Agronomy Library > Annual Weeds

Seeing Green: A New Tool for Predicting Nitrogen Rates in Spring Wheat and Canola
Author: Guy Lafond & Bill May, AAFC-Indian Head & Chris Holzapfel - IHARF
Date Created: November 21, 2007
Last Reviewed: November 21, 2007
Nitrogen is the most limiting nutrient in crop production and more N is applied as fertilizer in western Canada than any other nutrient. Nitrogen fertilizer also accounts for the largest portion of total energy required for production of non-N fixing crops on the Canadian prairies (Zentner et al.2004) and the manufacture of nitrogen fertilizers is an important source of greenhouse gas emissions, specifically nitrous oxide and carbon dioxide. Excessive use of N fertilizers can also have important negative environmental consequences such as surface and ground water contamination with nitrates and emissions of nitrous oxide form the soil (Malhi and Lemke 2007). Furthermore, producers are looking for ways to improve their ability to manage N fertilizers more effectively because of the recent large increases in N fertilizer prices. Nitrogen fertility management encompasses four major components, source, placement, timing and rate (Malhi et al. 2001). Research has demonstrated that there is very little difference between fertilizer forms, providing they are managed appropriately (Johnston et al. 1997; Grant et al. 2002). Placing the fertilizer in the soil, as opposed to on the surface, greatly minimizes losses from volatilization and immobilization and enhances overall N fertilizer recovery (Malhi and Nyborg 1991; Malhi et al. 2001; Grant et al. 2002). The timing of N application should be such that it is available close to the time of maximum crop uptake which in cereal grains extends from the start of elongation until heading with peak uptake during flag leaf extension (Bauer et al. 1987) and in canola from the start of flowering to the end of pod formation (Malhi et al 2007b). The current N fertilizer rate recommendations on the Canadian prairies generally consider factors such as soil texture, residual soil nitrate levels, soil moisture at seeding, average growing season precipitation, previous crop grown, crop to be grown, target grain yield, expected commodity prices and N fertilizer prices (McKenzie 1998; Anonymous 2007a). However there is much uncertainty with all of these factors due to year to year variations in climatic conditions and to spatial variability in soil nutrient levels and inherent fertility of the soil. Nitrogen release during the growing season and the major pathways of N losses (immobilization, volatilization, denitrification and leaching) are also greatly influenced by climatic conditions, making their amounts very difficult to estimate. Consequently much uncertainty exists in determining crop N requirements and the rate of application can easily be under or overestimated with important economic and/or environmental consequences in either case. There is interest in exploring post-emergent N applications in annual crops to refine our ability to arrive at more optimal rates of N fertilizer. Delaying some or all of the N fertilizer until after crop emergence may allow for a better sense of yield potential and expected growing conditions. With the recent introduction of commercial optical sensors as a N management tool, it is now possible to estimate crop yield potential early in the growing season in cereals (5-6 leaf stage), which is early enough to adjust the rates of N accordingly (Raun et al. 2002). A series of studies with spring wheat and canola were initiated in 2004 to develop the appropriate mathematical relationships relating values obtained from the GreenSeekertm sensors to actual grain yields. The first aspect of the study looked at ability of the sensor measurements and the mathematical relationships to to predict the potential grain yield of wheat and canola early in the growing season. The second aspect of the study was to determine the merits of this approach for fine-tuning N management during the growing season. In-field applications of liquid UAN solutions were used to see if the full yield potential of the crop could be realized based on measurements of NDVI with the GreenSeekertm sensor  open the attached PDF for the complete paper