Fine-Tuning the Amount of Nitrogen Fertilizer Applied to Canola (Brassica napus L.) in Western Canada Using Optical Sensors.

Although canola (Brassica napus L.) responds well to nitrogen (N) fertilizer, variability in both the soil’s capacity to supply N and crop demands make it difficult to determine how much fertilizer should be applied for a given crop-soil environment.  Optical sensors have potential to improve our ability to determine how much N to apply for canola and allow both spatial and temporal variability in fertilizer requirements to be addressed.  Previous research established the empirical equations required to estimate the yield potential of canola using mid-season normalized difference vegetation index (NDVI) measurements from GreenSeeker^TM optical sensors.  In the current research, these equations were tested over six site-years of plot trials for their ability to estimate N topdressing requirements and fine-tune the total amount of N applied compared with the predominant practice of banding the crop’s entire N requirements (based on yield goals and residual nitrate) beneath the soil surface during planting.  Fine-tuning the amount of N applied using NDVI and high N reference plots resulted in reductions in N fertilizer use ranging from 15-53 kg N ha^-1 and the total amounts applied were 34 kg N ha^-1 lower on average.  Grain yields of the optical sensor-based treatment were only lower than when fixed rates of N were applied at seeding at one site-year, Indian Head in 2006, because of dry conditions after the topdress N applications.  Nonetheless, this technology did not typically reduce post-harvest residual soil-nitrate levels and, due to high variability, increases in agronomic N use-efficiency were only significant at Indian Head in 2005 (P=0.026) and 2007 (P=0.054).  Overall, optical sensors showed good potential for improving N management in canola production; however, the practice is not without risks, especially in dry years.