Nitrogen is an essential plant nutrient that is needed in a relatively large amount to optimize yield. Nitrogen is the most expensive nutrient supplied as fertilizer by Canadian farmers annually. However, various pathways of nitrogen losses such as ammonia emission, nitrate leaching, nitrous oxide emission from nitrogen fertilizers, particularly urea, are a concern that limits crop nitrogen use efficiency. These nitrogen losses have negative agronomic and environmental consequences. A growing management practice to reduce nitrogen losses is the adoption of fertilizer additive- urease and nitrification inhibitors. Either of urease inhibitor or nitrification inhibitor may be used to coat urea as a single inhibitor or combined as a double inhibitor. While these inhibitors have successfully been used to reduce nitrogen losses to the environment, they have not consistently increased yield and the efficiency with which small grain crops such as wheat and canola use nitrogen fertilizer.

We hereby report the results of a two-year field study that was conducted at two locations (Carman and Portage la prairie) in Manitoba. The study was carried out to determine which of the four commonly used nitrification inhibitors when combined with the urease inhibitor, NBPT (N (n-butyl) thiophosphoric triamide) will maximize yields of canola and wheat and reduce nitrate leaching. We coated urea with either of the following nitrification inhibitor- Nitrapyrin, dicyandiamide, 3,4– dimethyl pyrazole phosphate, or 2-amino-4-chloro-6-methyl pyrimidine. Thereafter, we coated nitrification inhibitor-treated urea with NBPT using Agrotain Advanced formulation. Urea coated with NBPT only, uncoated urea, commercially coated urea (Super U), and a check with no nitrogen were also included in the study. These fertilizers were surface-applied at a rate of 80 kg N ha^-1 on plots seeded to canola and spring wheat in May of 2019 and 2020, respectively. Grain yields, grain N removal, crop N uptake, residual soil nitrate, and N use efficiency were assessed at both locations. The plots at Carman had field core lysimeters on each experimental unit that can measure the amounts of water and nitrate that are lost below the root zone. Weather information was collected from the nearest Manitoba Agriculture weather station at each site.

The two years of study were very dry with both sites receiving 35 to 44% less rainfall than the climate normal precipitation during the growing season (May – August). In each year, grain yields among the urea amended plots were not significantly affected by either NBPT only or any of the double inhibitor combinations at both sites. Also, grain N removal, crop N uptake, and N use efficiency were not significantly different among the urea amended plots irrespective of inhibitor types at both sites in each year. At both sites, the average residual soil nitrate sampled after harvest was not different between untreated urea and inhibitor-treated urea in each year. This showed that nitrogen was not a limiting factor for yield increase as the residual soil nitrate was high (28 – 115 kg N ha^-1) in all plots. The amount of water that flowed past the root zone during the two years of study was not affected by the addition of single or double inhibitors to urea. However, the addition of urea resulted in a greater loss of nitrate than the control plot with no urea amendment. The coating of urea with inhibitors did not reduce the cumulative loss of nitrate when compared to the uncoated urea. The lack of differences in the cumulative loss of nitrate may be due in part to the dry growing season and also because the conditions for nitrate leaching occurred after crop harvest when the effectiveness of the inhibitors had elapsed. Our inability to observe the benefit of the inhibitors on yield parameters might be due to low precipitation during the growing season that limited potential yield and the ability of the crop to use the applied nitrogen. While studies in other regions have shown yield responses to these inhibitors, particularly with crops that have high nitrogen demand such as corn, our study showed that it is a challenge to obtain yield responses from small grain crops on soils with high native fertility.