Late-season nitrogen application is still more art than science. However, practitioners and promoters emphasize basic realities behind the concept.
Even if you split apply your N, heavy rains after sidedress can lead to nutrient deficiencies when the corn crop needs it most.
Mineralization of N, while difficult to estimate during the growing season, can significantly reduce the need for a full rate of applied N – but you will only know it did if the full rate was not applied in the first place.
Access to late-season application equipment makes it possible to reduce costs given the environment. Like choosing not to apply in a dry year like Ohio in 2016, or choosing to apply additional N to increase yields in wet years like 2015 and 2017.
Two-dozen farmers and their agronomists involved in a 2016 Ohio State University (OSU) late-season nitrogen application study didn't see corn yield benefit from applying additional late season N. However, they did see a benefit from NOT applying additional N late season (V10 to VT crop stage). Excellent mineralization and a dry year in 2016 negated the need for the late-season treatment – something the farmers couldn't have benefitted from had all the N gone down earlier in the season.
"The best insurance policy is the one you don't have to use," says Andrew Klopfenstein, project coordinator, DecisionAg 2016.
Klopfenstein worked with Scott Shearer and John Fulton, OSU Department of Food, Agricultural and Biological Engineering, along with 17 industry partners to help farmers and their agronomists gauge the benefits of splitting their N applications. To judge this they used aerial imaging to guide scouting and soil sample collection to estimate available soil N.
Learning from aerial images
The 2016 season was the follow-up to 2015 research, based largely on use of aerial images. Growers involved that year used aerial imaging to identify stress areas in the field as well as specifically look at crops under nitrogen stress to evaluate the potential for late season application. Dave Scheiderer, Integrated Ag Services, was one of the growers in 2015, as well as a consulting agronomist to several other farmers in the 2016 study.
"2015 was an incredibly wet year, and we saw what denitrification was all about – as well as seeing a really good response to late season N," says Scheiderer. "OSU took what was learned and rolled that into 2016, focusing on late season N. In 2016, a dry year, we learned that late season application wasn't as simple as seeing red on an NDVI image and applying more N. My growers saw no response to additional N in 2016. The stress we saw on the image was all weather related."
The 2016 program involved more than 5,000 acres, with each field split into A, B and C zones. None of the farmers involved normally applied late season N. Zone A was the grower's standard program, such as a preplant anhydrous application, planter application followed by in-season sidedressing.
Zones B and C involved setting aside 10 to 25 percent of the planned total nitrogen application for a possible late season application. Late season was defined as V10 or as late as VT, partially to sidestep the impact of a heavy rain after application. The C zone was a late-season application at a straight rate to complete the growers total nitrogen program.
The late-season application in the B zone was completed using variable rates based on the aerial image and ground truthing plus soil samples. "The agronomists were asked to regularly review images received from AirScout, to identify ten to twenty areas in the field and scout them, pulling soil samples, observing plants and looking at roots and leaves to assess plant health and estimate what amount of N to put on," says Klopfenstein. "We wanted a yield estimate to compare with the soil N. Once we received the soil N test back, we helped develop a prescription for the B zones."
Late-season applications were all made using either a Hagie highboy equipped with 360 Yield Center Y-Drops or a Case/New Holland Miller applicator with a high clearance toolbar and coulters. Several farmers had their own equipment and did center drops.
Equipment, environment tell tale
When results for a focus field were reviewed at a December 2016 meeting, yields were comparable across all three zones, but costs for Zone B were 5% less than Zone A, and Zone C was 12% greater (see chart).
Klopfenstein notes several points that were clear to all. The first was that without the right equipment, true late season application obviously wasn't an option. The second was that having the option could boost yields or reduce costs through reducing the traditional planned N rates, depending on the year. A third point was that placing the N where it could be most effective was more difficult late season than with early season sidedressing, especially in a dry year with no rain to carry the N into the root zone.
"It was difficult to keep the coulter in the ground, and dropping N in the middle of two rows is like laying it on top of cement," says Klopfenstein. "You might as well go on vacation."
"We don't realize how good the plant is at funneling moisture to the roots at the base of the stalk," he says. "Putting N at the base with the Y-Drops where there is moisture is much better than in the center of the rows."
Small N amounts are a challenge
Scheiderer and Klopfenstein share skepticism about another aspect of variable-rate, late-season applications. Adding even 30 units of N, approximately 10 gallons of 32 percent, is difficult to apply effectively across an acre of ground. Klopfenstein likened the process to spreading N by the tablespoon across the rows.
"We had some farmers water down the N to raise the application rate; it was just too small an amount to get performance out of the sprayer," he says. "We also discovered there was no good boom height control for the Y-Drops, a problem which has since been fixed."
A bigger problem was realizing that technology is not yet to a point where variable rate prescriptions can be written without ground truthing, and ground truth soil sampling is not something anyone wants to do when corn is at tassel and heat and humidity are high.
"It isn't even a matter of will to do it," says Scheiderer. "There just isn't enough time to gather the information that way."
"There is zero-chance farmers will do this farm wide," agrees Klopfenstein. "In addition, soil sampling for N is hit or miss. You may hit the N band as you cross the field or not."
That doesn't negate the value of late season applications in his mind. It simply emphasizes the importance of having the option. "My take-away is that you need to use good, common, practical sense," says Scheiderer. "If it is a dry year, it's pretty likely late season N need is not there. If you have a wet year like 2015 and 2017 with big rain events in late June and early July, go ahead and add another 30 to 45 pounds of N because you know you will need to do so."
Remote sensing challenges
The next step is to provide a prescription for when and how much to apply based on remote sensing. Scheiderer is skeptical of current models based largely on weather. "There are probably five or six dynamics. Some of them we can't ensure, such as how late rains affect clay soil versus loamy soil with or without tiling," he says.
Climate Corporation was one of the corporate supporters of the project. They worked with OSU on late season N scripting using imagery and two other variable rate methods in 2015 and stayed involved in the 2016 project. Nick Koshnick, lead, Fertility Science Team, Climate Corporation, is confident the project was beneficial and will help reach an important goal.
"We saw it as a rich area for mutual learning and benefit," he says. "Our zone-level nitrogen management tool helps farmers understand some of the soil and weather dynamics in their fields so they can determine the optimal management of N and make better decisions. Combining it with imagery for in-season VRT N is a very exciting area for us and the industry as a whole. Working with programs like that at Ohio State helps bring information together to support profit for growers."
For the growers involved, 2015 and 2016 were a lesson learned. Klopfenstein reports that while funding ran out for the formal program, many of the farmers involved continued with late season applications in 2017.