Some producers may find it necessary this year because of prolonged wet field conditions to apply nitrogen (N) in the form of a urea-ammonium nitrate (UAN) solution or urea over the top of emerged corn.
Several years ago, Gyles Randall, a researcher at the Southern Minnesota Experiment Station in Waseca, studied the effects on corn growth and yields of broadcasting UAN on growing corn over a period of two years. He used UAN to provide N rates of 30, 60, 90, 120 and 150 lb. per acre broadcast over the top of the corn at the four- to five-leaf and eight-leaf stages of growth. Randall added supplemental N to equalize the rate so over-the-top effects could be evaluated. At the four- to five-leaf stages of growth, he included treatments of 2 lb. atrazine with the 60, 90, and 120 lb. N rates per acre.
Injury from urea applied over the top of emerged corn.
The study showed that UAN applied to growing corn had a phytotoxic effect on plants. When applied at the four-leaf stage, these effects were magnified as the N rate increased, but damage was not permanent and did not appear to reduce yields.
Injury from UAN applied over the top of emerged corn.
Based on results of this study, producers can expect heavy burning, some dead leaves, and reduced growth at a 120-150 lb. N per acre rate broadcast over the top as UAN. When 2 lb. of atrazine was included with the 60 lb. N per acre rate, leaf burning was similar to the 120-150 lb. N per acre rate applied only as UAN. No significant yield reduction occurred up to the 90 lb. N per acre rate with atrazine, but it did occur at the 120 lb. per acre rate.
When applied at the eight-leaf stage, the severity of leaf burn increased as the N rate increased. Leaf burn lasted longer when applied at the eight-leaf stage than when UAN was applied at the four-leaf stage. Decreased yields were noted when UAN rates were more than 60 lb. N per acre.
Based on this work, producers can expect some leaf burning and reduction in early growth when broadcasting UAN solution over the top of growing corn. Plant injury and any yield reduction will be determined by the N rate and subsequent weather and growing conditions.
For a conservative approach, we suggest you do not exceed the following amounts:
- 90 lb. N per acre applied as UAN alone at the four-to five-leaf stage of growth;
- 60 lb. N plus 2 lb. atrazine per acre at the four- to five-leaf stage of growth, and
- 60 lb. N per acre at the eight-leaf stage of growth.
Beyond the eight-leaf stage, sidedress UAN and do not apply to foliage. Check all herbicide labels to determine if post-emergence application with UAN is permissible.
Broadcasting urea is another alternative to get nitrogen to corn. It is not a general recommended practice, but when the decision has to be whether to apply N as broadcast urea over the top of growing corn, or to not apply needed N, it would be considered an acceptable alternative.
If urea prills or granules land in the whorl of the corn plant, the leaves emerging from the whorl will probably have white leaf margins, as shown in the photograph. These are symptoms of toxicity because more urea is absorbed through the leaves than the plant can use. This effect has not been shown to be particularly harmful, but expect some of this to occur.
If urea granules land in a leaf axil, a white-to-brown spot will occur and leaf margins of that leaf may turn white.
This article originally appeared on page 75 of the IC-476(11) -- May 27, 1996 issue.