Andrea Rice, Director of Research, Education, and Outreach
Nitrogen fertilizer is a major investment for tall fescue production and producers are seeking to control the risk involved. Ryan Lock, Principal Investigator on the research project, looked into commercially available products to mitigate volatilization and nitrification with these unbiased, replicated experiments.
The objective of the study was to evaluate urease and nitrification inhibitors versus urea and ammonium nitrate in spring/summer and autumn tall fescue forage production. Studies took place at the Southwest Research Center (SWC) at Mt. Vernon, MO due to its proximity to the heart of Missouri’s beef cattle industry. Urea, SuperU, Anvol, and ammonium nitrate were evaluated at 50 and 100 lb N/acre in spring application. The same N souces were evaluated at 75 lb N/acre for fall application. Spring applications took place in March with fall applications in August.
Spring Application Results: Of the four products tested, none of them resulted in better production at the 50 lb N/acre rate. This makes it hard to justify use of a nitrification inhibitor when the risk to N loss is low, as is often the case in the spring. The data from this study allowed researchers to conclude that the right rate for spring N application is 50 lb N/acre and use of protectants from N loss are likely unnecessary.
Figure 1: Forage yield of tall fescue averaged over three years at the Southwest Research Center near Mt. Vernon, MO. Products at two N rates were applied in mid-March and yields recorded in May.
Fall Application Results: Forages were allowed to accumulate until harvest around December 1 each year to simulate totals for stockpiled grazing. About 100 growing days were allotted for after fertilization at a rate of 75 lb N/A. Researchers expected urease and nitrification inhibitors would play a more important role in the fall than in the spring. However, the data show Ammonium nitrate out yielded Urea, but SuperU and Anvol did not provide more forage than Urea. Researchers noted the yields for autumn forages in southwest Missouri were paltry with forage growth about half the return on investment as typical with 75 lb N/A .
Nitrogen loss after fall fertilization was anticipated and researchers expected Anvol and SuperU to show benefit if it occurred. Reports exist showing up to 40% of N escaping the pasture system when urea volatilizes under hot, humid conditions. The data from this research project did not support that volatilization occurring. In fact, Anvol, SuperU, urea, and ammonium nitrate all provided equal results.
Figure 2: Autumn yield of tall fescue averages ove3r three years at the Southwest Research Center near Mt. Vernon, MO. Products were applied with a single N rate of 75 lb N/acre in mid-August and forage harvested around December 1.
Conclusions: Yields from Anvol were equal to those from SuperU and ammonium nitrate. This was the case with the two different yield rates (50 lb N/acre and 100 lb N/acre) in spring applications as well as the single N rate (75 lb N/acre) in the fall studies. Anvol suits well as a replacement for ammonium nitrate if availability issues exist. There was no volatilization after the fall application, but that risk is weather dependent.
Data from the project indicate the right rate is more important than right source for benefit/risk analysis. We could not show a clear need for urease or nitrification inhibitors in this three year study. The right rate for spring application rarely exceeds 50 lb N/acre. Applying 75 lb N/acre in autumn did not show environmental risk of soil N leaching, however precipitation was very low during the research period.