Responsible management of livestock slurry on farms across Northern Ireland was the primary focus during an industry open day at CAFRE, Greenmount last week.

As livestock slurry and farmyard manures can be a valuable source of nutrients for growing crops, they should be the starting point of any fertiliser plan.

Chemical fertilisers should then be used to either top up, or meet any undersupply in nutrients required for growth after organic manures have been applied.

However, the nitrogen (N), phosphorous (P) and potash (K) content of slurry varies depending on the animals that produce it and the diet fed. Dry matter (DM) is also an influencing factor.

Soils that are deficient in lime, or have a P and K below index 2-, will also see nutrients applied being locked up and unavailable for plant growth.

Nutrient levels

For cattle slurry at a typical 6% DM, it has potential to provide soils with nine units of N, 11 units of P and 20 units of K per 1,000 gallons/acre.

When diluting slurry down to 2% DM, N levels drop to six units/acre for every 1,000 gallons applied; P levels are reduced to five units/acre, with K dropping to 13 units/acre.

Higher dry matter slurry increases N, P and K levels. But with a lower water content, they are difficult to apply and unsuitable for low-emission slurry-spreading equipment (LESSE).

Pig slurry at a typical 4% DM can supply around 17 units of N, 13 units of P and 17 units of K for every 1,000 units of slurry applied using LESSE.

On soils at index 1 or index 0, only 50% of the P applied via fertiliser is available for plant growth, compared to the same application rate on soils at index 2- or above.

While the outlined figures are standard values, farmers can get slurry produced on their own farms analysed through DAERA direct offices at a cost of £35 for a basic DM and NPK content. Samples should be taken after tanks are agitated, provided they can be collected in a safe manner.

Application method

Nutrient content is also affected by the method used for slurry application, as well as the time of year it is applied.

For example, using a trailing shoe in spring should provide nine units/acre of N per 1,000 gallons of cattle slurry. That N availability falls to eight units when applied in summer using the same equipment.

In contrast, when using a splash plate for slurry spreading, every 1,000 gallons/acre in spring provides eight units of N, reducing to seven units for a summer application.

Targeted use

During the event, farmers were advised to alter the rate of slurry application to suit the crop’s requirement, based on soil fertility levels within individual fields.

For example, on soils at index 3 for P and K, first-cut silage would require 16 units/acre of P and 24 units/acre of K.

That requirement is met by spreading 1,500 gallons/acre of cattle slurry using LESSE, then topped up with straight nitrogen to drive grass growth. At index 2+, those requirements double to 32 units/acre of P and 48 units of K. The P requirement could be met by applying 3,000 gallons/acre, although there would be a small oversupply of K.

On soils with lower fertility, around index 1, the P and K requirement increases to 70 units/acre and 110 units respectively.

However, as P availability is reduced by 50% due to low fertility levels, applying 3,000 gallons/acre of slurry would only supply 16 units of P, along with 95 units of K.

That means 40 units of P have to be supplied via a chemical fertiliser, provided the aim is to maximise silage yields. Purchasing chemical fertiliser with a high P content comes at significant cost.

“Value the nutrients in your slurry and what they can potentially produce. There is no benefit to high application rates on soils at optimum fertility levels. Those nutrients will ultimately be lost. Knowing your soil fertility levels means slurry can be better directed where it will be used by the plant,” stated CAFRE technologist Aveen McMullan.

Nutrient planning

To assist farmers with calculating the nutrient requirements for various crops, CAFRE has an online calculator which can be used for individual fields.

Farmers simply input the results from soil analysis, along with the designated crop. The calculator outlines how much N, P and K to apply. It breaks the information down to outline the nutrient content supplied by slurry and farmyard manure, before topping up with chemical fertilisers.

Buffer zones

The closed period for slurry spreading ends at midnight on 31 January, meaning farmers can apply slurry from 1 February onwards, when conditions are suitable.

However, farmers were reminded that buffer zones to water courses are increased in February and again in October.

When using a splash plate in February, a minimum distance of 30m must be kept from lakes and 15m from rivers. If using LESSE, buffer zones reduce to 5m from rivers.

From 1 March to 30 September, buffer zones with the splash plate must be 20m from lakes and 10m from rivers, reducing to 3m when using LESSE.

“That buffer zone relates to the edge of the spreading width on the tanker. To keep 15m away from a river in February, the centre of the tanker needs to be even further away from the water course,” CAFRE’s Andrew Thompson reminded those attending.

Forward speed

Forward speed can also impact application rates, with higher speeds required for LESSE applications compared to a splash plate method.

For a 3,000 gallons/acre application using a 7.5m trailing shoe that empties in four minutes, the tractor requires a forward speed of around 8km/hour.

In contrast, the same application applied by a splash plate with a 12m spreading bandwidth, with the tanker emptying in six minutes, forward speed would be closer to 3km/hour.

Online tools are available to help farmers with calibrating forward speed.

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