Variable-rate application acknowledges risk to boost yields
To build a successful variable-rate nitrogen program, it’s essential to have a working knowledge of the nitrogen cycle, nitrogen-supplying power of your soil types and nitrate measurement tools.
"Zone management is very complimentary to variable-rate nitrogen because it factors in the plant’s demand for nitrogen, the soil’s supplying power and the risk of loss. It’s easy to visually identify areas of yellow corn in a field and see the need for more applied nitrogen, but it takes more time and effort when using zone management to assess risk of nitrogen loss and implement variable-rate nitrogen," says Farm Journal Field Agronomist Ken Ferrie.
Understanding how nitrogen and soil work together can help your farm pocket greater profits, both by saving inputs and by applying enough nitrogen to realize maximum yields.
"Some years, you make profit by putting on less nitrogen than you normally would have," Ferrie says. "Other years, like 2013 with heavy spring rains, we had to kick up nitrogen rates. It became profitable by finding areas where nitrogen got away from us then shoring them up with VRT [variable-rate technology]. You made more money by making sure you didn’t run out of nitrogen."
Before adjusting nitrogen rates, farmers must first master application timing. Due to risk of loss, variable-rate nitrogen is most successful in a sidedress application.
Timing is critical. When farmers split nitrogen applications, Ferrie says, they pick up 10% efficiency by applying closer to the time when the plant will use those nutrients—making it easier to manage loss. The key to splitting the timing of nitrogen applications is to sidedress green corn.
"I like to have half to two-thirds of nitrogen up front—enough for vegetative growth—then sidedress the ear-fill nitrogen," Ferrie says. "When corn shows a strong visual response right after sidedress, it’s telling you there wasn’t enough nitrogen for the vegetative stages and growth was slowed. In an ideal program, you’d sidedress dark green corn that doesn’t need that supply of nitrogen for weeks out. As a program, we never want corn to have a bad day, and we always want the plant to have the nitrogen it needs."
Variable nitrogen rates factor in the soil-supplying power as well as the risk of loss. Before sidedress, nitrate testing tells a zone’s nitrogen inventory.
After several years of playing catch-up with his nitrogen program, Aaron Gingerich, who grows corn and soybeans near Lovington, Ill., decided to switch from a rescue nitrogen program to a planned program.
"We were putting on all the nitrogen needed in the fall, and we’d lose those hot spots," Gingerich explains. "We would come back then put on extra in those areas."
Since 2009, Gingerich has modified his nitrogen program to include pre-emergence and sidedress applications. Today, 50% of his nitrogen is applied pre-emergence, and the remainder is with a variable-rate sidedress pass.
Planning a variable-rate nitrogen program should begin after harvest. First, use yield maps to pick out high-risk areas within management zones. "It’s trusting in the yield zones and management zones you created after years of collecting data for each field that make VRT nitrogen possible," Gingerich explains.
He estimates he saves 8% each year on his nitrogen costs using variable-rate application.
Prioritize zone management. A mix of nitrogen-friendly and nitrogen-thirsty soils, combined with varying topography, means farmers should test and document each zone to verify its supplying capacity.
"In fields with a lot of slope, you won’t have a lot of leaching on the slopes," Ferrie notes. "But when water runs down and ponds, there’s a risk of denitrification. If those lower areas are drained with tile, the nitrogen will still get away."
- Early Spring 2014