How much nitrogen to spread this summer?

If the first four months of the year are anything to go by, 2025 is shaping up to be a more ‘normal’ grass-growing season than in recent years. This time last year, farms were starved of grass and there was silage being fed on most dairy farms, as there had been all spring.

If anything, 2025 has been abnormal in the sense that it was very dry throughout February, March and early April, with more normal April weather returning over the last 10 days. The priorities over the next four or five months must be to maximise the amount of grass grown on farms and converting that grass to milk solids or silage.

Fertiliser

At this stage, most farmers will have in or around 60 units of N/acre (75kg N/ha) spread to date. That will obviously vary from farm to farm, depending on how much of the second rotation paddocks have been spread with nitrogen post-grazing. At an overall chemical nitrogen allowance of 212kg N/ha, that means there is 137kg N/ha left to be spread on the grazing platform between now and early September.

Converted to units/acre, that means there are 110 units/acre available to be spread over the next five months, which is 0.73 units/acre/day. The old rule of thumb was to apply one unit/acre of nitrogen per day during the main grazing season, but that was before there were cuts to allowances and it was before there was the same focus on water quality as there is today.

However, nitrogen grows grass and grass is the best feed for cows, so reducing N rates means less grass will be grown. Where there is good soil fertility and good grass varieties, spreading just 0.73 units/acre/day, which is 15 units/acre after each rotation, will in my view limit growth rates, particularly in May and June.

I believe we have entered into a new phase of grassland management in Ireland, whereby farmers with clover will be growing a lot more grass than those without. Rather than spreading all paddocks on the farm at the same rate, farms with clover can reduce the rate of chemical nitrogen on fields with clover and increase the rate on fields without clover.

At this stage everyone knows that 2024 was a bad year for clover, as it really only got going towards the end of the grazing season before it could fix much nitrogen. Some farmers have made the decision to abandon clover as it’s too unreliable.

That may be understandable, but those that ignore clover are effectively making the decision that they are forfeiting extra grass growth and extra milk solids production. The extra grass will come from the ability to apply extra nitrogen on the parts of the farm that don’t have clover present. How much extra nitrogen will be available for these areas will depend on how much of the farm has clover and at what level.

Clover science

According to Teagasc clover researcher Mike Egan, when and by how much to reduce chemical nitrogen on clover swards all depends on the clover content. He says that even though a field could have a high clover content of 20% in April/May, it is really not until June that the clover starts to fix nitrogen at a rate allowing for a reduction in chemical N.

In June, July and August a field with a high clover content can supply over 20kg N/ha per month. However, Mike says that if these high clover fields continue to receive chemical nitrogen, the clover won’t fix biological nitrogen. In other words, clover will only fix nitrogen if it needs to and will get lazy if it’s getting a ready supply of nitrogen in the form of bag fertiliser. Mike says that research has shown that white clover can fix up to 100kg N/ha per year.

On-farm experience

Last week, the Teagasc Clover 150 group met at Pat Walshe’s farm in Durrow, Co Laois. Over the last four years the Walshe farm has averaged a growth rate of over 16.2t DM/ha, but that was back to just over 14t DM/ha in 2024.

A recent clover score by Teagasc has shown that approximately two thirds of the 70ha milking platform has clover present. Half of this clover area, equal to one third of the entire milking block has medium clover contents in the 10% to 20% range, while the other half of the clover area has clover contents in the targeted 18% to 22% range.

The one third of the farm with less than 10% clover today is considered to be non-clover, and some of this area is the further away silage ground and ground that is out for reseeding. Pat’s plan is to manage the clover area for reduced chemical nitrogen, which will then allow him to spread more chemical nitrogen on the parts of the farm that are considered to have no clover, while still operating within the allowances set by the Department of Agriculture.

In line with the advice from Teagasc’s Mike Egan, Pat won’t reduce fertiliser on any of the clover paddocks until late May or early June, and those paddocks that are at close to 20% clover now will see a greater reduction in nitrogen compared to those at between 10% and 20% clover content today.

The high-clover paddocks will average about 112 units/acre (140kg N/ha) for the year. Only eight units/acre (10kg N/ha) will be spread for the entire months of June and August, while no chemical nitrogen will be spread on these fields during July – but Pat will target these fields with soiled water instead.

For the medium-clover paddocks that are not at 20% clover today, Pat will spread 12 units/acre (15kg N/ha) for the entire months of June and August, and go with eight units/acre (10kg N/ha) in July. Both the high- and the medium-clover paddocks will get 16 units/acre (20kg N/ha) in September. As a result, the medium-clover paddocks will end up receiving around 130 units/acre of nitrogen (160kg N/ha). Given that two thirds of the farm is to get around 150kg N/ha on average, that leaves Pat with a bigger nitrogen allowance for the part of the farm that have no clover, meaning he can spread these areas with up to 240kg N/ha.

By tweaking the rates on a paddock by paddock basis and by letting the clover do the work where it is present, Pat will increase the overall grass growth from the farm even though the average N use across the farm will be in the 180-185kg N/ha range. Pat is farming with his wife Chloe and mother Ella, along with full- and part-time help. The 220-cow herd is currently doing 2.3kg MS/cow from 3kg of meal and at the day of the walk last week Pat was about five days off starting the third rotation with a cover of 1,100kg in the first paddock of the third round. The plan is to drop back to 2kg of meal in the next few days.