Wed 24 Oct 2018 11:34PM

Adding urease inhibitors to Overseer

AR Ants Roberts Public Seen by 194

Ants Roberts Thu 25 Oct 2018 12:21AM

Having read the proposal here are my comments:

  1. Average effectiveness: While filed trials show quite variable effectiveness and internal review we did on published results from England, Ireland and NZ gave an average of 0.5, so happy to accept 0.45 as the figure to use here.

  2. Site specific: I agree that site specific soil characteristics are likely to have a small influence on UI performance, but that this should be ignored as proposed. However, I read no information about how the key drivers (rate, timing relative to rain/irrigation, plant cover etc.)r of volatilisation losses will be used to first calculate likely ammonia loss at each single N application over a year and then factor in the reduction if a UI is used. Given many people do not specify the timing of N applications in OS files (other than maybe a monthly amount) I will be interested to see how this will be implemented.

  3. Modelling: I agree that assuming N saved will equate to extra pasture grown is the way to model this. It will be relatively small.

  4. Other factors: I agree that including urine patch effects is too difficult at this stage particularly as OS has no information on grazing rotations etc. I agree that incorporating urea will reduce volatilisation to zero (See Bishop and Manning, FLRC paper 2011).

  5. Table 1: I agree that the UI use should be input as the Product category by brand name provided that the product is Fertmark registered with the amount of active ingredient stated. As discussed there are likely to be more products with UI, including \double inhibitors, and so future proofing to allow new entrants should be built in.


David Wheeler Tue 30 Oct 2018 7:57AM

Thanks for the comments.
I will look a Bishop and Manning to include as a reference in the document. The review you referred to would also provide additional justification for the method.

The method for calculating ammonia volatilisation is covered in the Characteristics of Fertiliser chapter of the Technical manual, section 3.1.4, available from:
The method is not included in the urease inhibitor description as it is a separate sub-model. Fertiliser inputs are on a monthly basis. Hence for each application of N fertiliser each month, volatilisation is estimated, and then reduced if that application included urease inhibitor.

The urine patch effects could be included without known the grazing rotations if the urease inhibitors are applied at each grazing, or followed a rotation as applied in the DCD model. The main concerns where that there didn't appear to be an accepted model on its effectiveness as for fertiliser, and that the technology to apply urease inhibitors is not in widespread use. There are systems under development, and the effect of urease inhibitors can be considered once these systems are in commercial use.


Richard Christie Mon 29 Oct 2018 8:27AM

One factor that may be also worth defining is what product the urease inhibitor is applied to and what it is mixed with. Due to the sensitivity of NBPT, it can very easily be deactivated by mixing with inappropriate fertiliser products.


David Wheeler Tue 30 Oct 2018 8:04AM

Good points. This is a case where Overseer may need to assume best practice, that is something like the product is stored in a condition such that the effectiveness of nBPT is not reduced, and that nBPT is not mixed with other products that reduce its effectiveness. Would this be an appropriate action?


Ian Power Tue 30 Oct 2018 10:50PM

Sounds appropriate to me