In most production systems, grain development (starting at pollination) begins about 75 to 95 days after planting, depending on hybrid maturity (CRM). However, the majority of N is typically applied just prior to planting or even 4 to 5 months ahead of planting (in the fall). N management programs are largely driven by workload management and N price, and partially by crop demand (e.g., split applications during the season), but rarely by accounting for late-season crop N uptake.

Ensuring there is a pool of soil N available during the late stages of grain development is the most difficult aspect of an N management plan. This is because late N availability is dependent on many complex and interacting factors, including:

initial N application rates and timing.

mineralization of soil organic matter, which provides N in plant-usable forms.

environmental factors that affect volatilization, leaching, denitrification and mineralization.

crop uptake during the vegetative period.

crop yield potential.

Losses due to volatilization and leaching may range from 0 to 50 lbs/acre/year depending on the growing environment. Denitrification losses may be as high as 2% to 5% of the total soil N per day if soils are saturated. Crop uptake during the vegetative period generally ranges from 1 to 2.8 lbs/acre per day. Because N is easily lost from the soil and may be taken up quickly by a fast-growing corn crop, planning for postflowering soil N availability is challenging. In fact, the confidence level in predicting the supply of soil N for grain filling declines as the interval between N application and grain development increases.

Current N application strategies target application in the fall or the spring prior to crop planting. Having a supply of N during vegetative development is clearly important, because greater than 60% of the N is taken up and stored prior to flowering (Figure 5). However, only a third of that N is generally remobilized to support grain development. In high-yield environments where the grain N demand exceeds 200 lbs N/acre, the importance of available late-season N is magnified.

Based on recent research findings regarding pre- and postflowering N uptake as well as N remobilization, a logical strategy is to build a nitrogen application plan that mirrors crop demand to limit losses and maximize N uptake:

Application of 70% of the total seasonal N requirement prior to planting should provide sufficient N for vegetative growth.

Applying the remaining 30% as late during the growing season as equipment allows would likely be advantageous most years. This planned late application also provides the option of replacing N lost due to high rainfall conditions typical in May and June in many locations.



Another option is adding a nitrification inhibitor to the late application to help delay N release until closer to flowering or after flowering. This option depends on soil N availability at the time of application, as well as the time of application relative to the crop development stage. A recent report documented that the addition of nitrapyrin (e.g., Instinct® nitrogen stabilizer) in preplant and side-dress applications of N increased N uptake during the season (Burzaco et al., 2013). Although the yield increase was non-significant, these N applications with nitrapyrin increased NUE by 17% and nitrogen recovery by 25%.