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Don't gut corn populations in dry soils -- expert

Jeff Caldwell 03/26/2013 @ 10:48am Multimedia Editor for Agriculture.com and Successful Farming magazine.

Though parts of the nation's center has taken big steps toward drought recovery after last year, much of Nebraska hasn't. Extreme drought conditions still persist in central and western parts of that state, especially, and while some may be considering cutting back corn plant populations to save on water, it's important to consider how much of a cut it would require and just how much benefit there really would be in doing so.

Farmers in Nebraska already face water allocations that dictate how much they can irrigate from the oft-thought rapidly declining Ogallalah Aquifer. Now, experts say more farmers in droughted areas are taking steps to making corn yields meet expectations under the constraints the crop's likely to face this growing season in areas where moisture's likely to remain on the short side, one of those being pared-back plant populations.

"Following the 2012 drought many Nebraska growers are faced with water allocations and looking for ways to reduce evapotranspiration or are wanting to ensure more efficient irrigation applications," says University of Nebraska Extension water/cropping systems educator Chuck Barr.

So, why isn't this the best strategy? It depends on evapotranspiration (ET). Optimal ET is the point at which it's not limited by short water supplies.

"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," says Barr. "For irrigated plant populations for corn, a LAI (leaf area index) value of 2.7 is reached when the crop has about 14 fully emerged leaves or is 5 to 6 feet tall. For many irrigated corn hybrids the LAI approaches 6.0 during the growing season."

Keeping that ET balanced at just the right point may require fairly steep reductions in the number of seeds planted. But, when you start to trim plant numbers too much, soil evaporation picks up the slack in ET created by less leaf area.

"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% to 20% greater and conserve water, but the principle is the same," Barr says. "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."

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