The research findings reported here are excerpted from a literature review performed by Alison Eagle of Duke University (Nicholas Institute for Environmental Policy Solutions) in February 2016 under contract to the Environmental Defense Fund.

AGROTAIN® PLUS is a fertilizer additive intended to reduce ammonia volatilization. Targeted specifically for liquid manure or UAN (urea ammonium nitrate) fertilizers, AGROTAIN® PLUS contains the nitrification inhibitor dicyandiamide (DCD) as well as the urease inhibitor NBPT.

Like AGROTAIN®, AGROTAIN® PLUS slows urea hydrolysis to reduce ammonia losses, but the DCD can also increase nitrogen efficiency by depressing the activities of the Nitrosomonas bacteria over a period of time, therefore delaying the bacterial oxidation of the ammonium-ion (NH4+) into nitrite (NO2-), which is then transformed into nitrate (NO3-) by Nitrobacter and Nitrosolobus bacteria.

Nitrification inhibitors, including DCD, have been used regularly in agriculture to slow down this typically rapid conversion of ammonium-N to the nitrate form.

AGROTAIN®, AGROTAIN® PLUS, and SUPERU™ are related products manufactured and marketed by Koch Agronomic Services. All three products are sold to agricultural producers with the claim that they will improve nitrogen use efficiency and thus improve productivity. The expectation is that when N is limiting, effective urease and nitrification inhibitors will improve crop yield.

Early laboratory experiments demonstrated DCD’s ability to limit nitrification (Reddy 1964) and early field trials showed that DCD could be as effective as nitrapyrin (another nitrification inhibitor) in limiting nitrification, while also being more soluble in urea (Ashworth and Rodgers 1981; Malzer et al. 1989).

The addition of DCD with the urease inhibitor NBPT into urea fertilizer can allow N to remain as NH₄⁺ for a longer time. Because ammonium N binds more to soil particles and doesn’t dissolve in water to the same extent as nitrate, this allows the mineral N to remain near crop roots for a longer period of time. The end result is more fertilizer available to the plant and less lost to the surrounding environment.

Early pot experiments with rice found more fertilizer-derived N in plants and the soil when both inhibitors were combined (Soliman and Monem 1995). Chinese scientists measured greater wheat N uptake with the combination of NBPT and DCD than that found when using the individual components alone (Jiao et al. 2004). The earliest reported field trial of corn in the USA (no-till, in Kentucky) found that crops treated with AGROTAIN® PLUS had much greater yield than those treated with UAN alone (Schwab and Murdock 2005).

A search of the scientific literature and grey literature (conference proceedings etc.) was conducted to locate data on crop yield responses to AGROTAIN® PLUS. Data were incorporated into an existing database on fertilizer management field trials, including all available management, climate, soil, N loss, and crop productivity information.

The table below summarizes the yield effects of AGROTAIN® PLUS for corn with discussion below the table. Weighted averages are calculated where possible, with results from each study weighted by the inverse of the number of trials in each location. This prevents studies from very well-studied locations from overwhelming the average results.

Overall effect sizes are reported for all studies, and also restricted to peer-reviewed data and non-generic products, as applicable.

In these studies, there is no overall significant crop yield response to supplementing UAN with AGROTAIN® PLUS. Even the two studies with statistically significant results disagreed with one another– one was positive and the other negative. Almost half of the trials that compared AGROTAIN® PLUS to non-amended UAN also monitored N₂O emissions. For these 12 comparisons, yield-scaled N₂O emissions may be lower, with an average decrease of 9.4% (±16.5), but again not a statistically significant effect. It seems that the variability in these data is simply too high to observe any significant differences. More targeted research may help reduce this uncertainty.

Why does the addition of DCD to NBPT not generate further yield improvements (and perhaps erases the yield improvements) of NBPT alone? Gioacchini et al. (2002) measured significantly higher NH₃ volatilization and NO₃ leaching losses with the combination, and suggested that by maintaining higher NH₄⁺ concentrations in the soil, inclusion of DCD with NBPT primed mineralization of soil organic matter, releasing mineral N then susceptible to loss. Higher NH₃ losses were also reported for NBPT+DCD compared to NBPT alone in another Italian winter wheat field trial (2000).

However, while yield and N₂O emission reductions were not significantly affected by the use of AGROTAIN® PLUS, this does not lead to an immediate conclusion that the product has no impact on nitrogen use efficiency. In fact, if the available N for the crop is not limiting, we might not even expect a positive yield response (or indeed reduced losses of N). More focused work at lower N fertilizer application rates may help shed light on these questions. Only one of the studies above tested AGROTAIN® PLUS at multiple N fertilizer rates, and while there was a clear positive yield response to increased N fertilizer, there was no indication of a yield response to AGROTAIN® PLUS, nor was there any relationship between N fertilizer rate and response to the product (Hatfield and Parkin 2014). However, certain regions, soil types, and cropping systems may be more responsive to the impacts of the inhibitors, so environmental and other management factors need to be considered carefully.

When looking at yield effects only and keeping fertilizer rates constant, evidence of positive yield impacts for corn is less compelling for AGROTAIN® PLUS compared to AGROTAIN® and SUPERU™.

Limiting the analyses to peer-reviewed data reduces the effect size (in almost all cases) and reduces the statistical significance of positive impacts (primarily because of fewer observations) that were seen in AGROTAIN® and SUPERU™.

Environmental benefits may be achieved even without economic or production gains if damaging forms of N losses are reduced. The measured 22% lower yield-scaled N2O emissions from SUPERU™ compared with urea in corn systems represent significant improvements. A similar trend also seems possible for AGROTAIN® PLUS and SUPERU™ replacing UAN in corn systems. Therefore, while more data would of course be useful, these products do show some evidence of improving crop yield and reducing negative environmental impacts related to N losses.

Asgedom, H., M. Tenuta, D.N. Flaten, X. Gao and E. Kebreab. 2014. Nitrous oxide emissions from a clay soil receiving granular urea formulations and dairy manure. Agronomy Journal 106(2):732–44.

Ashworth, J. and G. Rodgers. 1981. The compatibility of the nitrification inhibitor dicyandiamide with injected anhydrous ammonia. Canadian Journal of Soil Science 61(2):461–63.

Beyrouty, C.A., L.E. Sommers and D.W. Nelson. 1988. Ammonia volatilization from surface-applied urea as affected by several phosphoroamide compounds. Soil Science Society of America Journal 52(4):1173–78.

Bremner, J.M. and H.S. Chai. 1986. Evaluation of N‐butyl phosphorothioic triamide for retardation of urea hydrolysis in soil. Communications in Soil Science & Plant Analysis 17(3):337–51.

Dell, C.J., K. Han, R.B. Bryant and J.P. Schmidt. 2014. Nitrous oxide emissions with enhanced efficiency nitrogen fertilizers in a rainfed system. Agronomy Journal 106(2):723–31.

Ebelhar, S.A., C.D. Hart, J.D. Hernandez, L.E. Paul, J.J. Warren and F. Fernandez. 2007. Evaluation of new nitrogen fertilizer technologies for corn. In Proceedings of the Illinois Fertilizer Conference Proceedings.

Fox, R.H. and W.P. Piekielek. 1993. Management and urease inhibitor effects on nitrogen use efficiency in no-till corn. Journal of Production Agriculture 6(2):195–200.

Gao, X., H. Asgedom, M. Tenuta and D.N. Flaten. 2015. Enhanced efficiency urea sources and placement effects on nitrous oxide emissions. Agronomy Journal 107(1):265–77.

Gioacchini, P., A. Nastri, C. Marzadori, C. Giovannini, L.V. Antisari and C. Gessa. 2002. Influence of urease and nitrification inhibitors on N losses from soils fertilized with urea. Biology and Fertility of Soils 36(2):129–35.

Grant, C.A. 2014. Use of NBPT and ammonium thiosulphate as urease inhibitors with varying surface placement of urea and urea ammonium nitrate in production of hard red spring wheat under reduced tillage management. Canadian Journal of Plant Science 94(2):329–35.

Halvorson, A.D. and S.J. Del Grosso. 2012. Nitrogen source and placement effects on soil nitrous oxide emissions from no-till corn. Journal of Environmental Quality 41(5):1349–60.

Halvorson, A.D. and S.J. Del Grosso. 2013. Nitrogen placement and source effects on nitrous oxide emissions and yields of irrigated corn. Journal of Environmental Quality 42(2):312–22.

Halvorson, A.D., S.J. Del Grosso and F. Alluvione. 2010a. Tillage and inorganic nitrogen source effects on nitrous oxide emissions from irrigated cropping systems. Soil Science Society of America Journal 74(2):436–45.

Halvorson, A.D., S.J. Del Grosso and F. Alluvione. 2010b. Nitrogen source effects on nitrous oxide emissions from irrigated no-till corn. Journal of Environmental Quality 39(5):1554–62.

Halvorson, A.D., S.J. Del Grosso and C.P. Jantalia. 2011. Nitrogen source effects on soil nitrous oxide emissions from strip-till corn. Journal of Environmental Quality 40(6):1775–86.

Hatfield, J.L. and T.B. Parkin. 2011a. Evaluation of alternative forms of nitrogen fertilizer on agronomic performance and nitrous oxide emissions. National Laboratory for Agriculture and the Environment, Ames, IA. 4 pp.

Hatfield, J.L. and T.B. Parkin. 2011b. Nitrogen management: Unraveling the effects of timing and form. 41st North Central Extension-Industry Soil Fertility Conference 27:1–8.

Hatfield, J.L. and T.B. Parkin. 2014. Enhanced efficiency fertilizers: Effect on agronomic performance of corn in Iowa. Agronomy Journal 106(2):771–80.

Hendrickson, L.L. 1992. Corn yield response to the urease inhibitor NBPT–5-year summary. Journal of Production Agriculture 5(1):131–37.

Hendrickson, L.L. and E.A. Douglass. 1993. Metabolism of the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) in soils. Soil Biology and Biochemistry 25(11):1613–18.

Jiao, X., W. Liang, L. Chen, Y. Jiang and D. Wen. 2004. Effects of urease/nitrification inhibitors on soil available N and microbial biomass N and on N uptake of wheat. Yingyong Shengtai Xuebao 15(10):1903–06.

Lamond, R.E., W.L. Thomas, D.A. Whitney, L.D. Maddux, W.B. Gordon, and V.L. Martin. 1994. Nitrogen management for no-till production systems. Proceedings of the 24th North Central Extension-Industry Soil Fertility Conference 10:1–9.

Maharjan, B. and R.T. Venterea. 2013. Nitrite intensity explains N management effects on N₂O emissions in maize. Soil Biology & Biochemistry 66:229–38.

Malhi, S.S. and R.L. Lemke. 2013. Effectiveness of seedrow-placed N with polymer-coated and NBPT-treated urea for canola and wheat. Journal of Plant Nutrition 36(14):2205–24.

Malhi, S.S., E. Oliver, G. Mayerle, G. Kruger and K.S. Gill. 2003. Improving effectiveness of seedrow-placed urea with urease inhibitor and polymer coating for durum wheat and canola. Communications in Soil Science and Plant Analysis 34(11–12):1709–27.

Malzer, G.L., K.A. Kelling, M.A. Schmitt, R.G. Hoeft and G.W. Randall. 1989. Performance of dicyandiamide in the North Central states. Communications in Soil Science & Plant Analysis 20(19–20):2001–22.

Mohammed, Y.A., T. Jensen, J. Heser and C. Chen. 2013. Inhibitors, method and time of nitrogen application for improved winter wheat production in central Montana. In Proceedings of the Western Nutrient Management Conference, Reno, NV.

Motavalli, P.P., K.A. Nelson and S. Bardhan. 2013. Development of a variable-source N fertilizer management strategy using enhanced-efficiency N fertilizers. Soil Science 178(12):693–703.

Murphy, T.L. and R.B. Ferguson. 1997. Ridge-till corn and urea hydrolysis response to NBPT. Journal of Production Agriculture 10(2):271–82.

Nastri, A., G. Toderi, E. Bernati and G. Govi. 2000. Ammonia volatilization and yield response from urea applied to wheat with urease (NBPT) and nitrification (DCD) inhibitors. Agrochimica 44(5–6):231–39.

Reddy, G.R. 1964. Effect of mixing varying quantities of dicyandiamide with ammonium fertilizers on nitrification of ammonia in soils. Canadian Journal of Soil Science 44(2):254–59.

Rubin, J.C. 2013. Influence of Nitrogen Rate on Corn Grain Yield, Nitrogen Use Efficiency, and Nitrate Leaching on Coarse-Textured Minnesota Soils, University of Minnesota.

Schlegel, A.J., D.W. Nelson and L.E. Sommers. 1986. Field evaluation of urease inhibitors for corn production. Agronomy Journal 78(6):1007–12.

Schwab, G.J. and L.W. Murdock. 2005. Nitrogen transformation inhibitors and controlled release urea. College of Agriculture. Univ. Kentucky, Lexington, KY.

Sistani, K.R., M. Jn-Baptiste and J.R. Simmons. 2014. Corn response to enhanced-efficiency nitrogen fertilizers and poultry litter. Agronomy Journal 106(2):761–70.

Sistani, K.R., M. Jn-Baptiste, N. Lovanh and K.L. Cook. 2011. Atmospheric emissions of nitrous oxide, methane, and carbon dioxide from different nitrogen fertilizers. Journal of Environmental Quality 40(6):1797–805.

Soliman, M.S. and M.A.A. Monem. 1995. Effect of method of N-application and modified urea on N-15 recovery by rice. Fertilizer Research 43(1-3):143–48.

Tiessen, K.H.D., D.N. Flaten, C.A. Grant, R.E. Karamanos and M.H. Entz. 2005. Efficiency of fall-banded urea for spring wheat production in Manitoba: Influence of application date, landscape position and fertilizer additives. Canadian Journal of Soil Science 85(5):649–66.

Trenkel, M.E. 1997. Controlled-Release and Stabilized Fertilizers in Agriculture. International Fertilizer Industry Association, Paris.

Varsa, E.C., A.K. Leis, and J.M. Jemison Jr. 1989. Response of no-till corn to urease inhibitors and placement of N sources. Proceedings of the 19th North Central Extension-Industry Soil Fertility Conference 5:1–10.

Varsa, E.C., S.A. Ebelhar, P.R. Eberle, E. Gerhard, and T. Wyciskalla. 1999. Evaluation of nitrogen management practices in no-till corn production. 29th North Central Extension-Industry Soil Fertility Conference 15:1–18.

Venterea, R.T., B. Maharjan and M.S. Dolan. 2011. Fertilizer source and tillage effects on yield-scaled N₂O emissions in a corn-cropping system. Journal of Environmental Quality 40(5):1521–31.

Watson, C.J., H. Miller, P. Poland, D.J. Kilpatrick, M.D.B. Allen, M.K. Garrett and C.B. Christianson. 1994. Soil properties and the ability of the urease inhibitor N-(N-Butyl) thiophosphoric triamide (NBTPT) to reduce ammonia volatilization from surface-applied urea. Soil Biology & Biochemistry 26(9):1165–71.

Weber, H.S. and D.B. Mengel. 2009. Use of nitrogen management products and practices to enhance yield and nitrogen use efficiency in no-till corn. 39th North Central Extension-Industry Soil Fertility Conference Proceedings 25:1–7.

Wells, K.L., J.E. Dollarhide, and H.E. Burkwhat. 1999. Field evaluation of super urea for production of no-till corn. Agronomy Notes 31(1):1–6.