Farms.com Home   News

Topdressing Winter Grains With Nitrogen

By Douglas Beegle
 
Considerations for timing and materials.
 
Timing nitrogen applications as close to the time of crop uptake as practical is one of the best practices for improving N use efficiency by crops. Right now we need to be thinking about topdressing wheat. In many parts of the state we have had constant snow cover and now that there has been some melting, the soils are very wet making topdressing challenging at best. This is especially a concern if there was poor tillering in the fall, because in these situations, topdressing early, at green-up, is recommended. If you had good growth and tillering going into the winter delaying N until GS 5, just before stem elongation, is most efficient. Topdressing very early before green-up puts nitrogen out on the field before the crop can use it. With cold temperatures there probably won’t be much loss from volatilization of urea before the next rain. Ammonium forms of N are stable and won’t be lost from conversion to leachable nitrate under cold soil conditions.  The main risk of loss is under conditions where the fertilizer, in any form, is washed off the surface before it gets into the soil. Generally at our level of winter grain production we do not usually see a significant practical advantage to splitting spring N applications further.
 
The main sources for top dressing winter grains are UAN (30-0-0), Urea (46-0-0), and Ammonium Sulfate (21-0-0-24S). All are good sources of N for wheat. UAN and urea are both volatile forms, which should be a consideration with these sources, because of the potential for significant loss of the N. The potential loss from UAN is only half that of straight urea because UAN is only ½ urea. Losses are minimized if the urea or UAN can be applied just before around ½ inch of soaking rain. Also, the losses tend to be less at cold temperatures but even a brief warm up at the time or application can result in greater losses. A urease inhibitor can effectively reduce volatilization losses from surface applied urea or UAN when there are the conditions for volatilization loss. Not all N additives are urease inhibitors. Check the label carefully to make sure the additive contains an effective urease inhibitor. Ammonium sulfate is non-volatile and therefore there is no benefit to adding a urease inhibitor with ammonium sulfate.
 
One other common question is sulfur on wheat. Sulfur (S) deficiencies are still relatively rare in Pennsylvania but are becoming more and more common. Ammonium sulfate contains 24% S, therefore applying some of the wheat N requirement in this form will also supply S. Generally, 100 lb/A of ammonium sulfate in the wheat N program, which will supply 24 lb S/A, will supply adequate S for wheat in most situations. Ammonium thiosulfate (ATS) is often used as an S source with UAN. Three to five gallons of ATS will supply 10 to 15 lb S/A.
 
There have also been questions about injury to wheat top-dressed with UAN containing ATS. While ATS can cause crop injury, this is usually only when the ATS is applied with starter fertilizer close to the seed. I can find little indication that there is a problem with ATS applied with topdress nitrogen (N) on wheat. There has not been any research done in Pennsylvania on this. However, in looking at work done in other states and recommendations from many wheat producing areas there does not appear to be a problem directly related to ATS injury with topdressing. For example, in one study in Montana, ATS was applied straight with no dilution or other fertilizer and there was no injury. Remember that UAN itself can cause leaf burning which can be significant at higher rates, such as in overlaps. Usually, this burning is not serious and does not result in a yield reduction. However, if this injury occurs along with other stresses it could impact yield.
 

Trending Video

Weather with Eric Hunt

Video: Weather with Eric Hunt

We've seen several inches of snowfall and an arctic blast across Nebraska last week. Will we get any break from these frigid temperatures? Market Journal's Eric Hunt has the answers.