Following the 2012 drought, many Nebraska growers are faced with water allocations and looking for ways to reduce evaporation and transpiration (ET) or are wanting to ensure more efficient irrigation applications.
This year many growers have asked whether they should reduce irrigated plant populations to reduce ET. Let’s take a look at the dynamics of this suggestion.
First we must consider Leaf Area Index (LAI). The LAI defines the amount of leaf surface area available to transpire water. The leaf area index is the ratio of leaf surface area (one side) to land surface area. For example, if there is an average of 15 square feet of leaf surface for each 5 square feet of land surface, then the LAI would be 3.0.
When this value is greater than 2.7, the crop’s evapotranspiration is determined only by atmospheric demand and is not limited by leaf area, provided that the plant is not under water stress. For irrigated plant populations for corn, a LAI value of 2.7 is reached when the crop has about 14 fully emerged leaves or is five to six feet tall. For many irrigated corn hybrids the LAI approaches 6.0 during the growing season.
Figure 1 shows a typical leaf area index curve as it changes throughout the growing season for irrigated corn in various populations. Before the crop reaches the peak LAI value in the figure, it crosses the full ET threshold of 2.7. From that point on, leaf area becomes a non-limiting factor for crop water use.
The figure also illustrates the effect of plant population on leaf area index over the growing season. The curves represent four populations of corn planted on the same date and with the same hybrid. Once the “high” and “medium” populations cross the full ET threshold (LAI = 2.7), the rate of water use is essentially the same for both populations. Leaf area index is not the limiting factor in water use in either case; however, the high population crosses the threshold level sooner than the medium population. After the high population has passed a leaf area index of 2.7 and before the medium population reaches it, transpiration by the high population would be greater for a few days. While this is happening, soil evaporation would typically be greater for the low plant population so overall ET is about the same.
To get any significant reduction in water use under irrigation, populations of modern, upright leaf corn varieties (115-120 day maturity) would have to drop below about 18,000 plants per acre. Substantial water conservation would come only when populations are in the range of 8,000-10,000 plants per acre. For shorter season hybrids (with fewer leaves), populations can be 10-20 percent greater and conserve water, but the principle is the same.
The bottom line is that there may be good reasons to reduce plant populations on some soils or in certain areas of an irrigated field; however, water conservation is probably not one of them.