Deciding between Ammonium Nitrate (AN) and Urea is a crucial on-farm decision for livestock farmers so it is important to understand how the risks and benefits of each stack up.
Ultimately, both supply Nitrogen to the crop in the form of nitrate; however, the Nitrogen in urea must undergo more chemical transitions than AN to convert it into a crop ready form. As a result, it is far more dependent on favourable environmental conditions to perform well and more can be lost to the environment.
The Nitrogen in urea is first converted to ammonium and then onto nitrate whereas, the nitrate in AN is already crop available and the ammonium only undergoes one conversion.
Dr George Fisher, Grassland Consultant to CF Fertilisers UK explains why the two types of Nitrogen perform differently: “Success with urea is dependent on the rainfall and temperature in the three days after application, with dry and mild conditions increasing the risk of N loss to the air as ammonia which can reduce grass yields. On the other hand, ammonium nitrate is far less dependent on conditions.”
This year, CF Fertilisers funded trial work at Reaseheath College into Nitrogen applications for grazing and silage. The results show that the small increase in initial cost for AN in comparison to urea is more than made up for in the output of grass which equates to large amounts of energy for potential milk production, or savings in the cost of bought-in feed forage.
Trials looked at Nitram® (AN), urea and inhibited urea for first and second cut silage as well as grazing, which is simulated by taking small, regular cuts of grass from trial plots. Inhibited urea is given a coating that is designed to protect the conversion of urea to ammonium and then nitrate, reducing N losses as ammonia to the atmosphere. However, in these trials, AN outperformed both types of urea and the benefit of using inhibited urea was marginal.
“Over two cuts of silage, the AN gave 12 tonnes of dry matter per hectare, while urea and inhibited urea gave 9 and 9.5 tonnes respectively,” says Dr Fisher. “On the grazing plots, the differences weren’t as dramatic but there was still a clear benefit from using Nitram, dry matter yield was 6.4 tonnes per hectare over the first three grazings, both types of urea provided 6 tonnes.
When combined together, the results show AN providing at least 15% more dry matter than urea in a silage / grazing system. To understand what this means in practice, it is worth scaling up the results to see what it would mean for a typical 200 cow herd.
Over an 80 ha (200 acres) grassland block, from spring growth to second cut silage in July the total benefit from using AN compared to urea was 922,700 MJ ME, which is the equivalent of 427 tonnes of silage (fresh weight).
“The cost to replace the lost grass yield is far greater than any saving in fertiliser cost. In the 80ha example, the saving from using urea would be around £2,000. However, buying in silage to replace the yield loss would cost £8500. Or to look at it another way, the lost grass energy from using urea would have produced 170,000 litres of milk.
“So using urea and inhibited urea products can save money on your fertiliser bill, but end up costing your system a lot,” concludes Dr Fisher.