Rescue Nitrogen Applications for Corn

Rescue N applications to corn (Zea mays L) may be needed when wet conditions prevent N applications or when the loss of applied N is suspected due to wet conditions. Crop injury when N is broadcast applied may counteract the yield benefits of a rescue N application. The objective of this research was to determine the effect of corn height, N placement, and source on injury and yield response. Five site-years of research evaluated the impact of broadcast and between-row placement of NH4NO3 (AN), urea–NH4NO3 (UAN), urea, and urea plus N-(n-butyl) thiophosphoric triamide (NBPT), a urease inhibitor, at 170 kg N ha−1 either preplant or when the corn was 30, 60, 90, or 120 cm tall. Visual injury for broadcast-applied N sources was ranked urea = urea plus NBPT < AN ≤ UAN 7 d after treatment. Injury was dependent on plant height. Leaf injury resulted in reduced yield when UAN or AN was broadcast on corn that was 60, 90, or 120 cm tall. Broadcast urea or urea plus NBPT caused minimal crop injury and effectively supplied N to the corn crop. Application of NBPT-treated urea increased yield 260 kg ha−1 averaged across timings when compared with urea alone. Application when corn was 30 cm tall produced the highest yields, but excellent yield response to rescue N was obtained at all application heights. The placement and source of N should be considered when rescue N applications are made to corn >30 cm tall.