Long-term trials find solutions to nutrient loss

URBANA, Ill. — Extensive multiyear, in-field trials found a key to unlocking the challenge of meeting the final goal of the Illinois Nutrient Loss Reduction Strategy.

Lowell Gentry, who retired from the Department of Natural Resources and Environmental Sciences at the University of Illinois in 2022 and is now contracted by the Illinois Fertilizer and Chemical Association, led field trials on a Douglas County farm that began in 2014 and concluded last year.

The research, with funding support from the Illinois Nutrient Research and Education Council, Foundation for Food and Agriculture Research and the 4R Research Fund, focused on nitrogen applications timing and their impacts on tile nitrate loss in a corn/soybean rotation.

The farm had 36 monitored tile laterals — 18 for each crop. An Agri Drain structure featured a V-notch and pressure transducer and water sampler for analysis of nitrate ammonium, phosphorus and potassium. The initial steps in the research began on the farm in December 2014.

Gentry unveiled his findings during the recent Illinois NREC Investments Insight Live gathering.

The long-term objective of the NLRS is a 45% reduction in total phosphorus and total nitrogen loads originating in Illinois, with interim targets of a 15% decrease in nitrate-nitrogen and a 25% decrease in total phosphorus by 2025.

“Moving fall N application to the spring and growing cereal rye every other year ahead of soybeans can achieve the overall goal stated in the Illinois Nutrient Loss Reduction Strategy of reducing nitrate loss by 45%,” Gentry said.

He detailed the multiyear study’s protocols and findings.

N Treatments

Using the Maximum Return to Nitrogen calculator for a central Illinois corn after soybean rotation, the full rate was determined to be 180 pounds of N per acre with the following applications:

• Full rate of anhydrous ammonia with nitrapyrin in the fall.

• 50% applied NH3 with nitrapyrin in the fall; 25% as urea-ammonium nitrate at planting (2 by 2); and 25% UAN sidedress.

• Full rate applied as NH3 (no nitrapyrin) in early spring.

• Reduced rate (75%) of NH3 (no nitrapyrin) in spring.

• 50% NH3 applied early spring; 50% applied as UAN sidedress

• 50% NH3 early spring; 50% UAN sidedress; with cover crops — oats and radish after soybeans, cereal rye after corn; oats and radish winter-kill before corn.

Cover Challenge

As expected with any in-field trials, everything doesn’t always go as planned.

“We didn’t have any luck with oats and radish for various reasons. The soybeans were drill-planted in 7.5-inch rows and we aerially seeded the oats and radishes that essentially just got smothered by the leaf-drop of the soybeans. That was the first year,” Gentry said.

“In the second year, it was too dry. If you broadcast a cover crop and just get a little bit of moisture, radicle comes out and then it gets dry and dies.

“In year three, we waited until after the soybean harvest and used a drill to plant cover crops. It was very effective, but too late to get enough biomass to matter.

“So, we failed three times in a row with oats and radish. We then went with something that overwinters. We tried annual ryegrass and had a polar vortex in 2019 and it killed all of it.

“I think we were sort of fortunate because then it turned out that my experiment was nothing more than cereal rye ahead of soybean every other year.”

Corn Yields

The study documented average yields for 2016-2019.

“The 75% N application rate costs about 8% yield, but neither of the two splits significantly increased yields compared to the single rate treatment,” Gentry said.

“The best yield over a four-year average was 208 bushels per acre with 50% in fall, 25% early spring and 25% sidedressed. So, maybe the three-way split helped a little, but that wouldn’t be cost-effective with the small of a yield increase.”

The 100% fall applied yield averaged 199; full rate spring-applied averaged 202; 50% in early spring/50% sidedress averaged 204; 50% early spring, 50% sidedress with a cover crop averaged 202.

Soybean Yields

Average 2016-2019 soybean yields ranged from 67.3 bushels per acre with all the fertilizer applied in the spring to 70.5 (50% fall, 25% early spring, 25% sidedress).

“We wouldn’t expect any difference in soybean yields there. I don’t know why the three-way split somehow was the highest. I think that’s just a coincidence,” Gentry said.

“We do grow a good cover crop in front of soybeans and that did not help or hinder the crop yield (at a 68.6 bushels per acre four-year average).”

Tile N

Baseline tile nitrate concentrations were documented before the trials began in 2016 with a fall application of anhydrous ammonia and the first year of cereal rye.

“Clearly, there was more loss with fall N, and then there’s the cover crop effect with a 40% reduction in tile N load in the cover crop plots the first year,” Gentry said.

Results from the subsequent three years reinforced the first-year data.

“Tile nitrate concentrations for both 100% fall-applied and the 50% fall-applied treatments were the highest during the corn phases. They also stay higher most of the soybean years, as well, but clearly fall N is leakier than spring N,” Gentry said.

One of the many benefits of a long-term study is one fluke year won’t diminish the final analysis.

“We had a very cold winter in 2018 and tile nitrate was less for all plots, especially the fall-applied N plots, compared to other years. That was not a good year for cover crop growth, but I can say don’t fret if your cover crop didn’t grow in a cold winter. You didn’t need it as much anyway because there was less load loss,” Gentry said.

The four-year cumulative data showed a wider separation of tile nitrate loads.

“As we would have expected from year one, the fall 100% N treatments lost the most and the 50-50 split with cereal rye every other year lost the least,” Gentry said.

“But it’s interesting to point out that the reduced rate treatment of a 75% rate (early spring anhydrous with no inhibitor) lost just as much nitrate to the tile as did the 50-50 split with the full rate. So, that says something about the split rate working. We lost yield, but we didn’t significantly lower tile nitrate. It’s because we lose nitrate after soybeans, as well, so that sort of masks the effect of the efficiency during the corn phase.”

Conclusions

• Splitting fertilizer N applications did not significantly increase corn yields.

• Although tile nitrate trended down, splitting fertilizer N did not significantly lower tile nitrate.

• Tile nitrate loss was significantly greater with fall-applied N compared to spring-applied. A 100% fall N lost 12 pounds per acre more than 100% spring-applied. “That 12 pounds per acre is 38% of the tile nitrogen load, so that’s the rub. It’s not a big loss agronomically and it didn’t cost the farmer yield, but there it is, almost 40% of the tile load. It’s not more efficient than spring-applied N,” Gentry said.

• Spring applications of only 75% of the recommended fertilizer N rate decreased corn yields by 8%, but did not significantly reduce tile nitrate compared to the full-rate of fertilizer N applied in the spring.

• Cereal rye ahead of soybeans every other year in a corn-soybean rotation reduced tile nitrate by a total of 23% — averaging 33% during the soybean phase and 13% during the corn phase.

• Spring fertilizer N application reduced tile nitrate by a total of 21% compared to fall fertilizer application — averaging 32% during the corn phase and 11% during the soybean phase.

Considerations

“With the serendipity of only having cereal rye every other year thanks to our failure with oats and radish, having cereal rye only in front of soybeans avoids the potential yield drag from reduced plant N availability with cereal rye ahead of corn,” Gentry said.

“We know it’s more difficult to use a grass cover crop in front of corn. I’m not saying it can’t be done. I’m just saying we could avoid it and just worry about a good, healthy cover crop in front of soybeans.”

In addition, cereal rye only ahead of soybeans avoids the potential increase in nitrous oxide loss due to the interaction of fertilizer N with degradation of cover crop residue. That can be avoided by having only cereal rye every other year in front of soybeans.

“We continued four of the treatments through 2023. So, we have this 50/50 split and the 50/50 split with the cover crop now for eight years,” Gentry said.

“Regression analysis over eight years of cereal rye biomass production and tile nitrate reduction predicts that 1.5 tons of above ground cereal rye biomass ahead of soybeans can reduce tile nitrate by 45%, which is the overall goal of the Illinois Nutrient Reduction Loss Strategy.

“Based on studies from NREC and others, we see we don’t have much of an effect on tile nitrate until we reach an above-ground biomass of one-half ton.

“Also, if you’re more bold and want to try planting green, that’s a very good way to have enough time to get that cover crop biomass up to 1.5 tons or more.”