Managing In-crop nitrogen in cereals
In-crop nitrogen is a tool for many purposes. It can be used as a risk mitigation tool, when spring moisture condition is not ideal but got improved later on, the target yield increases and thus the require more N. It can also be used as a tool to reduce N losses and bump yield potentials by supplying the crop with N when the uptake is high. Besides yield, some growers apply in-crop nitrogen to increase grain protein, in the hope of the protein premium.
Each purpose requires different target timing and considerations. The following table is a brief summary of them.
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Risk mitigation:
When spring soil moisture conditions are not ideal, some producers may choose to hold back the N rate at seeding, with the concerns about overfertilizing a crop. After seeding, spring rain events may improve the moisture conditions and the target yield increases, meaning a higher nutrient requirement.
In this case, nitrogen can be applied in-crop to match the new target yield. Nitrogen uptake occurs early in the growth stage. Wheat takes up 50-60% of nitrogen by the end of tillering, when only 25% of the biomass has been produced. In addition, wheat yield potential is set during tillering, when the two most important yield components – heads/sq ft and kernels/head are determined.
For this reason, the window to top dress N is during tillering. Fertilizers applied after this stage can only maintain yield potential, rather than boosting it.
Reduce N losses/increase yield potential:
Split N application is a tool to reduce N losses during the early spring period, when crop N requirement is low and the N is susceptible to volatilization, leaching and denitrification.
The ideal application window for this purpose is also during tillering, when the crop is actively uptaking N and the N application can boost yield potential.
Figure 1. N uptake at different growth stages of cereals. Source: Sandhu, N., Sethi, M., Kumar, A., Dang, D., Singh, J., & Chhuneja, P. (2021). Biochemical and genetic approaches improving nitrogen use efficiency in cereal crops: a review. Frontiers in plant science, 12, 657629.
Increase grain protein:
From flag leaf stage to the end of flowering, N application during this window can increase grain protein. Application on the later end of this window tends to increase grain protein more effectively than earlier application.
A couple of important notes for those looking to boost grain protein.
- In-crop N application at this stage may not be economically beneficial. Grain protein response to in-crop N may be variable. Protein premiums are also variable depending on the year and region.
- The N rate required to achieve optimum grain protein is higher than that to maximize yield. See Figure 2 for the response curve of grain yield vs protein.
Due to the above considerations, please weigh the cost and benefit before applying an in-crop N to boost grain protein.
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Available N
Figure 2. Typical response of grain yield and protein concentration to available N. Source: McKenzie, R. H., Bremer, E., Grant, C. A., Johnston, A. M., DeMulder, J., & Middleton, A. B. (2006). In-crop application effect of nitrogen fertilizer on grain protein concentration of spring wheat in the Canadian prairies. Canadian Journal of Soil Science, 86(3), 565-572.
Rules of thumb
Despite their differences, there are universal rules of thumb for in-crop nitrogen. The goal is to reduce N losses, supply N when the crop needs it and avoid fertilizer burn from the application.
Reduce N losses
In-crop N can be applied in many forms and application methods. One of the common ways is broadcasting solid fertilizer, such as urea (46-0-0) or ammonia nitrate (34-0-0). Urea broadcast on soil surface is prone to N loss through volatilization. To reduce N loss, one can either use a urease inhibitor, or apply 1-2 days before rain events of at least 10-15mm. Rain helps incorporate urea into the soil profile and reduce volatilization risks.
Another common source of in-crop N is UAN (28-0-0) in liquid form. A quarter of N in UAN is in nitrate form (NO3-), readily available to the plants. However, 50% of UAN is urea, prone to volatilization. If applied in warm condition, a urease inhibitor is recommended to reduce N losses.
UAN can be applied through foliar application, dribble banding, or fertigation in the irrigated zone. Though foliar applied, plants absorb most of the N through the root system.
Moving the N to the root, either through rain or irrigation is important for improving N uptake.
For in-crop N application, the efficiency of N uptake is up to 35 to 40%. So have a realistic expectation while doing your best to reduce N losses.
Supply N when the crop needs it
If urea is one of the sources of in-crop N, keep in mind there is a delay between N application and N being available to the plants. The delay comes from needing rain to move the urea into the soil, hydrolyze urea into ammonia, and having soil microbials to convert ammonia into plant-available nitrate (NO3-). In warm and moist soil, this process takes 1-2 weeks. However, in cold and/or dry soil, the process could take 3-4 weeks.
Count this delay into timing the in-crop N application. If unsure, apply on the earlier side of the window to make sure the N supply matches up with plant N uptake.
Avoid fertilizer burn
Foliar applications of UAN can cause leaf burn in crops, particularly when applied at higher rates or during hot, dry conditions. To minimize the risk of leaf burn, one can use a moderate rate of N and dilute UAN with water. In addition, apply later in the day to avoid high temperatures and high sunlight intensity.
Resources:
In-crop Nitrogen Fertilizer Application – Ross McKenzie
Ross McKenzie - In-crop nitrogen applications, are they worth it? - 'In the Field' webinar
McKenzie, R. H., Bremer, E., Grant, C. A., Johnston, A. M., DeMulder, J., & Middleton, A. B. (2006). In-crop application effect of nitrogen fertilizer on grain protein concentration of spring wheat in the Canadian prairies. Canadian Journal of Soil Science, 86(3), 565-572.