Conservation Drainage Network

Controlled Drainage

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Controlled drainage may benefit soybean yields in dry years, but reductions are possible in wet years

Summary prepared by Jeff Strock and  Jane Frankenberger

Strock, J.S., Youssef, M.A., Moursi, H., Nelson, K., Ranaivoson, A.Z. and Poole, C.A., 2025. A synthesis of soybean yield response to controlled drainage under varying precipitation patterns in the US Midwest. Agricultural Water Management, 318, p.109707. https://doi.org/10.1016/j.agwat.2025.109707

Major Findings:

This synthesis of soybean yield comparisons under paired controlled drainage (CD) and free drainage (FD) fields at 11 Midwestern sites (Figure 1) found that while CD consistently benefits water quality, its effect on soybean yield is limited and variable.   Yields were similar under FD and CD in most years, and across all 31 site-years there was no significant overall difference in soybean yield between FD (average yield 58 bu ac⁻¹) and CD (average yield: 58 bu ac⁻¹) (Figure 2).

Figure 1. Soybean yield study locations (Strock et al., 2025)

Figure 2: Yields under controlled drainage (CD) were very similar to yields under free drainage (FD) in most years (Strock et al., 2025)

The authors analyzed those observations with yield differences of ±4% or more, and found that the main factors determining yield response were the timing of wet and dry periods, drainage system design, and management of water levels. Specifically, during mild to moderate drought years CD increased yields by 4–8%, while during wet growing seasons CD reduced yields by 4–9%. They also concluded that intensive management or automation of water control structure outlets is needed to avoid potential yield reductions.

Contributions to the conservation drainage community

The authors developed practical guidelines for management of the drainage outlet to maximize benefits during dry periods and minimize harm during wet periods, summarized as follows:

  • Planting to V5: The drainage system should be maintained in FD mode to maintain adequate water level for emergence and well-aerated rootzone for healthy vegetative growth.
  • June (R1, beginning flowering): A shift to CD during this time of the year would capture soil moisture in the soil profile, but the drainage systems should remain in FD mode if wet conditions persist throughout June.
  • July (R2, full flowering to R4, full pod): The drainage system should be operated in CD mode since dry stress is likely during this time.
  • August and September (R5, beginning seed to R6, full seed): Timely adjustments of the outlet depth with CD are particularly needed during this period to minimize  waterlogging during critical periods of seed development.
  • September and October (R7 to R8, maturity): Once soybean has matured, the drainage system can be changed from CD to FD in preparation for harvest. Managing the drainage system in FD mode improves trafficability for fall field operations.
  • When fall field operations have been completed, the system should be shifted into CD mode to minimize nutrient loss that may occur through drainage water flow.

Publicly available data

This is the latest paper using data from the Transforming Drainage database, which brought together research data spanning 39 experimental sites across the tile-drained U.S. Midwest. Data can be downloaded from the repository at the USDA National Agricultural Library Ag Data Commons, or at Drainagedata.org. Site Summaries are also available with maps, description of the data in the database, and citations for further information for each experimental site.

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Review of 31 drainage water management studies confirms large and consistent N loss reduction, but small and variable P loss reduction and economic benefit

Summary prepared by: Jane Frankenberger, Agricultural and Biological Engineering, Purdue University

Frankenberger, J., McMillan, S.K., Williams, M.R., Mazer, K., Ross, J. and Sohngen, B., 2024. Drainage water management: A review of nutrient load reductions and cost effectiveness. Journal of the ASABE. 67(4): 1077-1092. https://doi.org/10.13031/ja.15549

Major findings:

We synthesized results of drainage water management (controlled drainage) studies from all available studies that met criteria from 1979 to 2022, and determined the following average effects:

  • Nitrogen: The average nitrate reduction from DWM for 290 plot-years at 31 sites was 12 lb/ac/yr or 46%.
  • Phosphorus: Very limited studies showed that dissolved reactive phosphorus was reduced by 0.04 lb/ac/yr (34%) on average, and total phosphorus was reduced by 0.06 lb/ac/yr (36%).
  • Economics: Yield effects of DWM are highly variable, so economic analyses were done assuming no yield benefit and a 3% yield increase. With no yield benefit the cost of N reduction through DWM  is $1 to $11 per kg N removed, while a 3% yield increase would lead to a likely net economic benefit to the producer.

Sites used to synthesize the N or P reductions from drainage water management in the United States, Canada, and Europe. Symbol size indicates the number of years of comparison between controlled and free drainage, and shape/color indicates whether both N and P were included or only N.

Contributions to the conservation drainage community

These findings confirm and quantify what was already generally accepted, indicating that DWM can be an effective practice for decreasing nitrogen loading from drained landscapes. However losses through unmeasured flow paths such as seepage and surface runoff may  lead to overestimating the effectiveness of DWM, and complex interactions among soil organic matter, soil nutrient levels and water table depths need to be accounted for to ensure both N and P removal is achieved. This study did not evaluate effects on greenhouse gas emissions. The dataset used in this analysis is provided at https://doi.org/10.13031/23596572.v1 and also at https://conservationdrainage.net/resources/controlled-drainage/reviewdata so other researchers can continue to build on this analysis to better understand the impacts of DWM. Our goals in providing it is to provide easy access for drainage researchers in understanding our interpretation of published research, and to serve as the basis for future synthesis studies. We hope that others will add additional studies as they are completed and published, so that this version can be kept up to date with current research. Please contact Jane Frankenberger, frankenb@purdue.edu, with additions or modifications to this spreadsheet.

Distribution of NO3-N load reductions in percent of NO3-N load in free drainage. Sites are arranged from highest to lowest median. Boxes show 25th and 75th percentile, the whiskers are +/-1.5 x the interquartile range, and all points outside those values are shown. The blue lines show the grand mean and confidence interval from the mixed effect model.