By Dr Doug Edmeades

Dairy Exporter published a story about coated urea (August 2022) in which it was claimed using this type of product could reduce the volatilisation of nitrogen from urea by 50%. The associated editorial asks – should all farmers switch to these types of products?

Most farmers reading this would, I think, take the message that these products are very beneficial and economic. I express great caution.

To be clear; what we are talking about is urea treated with the chemical agrotain. Both fertiliser co-ops sell this product as either SustaiN (Ballance) or N Protect (Ravensdown). Agrotain slows the conversion of urea-N to ammonium-N and thus through to nitrate-N. Both companies claim adding agrotain to urea reduces the loss of N, via volatilisation as ammonia gas, by 50%, thus increasing N use efficiency (NUE).

Ammonia is not a green-house-gas and thus the only benefit a farmer can derive from switching from straight urea to SustaiN or N Protect is via an increase in NUE. In other words more pasture or crop growth per unit of N applied.

In a paper to the New Zealand Grasslands Association in 2011, a colleague and I reviewed all of the available trials in NZ comparing the effect of urea and agrotain-treated urea on pasture production (n = 16 trials).

The average response to agrotain-treated urea, relative to straight urea, was 4% (confidence interval 7%). We have subsequently updated this estimate by including trials from all around the world on both pastures and crops (n= 348). The average response was 3%.

Of particular relevance to NZ pastures, the results were rate dependent; at low rates of < 50 kg N/ha (108 kg urea/ha) the average response was 1.4% increasing to 7.5% where the rate of application was > 200 kg N/ha (434 kg urea/ha).

If, as both co-ops claim, adding agrotain to urea reduces ammonium volatilisation by 50%, then the only realistic conclusion, given the pasture and crop yield data above, is that the absolute amount of N volatilised is very small, in the range of 2-3kg N/ha, irrelevant in the overall scheme of things – 50% of a small number is a smaller number?