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® is an additive targeted to fertilizers that contain urea. The purpose of AGROTAIN® is to reduce ammonia volatilization after fertilizer application.

The active ingredient, NBPT, functions as a urease inhibitor. Urease inhibitors reduce the speed of conversion from urea in fertilizer to ammonium (enzymatic hydrolysis), which can then either rapidly convert to gaseous ammonia and be lost to the air or convert to nitrate (NO3-) and be subject to leaching losses. By staying longer in the urea form, the fertilizer can more slowly become available to plants, increasing the chance of being there when needed by the crop.

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.

AGROTAIN®’s active ingredient NBPT was identified in early laboratory studies to be a good repressor of urea hydrolysis, even when applied at low concentrations (Beyrouty et al. 1988; Bremner and Chai 1986; Hendrickson and Douglass 1993; Watson et al. 1994).

NBPT has been effective at reducing N losses in many US field research trials over the past two decades. Early field studies with corn identified NBPT (then called “TPT”) as the most promising phosphoroamide urease inhibitor relative to others when conditions existed for substantial NH₃ loss from volatilization (Schlegel et al. 1986). Later studies in ridge-till corn also showed that NBPT is useful for limiting yield loss due to ammonia volatilization under these conditions (Murphy and Ferguson 1997).

Extensive field studies also demonstrated that maximum corn grain yields can be achieved using an average of 80 kg/ha less nitrogen when NBPT is combined with urea in surface fertilizer applications (Hendrickson 1992).

Amending UAN and urea with NBPT also produced significantly greater N fertilizer use efficiency in sprayed, unincorporated broadcast, and banded fertilizer applications in no-till corn (Fox and Piekielek 1993).

NBPT became available for on-farm use in 1995 under the trade name of AGROTAIN®.

A search of the scientific literature and grey literature (conference proceedings etc.) was conducted to locate data on crop yield responses to AGROTAIN®. 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 two tables below summarize the yield effects of AGROTAIN® for corn and wheat with discussion below each 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.

For wheat, the overall weighted effect size for AGROTAIN® is significantly affected by very positive results from one study in Saskatchewan (Malhi et al. 2003), for which the strong yield impact was caused by poor seed emergence due to seed-row placement of the urea fertilizer. In this case, the benefit was not necessarily due to improved N use efficiency but the coating of the fertilizer, as less seedling damage with the AGROTAIN®-coated urea fertilizer enhanced crop establishment. This illustrates the importance of looking carefully at the results of individual studies, or at least at considering whether studies should be included in meta-analysis as representative of the typical product use. Therefore, the positive yield impact of AGROTAIN® for wheat should be approached with some caution, and further data collection would be beneficial.

Four separate studies testing AGROTAIN® reported significant increases in corn yield. However, while results of a number of many individual studies were inconclusive, the overall effect size indicates significantly increased yield. The inclusion of a large amount of data from non-peer-reviewed sources only affected the estimated impact by a small amount, but reduced variability in the estimated mean effect on corn, thus increasing certainty. Because of the similar results, it seems likely that these data from the grey literature are reliable and useful for determining the effect of AGROTAIN® on corn crop yields. Average increases in corn yield of between 7% and 14% can be anticipated within the regions for which data are available. We were unable to find any studies that measured N₂O or NO₃ loss impacts of AGROTAIN®, so the loss impacts are uncertain.

What contributes to the variability between studies? While theoretically it would seem that crop yield would respond more to urease inhibitors when N is limiting (and thus would be more obvious in fields with a yield response to N fertilizer), an examination of yield effects in these experiments found no correlation between yield response to additional N fertilizer and yield response to the use of AGROTAIN®. Murphy and Ferguson (1997) hypothesized that the response to NBPT with urea, but not UAN was related to limited precipitation and low humidity shortly after fertilizer application, conditions which encourage heightened ammonia volatilization loss from urea. While climatic conditions were not provided, this may explain the different responses in UAN versus urea in Illinois as well (Varsa 1989; Varsa 1999). Additional differences between studies might also be caused by variation in pre-plant available N, soil organic C, or other factors.

When looking at yield effects only and keeping fertilizer rates constant, positive yield impacts for corn are evident for AGROTAIN®. 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). Overall, yield benefits for wheat from AGROTAIN® seem to be more an effect of reduced seedling damage by fertilizer than any reduced losses. Thus, the benefits were affected by fertilizer placement.

Beneficial impacts of products containing NBPT and DCD were further modified by climate. For example, adding AGROTAIN® to urea was especially effective in dry conditions, when ammonia losses would be significant.

However, yield impact of a product without changing N fertilizer rate may not be the optimal measure of whether or not these products positively affect nitrogen efficiency. Perhaps studies with multiple rates would find similar yields from these products even with less overall N applied. Such results would be expected when a product improves N use efficiency. Many of the experiments from which data are currently available are not in systems where N is limiting the crop productivity, so the product may not have had opportunity to demonstrate its potential.

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