Instinct® II and N-Serve® (Nitrapyrin)

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.

Nitrapyrin was first identified as a nitrification inhibitor in 1962, where it was shown that nitrapyrin activity appears to select for Nitrosomonas and does not have a significant effect on other nitrifiers or soil heterotrophs (Goring 1962; Laskowski et al. 1975; Shattuck and Alexander 1963). Nitrapyrin was first registered as a product in 1974, and early laboratory studies with field soils showed that nitrapyrin volatilizes if broadcast-applied to the soil surface. (McCall and Swann 1978). It was demonstrated in these early lab studies that nitrapyrin is significantly more effective at reducing leaching losses of nitrate when incorporated into the soil (Briggs 1975), and Dow AgroSciences emphasizes the importance of this application method in their product literature.

Product Description and Mode of Action

Nitrapyrin is a chlorinated pyridine compound that functions as a nitrification inhibitor. It delays the nitrification process by inhibiting the Nitrosomonas spp. bacteria that convert ammonia to nitrite, thus retaining more fertilizer-applied nitrogen in a form readily available to crops. This can prevent the loss of soil nitrogen through leaching or runoff of nitrate (NO3), or gaseous emissions of nitrogen (N2) and nitrous oxide (N2O) gases. For over 40 years, nitrapyrin has been used and studied internationally as a nitrification inhibitor with the goal of increasing crop yields and reducing the environmental impacts of nitrogen fertilizers used in agriculture.

Later lab studies indicated that nitrapyrin could also reduce N losses from NO3 leaching by an average of around 11% (Owens 1981; Timmons 1984). These observations were also seen in field lysimeter studies with corn with average NO3 leaching loss reductions ranging from 7 to 28% over 3 and 6-year periods, respectively (Owens 1987; Timmons 1984). Other early studies of nitrapyrin in the field clearly demonstrated nitrapyrin’s ability to conserve soil N under certain conditions (Nelson and Huber 1980).

Product Efficacy in the Field

A search of the scientific literature and grey literature (e.g., conference proceedings and extension reports) 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 summarize the yield impacts of nitrapyrin, 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 separated out by specific products, as applicable. All experiments included in these tables either reported using N-Serve® or did not indicate any brand-name for the nitrapyrin used.

Nitrapyrin applied with either anhydrous ammonia or urea has a long history of field trials, especially with corn. For both anhydrous ammonia and urea, all wheat experiments reported using N-Serve®, but for corn experiments this was far less common. Results for wheat were consistent across both fertilizer sources, with average yield increases of 4.8% with the nitrification inhibitor. Across a large number of research studies, we see average corn yield increases of over 2% with anhydrous ammonia and almost 6% with urea. Exclusion of data from the grey literature had little impact on overall results. For corn, restricting the analysis to only those studies that reported using the brand-name N-Serve® product seemed to erase the positive effect for anhydrous ammonia. However, the active ingredient (nitrapyrin) is no different among these experiments, and further testing with regression models did not find any yield response difference between N-Serve® and generic nitrapyrin. Therefore, it seems reasonable to look at the full dataset, concluding that there is indeed a positive yield impact when N-Serve® is used with anhydrous ammonia or urea on corn.

Field researchers have also tested corn yield response to nitrapyrin with other N fertilizers, including UAN, ammonium nitrate, and ammonium sulfate, with insignificant results. On the other hand, wheat yields average 8.3% higher with nitrapyrin for UAN and ammonium sulfate, although these data are limited. Including these “miscellaneous” fertilizer types with the anhydrous ammonia and urea research studies, there is broad regional coverage across much of the U.S. and Canada for nitrapyrin yield experiments (except for the western states).

Putting together all mineral fertilizer sources (anhydrous ammonia, urea, UAN, ammonium nitrate, and ammonium sulfate), nitrapyrin resulted in average corn yield gains of 3.3% (±1.5) and average wheat yield increases of 5.5% (±1.8). Responses seem to be consistent between anhydrous ammonia and urea, and regression models find no significant response differences between these two fertilizer types.

Preliminary modeling finds that the responses to nitrapyrin are not affected by tillage, irrigation status, or crop rotation.

Summary of Evidence

While holding fertilizer rate and other management characteristics constant, nitrapyrin treatment increases average grain yield for both corn and wheat in North America. These yield benefits suggest that nitrapyrin increases nitrogen use efficiency, so that it may also be possible to use nitrapyrin and reduce fertilizer N rates while not negatively affecting yield. Such action could reduce environmentally harmful N losses and fertilizer costs.

In addition, studies have found some potential for N loss reduction with nitrapyrin even without changing fertilizer N rates. Researchers in Minnesota tested NO3 leaching losses in corn fertilized with urea, with and without nitrapyrin (12 observations), finding no statistically significant difference in total NO3 losses, but these losses were delayed with nitrapyrin use (Walters and Malzer 1990). Thus, with N available to the plant later in the season, it may be possible to reduce N rates (and leaching losses) while still achieving the same crop yield. Further south, but also in Minnesota, other scientists found that nitrapyrin with anhydrous ammonia reduced total yield-scaled NO3 losses by 13.1% (±9.3; n=18) (Randall and Vetsch 2005; Randall et al. 2003). As these results are limited to one region, it would be prudent to confirm whether such benefits also exist in other climatic and soil conditions. Given its demonstrated ability to increase crop yield while simultaneously decreasing fertilizer N needs as well as damaging N losses to the air and water, nitrapyrin promises to be an important component of improved management practices.

Complete references for papers cited in all above tables and links to papers available as open access are here.