Two years ago, Decatur, Ill., farmer Adam Brown began dropping nitrogen rates, from 230 pounds per acre to 200 pounds.

With yield monitor data from two years of harvest, he says he’s confident the decision to reduce N rates on corn isn’t impacting his yields. In 2020, yields are averaging 226 bushels per acre — 10 bushels above his farm’s three-year average production history from 2016-18.

“I think that’s one place where my operation has thrived,” Brown says. “I’m not necessarily the best at any particular practice, but what I try to do is replicate success that I see in trials.” Brown is referencing the dozens of rate trials that provide data for the tool he uses to calculate rates, the Maximum Return to N calculator from the University of Illinois. Data from across the state extends back more than six years, with older data deleted as newer data comes in. Learn more at ifca.com/4R/Trials.

Brown says he’ll likely reduce N rates even more in 2021. He’s got room to move, since his current rates are at the top end of the MRTN-recommended rate.

“I’ve done a lot of tweaking in the operation as far as seeds and so on. I’m trying to be careful not to change too much at once, so I can see what the effect of each tweak is more clearly,” Brown says. He notes that on most of his acreage, he uses micronutrient seed treatments that help improve emergence.

Data suggests Brown is not alone in reducing N rates. In 2019, retailers surveyed by the Illinois Fertilizer and Chemical Association said close to 90% of their customers use rates that fall into the MRTN range.

Finding the right rate

Emerson Nafziger, University of Illinois agronomy professor emeritus, says farmers used to multiply every bushel of expected yield by 1.2 pounds of nitrogen and then deducted a “soybean N credit” of 40 pounds if the previous year’s crop was soybeans and not corn.

But that’s not necessary with the MRTN calculator.

The calculator already includes the N credit, and factors in the prices of corn and fertilizer to determine an economic rate, offering up a range of recommendations on either side of the MRTN value.

The old system stopped working as yields went up and genetics improved, to the point where farmers were putting on too much N in higher organic matter soils in central and northern Illinois and not enough in southern Illinois, Nafziger says.

“This is because plants were better able to take up N produced from soil organic matter, and this meant less need for fertilizer N per bushel of yield,” he explains.

Nafziger notes that lower organic matter soils produce less nitrogen for the crop. By tracking nitrogen responses across the region, the MRTN calculator takes the soil-supplied N into account.

He says regardless of whether a farmer has heard of MRTN, the rates the calculator recommends are becoming more acceptable over time. This is because hybrids experience diminishing returns to N as the rate increases, and once past 150 to 175 pounds of applied nitrogen, yield advantages decrease and eventually disappear. At yield levels of 225 to 250 bushels per acre, taking each bushel times 1.2 pounds of nitrogen is too much N, given the amount that the soil provides.

When it started to turn dry in July this year, hybrids from a couple of decades ago would have started to lose color and the ability to fill grain, Nafziger says.

“But this year, most of us found that the corn crop stayed green pretty much up until harvest. That means that the plant was able to keep taking up water from the soil, and the N that comes with it. We can attribute a large part of that to improved genetics,” he says.

Nafziger says as long as today’s hybrids hit pollination and set kernels before a dry spell, they “have a remarkable ability to keep pulling water and nutrients from deeper in the soil.” It helps that most of the crop’s nitrogen is taken up by pollination, so the daily demand during grain fill may be less than a pound per acre per day.

Weather influences rates

Weather changes how much nitrogen is provided to the crop by soil. For every trial in the MRTN program, farmer-cooperators record how much nitrogen the soil provides by harvesting unfertilized rows of corn.

In an ideal season, a crop growing in a high organic matter soil that warms up early and doesn’t get flooded may require as little as 100 pounds of N per acre to reach its full yield. In the same field in a different year, with a cool start to the season or heavy rainfall, the crop may need more than 200 pounds of fertilizer N.

“If there’s heavy rainfall, like we had in June of 2015, some N may be lost for the soil, and roots often don’t work very well, so how much fertilizer N the crop needs may go up considerably,” Nafziger says. “But we think the best approach is to not apply extra N in case this happens, but rather to be ready to put on some more if those conditions show up.”

The nitrogen story varies by soil type — and region in Illinois. The supervisor of the southern Illinois MRTN trials, John Pike of Pike Ag, says farmers he works with have adjusted their N rates, in some cases higher. The average optimal rate for southern Illinois farmers is about 195 pounds per acre, while in central Illinois, higher-quality soils in trial data show its 180. It’s about 170 pounds per acre in northern Illinois. 

“The MRTN is the best guess we’ve got for how much N that’s needed in any given year,” Pike says. “We give people a range where they can adjust to what the weather throws at them.”  

Brown agrees and thinks he could drop N rates 10 more pounds after the results of his 2020 rate reduction success.

“Now that I’ve got two years under my belt, I’m confident it wasn’t just a one-off. I’m eager to continue to use the MRTN and try to find that right balance between monthly prices and N rate,” Brown concludes.