Irrigation

Hamid Farahani and William B. Smith 

Making the Case for Irrigated Corn

One of the major factors affecting corn production in the southeastern Coastal Plain region is available soil water.  Even though this region enjoys average rainfall amounts of about 50 inches per year, the amount and distribution of rainfall is highly unpredictable, making rainfed corn production risky with large variations in yield from one year to the next. 

Production is further challenged by the prevailing sandy soils with low water holding capacities of 0.5 to 1.0 inch per foot overlying a difficult-to-manage hard pan that restricts root penetration into the more clayey soils below. This limits the root zone depth and thus the effective soil water storage volume. 

Because of the high evaporative demand during mid-summer, available soil water is quickly depleted in sandy soils and lack of rainfall can result in yield reductions due to water stress.  This was evident in the 2008 drought in South Carolina, as rainfed corn yields were miserable.

During peak water use (i.e., mid June to Mid July that includes the critical silking to milk stage), corn needs about two inches of water per week (0.3 in per day), but the probability of receiving two inches of rainfall in one week in mid season is only 20%. During this peak water use, yield loss due to water stress is substantial and estimated at 6-8% per day of stress. 

It is thus obvious that corn producers must supplement rainfall with irrigation to meet full crop water needs for high yield.  Although supplemental irrigation minimizes crop water stress due to inadequate and/or untimely rainfall during the season, it is costly and energy intensive and thus must be managed most efficiently to be profitable.

 Irrigation in South Carolina

In South Carolina supplemental irrigation accounts for about quarter of the State's water usage during summer months.  Currently, of the total harvested cropland of about 1.6 million acres, close to 170,000 acres are irrigated, of which about a fourth is in corn and mostly under center pivot irrigation. 

Irrigation requires a high initial equipment cost and substantial yearly operating and maintenance costs.  Another important requirement is the need for on-farm irrigation management capabilities and time.  In return, irrigation offers a significant potential to increase yield (and thus profit) and to stabilize yield over time.

With the rising commodity prices and the potential devastating effect of drought on yield, an increasing number of rainfed corn growers are interested in adopting irrigation.  However, many producers are raised in a rainfed culture and tend to lack familiarity with the basics of irrigation water and systems management.  This limits their ability to take full advantage of irrigation and its higher input requirements for maximum profit and high water and energy use efficiency. 

Over-irrigation is the natural tendency to guard against risks associated with inadequate water management plans, especially in places where water is inexpensive.  Another problem is under- or late-watering which limits yield and a failure to return a profit above the cost of the irrigation. 

In South Carolina irrigators need up-to-date information and know-how as well as simple and practical methods and technologies to efficiently utilize the advantages of irrigation to remain competitive. One of their main challenges is to operate as efficiently as possible to ensure that their irrigation investment will pay for itself and more. 

Good water management will go a long way to meet this challenge and help reap the benefits of irrigation by avoiding over-watering that wastes water and energy, and increases disease, nutrient leaching, and contamination.  As water resources become limiting due to population growth, competition, regulation, drought, and quality degradation, efficient and wise use of irrigation water in agriculture will be increasingly more important. 

A good irrigation water management scheme starts with sound irrigation scheduling.  A general discussion of irrigation scheduling follows, but the discussion is less than exhaustive due to lack of space and thus should be viewed only as a rough guide.

Irrigation Scheduling of Corn

In arid environments there is usually limited to no rainfall during the season, and thus a strategic scheduling at regular intervals and in routine amounts usually works.  Irrigation in humid environments like South Carolina where rainfall is plentiful, but unpredictable, requires a more tactical scheduling strategy.  In this environment, irrigation must be planned in conjunction with rainfall.  Any irrigation strategy simply using regular intervals is doomed to waste water, energy, and inputs. 

Unfortunately, it is not uncommon in South Carolina to observe on-farm strategies of 2 inches per week applications to corn for an extended period of about 10 weeks.  This is too much water with a high cost of about $8-12 per inch per acre if pumped from a few hundred feet deep well. A

s evident in Fig. 1, corn peak water use rates are about 0.3 to 0.35 inches per day in mid season (during tasselling), with smaller rates before (vegetative stage) and after (senescence).  Sound scheduling is needed to match crop water use with irrigation amounts to prevent over-application while minimizing yield loss due to stress.

Any irrigation scheduling method must answer two basic questions: when to irrigate and how much to apply (or basically when to stop).  We should irrigate when crop comes under water stress large enough to reduce yield.  This critical level of stress varies with stage of growth. 

Generally, corn is more sensitive to water stress during reproductive (especially silking stage) than vegetative stages.  There are three ways to decide when to irrigate: measure crop stress, measure the amount of water in the soil, or estimate the amount of soil water using an accounting approach (i.e., the check-book method).

There are tools to measure crop water stress, but they are not yet suitable in practice. The common practice of visual observation of crop water stress in the form of leaf darkening or rolling is not wise because by then some yield damage has already been done.

Most other methods of estimating the timing of irrigation require determining the amount of soil water in the crop root zone.  Since the crop root zone elongates as the season progresses, it is important to know its effective depth at each irrigation. 

The amount of soil water in the root zone can be determined by varying methods ranging from using a shovel or a soil probe for hand feel determination of soil water to installing mechanical or electronic soil water probes, with some capable of relaying the data to a nearby data logger or to remote computers for retrieval and analysis. 

The most practical and economical soil water sensors are tensiometers and soil water blocks (i.e., WATERMARK sensors from Irrometer, CO., http://www.irrometer.com/agcat.htm), both measuring soil water tension.  Calibration is needed to relate the tension reading to the amount of soil water. Tensiometers are easiest to read and are best suited for sandy soils.

Soil water blocks work in most soil and are read using an electric meter. The hand-feel technique is inexpensive, but is much less precise and takes time to learn.  Generally, several sites in each field should be monitored frequently enough to start irrigations on time.  Consult your local county Cooperative Extension agent for assistance in using a suitable soil water monitoring system.  If available, it pays to seek the services of an irrigation consultant.  This is a quite common practice in the West, but there are hardly any service providers in South Carolina.

In humid regions and because of the significant rainfall, the most reliable method for determining when to irrigate is based on frequent measurement of soil water.  In comparison to other methods like the check-book method, a direct measurement of soil water is most accurate as it eliminates the need to measure rainfall and estimate runoff, leaching below the root zone, and crop water use on daily basis.

Regardless of the method used to determine soil water status, the farmer needs to know how much water the soil profile (i.e., top 2 to 3 ft) can hold between field capacity and wilting point.  This is the plant-available soil water, and as shown in Table 1, it varies with soil type.  Most soil profiles in the Coastal Plains region are sand to loamy sand in the top foot and loamy sand to sandy loam and loam below, with available soil water of about 2 inches in the top two feet of soil.  Consult your local NRCS or county soils maps for estimates of available soil water in your field.

Irrigating the corn when 50% of the plant available soil water is depleted is a recommended practice that ensures adequate soil water without a yield loss.  As a general rule, when soil water drops to 50% available in the crop root zone, simply irrigate to bring the root zone back up to full capacity. 

Corn is a water sensitive crop and has a low tolerance for drought with the danger of yield loss regardless of the timing of drought.  The amount of yield loss due to drought stress depends on the corn growth stage and the severity of the drought. 

The sensitivity of corn to drought stress declines in the order of silking to grain filling (blister to dent) to vegetative development (pre-silking). However, corn grown on sandy soils is more prone to yield-reducing effects of pre-silking water deficits than is corn grown on finer-textured soils, and should be monitored for stress that may limit vegetative growth. 

On sandy soils, irrigation may be required to germinate the seed and maintain proper development.  During the rapid vegetative growth stage, corn is reasonably tolerant of soil water stress and soil water depletion may approach 60% before irrigating.  The most critical period for corn is during tasselling and kernel fill, particularly one week before to couple of weeks after silking stage.  During this latter period, keeping the available soil water depletion level to less than 50% (i.e., 30-40%) is wise to stay ahead of the rapid water depletion due to high demand.

In sandy soils it is also a good practice to anticipate silking and ensure near full soil water capacity just prior to this critical stage. Adequate water is still needed in the late reproductive stage of kernel denting to black layer, but it is not as critical as the silking and tasselling stages and thus available water depletion may approach 60% without yield damage. Following dent, soil water depletion can fall to the 70 to 80% range without danger of a yield loss.  Use the "milk layer" or "milk line" as the criterion for estimating days to maturity and to determine the last irrigation.

It takes about 20 to 25 days for the milk-line to progress from the top of the kernel (beginning dent stage) to the bottom or base of the kernel (maturity or black layer) where it is attached to the cob. Thus, a final irrigation when the milk-line is halfway down the kernel coincides with 10 to 12 days to maturity. Furthermore, since rainfall forecasts are now relatively reliable for the next 24 to 36 hrs, it is an efficient practice to anticipate this rainfall and thus allow some soil water storage in case the rain should come soon after irrigation. Using depletion of available soil water as a guide removes the guess work as to when to begin watering in the growing season and after each rainfall event.

By knowing soil water capacity and the current soil water status, correct amounts of water can be applied so that losses to runoff and deep percolation are minimized while crop needs are met.  The common practice of running the pivot continuously during an extended period in the season is costly and should be avoided.

On rainy days, stop the system and then restart when soil water measurements indicate the need for irrigation.  Under conditions of frequent rain, there is really no easy way to estimate soil water in the profile than to simply measure it. 

Investing time in scheduling irrigation based on soil water monitoring will eliminate the guess work and help promote wise use of water.  In summary, measuring soil water can help determine not only when to start irrigating, but also when to stop.

The above guidelines are partially based on authors' experience, but most credit should be given to water management in corn articles by Dr. Larry G. Heatherly (retired USDA-ARS Research Agronomist) and Dr. Ronnie W. Heiniger, Professor, Crop Science Department at North Carolina State University.

daily water use 

Figure 1.  Daily corn water use during the growing season (reprinted with permission from NC Corn Production Guide at http://www.ces.ncsu.edu/plymouth/cropsci/cornguide, Irrigation and Drought Management Chapter by Dr. R.W. Heiniger).

 Table 1. Approximate available soil water holding capacity for various soil textural classifications.

 

Available Moisture

Soil Texture

Inches/Inch

Inches/Foot

Coarse sand and gravel

0.02 to 0.06

0.2 to 0.7

Sand

0.04 to 0.09

0.5 to 1.1

Loamy sand

0.06 to 0.12

0.7 to 1.4

Sandy loam

0.11 to 0.15

1.3 to 1.8

Fine sandy loam

0.14 to 0.18

1.7 to 2.2

Loam and silt loam

0.17 to 0.23

2.0 to 2.8

Clay loam and silty clay loam

0.14 to 0.21

1.7 to 2.5

Silty clay and clay

0.13 to 0.18

1.6 to 2.2