Why it pays to make most of February nitrogen window

File photo
Joe Kelleher

Joe Kelleher

There is an abundance of grass on most farms this spring due to the mild winter which led to almost a doubling of grass growth rates over the closed period. As a result many may be contemplating skipping an early fertiliser application. But is this wise?

Many farmers regretted having missed the window of settled weather in mid to late January to get some early nitrogen (N) out.

But it now appears that another opportunity has presented itself to get nitrogen out. Does it pay to put out N this time of year?

There have been numerous trials carried out across Ireland over the past 35 years to examine this and they have all come up with a similar conclusion - the best response to early N is achieved by spreading in January.

The next best time is February. This is on the assumption that the grass is going to be eaten by the cow by mid-April, therefore, the longer length of time the N has to work (before the cow eats the plant), the greater the response will be.

For early N to pay for itself, it has to return a yield greater than 5kgs/ha.

The trials which were conducted on numerous sites across the country demonstrated that fertiliser spread in February on average yielded a response of approximately 10kgs of grass dry matter grown for every 1kg of N applied, based on an application rate of 30kgs/ha (24 units/acre). This clearly demonstrates that early N pays for itself. Another interesting point is that statistically, February is a drier month than March, so there should be a reduced risk of N being leached in February than by waiting to spread in March.

Should every farmer across the country put on the fertiliser spreader this week and start spreading?

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The responses above are more likely to be achieved on swards with high ryegrass content, so these fields should be targeted.

Colder soils are slower to respond, and a rule of thumb of soil temperatures reaching 5-6°C and rising as a guideline for first N application is worth noting when deciding on timing in early spring.

Likewise, soil drainage plays a big role as land that is more prone to extended waterlogging and poor trafficability for extended periods in most springs is less likely to respond to early nitrogen.

Attention to deciding on the early spring N strategy is important, and may result on holding back on N applications in some areas of the farm. It is still important to apply N fertilisers on fields where a response is more likely.

The next question is, what do I spread? It is probably still too early to be considering spreading compound fertiliser and we would rather wait for soil temperatures to be in double digits before going with these products.

This narrows the choice down to straight nitrogen of which we have a choice of 3 products; urea (46pc N); CAN (27pc N) or protected urea (46pc N).

Clay particles

Typically urea has been the product of choice for early spring N. Urea is made of 46pc nitrogen in the form of Ammonium.

Urea is positively charged, while clay particles are negatively charged, and therefore urea is held by clay particles provided moisture is present. It is readily absorbed by plants, and is the dominant source of N for plant growth. Calcium ammonium nitrate (CAN) is made of 27pc N, in the form of 50pc nitrate and 50pc ammonium.

Best results from urea fertiliser will be obtained when application is followed by approximately 10mm of rainfall within three days.

Obviously, farmers need to be vigilant to avoid heavy rainfall immediately after spreading to avoid run-off/leaching.

Numerous trials have shown very little difference in terms of herbage production responses between CAN and urea, however, as urea is cheaper on a per kilogram basis, urea is the more cost-effective fertiliser to apply during the spring or as long as weather conditions allow.

Joe Kelleher is a Teagasc advisor based in Newcastle West, Co Limerick

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