Irrigation has not been a common practice for North Carolina corn, but the value of the 2008 crop and recent memories of the 2007 drought may have influenced some growers to plant under irrigation.

Despite the high prices growers are receiving for corn, the wise use of inputs, including irrigation water, may mean the difference between profit and loss in these boom times.

With the cost of diesel fuel topping $4 per gallon, growers need to be keenly aware of how much water a corn crop needs, when it needs the water, and how to best apply it.

The main objective of irrigation scheduling is to manage irrigations for greatest effectiveness. Proper scheduling will minimize yield loss due to crop water stress, maximize yield in response to other management practices, and optimize yield per unit of water applied. The net result is a positive contribution to the bottom line.

Irrigation scheduling that results in excessive or inadequate water application usually reduces profitability. For example, too much irrigation water during peak times in the growing season can reduce crop water stress. However, it is likely to reduce both irrigation efficiency and corn response to good management practices, like fertilization, planting date, plant population and weed control.

Too little irrigation water can cause corn water stress and less yield response to the same good management practices, primarily by reducing deep percolation into the soil.

Last year's drought caused many growers to be reluctant to turn off irrigation systems. The high cost of fuel and ever-increasing concerns over water availability are good reasons to apply science rather than emotions when it comes to decide whether to run irrigation systems.

Allowing the center pivot to run continuously during the bulk of the season is a common but costly procedure.

Throughout much of the upper Southeast 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.

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.

North Carolina State University Corn Specialist Ron Heiniger says, “Irrigation scheduling should be used to determine exactly when to irrigate and how much water to apply.

Heiniger stresses that scheduling can take many forms: Calendar date, plant growth stage, crop condition, soil water status, and scheduling using evapotranspiration (ET).

North Carolina's highly variable weather patterns prevent calendar dates or plant conditions from being good criteria for irrigation scheduling, Heiniger says.

Instead, he contends for medium and fine textured soils or on organic soils, using plant growth stage can work well. Heiniger stresses that regardless of the scheduling method used — don't wait. Waiting for the crop to show signs of wilt means it is too late to prevent damage, particularly during the reproductive period, he says.

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.

The most reliable scheduling method is monitoring soil water, which can be measured periodically using soil water blocks, tensiometers, or the hand feel technique, he says.

“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,” Heiniger says.

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 Heiniger suggests contacting local county cooperative Extension personnel.

The North Carolina State specialist says 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,” Heiniger says.

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, he adds.

In addition to proper irrigation scheduling, Heiniger says there are some management practices that can maximize irrigation capabilities, including:

  • Hybrid selection. On fields that will be irrigated growers should select medium maturing, disease resistant hybrids that will tolerate plant populations of 31,900 plants per acre or higher.

  • Adequate fertility. Potash at 100 to 150 pounds per acres on irrigated fields when soil tests indicate a need is particularly important to prevent lodging.

  • Starter fertilizer. To maximize yields in the upper Southeast, a 1:1 ratio of N to P is a good bet.

  • Multiple N applications. A proven application is 40 pounds at planting, 120 pounds at side-dress and 40 pounds at pre-tassle.

  • Additional sulfur. To keep N and S ratios in a 12:1 ratio or less, additional sulfur may be needed.

Along with high corn prices comes high input cost — not the least of which is irrigation water. Diesel fuel at $4 per gallon may make it economically questionable to irrigate. When irrigation is called for and is an economically sound crop input, scheduling is the best way to insure maximum value of this high-cost input.