The use of granular urea as a nitrogen fertilizer for corn production in Iowa is fairly low compared to the amount of anhydrous ammonia (NH3) and UAN (urea-ammonium nitrate) that is applied. "In evaluations by the On-Farm Network, the number of farmers who have tested urea is so low that we don't have good comparisons over a number of years, so it's not possible for us to talk about the long term odds with the product," says Tracy Blackmer. He is director of research for the On-Farm Network, an on-farm testing program sponsored by the Iowa Soybean Association.
"However, a few of the long-time cooperators in our trials agreed to put out trials this year using a urea product called ESN or Environmentally Smart Nitrogen, which has a coating designed to delay N availability. These trials gave us some very interesting results," says Blackmer.
Fall anhydrous vs. spring-applied granular urea?
Blackmer cites the results of two of the On-Farm Network's replicated strip trials that compared alternating strips of fall applied anhydrous ammonia and spring-applied ESN. Both treatments received the same rate of actual N per acre and a light spring tillage required to incorporate the urea was used across the field. As in all trials of this nature, the same hybrid, planting rate, herbicide, etc., was used for all of the strips in the field trials.
There were very pronounced visual symptoms of sulfur deficiency in the ESN strips (see photo accompanying this article). "We were not expecting this," says Blackmer. "We wrote about this in our June 28 ADVANCE newsletter and promised to follow-up and explain it this fall."~~~PAGE_BREAK_HERE~~~
In investigating further, Blackmer and his research team verified with the growers that there were no other differences in applied fertilizer or other products between the strips that could have added different amounts of sulfur. The team also pulled leaf samples from both the NH3 and ESN strips for tissue testing. While both sets of strips had adequate N, the fall NH3 strips tested somewhat lower in N, but higher in Sulfur than the ESN strips.
Shortly after the researchers inspected the fields and pulled the leaf samples for testing, all of the fields received varying amounts of rain. After the rain, the growers all reported that the visual differences between the strips disappeared. "However, in late season aerial imagery of the fields (see photo with this article), we were able to see differences in the strips that followed the patterns seen earlier in the season," says Blackmer.
Combine yield monitor data from two trials shows a 5.6 bushel per acre difference in one and a 10.8 bushel per acre difference in the other, both favoring fall NH3. "We do not know if this was due to a single factor such as heat, drought, fertilizer product, etc., nor can we say whether we'd see the same results in similar trials next season," says Blackmer. "What we can say definitively is there was a difference this year that most people would not have expected. We'll continue to follow this subject and report any additional results or information we find that might help explain what we saw in these trials."
He adds, "Watch our website for final results of these and other On-Farm Network replicated strip trials and guided stalk nitrate sampling."