It’s difficult to find much current research on the interaction of corn with nitrogen and irrigation in the South, but if recent crop years are any indication, there could be more coming.
“This past year, we averaged about 154 bushels per acre on corn this year in Mississippi, which was unbelievable,” says Jac Varco, Mississippi State University Extension soil scientist.
“When I first arrived in Mississippi, we were averaging 68 bushels per acre. In the beginning, corn was being grown in the hills, but now it’s being grown on cotton ground in the Mississippi Delta, and most is irrigated. We’ve switched soils, so yield potential has increased,” he says.
Speaking to a group of Alabama crop consultants recently, Varco says rates, timing and placement of nitrogen fertilization all are important considerations when producing corn.
“I don’t worry much about fertilization until I see the crop start to come up because the crop isn’t using any during germination. It has enough nitrogen in the soil and in the seed to get it through a couple of leaves of corn. By the time it has two leaves or so, I put on my first fertilization — usually about 50 percent.
“I would wait even longer if I could, but the corn is getting high and we’re usually using ground equipment, so we don’t have high-clearance equipment yet for side-dressing. Depending on how many acres you have to cover, the corn might be at V6 when you start and V8 when you finish,” says Varco.
In Mississippi, he says, growers have gotten away from using anhydrous ammonia. They’ve been weaning us off ammonium nitrate because people are using it for purposes other than to fertilize, and we’re losing it,” he says.
While fertilizer prices have decreased some, they still comprise about 40 percent of the cost of producing a crop of cotton or corn, he says. The next largest input crop is fuel.
Anhydrous is cheapest source per pound
“Anhydrous ammonia is the cheapest source of nitrogen per pound of nitrogen, urea is next, then urea ammonium nitrate solution, and then ammonium nitrate is the most expensive,” says Varco.
“With cotton, at 100 pounds of nitrogen from anhydrous ammonia or ammonium nitrate, you’ll spend $29 or so more per acre if you go with the liquid over anhydrous ammonia.
“If you switch to corn at 200 pounds of nitrogen, you’ll spend close to $60 per acre. When you do it on paper, anhydrous ammonia looks good.”
Mississippi has been changing, he says, in terms of the crops being planted.
“Not too long ago, we had 1.25 million acres of cotton. Now we have 800,000 acres of corn, 500,000 to 600,000 acres of cotton, and cotton is expected to go down.
“When you switch to corn, it doesn’t take as much labor as with cotton. But we’re trading labor costs for fertilizer nitrogen costs. You’re spending $59 more acre for that extra 100 pounds of nitrogen required by corn. You go from a 100-pound nitrogen rate with cotton to a 200-pound rate with corn,” says Varco.
Ammonium nitrate is efficient on most soils in Mississippi as a broadcast spread, he adds. “Urea usually takes a higher nitrogen rate to get the same yield as ammonium nitrate.
“If you did an economic comparison, ammonium nitrate is probably at about 175 to 180 — the nitrogen rate you should apply — whereas urea is closer to the 200-pound rate.
“UAN solution is knifed into the soil, and it’s very efficient, producing almost 200-bushel corn with 150 pounds of nitrogen.
Mississippi State University has a general recommendation of 1.3 pounds of nitrogen per bushel of yield goal for corn, says Varco.
“More and more wells are being dug in Mississippi each year, and we’re depleting the aquifer, even in the state’s Delta region. With furrow irrigation in the Delta, it seems that when we flush our soils, we’re flushing the nitrogen.”
Since Mississippi growers are banding nitrogen, Varco says research has been conducted to look at the best place for putting that band.
“We’re spacing nitrogen 6, 12 and 18 inches from the row, so with 38-inch rows, we’re almost in the middle.
“A lot of our growers and the Extension Service recommend that we put nitrogen in the middle. Part of the reason for that is that it’s easy for tractor drivers to do it and not damage the crop. The knife will be in the middle and won’t prune roots.”
Treatments included 15N labeled fertilizer applied at 6, 12 and 18-inch distances from the corn row using both surface and subsurface-banded application.
Following university recommendations, fertilizer nitrogen was split-applied, with 50 percent put down at planting followed by 50 percent at the V6 corn growth stage.
The research showed that with subsurface banding, there wasn’t much difference in banding at 6 and 12 inches from the row, says Varco.
“But when we got in the middle, nitrogen use efficiency dropped because we’re placing it farther away from the plant. This past year, with surface dribble, we also saw declines as we got farther out.”
When researchers looked at yield influenced by irrigation, it went from 160 bushels per acre down to about 135 as the nitrogen got farther from the row and it was placed in the soil, he says.
“That’s 25 bushels, and a lot of money at today’s prices. There’s a general decline with the surface placement. Even though it’s out in the middle, irrigation washes it into the soil, but the band is already 3 to 4 inches deep, giving it a head start. This is all furrow irrigation on every row.”
In 2012, yields dropped off by 40 bushels from a high of 220 bushels per acre when it was applied closer to the row, says Varco.
“One factor too is that today’s varieties are yielding much better than in the past. There has definitely been a yield boost. If you’re banding your fertilizer, this research is telling us to get it closer, even to go from broadcasting to banding the UAN.”
Another study looked at tracer fertilizer and the recovery of nitrogen, explains Varco.
“With nitrogen surface-banded, we saw a decline in recovery and use efficiency goes down by about 15 percent. So we recover about 15 percent less of the fertilizer that we put out by going closer to the row.
“That tells us the corn roots aren’t always getting the fertilizer. In 2012, recovery declined by almost 20 percent when we got farther away from the row. The closer you get that nitrogen to the row, the greater the efficiency.”
In a study conducted on cotton, Varco says about 36 percent of the ammonium nitrate applied was recovered while 20 percent of the urea was recovered.
“When we put in Agrotain, the recovery was almost equal to ammonium nitrate, at 32 percent. UAN banded, about 9 inches from the row, was most efficient. There is some loss when UAN is applied surface-dribbled. UAN dribbled with Agrotain is as good as UAN banded.”
When growers are spending 40 percent of their crop input costs on fertilizer, they’ve got to find a way to maximize the efficiency, says Varco.
“What other input produces a growth stimulus as great as fertilizer nitrogen? Irrigation does, sometimes, and it costs a lot.
“The boost we get from fertilizer nitrogen is tremendous, so strategic application can make a difference. If it’s in the bed or if it’s flat ground, get it about 6 or 8 inches from the row, and it doesn’t have to be too deep.
“It’s important to split applications, anywhere from about 25 percent at planting and get the rest on by V7. Put your nitrogen out and then water it in, but don’t do a heavy irrigation. Just get the nitrogen into the soil, about one half inch. Our center pivots run about 7/10 inch, and one pass is a pretty good break.”
Growers might consider variable-rate application, says Varco, and the use of inhibitors should be on a case-by-case basis.
“If you really want to buy fertilizer insurance every time you plant a crop, then you should use an inhibitor. But I believe it should be on a case-by-case basis, considering the conditions.”