A: A rescue treatment is when an additional and previously unplanned nitrogen treatment is applied. The need for more nitrogen may be indicated by yellowing of corn from environmental factors, most likely too much water, or loss of N from volatilization of surface applied N. In 2016 the yellowing reported in many areas of Nebraska was likely caused by too much water early in the season moving nitrogen lower in or below the root zone.
Q: Do rescue treatments work?
A: Sidedress N has been a standard practice for years. The difference between it and a a rescue treatment is that it goes on before plants are yellow. While much research has shown that sidedress is effective, new research is studying "real-time N application," which means applying N during the season to meet plant needs. Corn needing a rescue treatment is past the point of "hidden hunger" used for real-time applications.
Research in Missouri shows that visibly yellow corn will respond to N when applied up to tassel. Across a number of tests, the average yield increase in corn from the nitrogen rescue treatment was 34 bu/ac. The yield increase from the rescue treatment for various stress categories was: high stress - 57 bu/ac; medium stress - 42 bu/ac; low stress - 14 bu/ac. (For more details see Nitrogen Loss and Late-Season N Application for Corn by Peter Scharf of the University of Missouri.)
Q: Are there any concerns with rescue treatments?
A: In some instances, depending on when and how much N was applied, leaf burn can result from a rescue treatment. This can look pretty ugly in the short-term, but corn will usually grow out of it. Scharf found that when applying a rescue treatment of 150 lb N, which would be a fairly high rate, there was no significant damage to 1-foot corn plants. UAN solution damage started at 2-foot high plants (14 bu yield reduction), increasing to 61 bu/ac yield loss in 4-foot corn. Urea caused less damage at all heights; loss was only 4 bu/ac in 4-foot corn.
To estimate possible yield loss from leaf burn, see the charts used to estimate potential yield loss from hail in Nebraska Extension EC 126, Evaluating Hail Damage to Corn.
Use Table III on page 6 to estimate corn yield loss due to leaf damage at various growth stages. Determine growth stage, using the method outlined in the resource, and estimate percent leaf area destroyed. Be sure to do this over the whole plant not just the injured leaves. For the earlier leaf stages the loss will be minimal. For example at the 14-leaf stage, there needs to be 45% leaf area loss to predict a 10% yield loss.