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Dig in on cover crop N contributions

mvburling/Getty Images Close-up of cover crops growing between rows winter wheat
COVER CROP PAYBACK: Rye can be a good nitrogen source for your cash crop, but research done in Pennsylvania in 2021 shows the importance of paying close attention to the termination date.
A project focuses on using modeling software to pinpoint nitrogen contributions from cover crops.

No two farms use cover crops the same way. But if your main goal is getting nitrogen from that cover crop, keep a close eye on termination date and be aware of the tools that can help you decide what species are best.

In Pennsylvania, a project involving 20 farms is looking at nitrogen modeling to see how much N certain cover crops will produce.

“As farmers continue adopting no-till and cover crops, we want to show that those practices can lead to reduced nitrogen applications. With N prices being the way they are right now, anything that we can do to capture N contributions from things you’re currently doing, we want to look at,” Eric Rosenbaum, owner of Rosetree Consulting in Shillington, Pa., told a group of New Jersey farmers recently gathered for a nutrient management field day in Pittstown.

Rosenbaum, working through a $45,000 USDA Natural Resources Conservation Service Conservation Innovation Grant, partnered with Granular, which contributed $60,000 to the project, and the 20 farms — mainly in the eastern part of the state — on using tools to forecast the amount of nitrogen cover crops can provide to the subsequent corn crop.

Calculating nitrogen cycling and the amount of nitrogen mineralization from cover crops is site-specific, Rosenbaum said. Also, the amount of nutrients a cover crop takes up doesn’t necessarily mean the same amount will be available to the corn.

Digging in

Using Granular’s satellite image service, Rosenbaum said he and his collaborators looked at each field to home in on average spots. They then went to each field to harvest square-meter samples of the cover crops. The samples were weighed and then sent to a lab for forage analysis.

It might sound counterintuitive to have a forage analysis done when the sole focus of this project was on nitrogen availability, but Rosenbaum said a forage analysis can provide helpful clues.

The percentage of lignin and non-fiber carbohydrates are important things to look at, he said, because the higher the NFC percentage in a plant, the more nitrogen will be able to work its way out of a cover crop and into corn.

When the lignin percentage is high, less nitrogen will be available, he said.

Rosenbaum presented a few examples from the project. On one site in Lancaster County where triticale was sampled April 26, and then corn was planted April 28, the crop produced 5,000 pounds of dry matter, had an NFC of 28.7%, lignin of 2.2% and was knee-high.

Another example, a rye field in Adams County that was sampled May 7 and planted May 21, had 5,200 pounds of dry matter and 2.2% nitrogen, but its NFC measured 11.6% and lignin was 5.9%.

Letting the rye grow yielded a little more biomass, but the big difference was NFC.

“The NFC dropped like a rock. This is rapidly available nitrogen from the cover crop,” Rosenbaum said. “So if you’re going to manage your cover crop this spring and try to get the max amount of N out of it that you possibly can, termination date is going to matter a whole lot. Don’t let that cover crop get too wild on you, or else the N contributions to that cover crop are going to be a little less.”

Multispecies cover crops also were evaluated. These are more common in the northern parts of Pennsylvania, New Jersey, and into New York and New England, because the window for double-crop soybeans after winter wheat is much shorter. Farmers often plant these multispecies cover crops to go after winter wheat.

On a farm in northern Pennsylvania, the multispecies mix had triticale, clover, winter peas and hairy vetch. It was sampled May 13 and had 4,708 pounds of dry matter, 14% crude protein, 26.3% NFC and 5.2% lignin.

A second farm from Cumberland County, which is much closer to the Mason-Dixon line, was sampled May 5. It included a mix of wheat, barley, annual ryegrass, radish and clover, and yielded 5,171 pounds of dry matter and 19.9% crude protein, but only 12.9% NFC and 5.9% lignin.

“So if you’re looking for cover crops you can plant that are going to return nitrogen to you when your corn actually needs them, species are going to play a big part in that. Also, termination date,” Rosenbaum said. “There’s a lot of different moving parts.”

Nitrogen contributions

This is where the Granular modeling software comes into play. The model took in all the data from each farm and included weather analyses from the past 20 years to forecast the amount of available nitrogen from the cover crop to corn.

On the Lancaster site, Rosenbaum said the model forecasted 42.7 pounds of N would be available to the next crop. “I’m fairly confident that any N that mineralizes from your cover crop up to silking is going to be 100% available to your corn crop,” he said.

The later-terminated field in Adams County showed 33.2 pounds of N mineralization. Both these sites had nitrogen uptake levels around 118 pounds. “So it shows that uptake doesn’t necessarily mean mineralization, and this was just the first year,” he said. “So some of that mineralization is occurring either at a longer process, or we have some loss occurring with that nitrogen as it mineralizes.”

More importantly, however, it showed how termination date can affect nitrogen availability. Terminate early, more N; terminate late, less N.

“So the termination date here cost you almost 10 pounds of N,” Rosenbaum said.

In the multispecies mixes, the grass-dominant cover crop yielded 47.8 pounds of nitrogen, higher than the legume-dominant mix that yielded 36.8 pounds.

Many other factors can affect nitrogen mineralization, Rosenbaum added, including soil characteristics, planting date, species selection, and if any manure had been previously applied.

“So if you apply manure, the cover crop will retain that ammonium that would otherwise escape,” he said. “So if you have a cover crop and add manure, it’s a good bonus, and residual fertility also plays into it.”

The project also didn’t consider belowground nitrogen fixation that can come from, say, a legume cover.

Still, any edge you can get on lowering inputs — especially fertilizer — will help boost the bottom line, hopefully without undercutting yields.

TAGS: Fertilizer
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