Rainfall and on-farm practices have biggest impact on N leaching

Spring nitrogen (N) application strategies on dairy farms and rainfall have the biggest impact on N leaching, according to a Teagasc study.

Speaking at the Irish grassland association dairy conference, Teagasc researcher Michael Egan outlined that for every 1kg of N applied, a response of between 11-15kg of herbage production is required to make it economically viable.

Spring N

With the recent changes to regulations and environmental concerns, there is currently no research available on optimal spring N application that coincides with current environmental regulations, which aim to reduce the amount of N fertiliser used.

It is important for Irish farmers that any changes to regulations do not impact on herbage production or nitrogen use efficiency (NUE).

Egan discussed a recent study that was conducted by Teagasc, which looked at the impact of applying three rates of spring N fertiliser (30kg/ha, 60kg/ha or 90kg/ha) by April 1.

This N was applied on two dates, February 3 and March 19, with three different application strategies (0:100; 50:50 and 33:66).

The greatest spring herbage production was seen with the 90kg of N/ha followed by 60kg of N/ha and then the 30kg of N/ha.

However, the different rates of fertiliser applied had different rates of N recovery and lower N response/kg of N applied.

This had the same trend as herbage production, with 90kg of N/ha having the lowest recovery and response followed by 60kg of N/ha and then 30kg of N/ha

Herbage production

When the impact of application strategy was looked and compared to herbage production it was determined that when N was not applied in February and all was applied in March, herbage production was reduced by 200kg of dry matter (DM)/ha.

This was compared to either the 33:66 or the 50:50 strategies.

The 33:66 had the greatest N response with 27.2kg of dry matter (DM)/kg of N spread, with 0:100 having an N response of 14.3kg DM/kg of N spread and 50:50 having a response of 21.1kg DM/kg of N spread.

Egan said that this means that a chemical fertiliser application that is too high in spring results in a level of available N that is too high for the plant to utilise and does not result in increased herbage production.

This ultimately highlights that spring N application strategies can have a significant impact on N response and recovery – which can result in an increase of N loss to the environment.

But herbage production and response based on chemical N are not the only factors that need to be considered – according to Egan, N leaching must be considered when applying spring fertiliser.

N leaching

Egan explained that they looked at the level of N being leached from these systems over the entire year.

The data had been collected over a 20-year period, when chemical fertiliser was spread on different dates throughout the month of February.

He noted that as N is increased there is an increase in N leaching, but that higher levels of N in February resulted in increased levels of leaching.

Egan told the crowd in attendance that the date that fertiliser is spread in February has no impact on the level of leaching – rate does, but date does not.

Commenting, Egan said: “The biggest effect here is the year and the big effect on year is rainfall.”

Egan said that in figures from 2010 and 2011 when the same amount of chemical N was applied, but there was a huge amount of variation in N leaching, this was driven by overall rainfall.

“Rainfall is really the driving factor of overall N leaching, particularly in that spring period,” he said.

Egan added that rather than looking at spreading on a particular date, farmers need to look at precision N application in the spring and the summer period with droughts.

He advised farmers to look at the predicted grass growth models, rainfall and soil temperature.

Based on the research conducted by Teagasc, Egan advised farmers to use between 60kg and 70kg of N in the spring period to achieve the highest economic response and highest herbage response – while also reducing N leaching.