Production: Water Works

Irrigation timing and frequency impact yields
and operating costs.

By Kerry Harrison and Dewey Lee
CS  Cover 2Irrigation requires a relatively high investment in equipment, fuel, maintenance and labor, but offers a significant potential for increasing net farm income. Frequency and timing of water application have a major impact on yields and operating costs.

To schedule irrigation for the most efficient use of water and to optimize production, it is desirable to frequently determine the soil water conditions throughout the root zone of the crop being grown. A number of methods for doing this have been developed and used with varying degrees of success. In comparison to investment in irrigation equipment, these scheduling methods are relatively inexpensive. When properly used and coupled with grower experience, a scheduling method can improve the grower’s chances of success.

Any plan typically is better than no plan or method at all, particularly with corn. A good plan pays dividends in terms of yield, water-use efficiency and net returns. Growers who take a “wait for the crop to tell me” attitude never get the greatest benefits for their irrigation. In corn, irrigating too late causes yield loss, while irrigating too much wastes energy, water, money and can leach nutrients beyond the root zone.

Moisture Balance Or Checkbook Method
The most simple and practical way of scheduling corn irrigation is to use the moisture balance or checkbook method. This helps a grower keep up with an estimated amount of available water in the field as the crop grows.

The objective is to maintain a record of incoming and outgoing water so that an adequate balance amount is maintained for crop growth. Growers will need certain basic information to use a checkbook method. The soil type of the field, expected daily water use of corn, water-holding capacity of the soil and a rain gauge or access to nearby rainfall information are the basic starting point items.

Checkbook-type methods can be enhanced with other different tools or methods, such as the EASY Pan method. The UGA EASY (Evaporation based Accumulator for Sprinkler enhanced Yield) Pan is designed to be easy to operate, economical and representative of the water used by the crop in humid areas. A couple of the unique operating characteristics are the ability to read the unit from a distance and the fact that no record keeping is required. This makes the EASY Pan a simple tool for scheduling irrigation. The float-based mechanism is designed to represent both the effective root depth of a crop and the soil water-holding capacity. The covering screen on the pan unit is designed to limit evaporation to a level similar to the evapotranspiration rate (water use) of a crop.

Also expert systems, such as Irrigator Pro (software by USDA) or other scheduling software, are available to help you make decisions regarding when to irrigate. Soil moisture measuring devices such as Echo7 and Watermark7 can be used in conjunction with corn growth curves to enhance irrigation scheduling as well.
These devices provide instant readings of either soil moisture content or tension in the root zone and can identify exactly when water is needed to replenish the root zone.

Less Than 150 Bushels Per Acre
Probably the most important management decision about irrigation scheduling is your yield expectation and water availability.

For growers targeting yields of less than 150 bushels per acre or with less than five inches of water available, watch for visual signs of stress that occur just prior to tasseling – “leaf curling” that occurs before noon.

In this case, a thorough application of water (up to 2.5 inches, depending on soil type) should be made as tassels begin to emerge and another application made two weeks later. Do not change this timing unless very heavy rainfall occurs. If water is still available, a third application of the same amount could be made two weeks after the second application.

In short, if you are targeting a yield of less than 150 bushels per acre, or if you have a limited amount of water to apply, you should not irrigate until you see “leaf curl” at the tassel stage. At this point apply 2 to 2.5 inches of water every 14 days (1 to 1.25 inches back-to-back may be necessary) until a heavy rainfall occurs or you run out of water.

Greater Than 150 Bushels Per Acre
Corn growers who are targeting yields of greater than 150 bushels per acre and have an adequate amount of water available may want to consider other methods to schedule irrigation that will help eliminate all periods of drought stress. To effectively schedule irrigation, determine the soil moisture conditions in the rooting zone frequently, and keep a record of your findings.

Devices such as soil water potential sensors can be used to monitor water use and increase water-use efficiency by more accurately helping to schedule irrigations. Irrigate whenever soil moisture falls below the desired level. This system allows efficient water use and top yield potential.

Make soil moisture determinations daily during peak moisture use periods. During other periods, make readings frequently enough to detect irrigation needs before stress occurs. Usually three times weekly will be often enough for the first 50 to 60 days after planting.

Contact Kerry Harrison, University of Georgia (UGA) Extension Service, at (229) 386-3442 or Contact Dewey Lee, UGA Extension, at (229) 386-3006 or Consult the 2009 University of Georgia Corn Production Guide for more information.

Will Center-Pivot Irrigation Hurt Pollination?

Corn possesses a vast overabundance of pollen and several traits that make the pollination process relatively immune to overhead irrigation or rainfall disturbance.

Corn produces more than enough pollen grains (three to five million per plant), which fertilize about 600 to 700 kernels per ear (greater than 4,000 pollen grains per silk). Physical disturbance caused by overhead irrigation occurs over a very short time period in relation to corn pollination capacity.

Pollen shed normally lasts five to eight days, during which pollination may occur at any time. Corn plants also have an innate ability to stop pollen shed when the tassel is too wet or dry and trigger pollen shed when conditions are favorable. Additionally, silks are quite sticky, which makes pollen grains hard to wash off after they land on a silk.

Thus, the physical disturbance caused by overhead (center-pivot) irrigation will not reduce corn pollination in a normal field/agricultural environment.

Insufficient irrigation during the pollination period is actually much more likely to hurt pollination and kernel set. Corn’s water use rate is highest during a four-week period following silking. Corn grain yield is extremely sensitive to any type of stress, including water deficiency, during this time.

Ample moisture helps plants maintain photosynthesis and transpiration rates at high levels. These processes are responsible for producing energy and cooling plants, respectively. Thus, if water deficiency limits corn growth during pollination and early kernel fill, considerable grain yield reduction will occur.

Source: Mississippi State University Extension Service

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