Research focuses on management systems, nitrate loss

URBANA, Ill. — Multiyear field trials in Douglas County were conducted to find answers to the complexities of nitrogen management in tile-drained fields.

The trials were funded by the Illinois Nutrient Research and Education Council and led by University of Illinois and Illinois Fertilizer and Chemical Association researchers and participating farmers, Dick Searls and Eric Miller.

Lowell Gentry, U of I principal research specialist in agriculture, natural resources and environmental sciences and a project lead, presented five years of data at NREC’s Investment Insight forum.

“The objective of this NREC study is to evaluate when and how tile nitrate losses occur in corn/soybean rotations,” Gentry said.

Treatments

The nitrogen treatments for corn with three replicates each were:

• Full rate of NH3 (180 pounds nitrogen/acre) applied in the fall with nitrapyrin.

• 90 pounds of N applied as NH3 in the fall with nitrapyrin followed by 45 pounds N/acre as UAN at planting followed by 45 pounds side-dressed as UAN.

• Full rate applied as NH3 (no nitrapyrin) in early spring (before planting), with placement between rows by RTK.

• Reduced rate (135 pounds N/acre) applied as NH3 (no nitrapyrin) in early spring (before planting), with placement between rows by RTK.

• 90 pounds N applied as NH3 early spring (before planting) followed by 90 pounds N as UAN side-dressed.

• 90 pounds N applied as NH3 early spring (before planting) followed by 90 pounds N as UAN side-dressed with cover crops (oats-radish mixture after soybean crop; cereal rye after corn).

“Tile nitrate is not simply a matter of excessive N fertilization.”

—  Lowell Gentry, University of Illinois principal research specialist in agriculture, natural resources and environmental sciences

Gentry noted the nitrogen rate was increased to 180 pounds per acre for the 2019 cropping year as determined by the Maximum Return to Nitrogen calculator. The nitrogen rate had been 160 pounds per acre from 2016 to 2018.

Cover Crops

Initially the trials used an oat/radish cover crop mix after soybeans, but there was not enough biomass to decrease tile nitrate load. In 2018 researchers switched to annual ryegrass and the polar vortex that year ago killed the annual ryegrass.

“Two winter crops survived the polar vortex — cereal rye and winter wheat. So, we are now trying winter wheat as our cover crop and will terminate it just like we would a cover crop. We have some bin run winter wheat out there, very cover-effective,” Gentry said.

“The first year of the trials, the plot with cereal rye cover crops after corn and before soybeans lowered the tile nitrate by about 10 pounds per acre. That’s a very significant drop. That was about a 45% decrease in tile nitrate loss in the very first year we tried it. That happens to be the number that we’re trying to get to with the Illinois Nutrient Loss Reduction Strategy.”

The three-year cumulative tile nitrate load — average across both corn and soybean plots — for applying 80 pounds of nitrogen in the spring and 80 pounds side-dress in a field where cover crops were used was 41% less than applying all 160 pounds in the fall.

The tile nitrate loads for the plot with 120 pounds applied in the spring and the plot with no cover crops and 80 pounds applied in the fall and 80 pounds side-dress were nearly identical.

“Timing matters and it shows that tile nitrate is not simply a matter of excessive N fertilization. Just cutting back is not the answer and that’s the only treatment that gives us a yield hit. We’ve lost yield by not putting enough N on and we didn’t reduce the tile load any more than we did if we just split it more in-season and not put any on in the fall,” Gentry explained.

“The other interesting thing is there is no yield difference among the other five treatments. So, it doesn’t matter, at least for yield, that we lost more fall N than we did spring or side-dress N. It did not hurt yield. Why is that? Are we over-fertilizing? No, we’re not. Mineralization probably makes up for the small amount of tile nitrate that we lose with fall N. Environmentally it’s not the best, but it’s not hurting us as far as yield goes.”

Research Findings

Gentry summarized the findings from the trials:

• Fall N application loses only 10% of the fertilizer, but this amount of loss represents 30% of the annual tile load, yet no yield loss. “The economics of that situation is not going to solve itself. We’re going to continue to use fall N and we’re going to be a little leakier, but we’re probably not going to see any yield decease unless it was an extreme year for N loss,” Gentry said.

• High carbon-nitrogen ratio of corn residue promotes N immobilization.

• Mineralization following soybean adds to the tile nitrate load.

• Quantity and quality of residue influences net N mineralization.

• Need to tie up mineralized N during non-crop growing season.

• Cover crop may be the best strategy.

“Looking back at the warm weather in February of 2017 when the temperature was 12 degrees above normal, that’s the year we needed less nitrogen to reach our economic optimum N rate and the cover crop grew well — everything points to lots of mineralization,” he continued.

Warm winters enhanced mineralization before the row crop growing season. Over-wintering cover crops will capture mineralized N and release it during the growing season. Cover crops may be the best strategy for preventing this type of loss.

“In 2016 we had record crop yields in the Midwest and in 2017 we had record crop yields, but sandwiched in between these two record yields is the largest hypoxic zone ever measured in the Gulf of Mexico. So, I’m worried that warm winters are slowly draining our ecological capital from the prairie and I think we’ve got to be concerned about N loss following soybean and that cover crop might be the best strategy,” Gentry said.

“Tile nitrate is not simply a matter of excessive N fertilization. It’s much more complicated than that and if nothing more I’m here to explain it to the people on the outside looking in who want to think that farmers are just over-applying and it’s just a simple matter of putting too much nitrogen on. That is not true. It’s much more complicated than that.”