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Tile Water Monitoring 2018, Part 3: Cover Crops

Article cover photo
Cover crops can provide a benefit to improved water quality, according to ISA's water monitoring analysis. (Photo: Joseph L. Murphy/Iowa Soybean Association)

This is the third installment of a four-part series on the Iowa Soybean Association’s tile water monitoring program and aggregated results from 2018.

By Anthony Seeman, ISA environmental research coordinator

Results from the 2018 Iowa Soybean Association (ISA) tile water monitoring program show cover crops have an important role in providing water quality benefits and achieving goals set forth in the Iowa Nutrient Reduction Strategy (INRS).

The ISA Environmental Programs and Services team analyzed nitrate-N concentrations in fields with cover crops to those without, collecting more than 900 water samples from 144 sites. The 2018 results were similar to 2017: showing that cover crops can provide a water quality benefit. 

There were 568 water samples taken from 93 fields without cover crops. These samples were compared with 357 water samples taken from 51 fields with cover crops.

Fields without cover crops had an average nitrate-N concentration of 14.8 milligrams per liter (mg/L). Fields with cover crops averaged 11 mg/L, for a difference of 3.8 mg/L, or a 26 percent advantage. The box plots in Figure 1 show the range of results. There was a higher difference between cover crop and no cover crop fields after soybeans (4.4 mg/L) than corn (3.2 mg/L).

Figure 1: The average nitrate-N concentrations in tile drained water compared in fields with cover crops and without cover crops.

While the concentration of nitrate-N in water is important from a water quality perspective, many farmers want to know how much nitrogen is being held in their fields when using cover crops. To calculate the pounds of nitrogen leaving fields it is important to know the volume of water leaving the fields.

Fewer fields were monitored for this level of information: 31 fields with cover crops and 52 without cover crops were measured for how much nitrogen was exiting the fields between April and September. Results showed that fields without cover crops lost an average of 0.13 pounds of nitrate-N per acre per day. Fields with cover crops lost an average of 0.07 pounds of nitrate-N per acre per day. This difference, when interpolated to the six-month monitoring period, equates to 11 pounds of nitrate-N per acre leaving the field which could have been used by a cash crop or cover crop. While this alone does not offset the expense of cover crops, it is an important point when determining a return on investment. 

Cover crops have been studied as a key in-field practice to help achieve the INRS goals. Research is ongoing across Iowa to fine-tune cover crop management to maximize benefits and return on investment.

According to the USDA’s 2017 Census of Agriculture, cover crop acres in Iowa have increased 156 percent since 2012. While this is a positive step, farmers will need to use cover crops on a significant number of acres to reach the INRS goal of a 45 percent reduction in nutrients entering rivers and streams. Cover crops, precision nutrient management and edge-of-field practices like bioreactors and saturated buffers all play a part in achieving the goals of the strategy.

More information about cover crop research results can be found on the ISA website:

Presentation from 2019 Research Conference on cover crop trials

2018 Long-term cover crop trial report

What is the return on investment for cover crops?

In all, ISA, partner organizations and farmers collected more than 4,000 samples in 2018 from Iowa streams, drainage tiles and edge-of-field practice locations for analysis in ISA’s certified water lab.

The ISA tile water monitoring program does not sample from a controlled experimental design, therefore results presented in this article are observational and additional scientific study may be warranted. Findings from the monitoring program generally align with the Iowa Nutrient Reduction Strategy and other published research.

The final article in the series will provide a summary of results from edge-of-field water quality practices.

Contact Anthony Seeman at

For media inquiries, please contact Katie James, ISA Public Relations Manager at or Aaron Putze, ISA Communications Director at

For permission to republish articles or to request high-res photos contact Aaron Putze at Iowa Soybean Association | 1255 SW Prairie Trail Pkwy | Ankeny | IA | 50023 | US

©2018 Iowa Soybean Association On-Farm Network®. All rights reserved. On-Farm Network® is a registered trademark of the Iowa Soybean Association, Ankeny, IA.Portions of some On-Farm Network trials are paid for in total or in part by the soybean checkoff.