ESN® (Environmentally Smart Nitrogen)
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 December 2015 under contract to the Environmental Defense Fund.
Environmentally Smart Nitrogen (ESN®) is an enhanced efficiency nitrogen (N) fertilizer produced and distributed by Agrium. It is a polymer-coated, controlled-release urea product (often referred to as PCU or CRU), designed to protect against environmental N losses through leaching, volatilization, and denitrification. The polymer membrane encapsulating the urea allows water to diffuse into the granule, creating a urea solution that slowly diffuses into the soil over a longer period of time, so it is promoted as an ideal pre-plant, single-application N fertilizer. The slow-release increases the amount of N available to the crop when it needs it, but reduces the amount of N exposed to loss mechanisms during the time the polymer is protecting the urea. Based on these properties, ESN® is marketed to producers as a tool to increase N use efficiency (NUE) while decreasing the number of seasonal fertilizer applications needed, thus reducing supply and labor costs as well as soil compaction that results from multiple applications (Agrium Inc. 2014).
Though only distributed wholesale in the U.S. since 2014, ESN® has been on the retail market since 2004 and has been studied in North America since the 1990s, providing almost two decades of research on its efficacy as an enhanced efficiency fertilizer. Grain protein results from an early laboratory study with barley suggested that coated urea allowed for a slower fertilizer release, providing a higher N supply in later growth stages (Zhang et al. 2000). Coated urea applied alone had the greatest reduction in potential N loss compared to non-coated urea and a mixture of both. Early field trials conducted on spring wheat across many sites in western Canada from 1998–2000 found similar grain yield and higher grain N content from seed-placed ESN® compared to side-banded urea applied at equivalent rates (Haderlein et al. 2001). These trials also demonstrated an average nitrogen use efficiency increase with ESN® of 4.2% across all sites, with a maximum NUE increase of 35%.
Later field trials with spring wheat and canola found that ESN® also increased the safety of seed-placed urea as it reduced potential for both salt and free ammonia injury, providing a margin of safety equivalent to side-banded urea without all of the specialized equipment and a margin greater than that provided by Agrotain (Brandt et al. 2004). Similar plant survival results were also seen in winter wheat trials in Southern Alberta at high N application rates, and later studies at these locations demonstrated a slight grain yield increase with PCU (these were early versions of ESN®, according to the authors) compared to non-coated urea (McKenzie et al. 2010; McKenzie et al. 2007). Finally, a field lysimeter study from Ohio measured lower nitrate losses, with total N losses from spring-applied urea and UAN approximately 3 lbs N per acre greater than from spring-applied ESN® (Blaylock et al. 2004).
A search of the scientific literature and grey literature (conference proceedings etc.) was conducted to locate data on crop yield responses to the products of interest. 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 following tables and discussion summarize the yield impacts of ESN®. 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, as applicable.
The positive overall weighted effect size for ESN® is strongly affected by very positive results from one study in Saskatchewan (Malhi et al. 2003) and another in Montana (Mohammed et al. 2013), for whom the significant grain yield impact was caused by poor seed emergence due to seed-row placement of the urea fertilizer (for the same reason, Malhi et al. 2003 also found AGROTAIN® to perform much better than urea). In these cases, the benefit from ESN® was not necessarily due to improved N use efficiency but to the coating of the fertilizer that reduced seedling damage and enhanced crop establishment. Excluding these outlier and the observations from the grey literature reveal a very different overall average yield impact, one that is not statistically significant. Therefore, while ESN® replacing urea could certainly improve yield in a seed-row fertilizer placement scenario, generally improved NUE leading to greater wheat yield is not overwhelmingly supported with the data available.
In other related work, Malhi et al. (2011) compared ESN® with urea at two locations in Alberta, Canada, over a 27-year long crop study that included barley, wheat, canola, and field pea. They found that ESN® outperformed urea in terms of yield and N uptake at a site with lower soil carbon and greater N loss potential. However, urea generated greater straw yield and higher total N uptake at the other location. Thus, variability of ESN® performance by location seems worthy of further exploration, perhaps due to differences in soil C, texture, or climate.
None of the wheat field studies comparing ESN® and urea also measured N2O or NO3 losses, so the only indicator of NUE is the yield response for this comparison. Some promise for improved NUE was found by Soon et al. (2011), who – while not reporting crop yield effect – measured higher N uptake and lower N2O losses from spring-applied ESN® versus urea in northern Alberta and Saskatchewan wheat cropping systems. In those field studies, there was little difference between sources that were fall applied, and the timing effect was much stronger than any impact of source.
For corn, ESN® has been compared with urea in many different locations in Canada and the United States. Individual experiments have found both positive and negative yield impacts with ESN®, and the overall weighted average difference is not significantly different from zero. Thus, ESN® does not seem to consistently provide the anticipated yield benefits when applied with the same timing as urea. The expectation that ESN® could allow for fewer fertilizer applications has also not been borne out in research studies. Maharjan et al. (2014) tested corn production with a single application of ESN® compared to split urea, finding higher yields and lower NO3 losses with split urea compared to a pre-plant ESN® application. On the other hand, they did observe some N2O emission reductions with the ESN® treatment.
Twelve of the recent studies testing ESN® versus urea in corn also measured N2O emission losses, but while average N2O emissions from ESN® were perhaps lower (by 6.4%), there was no statistically significant effect due to high variability in the 38 observations (95% confidence interval is ±12.5%). The only available data on NO3 losses are from the above-mentioned Maharjan et al (2014) for whom timing was also different between sources.
As was observed when compared to urea, ESN® does not consistently provide any yield benefits over UAN fertilizer when applied with the same timing. Further, ESN® may even result in greater N2O emission losses than UAN in corn systems, averaging 28% (±31%) higher emissions in the 10 observations reported.
A number of corn and wheat field studies also compare ESN® to other fertilizer sources. For example, Warncke (2008) no significant improvements in wheat yield or economics when using ESN® instead of UAN in experiments from 2004–07. In addition, no yield differences were found when ESN® was compared with ammonium nitrate in both corn in Kentucky (Sistani et al. 2014; Sistani et al. 2011) and wheat in Missouri (Nash et al. 2012b). Studies in both Missouri and Iowa also reported no corn yield difference between ESN® and anhydrous ammonia (Hatfield and Parkin 2014; Nelson et al. 2014). In another Iowa experiment, Parkin and Hatfield (2014) measured N2O emissions for three different years, finding either no difference or increased N2O emissions from ESN® when compared to both urea and UAN (no yield reported, so emissions are not yield-scaled).
Based on the research data available from North America, ESN® does not provide consistent yield benefits over commonly used N fertilizers when applied at the same rate and timing. Improved yield in wheat experiments were mostly evident in situations where seed-placed fertilizer urea harmed seedling growth. Some variability in the data suggests that while there is no broadly applicable benefit, there may yet be climate, soil, or cropping system conditions where ESN® is preferable to other sources. For example, Malhi et al. (2011) tested two sites, and observed better ESN® performance on the soil with lower organic C content and greater N loss potential. Regression models using the data provided in the studies examined above were unable to pinpoint any specific controlling factors. However, since weather and soil data are inconsistently provided in these articles, gathering comparable climate and soil information from other sources (such as nearest weather stations) and using them for further assessment is likely to provide useful insight.
Complete references are here.