Trial work on protected urea at Teagasc sites in Moorepark and Clonakilty has shown reasonably good results. Protected urea is ordinary urea with an added urease inhibitor. The inhibitor slows down the conversion of urea to ammonia and thus reduces ammonia losses.

Up to recently, there were just two forms of nitrogen available to Irish farmers – CAN and urea. The introduction of protected urea has added a third element.

There is also an increase in the availability of other nitrogen forms, such as liquid nitrogen and enhanced granular nitrogen. However, protected urea is the only form of nitrogen that Teagasc has been promoting for the past few years. The existing research says it has much lower greenhouse gas emissions than CAN and much lower ammonia emissions than urea.

Initial trial work on the effects of protected urea on grass growth were criticised because they weren’t conducted under a grazing scenario. For the past two years, a grazing study has been under way at Teagasc research centres in Moorepark and Clonakilty.

Experiment

The experiment involves applying the three different types of fertilisers (CAN, protected urea and urea) at 200kg N/ha to plots and grazing these when pre-grazing yield reaches around 1,500kg DM/ha. There is also a side experiment looking at two different rates of protected urea, 250kg N/ha and 150kg N/ha.

At this stage, data has been collected on the two sites for the past two years. In 2020, grass growth was highest at Clonakilty for all three treatments. CAN and protected urea had similar yields in Clonakilty at 15.052t DM/ha and 15.009 tDM/ha respectively, while ordinary urea grew the least at 14.742t DM/ha. The 250kg N/ha plots grew 15.872t DM/ha, while the 150kg N/ha plots grew 13.997t DM/ha. Total rainfall in Clonakilty to mid-October was 690mm.

In Moorepark, ordinary urea produced the most grass at 13.052t DM/ha, followed by protected urea at 12.863t DM/ha and CAN at 12.689t DM/ha. The 250kg N/ha plots grew 13.838t DM/ha, while the 150kg N/ha plots grew 11.899t DM/ha. Total rainfall in Moorepark up to mid-October was 724mm.

The combined data for the two sites for 2019 and 2020 is presented in Table 1.

NBPT refers to the type of protected urea used. This shows that CAN grew the most grass at 14.638t DM/ha, followed by protected urea at 14.373t DM/ha and urea at 14.283t DM/ha.

The number of grazings across all plots for each year was nine and the average pre-grazing yield for the two urea-based treatments was almost identical at 1,519kg DM/ha but it was 30kgDM/ha higher for the CAN treatment at 1,550kgDM/ha.

Differences

There were small and not significant differences in pre-grazing yields recorded across the different treatments, see Figure 1. The largest difference between the treatments was observed in rotations seven and eight, corresponding to September/early October, when the pre-grazing yield of CAN and urea was approximately 150kg DM/ha higher than protected urea in both instances.

This is unexpected, as untreated urea performed as good as CAN during this time. There was no major difference in pre-grazing yield at the ninth and final rotation.

The pre-grazing yield of ordinary urea was lowest in the fourth, fifth and sixth rotations, but was highest in the second rotation. CAN performed well throughout, recording a pre-grazing yield only marginally lower than the other two treatments once, and that was at the second rotation. The study will be repeated again next year.

What does this mean for farmers?

These results show that protected urea is comparable to CAN when it comes to cumulative grass growth under a grazing scenario. The difference in growth recorded between CAN and protected urea across the two sites over the two years is 0.265t DM or 1.8% less grass with protected urea.

While there is a numerical difference, this is not significantly different from a statistical perspective. In other words, on the law of averages, there will be no difference in yield if using any of the three products.

Moving away from CAN-based fertilisers will reduce the amount of greenhouse gas emissions credited to agriculture

Cost-wise, protected urea is usually between 10% and 15% cheaper than CAN on a kilo-of-nitrogen basis, with similar price differentials between urea and protected urea.

However, it’s not just economics or agronomy that will drive farmers to use protected urea. Moving away from CAN-based fertilisers will reduce the amount of greenhouse gas emissions credited to agriculture. This is something that’s very easy to track and record at a national level. Protected urea has the single biggest impact on reducing greenhouse gas emissions on the Teagasc marginal abatement cost curve (MACC).

The issue with CAN is that it is a major contributor to nitrous oxide emissions from agriculture. Nitrous oxide is a potent greenhouse gas and it is the second largest contributor to agricultural greenhouse gas emissions at 30% of total emissions. An invisible gas, it is released when CAN fertiliser is spread on the land.

Issues with protected urea

While these research results shed a positive light in terms of the growth response to protected urea, some issues remain. Firstly, the scope of the research to date has been limited, focusing on just two sites in Co Cork. You would like to see this research carried out on a wider range of soil types and climates, all the while with grazing animals.

Farmers are being told that they should be using protected urea, but very little research has been done on different soil types. The second issue concerns using protected urea along with other nutrients.

The only other nutrients that protected urea can be used with are potash and sulphur. Phosphorus cannot be included in a compound containing protected urea.

The shelf life of protected urea is another issue. Teagasc advice is to use protected urea within six to 12 months of the date the urease inhibitor was added, although BASF claim it’s Limus treatment has a shelf life longer than 12 months. However, Teagasc also says that how long the inhibitor lasts depends on whether or not other nutrients are included.

On some bags, the date of bagging is clearly identifiable

Protected urea with potash is said to be stable, but protected urea with added sulphur can be unstable, depending on how dusty the ammonium sulphate is.

Alongside this, there is inconsistencies between different manufacturers of protected urea regarding labelling. On some bags, the date of bagging is clearly identifiable when you know where to look, but on others it is coded information.

Teagasc has found no traces of urease inhibitor in milk from cows in Johnstown Castle, where protected urea has been applied since 2016

The quantity of inhibitor applied is also missing off the label, leading to claims that some manufacturers are not adding the same amount of inhibitor as was used in the trials. For farmers to have confidence in protected urea, they will need to know that the product they have is ‘in date’ and has the required amount of inhibitor to be effective.

In relation to residues, Teagasc has found no traces of urease inhibitor in milk from cows in Johnstown Castle, where protected urea has been applied since 2016.

Work is continuing in Johnstown Castle to assess the impact of applying different fertilisers on soil microbes and results should be available by spring 2021.