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Which type of Nitrogen delivers most for grass?


If your local merchant offered “buy eight tonnes, get one tonne free”, how would you respond? I suspect most farmers would go for it, especially given the current high fertiliser prices. Yet this is exactly the benefit that protected urea offers in comparison to straight urea. Read more here

Article Authors, Tom O’Dwyer, Head of Teagasc Signpost Programme and Patrick Forrestal, Teagasc Research Officer - Soil Science Agronomy, point out the cost benefits of using protected urea as increased N fertiliser prices are likely to drive farmer interest in which type of Nitrogen delivers most for grass.

Previous research work by Teagasc has measured ammonia-N and nitrous oxide-N losses from applied fertiliser N. The percentage of N lost varies depending on type of N fertiliser used (CAN, urea, and protected urea).  Given the current high price for fertiliser nitrogen (N), it is reasonable to expect that farmers will place more focus on using N products that can provide more effective N for plant uptake for a given quantity of fertiliser N applied.

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.  Both protected urea and CAN havelower rates of N loss (79% reduction for protected urea = 3.3% loss; 85% reduction for CAN = 2.3% loss).  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:

  1. ammonia N emissions compared with straight urea, and
  2. nitrous oxide N emissions compared to CAN.

Table 1: Percentage N lost from three N fertiliser products through two loss pathways

 

So, what does this all mean? 

Let’s look at an example application of 40 kg/ha fertiliser N of each of the three fertiliser types.  Table 2 shows that the effective N net of ammonia and nitrous oxide N loss varies from 33.7 kg/ha (urea) to 38.5 kg/ha (protected urea). By using protected urea (rather than urea), you are increasing the amount of effective N by 4.8 kg/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 35 kg/ha) and still apply the same amount of effective N as would have been applied using 40 kg 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.

Table 2: Effective N for three fertiliser N formats

Johnstown Castle Grass Growth experiments

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 cut plots, significantly lower N use efficiency was detected where standard urea was used.  While in 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).  David Wall, Teagasc has also indicated that there is sufficient N within the system to grow the grass even with the losses occurring from either urea (straight) or CAN.

Finally, while fertiliser N prices are very high at present, and it may be risky to use current prices for comparison, the following example is informative.

EXAMPLE:

  • Costs assumed: urea @ €950/t, protected urea @ €1,000/t, CAN @ €750/t
  • In both comparisons, a similar level of fertiliser application rate is applied to achieve the same rate of effective N.

1.  Urea v Protected Urea

40 kg/ha N as urea (34 kg/ha effective N) @ €950/t  =  €82.60/ha

35 kg/ha N as protected urea (34 kg/ha effective N) @ €1,000/t  =  €76.10/ha

At these prices, the farmer will get better value for money by using protected urea (as opposed to urea).  The example above, using current fertiliser prices, shows that the extra cost of the urease inhibitor more than covers its cost if it saves 5 kg of N (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. 

2. Protected Urea v CAN 

40 kg/ha N as protected urea (38.5 kg/ha effective N) @ €1,000/t  =  €86.95/ha

40 kg/ha N as CAN (38.5 kg/ha effective N) @ €750/t  =  €111.11/ha 

At these prices, the farmer will get better value for money by using protected urea (as opposed to CAN)

For more information on protected urea see the Teagasc Protected Urea page. This page contains information on protected urea fertilisers and their efficient use.  Protected urea is the number one technology to reduce greehouse gas emissions on grassland farms and help achieve ammonia reduction targets. 

Contact your Teagasc advisor for fertiliser advice. Find your local Teagasc office here