As nitrogen prices hit record highs farmers will no doubt be focusing on using nitrogen (N) products that will be most efficient.
Research by Teagasc measuring ammonia and nitrous oxide losses from applied fertiliser N has shown that the percentage of N lost varies depending on the type of N fertiliser used – CAN, urea, or protected urea.
Table 1 summarises the N lost from the three N fertiliser products as ammonia and nitrous oxide nitrogen gases. The EPA estimates that ammonia loss from urea is 15.5% on average, with both protected urea and CAN having lower rates of N loss.
The use of protected urea can result in a 79% reduction when compared with standard urea and results in ammonia losses of 3.3%.
When CAN is used instead of standard urea it results in an 85% reduction in ammonia losses to 2.3%. Published research has quantified direct N loss as nitrous oxide from urea (0.25%), protected urea (0.4%) and CAN (1.49%). In summary, protected urea curtails N losses by reducing ammonia N emissions compared with straight urea and nitrous oxide N emissions compared to CAN.
So, what does this mean for artificial fertiliser rates?
Let’s look at an example application of 40kg/ha fertiliser N of each of the three fertiliser types. Table 2 shows that the N available after ammonia and nitrous oxide loss varies from 33.7kg/ha with urea to 38.5kg/ha with protected urea. By using protected urea (rather than urea), you are increasing the amount of effective N by 4.8kg/ha. The effective N figures for protected urea and CAN are similar.
One advantage of this is that the farmer can reduce the amount of fertiliser N applied as protected urea to 35kg/ha and still apply the same amount of effective N as would have been applied using 40kg N/ha as standard urea.
By this calculation, based on average gaseous losses and without considering leaching, the protected urea N rate could be reduced by 12.5% while still delivering the same amount of “effective N” as standard urea.
Finally, a question that is asked is how come the grass growth experiments are not showing up a reduction in grass growth on the urea treatments (in comparison to the protected urea and CAN treatments) if the same amount of fertiliser N is applied across all treatments.
In experiments where grass is cut, significantly lower N use efficiency was detected where standard urea was used.
While in a single year trial yields were indeed similar, over multiple years of repeated application in the Teagasc long-term protected urea plots at Johnstown Castle, a trend of reduced yield with standard urea use is emerging in addition to lower NUE. This is due to the lower level of effective N delivered (as described above). Researcher David Wall has also indicated that there is sufficient N within the system to grow the grass even with the losses occurring from either standard urea or CAN.
This fertiliser N adjustment is a key principle as we go forward
Finally, Tables 3 and 4 show the differences in effective N rates and costs of rates between, urea, protected urea and CAN using assumed costs for the season ahead. Prices used are for explanatory purposes.
At those prices, Table 3 shows that the farmer will get better value for money by using protected urea as opposed to urea. The table shows that using recent fertiliser prices as an example the extra cost of the urease inhibitor more than covers its cost if it saves 5kg of N/ha (protected urea v urea). The value of retaining N that had previously been lost as ammonia has increased dramatically in line with the increased fertiliser cost.
Table 4 shows that the farmer will get better value for money by using protected urea as opposed to CAN.
According to Patrick Forrestal: “Where farmers adopt ammonia reduction technologies such as protected urea in place of urea or LESS slurry spreading in place of splash plate they can and they should reduce their fertiliser N rate to avail of a cost saving.
“Making the fertiliser N reduction accounting for the ammonia saving is also key to realising the full environmental benefit of the new practice.”
This fertiliser N adjustment is a key principle as we go forward with ammonia saving measures over the next decade and beyond.