In 1985, Wadell Altom, Jerry Rogers (both previous soil and crops specialists) and Bret Flatt (senior research assistant) initiated a research project northwest of Ardmore, Okla., on the Pasture Demonstration Farm to evaluate nitrogen fertilizer source and timing of application. The objectives of this study were to compare nitrogen fertilizer sources and to compare applying all of the required nitrogen in April versus splitting the application with one-half applied in April and one-half in June in a hay production system. The nitrogen fertilizer sources used included 28% UAN (28-0-0), ammonium nitrate (34-0-0), urea (46-0-0) and anhydrous ammonia (82-0-0). Each treatment received a total of 150 pounds actual nitrogen per acre either all upfront in April or split applied (half in April and half in June). This article will compare urea and ammonium nitrate, since they can be used interchangeably.
The first objective regarding nitrogen fertilizer source was easily answered. There was no difference observed in yield as a result of the N fertilizer source used in April. However, there were big yield differences noted in the timing of application. For example, Figure 1 shows that about 1,000 pounds more bermudagrass forage was produced when the 150 pounds of N was split applied than when it was all applied in April using 34-0-0 as the N source.
Another important item to note from this work is that ammonium nitrate produced 518 pounds more bermudagrass per acre than 46-0-0 when split applied as noted in Figure 1. The reasoning for this yield difference is due to the fact that nitrogen can be lost as ammonia gas (urea volatilization) when urea fertilizer is surface applied in June. The environmental conditions are perfect for urea volatilization in June. The soil and air temperatures are high, heavy morning dews are likely and the likelihood of rainfall decreases. There is no threat of nitrogen loss with ammonium nitrate fertilizer.
As a result of this study, we usually recommend applying no more than 100 pounds of actual N per acre in May. If more forage is needed, then apply an additional 50 pounds N per acre after the first cutting of hay or any time through the growing season when adequate soil moisture is available to warrant a forage response. Following this recommendation will not only improve nitrogen use efficiency, but will improve forage quality as well.
If you plan on splitting your nitrogen fertilizer into multiple applications through the growing season, use ammonium nitrate on pasture after May 15. Urea fertilizer sources can be used, but a minimum of one-quarter inch of rainfall is required within three to five days of application to negate the risk of nitrogen loss. Prior to May 15, any nitrogen source can be used since rainfall is more likely. Typically, urea is cheaper than ammonium nitrate per unit nitrogen (not per ton fertilizer) and is the best buy for late April to early May application.
We realize nitrogen fertilizer prices have increased this year, but nitrogen may still be required by bermudagrass to meet your desired yield goal. This information will assist you in choosing the right nitrogen fertilizer source for the right time of year.