Teagasc has assessed the chemical nitrogen allowances for the tillage sector in the context of a new Nitrates Action Programme (NAP).

The Teagasc report is entitled 'An environmental and economic assessment on the impact of possible reductions in the maximum chemical nitrogen allowances for main arable crops'.

According to Teagasc, nitrogen (N) rate recommendations for arable crops are based on an economic optimum (Nopt), above which further yield increases do not cover the cost of the additional fertiliser. 

They are therefore below the rate needed to achieve maximum yield.

The report highlights that research also shows the residual nitrate-N at harvest of a crop remains relatively stable until applications exceed Nopt, and then increases steadily as additional fertiliser N is applied.

According to the report: "Reducing recommended N rates for arable crops is, therefore, unlikely to provide any environmental benefit but risks reduced yield and profitability in a farming system with already tight margins."

Arable systems are at risk of N leaching in the autumn and winter due to the mineralisation of organic N residues when there is no crop to take up the nitrogen, the report said.

Establishment and maintenance of over-winter green cover is "therefore the key action in arable systems to reduce the risk of over winter N leaching".

The date the green cover is established is "critical to minimising N loss", with the report stating that earlier established covers maximise growth and N uptake.

So, drilling dates of these crops are generally is more important than the type of cover or the method of establishment.

In addition to the establishment of green cover, the level of organic N present in the soil in autumn is important, according to the report.

"Autumn application of low C:N ratio manures can increase the risk of N loss unless a high N uptake crop such as early sown oilseed rape is established" the report stated.

Trials have repeatedly confirmed that the response of cereal crops to fertiliser N is curvilinear in nature (usually referred to as an N response curve), according to the report.

As a consequence, there is a large response in grain yield to low levels of N input - as N rates increase, the marginal increase in grain yield decreases until eventually there no increase in yield.

At this N rate (Nmax), the maximum yield is said to have been reached. Above this rate, yield is likely to decrease - particularly if lodging becomes an issue.

Fertiliser nitrogen advice for cereals in Ireland is based on maximising the economic return on investment in fertiliser N rather than maximising yield.

Given that there is always a cost associated with fertiliser N, the N rate that maximises the economic return - the economic optimum N rate - will always be lower than the rate which maximises yield.

This is because for N rates at or close to the rate that gives maximum yield, the cost of each kg of N is greater than the value of the additional grain yield achieved.

The economic optimum N rate (Nopt) is the N rate where the additional cost of a 1kg N/ha increment is equal to the value of the additional grain yield achieved.

Typically, between 5-10kg of grain will be required to pay for 1kg N, depending on grain and fertiliser prices.

As a consequence, the Nopt figure will be located at the point on the curve where one kg N gives an increase in yield equal in value to the cost of that fertiliser input.

This point will typically be 20-50kg N/ha below the rate that would give the maximum yield, depending on the shape of the response curve and the relative prices of grain and N.

There is very limited published research in relation to the impact of N fertiliser rate on cereal crops and subsequent water quality, according to the Teagasc report.

However, the work of Keith Chaney at Harper Adams University College some 35 years ago is generally accepted as the best in a north-western European context.

This research indicates that residual nitrate-N in the soil at harvest changes little for increasing rates of N until Nopt is exceeded.

There has been no similar work completed in as much detail in Ireland. However, tillage specialists reference  a study carried out 20 years ago by Teagasc research staff.

This study compared soil mineral N concentrations under spring barley and winter wheat over two years grown with normal and reduced N rates (20% less).

The low N fertiliser input treatments in both crops tended to have higher soil water N concentrations than high input treatments and was significantly higher for winter wheat in the first year.

Statutory Instrument (SI) 113 of 2022 references N fertiliser application rates for cereal grain yields that are lower than the national average yield as indicated by the Central Statistics Office (CSO).

Achieve such yields with the N rates given in SI 113 of 2022 on a consistent basis with current varieties is unlikely.

These figures relate to yields that are much lower than those achieved by the best Irish growers - yields of 8.5t/ha of spring barley and 11t/ha of winter wheat are common in good years.

Therefore, an additional N allowance where high yields are being achieved is critical to maintain productivity of cereal crops in Ireland.

Applications of quantities of N in excess of crop requirements on lower yielding farms is avoided under the regulations.

The Teagasc report also noted that over time. as new varieties with higher yield potential were introduced, the grain yield potential of crops increased.

This higher yield potential usually requires additional fertiliser N to support the additional yield.

The yield-based adjustments allowed under the regulations can lead to exploitation of additional N allowance for high yields.

Growers can use the best yield in the previous three years as evidence of higher yield to justify additional N allowance.

However, new growers of a crop that do not have historic yields to demonstrate higher yields may therefore be restricted from achieving the yield potential of their farm.

In addition, if three low yielding years occur in a row, growers who have previously demonstrated higher yields may be unable to attain those yields as a result of natural processes.

In this situation, growers cannot use the higher rates of N even though they have demonstrated that they can achieve higher yields when seasonal attributes allow.

Using sub-optimal N rates can also cause low grain protein, which is an issue if growing for a premium market such as malting barley. where crops can be rejected for low protein.

Teagasc trlals confirm that increasing N use above the optimum recommended rates on tillage crops leads to higher costs that are not offset by additional yield.

This increased N use will result in no extra margin for the grower, and can increase costs lodging and crop losses occur.

Conversely, reducing N use below the optimum recommended rates lowers costs but also reduces yield, leading to lower overall margins.

Since recommended N rates are already optimised for both economic and environmental impact, a reduction in applied N will have little impact on the residual nitrate-N at harvest.

However, according to the Teagasc report, it increases the risk of significant yield losses and financial losses for tillage farmers who are already operating on narrow margins.

Higher nutrient loss risk from tillage soils over the winter period is reflected in data from the Agricultural Catchment Programme (ACP).

This data shows that leaching of nutrients to water bodies generally starts in late October to early November as soils become saturated.

Leaching of nutrients then stops in spring as crop N uptake accelerates and soils begin to dry.

For tillage cropping, the demand for N in the autumn for winter cereals can mostly be supplied from soil reserves.

Additional applications of N during the late autumn and winter period will potentially increase the risk of nitrate leaching, especially on free draining soils.

The report highlights research confirming that reducing nitrate-N losses from tillage systems is most effectively achieved by one of two ways: reducing the organic N available for mineralisation in the autumn; or ensuring the maximum uptake of N in the autumn and holding it in a green crop until soil temperatures have dropped and the risk of remineralisation has dropped.

According to Teagasc, establishing over-winter green cover on tillage soils as soon as possible post-harvest on fields planned for late autumn planted crops and spring crops is key to reducing the risk of nutrient loss during the fallow period.

Cover crop species vary in terms of N uptake, with brassicas generally fast-growing and among the most efficient at taking up residual N.

However, cover crop establishment on tillage farms often needs to be carried out at one of the busiest periods of the year - during harvest, when there may be limited labour and machinery available, which can lead to delays in planting and establishment.

Investigating alternative rapid methods of cover crop establishment could aid the broad uptake of over-winter green covers.