A three-year study of weather patterns and fertiliser applications at Cheshire’s Reaseheath College reveals some significant differences in performance.
Analysis of weather from 2015 - 2017 shows urea would only match the performance of Ammonium Nitrate (AN) in around 20 per cent of grassland application scenarios.
Trials carried out at Reaseheath College, Cheshire show that AN produced, on average, 15 per cent more grass over first and second cut silage compared to urea for the same three-year period.
CF Fertilisers’ North Regional Manager Mark Garrett says: “To get the most economic response to Nitrogen you need to apply it when the crop needs it, not when the weather forecast is favourable to urea.
“With urea you need significant rainfall within three days of application to wash the fertiliser into the soil where the microbial urease can convert it into nitrate. Without that, losses to air through volatilisation of ammonia make it risky and uneconomic.”
Analysis of weather patterns for the last three years at Reaseheath shows that in March on average there were seven days where urea would achieve a similar level of Nitrogen utilisation efficiency as Ammonium Nitrate.
On some days, the predicted potential yield loss from urea compared to AN was 18 per cent with the average over the days being around an 8 per cent deficit.
For after cut fertiliser application, none of the first 15 days of May were suitable for optimum urea utilisation with the resulting average potential loss in grass yield being over 10 per cent and up to 20 per cent in four of the 15 days, compared to AN.
In the silage trials in 2017 210kg N/ha was applied as AN (34.5 per cent N) at 120kg N for first cut and 90kg N for second cut, and the same quantities were applied, at the same time, as urea (46 per cent N).
On AN plots an average of 10.2t DM/ha was produced for first and second cut whilst on the urea plots this was 8.9t DM/ha.
Mr Garrett says: “The three years represent a range of growing conditions from very dry in 2017 to the relatively cold and wet spring in 2015 and the mild and wet spring of 2016, so the AN advantage is not tied to one set of conditions.
“The performance of urea was very variable against the steady consistent performance of AN.”
In the grazing work, 50kg N/ha was applied for each of the first and second grazing rounds and 40kg N/ha for the third and fourth rounds. These plots were in the same field as the silage trial and the same amount of Nitrogen was put on either as AN or urea.
The three rotation yield average for the AN treated grass was 6.4 tonne DM/ha whilst for the urea it was 5.9 t/ha, a benefit of plus 8 per cent in favour of AN.
Mark Garrett says: “Farmers think they are saving money purchasing urea as per unit of Nitrogen it often appears cheaper, although the gap has closed in recent years.
“If you look at the Reaseheath trials, initial savings of £1,000 on a 80ha system were possible when using urea in place of AN, but the potential loss in production in grass yield was around £11,500 in replacement concentrate energy.”
A nationwide weather analysis of over 15 sites from Cornwall to Glasgow for the last five years reveals a similar picture to the Reasheath results, says independent grassland consultant Dr. George Fisher.
“At 0oC urea needs at least 5mm of rainfall in the three days after application to work effectively. Analysis shows the chance of getting this is just 1 in 3, whilst at 5oC, 7mm rainfall is needed and the likelihood of this is 1 in 4.
“At 10oC urea needs at least 10mm rainfall in the three days after application to work and there is only a 15 per cent chance of this occurring.
“Looking at spring weather on a national basis in the grassland west over the last five years, there is generally only a 30 per cent chance throughout the west of the UK of getting an economic benefit from using urea over AN. This adds unnecessary risk to a grass-based system.”