Considerations For Planting Small Grains In The Narrowing Planting Window

Considerations For Planting Small Grains In The Narrowing Planting Window

Rain keeps pushing the plantings season back even further and make what was a narrow planting window even tighter.  Coupled with the narrow time frame to get a crop planted is a concern that nitrogen fertilizer sources, especially anhydrous ammonia, will be in short supply. Since much of the Great Plains will likely be applying nitrogen simultaneously, keeping pace with the demand will be a logistical challenge.   This situation may cause growers to consider a scenario where they fill their drill tanks with seed and starter fertilizer in an attempt to cover as many acres as possible and come back at a later date to apply nitrogen fertilizer.  This approach can certainly increase efficiency in terms of getting the crop planted at an earlier date but there are some concerns regarding the risks of post-applied nitrogen. 

Perhaps the first risk to consider is timing.  Regardless of nitrogen form or application method it is important to get the nitrogen applied early in the season during the vegetative growth stages of a small grain crop.  It is generally accepted that nitrogen should be available to the wheat plant prior to jointing, or six leaf stage, to have an appreciable effect on yield.  Jointing typically occurs roughly one month after planting but there must be some time allowed to move the nitrogen fertilizer into the crop’s root zone so it can be utilized by the plant.  So, the realistic time for applying nitrogen post planting may actually be around 20-25 days depending on how cooperative the rain events are. Basically, the earlier the nitrogen is made available to the plant the more yield components will be affected. For example, wheat begins to initiate tillers at the three leaf stage, or roughly 20 days after planting. If nitrogen is deficient at this time the plant may not develop as many tillers as if there was a good supply of nitrogen available.   

Other risks associated with post applied nitrogen revolve around the form or nitrogen used, the application method, or both.  Let’s first look at applying urea in a post plant application.  

Spreading urea always comes with some risk as it takes a good rain (roughly ½”) to thoroughly dissolve the urea pellets and move them into the ground.  If an adequate rain is not received the urea pellets can begin to partially dissolve due to surface moisture, mist, or even high humidity and nitrogen losses can occur through volatilization to the atmosphere.  The amount and speed of nitrogen loss can vary due to environmental conditions, soil pH, and soil residue.  Acid soils (pH 7.0 or less) will typically exhibit slower nitrogen losses than basic soils (pH over 7.0).  On a bare, acid soil, substantial nitrogen losses due to volatilization typically don’t begin occurring until seven days after urea application, but losses as high as 30% have been observed two – three weeks after application.   When urea is spread on an acid soil that contains a lot of residue, as in the case of no-till, nitrogen losses can begin occurring more quickly and losses have been seen as soon as 3-4 days after application.  Soils with a pH greater than 7.0 which contain little residue will behave similar to no-till acid soils.  One way to help minimize the risk of nitrogen losses with spread urea is to treat the urea with a urease inhibitor such as Agrotain®. Agrotain® has shown to help provide protection against nitrogen losses for 7-10 days.  Another method to virtually ensure no volatility losses would be to place the urea beneath the soil surface.  To accomplish this in an established crop will most likely require the use of a low-disturbance disk opener either operated at an angle to the planted rows or in a mid-row application utilizing RTK GPS positioning.  Care should be taken to monitor the amount of mechanical damage to the crop so as to not reduce the stand.   

Another option for post applied nitrogen may be liquid 28% urea ammonium nitrate (UAN).  UAN contains about fifty percent ammonium nitrate and fifty percent urea.  Since ammonium nitrate will not volatilize, the risks of nitrogen losses associated with spread urea can be roughly cut in half.  Also, a banded application of UAN through the use of streamer bars has also been shown to slow the rate of volatilization of the urea portion of UAN.  The use of streamer bars with UAN can also help to reduce the amount of leaf burn as most of the product will be driven to the soil surface rather than on the plant leaves.  It is advisable to avoid windy days when applying UAN as the wind can shear the solution stream, causing it to be deposited on the plant leaves and increasing the amount of leaf burn.  Do not apply early season, in-crop UAN with broadcast nozzles as yields will most likely be reduced due to leaf burn.  Another consideration with UAN is the infrastructure required to handle and apply the product. A set of streamer bars is easy enough to come by but one should consider the volume of product that will be applied. UAN contains roughly 3 pounds of nitrogen per pound of product so an application of 33 gallons of product per acre will be required to supply 100 pounds of nitrogen per acre.  Surface applied UAN will also require a good (1/2”) rain event to move the nitrogen into the soil where plant roots can utilize it. Growers may also want to consider the