Late-season N applications could improve corn yields

Corn farmers get kidded about how they supposedly work only a couple of months of the year. They plant the crop, spray it with herbicides once or twice and then go to Florida until it's ready for harvest. Or so the story line goes.

Louisiana State University AgCenter researchers may have found something else for corn producers to do in the summer — sample leaf tissue to determine if conditions warrant a late-season nitrogen application. (Don't write me. I know growing corn isn't that easy.)

Preliminary research at the AgCenter's Northeast Research Station near St. Joseph indicates that fertilizer N applied as late as tassel emergence in corn can increase yield, researchers said at the Northeast Station's Crop Production and Pest Management Field Day.

“Sometimes nitrogen applications are delayed or omitted due to inclement weather,” said Rick Mascagni, agronomist with the AgCenter. “At other times, growers apply the recommended N rate for an expected yield potential; however, as the crop develops, yield potential may be higher than expected and additional N may be required.

“In each of those situations, the question arises: Can N applications as late as reproductive growth stages be effective?”

LSU researchers compared a number of N rates and application timings on two Mississippi River soils — a Commerce silt loam and a Sharkey clay — at the Northeast Station in 2005.

They “knifed in” a 32-percent URAN solution at the two- to three-leaf stage of Pioneer 32D99 variety corn that was planted on April 10. The rates were 120 and 180 pounds of N per acre on the Commerce silt loam and 180, 210 and 240 pounds per acre on the Sharkey clay.

The late-season application included a no-nitrogen control, 30 and 60 pounds of N per acre as ammonium nitrate and 30 and 60 pounds of N per acre as urea. The late N treatments were hand-broadcast at tassel emergence.

The researchers seeded the plots on both of the soil types at 30,000 seed per acre with a John Deere 1700 vacuum planters. The Sharkey clay plots were furrow-irrigated seven times beginning May (about the seven-leaf stage) and the Commerce silt loam plots were not irrigated.

Mascagni said the corn yields on the Sharkey clay plots were “exceptional,” ranging from 213 to 242 bushels per acre. Yields ran from 213 bushels per acre for the 180-pound N early-season application to 231 bushels per acre for the 240-pound N early-season application.

“Late N applied at tassel emergence increased yield at each early-season rate,” said Mascagni. “Since there were no differences between ammonium nitrate and urea N fertilizer sources, late-N data were averaged across fertilizer sources.”

At the 60-pound N late rate, yields were increased 7.9 percent, 7.0 percent and 4.7 percent for the 180-pound, 210-pound and 240-pound N early-season rates, he said. “Yields for total N applied (early-season plus tassel emergence) were similar regardless if applied in a single application at early-season or split between early-season and tassel emergence.”

On the Commerce silt loam, yields ranged from 154 bushels per acre for the 120-pound N early-season application to 174 bushels per acre for the 180-pound N early-season application. The 60-pound late N rate (averaged over the fertilizer N sources) increased yields 12.3 percent and 22.4 percent for the 120- and 180-pound N acre early-season applications.

As with the results on the Sharkey clay, yields for total N applied were similar regardless if applied in a single application at early-season or split between early-season and tassel emergence, Mascagni noted.

“Findings from this one-year trial indicate that fertilizer N applied as late as tassel emergence can increase yield,” he said. “But further studies are needed over time to better quantify yield responses to late-applied N on different soil types with different yield potentials.”

Researchers are conducting more trials on different soil types at the Northeast Station, including a Moohn silty clay. Mascagni said dry conditions that have prevailed much of the season could have an impact on this year's tests.

“We could use a rain,” he said. “Two years ago, we had 20 inches of rain in May and June, which caused a completely different set of problems.” (The 40-year average rainfall for St. Joseph is 5.27 inches in May and 3.89 inches in June.)

He said LSU researchers are also evaluating the economic feasibility of applying additional N late in the season, given last winter's high natural gas prices and the resulting increase in fertilizer costs.

“At one point, we were looking at nitrogen costs of 40 cents per pound,” he said. “But, fortunately, they came down.”

Steve Moore, resident director at the Dean Lee Research Station and coordinator of the state corn hybrid trials, told field day participants that 77 hybrids, most of them transgenic varieties, have been entered in the 2006 hybrid trials, which are being conducted at five locations around the state.

“About 70 percent of the varieties contain the Bt gene, about 70 percent are Roundup Ready and 8 percent Liberty Link,” he said. “With all of the Roundup Ready crops being planted, many of our growers have begun planting Roundup Ready corn as an insurance policy against glyphosate drift.”

Moore said LSU AgCenter researchers are continuing their efforts to identify and develop corn varieties that are more resistant to aspergillus flavus, the fungal disease in corn that produces aflatoxin.

“We think we could be two years away from a commercial hybrid with true resistance to the disease,” he said. “Researchers at the University of Illinois are conducting trials with about 20 hybrids. Efforts are also being made to insert biotech traits in those varieties.”

LSU researchers are also conducting field tests with applications of glufosinate, the active ingredient in Liberty herbicide, which is believed to trigger ammonia production that, in turn, breaks down the aflatoxin by-product of aspergillus.

“Glufosinate had little or no effect on aflatoxin in Liberty Link corn, but reduced aflatoxin in non-Liberty Link corn,” he said. “We're still trying to figure that one out. We're continuing to research the technology.

“Glufosinate application to corn threatened by aflatoxin contamination appears to be a hopeful economic tool for producers,” he said. “Use of glufosinate in this manner would probably require expanding the label. More field-plot and field-scale research is needed to confirm benefits and determine best practices.”

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