AGROTAIN® PLUS Research Findings
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.
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