afternoon may be tolerated without harm if the plant regains its turgor by early morning. Available soil water can be depleted by 70 percent of capacity before yield losses occur. On medium and fine textured soils or organic soils, irrigation is seldom required during this period. On sandy soils in the coastal plain, some irrigation is usually necessary in the middle to late part of this growth period.

Reproductive Stage (1 week before silking to 2 weeks after the tassel appears). This is the most critical period for corn. The water use rate is near its high point. If the weather is hot, the plants need plenty of water to keep wilting to a minimum. Holding the soil water storage capacity in the top 20 to 30 percent of its available range is important to stay ahead of water depletion during a prolonged period of drought. On all types of soils in North Carolina, it is critical that they be near soil water capacity at the beginning of this period. Often this means that a single irrigation one week before silking is required and, in a good year, this may be the only irrigation necessary. On sandy soils, this early irrigation will, most likely, need to be followed up with supplemental applications of water every 2 to 3 days depending on the rainfall received. On low capacity systems or if the grower is using a traveling gun or cable-tow machine, water should be applied throughout this period whenever a rainfall event has not occurred for 3 days.

Grain Fill (2 weeks after silking to black layer). While the corn plant is more stress resistant during this period than it was during the reproductive period, adequate water is still necessary to complete kernel development. Holding soil water in the upper 50 percent of the soil availability range until dent occurs is recommended. Following dent, soil water availability can fall to the 20 to 30 percent range without danger of hurting yield. Medium and fine textured soils and organic soils may require a single irrigation during this period depending on the amount of rainfall received. On sandy soils, the corn crop will most likely need to be irrigated at least once or twice. In summary, the amount of soil water in the root zone should be viewed as an insurance supply. The time to rely on this insurance is when the rate of removal is slow and rainfall is adequate. This means that the time to use the soil supply is either early or late in the growing period when the use rate is low and the consequences of soil water stress on yield are also low. Irrigation should be considered the primary source of water during the reproductive stage and early grain fill stage.

Fig. 4-1. Amount of water used per day by a growing corn crop planted on April 1 at a plant population of 30,000 plants per acre.

Irrigation Scheduling

Irrigation scheduling should be used to determine exactly when to irrigate and how much water to apply. Scheduling can take many forms: calendar date, plant growth stage, crop condition, soil water status, and scheduling using evapotranspiration (ET). Scheduling using calendar date or plant conditions does not work well in North Carolina. The weather is highly variable and waiting for the crop to show signs of wilt means that it is too late to prevent damage, particularly during the reproductive period. Scheduling using plant growth stage works well on medium and fine textured soils or on organic soils. Unless adequate rainfall is received, water should be applied one week before silking and every week thereafter until dent occurs. Unfortunately, plant growth stage does not work as well on sandy soils with low capacity irrigation systems because crop damage can occur before there is enough time to apply adequate water.

Monitoring soil water is a safe scheduling method with universal application. Soil water may be measured periodically using soil water blocks, tensiometers, or the hand feel technique shown in Table 4-1. Tensiometers are easiest to read, but are only meaningful in sandy soils. Soil water blocks will work in any soil, but the blocks take time to place and must be read with an electric meter attached to wires that lead from each block. The hand-feel technique is rapid and inexpensive, but is less exact and takes time to learn. Several sites in each field should be monitored and the evaluations must be made frequently enough to start irrigations on time. For assistance in using a soil moisture monitoring system consult your local county cooperative extension agent.

Scheduling can also be done using crop water use or ET calculations based on temperature and rainfall. Making use of estimated water use rates using a checkbook type routine is an excellent method of determining when to irrigate. A soil water estimate is necessary at the start of the scheduling period for each field. This soil water measurement is treated like money in the bank. Daily use amounts are deductions and rainfall and irrigation amounts are deposits. This way the amount of soil water is known at all times. Observing the trend in values can help growers anticipate precisely when to irrigate. Software programs are available to calculate water use rates from weather data.

By knowing soil water capacity, the current soil moisture status, and crop use, precise amounts of water can be applied so that limited moisture supplies are not wasted and crop needs are met. This is where scheduling can be a real asset. Scheduling will indicate when the irrigation system can be shut down. Allowing the center pivot to run continuously during the bulk of the season is a common but costly procedure. In North Carolina, rainfall will often allow soil water storage to catch up with crop demand. On these days the system may be safely stopped and then restarted when soil moisture declines to indicated levels. Without scheduling, the grower is never sure when these periods occur and may be afraid to shut the system down or may fail to restart the irrigation system in time to meet a period of peak water demand during a long dry spell.

Table 4-1. Chart for interpreting the amount of soil water in a given soil type.
Soil Water Remaining Very SandySoil Sandy Soil Medium Texture or Organic Soil Fine and Very Fine Texture
0 percent Dry, loose, single-grained. Flows through fingers Dry, loose, flows through fingers Powdery, dry, sometimes slightly crusted but easily breaks down into a powdery condition Hard, baked, cracked, sometimes has loose crumbs on surface
50 percent or less Still appears to be dry. Will not form a ball1 with pressure. Still appears to be dry. Will not form a ball. Somewhat crumbly, but will hold together with pressure Somewhat pliable, will ball under pressure.
50 to 75 percent Still appears to be dry. Will not form a ball. Tends to form a ball under pressure, but seldom will hold together once released Forms a ball, somewhat plastic, will sometimes stick slightly with pressure Forms a ball and is very pliable. Sticks readily if high in clay content.
75 percent to field capacity Tends to stick together, sometimes forms a very weak ball under pressure Forms a weak ball, breaks easily, will not stick. Forms ball, will ribbon out between thumb and forefinger Easily ribbons out between fingers, has a slick feeling
At field capacity (100 percent plant available water) Upon squeezing, no free water appears on soil but wet outline of ball is left on hand Upon squeezing, no free water appears on soil but wet outline of ball is left on hand Upon squeezing, no free water appears on soil but wet outline of ball is left on hand Upon squeezing, no free water appears on soil but wet outline of ball is left on hand
Above field capacity Free water appears when soil is bounced in hand. Free water will be released with kneading Can squeeze out free water Puddles and free water form on surface
1 Ball is formed by squeezing a handful of soil very firmly with fingers.

Management Practices that Maximize Irrigation Capabilities

Hybrid Selection. On fields that will be irrigated corn growers should select medium maturing, disease resistant hybrids that will tolerate high plant populations. With ample water, medium maturing hybrids yield as well as full-season hybrids and may be harvested at lower grain moisture. Hybrids should tolerate plant populations of 31,900 plants per acre or higher. Final plant populations of 31,900 plants per acre produce optimum yields with irrigation.

Fertility. For optimum yields under irrigation, corn requires adequate fertility levels. Potash is particularly important to help prevent lodging and plant diseases associated with high humidity and plant populations. Apply 100 to 150 lbs/acre of K on irrigated fields whenever soil test levels indicate a need. Starter fertilizer with a 1:1 ratio of N:P is also critical to maximizing yield potential on irrigated fields. North Carolina research has shown that starter fertilizer on irrigated fields lowers grain moisture and increases yield even when soil test levels of phosphorus are high. Corn growers should plan on applying 200 to 225 lbs of N per acre in multiple applications. A proven approach is to apply 40, 120, and 40 lbs N per acre at planting, sidedress, and pe-tassel stages, respectively. Finally, additional sulfur may be necessary for irrigated corn. Additional S may be added with N applications to maintain a N:S ration of 12:1 or less.

Summary

To insure profitable production from irrigated corn, it is necessary to maintain soil water in the upper 50 percent of the soil's water availability range during the critical reproductive stage (just before and after silking). Knowing the irrigation system capacity and soil water storage capacity, it is possible to evaluate and manage irrigation requirements to maintain soil moisture levels. A corn crop uses a lot of water, but it is very good at turning water into yield. To maintain high corn yields, the grower must insure that adequate water is available.

Irrigation scheduling helps determine when to irrigate and how much water to apply. Scheduling reduces the amount of water applied and insures that adequate soil moisture levels are maintained at all times. Scheduling depends on the grower knowing the crop growth stage, the rate of water use, soil water status and holding capacity, and irrigation system capacity.

Drought Management

A prolonged period of dry weather (drought) is the most difficult and damaging problem a corn grower faces in North Carolina. Shallow root zones caused by low pH subsoils or hardpans restrict the amount of soil water available to the corn plant. Therefore, consistent rainfall where a significant rain event occurs every 10 to 14 days is required to prevent damage to the corn crop. Unfortunately, summer weather patterns are anything but consistent and North Carolina corn growers usually experience drought conditions at some point during the growing season. This means that without irrigation corn yields will be limited by lack of water. The amount of yield lost will depend on when the drought occurs and the amount of available soil water that the soil can hold. The keys to managing drought are to understand the effect of drought on corn at different times in the growing season and to find ways to improve drought tolerance either by increasing available soil moisture or through crop management.

The Impact of Drought at Different Stages of Development

Corn is very susceptible to drought damage due to the plant's requirement for water for cell elongation and it's inability to delay vegetative growth. Therefore, there is always the danger of yield loss regardless of the timing of dry weather. In fact, the golden rule of corn production is that highest yields will be obtained only where environmental conditions are favorable at all stages of growth. The amount of yield loss that occurs during dry weather depends on what growth stage the corn is in and how severe the dry conditions become.

Fig. 4-2. Corn yield loss resulting from loss of leaf area due to drought or other factors.

Fig. 4-3. Corn yield loss resulting from continuous moisture stress during the period just prior to tasseling and silking.

From emergence to V8 (eighth leaf full emerged or about 4 weeks after planting). Growth during this period determines the size that the plant achieves and the size of the individual leaves. Dry weather during this period will reduce plant and leaf size. Impact on yield will be based on the reduction in leaf area available for photosynthesis. Minor reductions in leaf size will have little impact on yield while major reductions (all leaves removed from the plant) could reduce potential yields as much as 20 percent. Extended dry weather that results in leaf burning and loss will have the greatest impact on yield. Figure 4-2 shows the relationship between leaf area and yield loss during this growth period. Corn growers should keep in mind that even though there is little or no leaf burn, leaf size can be affected by drought and result in reduction in leaf area.

V8 to V16 (All leaves emerged, start of tasseling; from 4 weeks to 66 days after plant emergence): Plant growth in this period determines ear size and the number of kernels set. From V8 to V14, ear size is set. Drought during this period will reduce ear size and potential yield. Yield losses will be related to the length and severity of drought. Potential yield losses can range from 10 percent to 30 percent. From V14 to tasseling, the number of kernels that can be fertilized are determined. Drought during this period can reduce corn yields 10 to 50 percent.

Throughout the V8 to V16 period the key question is how long the drought stress is present. Figure 4-3 shows the relationship between the length of drought during the V8 to V16 period and the loss of potential corn yield.

Silking: This is the most sensitive stage for drought stress. Drought during silking coupled with high temperatures can result in 100 percent loss. This occurs most frequently in the midwest where high daytime temperatures can kill pollen before it can reach the silks. In the southeast, high humidity often results in heavy dew which can help pollen reach the corn silk. However, severe yield reductions can occur due to incomplete pollination and the loss of kernel number. There are no good measures of yield loss. Scouting of the corn crop can determine the number of kernels set following pollination and can help determine yield potential. To determine if pollination has occurred, remove the shucks from the ear and take your hand and run it over the surface of the ear. If the silks brush off easily, then the kernel has been pollinated. If the silks stay attached then pollination has not occurred. Another good measure of pollination is to examine the length of the silks. The silk will continue to grow until pollination occurs or until it becomes damaged. The longer the silks, the less efficient the pollination process was.

Silking to Maturity: Drought during this period effects kernel weight. Severe drought can reduce corn yields during this period by 20 to 30 percent. Again, the key factor is how long the drought occurs and how late in this period it occurs. Drought immediately following silking has the largest impact, and can reduce yield substantially. Drought latter in this period is less damaging, but can hasten maturity. While each stage is important, drought during some stages can be especially devastating. The key stage is silking followed by the V8 to V16 period and then the grain fill period from silking to maturity. Dry weather that starts early and covers several growth periods will have a compounding effect with severe reductions in corn yields.

Managing Drought Stress

While there is no way that a producer can avoid drought in corn production, there are two areas where careful management can improve drought tolerance. One way to improve drought tolerance is the use of soil and nutrient management practices that improve the soil's water holding capacity. The other is to carefully select corn hybrids that have the capacity to withstand short periods of drought.

Nutrient Management. One of the best practices for improving the soil's water holding capacity is to increase rooting depth. The major limitation to rooting depth is acidic subsoils. The careful application of lime with incorporation to depths of 8 to 12 inches can help increase pH in the soil profile and improve rooting depth. A thorough soil sampling program that covers all areas of the field and includes a few samples at depths from 8 to 12 inches can help a grower determine the overall pH of the field and how subsoil pH is impacting crop rooting depth. A grower must be careful to not over apply lime in the top 6 inches of the soil while trying to increase pH in the lower profile.

Adequate Potassium is a nutrient often related to drought tolerance. Potassium (K) is an important element in water movement within the plant and in stalk and root health. Sandy soils are often low in available K and should be fertilized to maintain adequate levels.

Tillage Practices. One of the most practical ways of improving plant available soil moisture is the use of no-till practices centered on planting corn into a residue mulch. Studies have shown that planting corn into a residue mulch on sandy loam soils has the potential to increase yields 10-20 bushels per acre. Studies on other soil types have found similar increases. However, before considering the use of no-till practices, the potential for subsoil compaction should be considered. On sandy loam soils with clay subsoils (common in many areas of the coastal plain and tidewater regions), the potential for subsoil compaction is high. In-row subsoiling on these soil types increases nutrient uptake and water use from beneath the compaction layer. The most important step toward improving the odds for successful corn production on droughty soils is to plant no-till into a mulch with appropriate tillage or under-row ripping if necessary to reduce compaction.

Hybrid Selection. Corn hybrids for droughty soils should have the ability to produce high yields at low plant populations under favorable growing conditions. This requires a hybrid that can support a single ear under poor growing conditions and put on multiple ears when growing conditions are good. These type of corn hybrids are known as stretch hybrids. Corn hybrids for droughty soils should also have good drought tolerance and staygreen ratings (See Table 2-1 in Chapter 2). Good drought tolerance and proven high yield potential when planted at 18,000 to 20,000 plants per acre is more important than whether a hybrid tends to be prolific or has the potential to make one big ear.

Another hybrid characteristic that is important under drought conditions is the synchronization between silking and pollen shed. Delayed silking in relation to pollen shed is commonly observed when drought stress is present during the silking period. Selecting hybrids that pollinate over a longer period of time can improve corn yields by insuring that potential kernels are fertilized.

Plant Populations. Chapter 2 discusses the selection of seeding rates for different soils and different conditions. It is critical on drought prone soils to reduce plant populations. On most droughty soils, 18,000 to 20,000 plants per acre is the maximum population that can be sustained.

Summary

While there is little that corn growers can do to avoid drought, they can improve their chances of success by knowing the critical periods of plant growth in relation to drought stress, by using management practices that improve the amount of plant available water stored in the soil, and by proper hybrid selection and plant population management. In particular, growers planting corn on droughty soils should:

  • Plant corn no-till into a residue mulch.
  • Use in-row subsoiling on soils prone to claypans or hardpans.
  • Plant drought tolerant, stretch type hybrids.
  • Plant at populations of 18,000 to 20,000 plants per acre.



This page created by: Alan D. Meijer, Agricultural Research Technician II on 07/24/00 and last revised on 7/26/00