Written by Jonathan Eisenthal

The monitoring of tile water on 48 Minnesota and Wisconsin farms from 2018-20 has established that biweekly testing is comparable in accuracy to real-time testing when measuring nitrate levels, according to Tim Radatz, coordinator of Discovery Farms Minnesota and Discovery Farms Wisconsin. The Discovery Farms programs are farmer-funded and farmer-directed programs that conduct on-farm research, outreach and education about conservation farming methods, with a focus on improving water quality.

“One of the more important things to come out of this study is looking at how accurate bi-weekly samples of tile water are, versus if we are sampling 24/7, 365 days a year,” Radatz said. “Nitrate, typically what we are most concerned with in the tile water… (has) a very tight relationship, meaning we can get really good results from our biweekly samples. If you are interested in knowing the levels of the nitrates flowing off your field, taking samples every two weeks — when the tiles are flowing — is going to give you very good information that you can learn from.”

The Discovery Farms researchers also studied how factors such crop rotations, nitrogen-application rates and cover-crop use affect nitrate levels.

Data from all the sites indicate that certain crop rotations and certain nitrogen rates can reduce nitrate concentrations.

When corn follows dairy-forage alfalfa, the mean nitrate concentration drops by more than half, falling well below the threshold for maintaining water quality, of 10 milligrams per liter. Fertilizer rates also have an impact. Comparing sites where from 200 pounds per acre was applied, versus 150 pounds per acre, the study found mean concentration of nitrates dropped by more than 9 milligrams per liter at the lower rate.

Combining best practices for both rotation and nitrogen-application rate further reduces nitrate levels, Radatz reported.

Through this study, Discovery Farms developed three main recommendations, according to Radatz. Updating and maintaining tile systems will reduce sediment flows in tile water, and with it phosphorous concentrations. Using soil-test phosphorous levels to determine the right phosphorous rate can also lower phosphorous concentrations. And for nitrogen, evaluating the 4 Rs — rate, timing, placement and form of nitrogen fertilizer — and comparing results using grab tests of tile water, can help reduce nitrate concentrations in the water leaving the farm field.

As part of its mission to boost on-farm sustainability, the Minnesota Corn Growers Association (MCGA) has long supported Discovery Farms in Minnesota. The program is run by the Minnesota Agricultural Water Resource Center.

The tile-monitoring research was supported by a federal Conservation Innovation Grant. In February, Radatz presented the project results at the MCGA-supported Nitrogen: Minnesota’s Grand Challenge and Compelling Opportunity Conference.

To learn more about Minnesota Corn’s support of Discovery Farms Minnesota, visit mncorn.org/research.