Shifting nitrogen fertilizer applications to later in the growing season pays off in higher yields as well as more efficient fertilizer use, according to a study by a Dumas, Texas, crop consultant.
David Reinart compiled data from 781 corn fields for 11 years. He and his partner, Chris Cogburn, consult for about 25 growers as a sideline to their full-time jobs—Reinart with Wilbur-Ellis Company and Cogburn with National Sorghum Producers. Their company is called Better Harvest.
Because Reinart’s clients irrigate, they can feed UAN (urea-ammonium nitrate) solution through their center-pivot systems all season long. Although conditions vary around the U.S., the principle of spoon-feeding nitrogen as late as possible applies to dryland farmers, too. (See “Adjust Nitrogen Principles to Your Environment,” page 50.)
The numbers tell all. Farmers who applied most of their nitrogen before planting averaged yields of 208 bu. per acre during the 11 years. They applied almost 1.2 lb. of nitrogen per bushel of corn produced. However, those who spoon-fed nitrogen through their center-pivot irrigation system, with the application extending through pollination, averaged yields of 225 bu. per acre. They applied less than 0.8 lb. of nitrogen per bushel of corn.
In addition to the economic benefits of higher yields, spoon-feeding has positive environmental implications. The less nitrogen fertilizer applied (and the closer to time of usage by plants), the less chance there is of unused nutrients finding their way into water supplies. Leaching is less of a risk in the arid Texas Panhandle, with 17" of annual rainfall, but it’s a concern in higher-rainfall areas.
“The total amount of nitrogen we apply does not increase,” Reinart points out. “The study does not suggest applying more nitrogen than you otherwise would. We just apply less of it preplant and more of it late in the season. In fact, other studies we have done show that if you apply too much nitrogen, corn yield will decline.”
Maximizing nitrogen efficiency hinges on knowing your soil conditions and flexing your nutrient application plans based on various factors. Reinart’s preplant soil test, in addition to residue cover, organic matter content and whether manure was applied, determines how much nitrogen a client will apply before planting.
“Whether to apply any preplant nitrogen at all depends on the individual field,” Reinart says. “But we never apply more than 100 lb. per acre.”
Midseason adjustments. In-season tests reveal how much nitrogen is already available for the crop, preventing excessive application. Reinart also adjusts each client’s recommendation based on soil and plant tissue tests and growing conditions.
At the four-leaf stage, Reinart analyzes soil and plant tissue for nitrogen, then formulates a nitrogen recommendation for the rest of the season. “Whatever the nitrogen recommendation at the four-leaf tissue testing, we always recommend applying at least 70 lb. of it after pollination,” he says.
In an ideal program, Reinart also pulls soil and tissue tests at tasselling and again adjusts his nitrogen recommendation. “If the corn is doing well, by the time it gets to tasselling it will have used more nitrogen, so soil levels will be lower,” he says. “That warrants an increased late-season nitrogen application if the remainder of the growing season will be favorable.”
Midseason rate adjustments have a big impact on nitrogen efficiency. “In 2004, a very good growing season, my average nitrogen recommendation at tasselling was 63 lb. per acre,” Reinart says. “My grower’s fields averaged 236 bu. per acre. In 2005, a hot, dry season, my average recommendation after tasselling was only 15 lb. per acre, and my growers averaged 202 bu. per acre. More nitrogen wouldn’t have done any good because we didn’t have the growing conditions to maximize yield.”
For Dumas farmer Harold Grall, using this spoon-feeding program gives him the flexibility to cut back on nitrogen rate if the corn gets damaged by hail, other weather events occur or the crop loses yield potential in any way.
Harvesttime test. For growers who request it, Reinart will test soil and plant tissue at harvest as well. In his low-rainfall area, a fall nitrate test usually provides a reliable indication of how much nitrogen will be available next spring (unless the nitrogen gets tied up by bacteria decomposing the crop residue, another factor that Reinart keeps in mind).
The harvesttime tissue test reveals how effective the current year’s fertilizer recommendation was. “Iowa State University recommends 700 ppm [parts per million] to 2,000 ppm in the stalk at harvesttime,” Reinart says. “I’ve found that the best yields tend to occur in that range.”
On-farm impact. Dalhart, Texas, grower Will Allen has followed Reinart’s recommendations since 2003.
- December 2010