Farmers may be able to increase corn yields by splitting their nitrogen applications, but they could help the cause considerably if they can water the nitrogen in, LSU AgCenter researchers say.

Studies at the Northeast Research Station near St. Joseph, La., and other locations have shown growers can apply supplemental nitrogen as late as tassel emergence and improve yields. But the amount of increase can vary by soil type, year and whether the field can be irrigated.

Last year, LSU AgCenter scientists at the station applied 0, 150, 180, 210, 240 and 270 pounds of nitrogen at the two-leaf growth stage and followed that with 0 or 60 pounds of N (ammonium nitrate) at tasseling to corn planted on a Sharkey silty clay.

“When we irrigated the fertilizer in, at the 150-pound early season rate we increased yield from 148 to 178 bushels per acre with that 60-pound late application,” said Rick Mascagni, research agronomist with the LSU AgCenter. “But, if you look at the total N applied, yields were similar regardless of how you made the split.”

Speaking at the Northeast Station's annual field day, Mascagni said researchers at the station have been conducting studies using precision agriculture technology — with a twist.

“We know corn fertility or soil fertility, in general, is site specific as well as year specific,” he said. “In other words, each year is different, and the soils and what have you are different. So we're putting on a base amount early and monitoring the plant to see if we can add more later to maximize yield.”

The monitoring tools include the GreenSeeker optical sensing and variable-rate application system, SPAD or chlorophyll meter and pulling leaf samples to determine total N in the leaf tissues.

AgCenter researchers, who have been looking at “late” nitrogen applications in corn since 2003, say some of their best yields were harvested on irrigated corn on the Sharkey clay plots in 2005. (The corn, Pioneer 32D99, was planted April 10 with a seeding rate of 30,000 plants per acre.)

That year, researchers applied 180, 210 and 240 pounds of nitrogen in a 32 percent URAN-solution. The nitrogen was knifed in at the two-leaf stage. At tassel emergence, the researchers broadcast 0, 30 or 60 units of nitrogen as ammonium nitrate.

“We had some outstanding yields,” said Mascagni. “The corn was irrigated, and it didn't seem to matter how the nitrogen was applied, we basically had the same yield whether it went out at the two-leaf stage or at tassel.”

When they applied 240 pounds of N — 180 pounds early season and 60 at tassel — for example, the plots harvested the equivalent of 230 bushels per acre. If they put out 210 early season and 30 at tassel, the yield was 228 bushels per acre. “In other words, the split application did not enhance the nitrogen fertilizer efficiency, at least in 2005.”

The researchers applied 120 pounds and 180 pounds of nitrogen per acre to a Commerce silt loam soil in a non-irrigated test that same year. Yields ranged from 155 to 200 bushels per acre, but produced no significant differences between single and split applications.

In 2006, the researchers encountered some yield differences between the two methods in a non-irrigated and irrigated test. Planting a different hybrid, Pioneer 31R88, the researchers applied rates of 150, 180, 210, 240 and 270 pounds of nitrogen at the two-leaf stage. Late applications included 0 or 60 pounds of N as ammonium nitrate.

“In the non-irrigated, we applied the ammonium nitrate right at tassel, thinking that surely we would get some rain,” said Mascagni. “It was about 30 days before we got any rain so the late nitrogen was completely ineffective in 2006 because of the lack of rain to activate it.”

In the irrigated plots, applying 60 pounds of nitrogen at tassel emergence increased the yield with the 150-pound early season rate from 148 bushels (with 0 late N) to 178 bushels per acre.

“Nitrogen moves with the soil water to the roots via mass flow,” he said. “If you don't have any soil water, then nitrogen is not being transported to the roots. The plant itself is also being affected, shutting down somewhat in the heat resulting from the lack of moisture.

“But, if you look at the total nitrogen applied, the yields were similar no matter how you made the split.” (That is, 210 pounds applied early season with no late nitrogen produced 185 bushels per acre, a yield not significantly different from 178 bushels.)

Researchers took seed samples and had them analyzed for the percentage of nitrogen and total seed nitrogen. They also calculated nitrogen fertilizer uptake efficiency percentages for the different rates of early season and late season nitrogen applications and correlated those numbers with grain yields.

“We increased our yields up to 175 bushels,” he said. “You can see the optimum rate on the silt loam is probably between 180 and 210 pounds of nitrogen at least for these particular tests.”

Irrigation is proving to be a key ingredient in this year's tests, as well, he noted.

“We've watered this test about four times,” said Mascagni, who spoke in front of the irrigated plots in the Sharkey clay portion of the study at the field day on June 13. “Our last rain was about 1.3 inches on May 3. That's been about 40 days we basically have had no rain. The non-irrigated corn is really starting to show the effects of drought stress.

“Unless we get some rain this week — I mean 2 or 3 inches — a good rain, we're probably not going to have a test at all.”

Mascagni said the researchers will attempt to harvest the plots and continue to analyze the data from the GreenSeeker technology, SPAD meter and leaf and seed samples.

“We will correlate all that back to the yield response at a particular rate,” he said. “Hopefully, we'll also do an economic analysis of the different treatments. When you consider the cost of fertilizer and application expense with the airplane, anything you can do to reduce rates without cutting yields can help.”