To maximise the nutrient value of cattle slurry, a number of decisions should be made over the coming weeks as to where on the farm slurry is required and consider application timing to maximise nitrogen (N) recovery.

The typical value of 4,500 litres (1,000 gallons) of cattle slurry applied by splash plate in spring has an available N-P-K content equivalent to a 50kg bag of 6-5-30.

The nutrient content of cattle slurry will vary with animal type and diet, and especially with slurry dilution with water. Knowing the nutrient content will help ensure that crops receive the planned levels of N, P and K to maximise grass growth for either silage or grazing. A slurry hydrometer is a low-cost and useful tool to estimate the N-P-K value based on the dry matter of the slurry.

P and K

Cattle slurry is a good source of P and K fertilizer. It should be applied to parts of the farm that have either low soil P or K levels, or to crops with high P and K demands, such as grass or maize silage. Targeting these areas will help reduce fertilizer bills and replenish soil P and K reserves. Research shows that fields around the farmyard tend to have higher levels of both P and K due to more regular applications of manures. Silage fields tend to be the furthest fields away from the yard and have low soil fertility levels, plus the largest demand for both P and K. Slurry is a valuable fertilizer and the extra transport costs in moving slurry to fields further from the farm may offset the extra spreading charges associated with transport.

Slurry is also a well-balanced fertilizer (P to K ratio) for grass silage crops. For grazing ground, the P and K demand will be lower and will depend on the stocking rate and the soil test results.

The P in organic manures, such as cattle slurry, is 100% available relative to chemical fertilizer at soil P Index 3 and 4. However, if a soil is P Index 1 or 2, the availability of the P will only be 50%. A soil test will confirm the P status of the soil and help with targeting slurry to Index 1 and 2 soils for more efficient crop fertilisation and P utilisation.

Reduction in slurry K value

Higher levels of K content in cattle slurry were assumed in the past compared with the levels shown in Table 1. Recent research surveying nutrient content in cattle slurries has shown that the level of K in slurry has fallen by approximately 25% compared with levels that were previously assumed based on older studies from the early 1990s. This reduction in K content is not surprising given the decline in K fertilizer inputs over the same period.

Nitrogen content

The form of N in cattle slurry is ammonium N and is the same as the N in urea fertilizer. This form of N is readily available for plant uptake. Losses of ammonium N occur when there are drying conditions, such as warm, sunny and windy days. It is recommended to apply as much slurry as possible in the spring to maximise the fertilizer N value of slurry. Spring-applied slurry is worth approximately three units of N per 4,500 litres (1,000 gallons) – worth approximately €2 per 4,500 litres – extra compared with summer application, due to better N recovery at that time of the year (Table 1). However, irrespective of timing, applying slurry in the right weather conditions (cool, overcast, misty conditions) is advised rather than hot, dry weather.

Dilution of cattle slurry will also improve the N uptake as the slurry will infiltrate faster into the soil compared to thick slurry.

Remember, dilution will increase the N efficiency but will reduce the P and K content of the slurry and this needs to be accounted for in balancing the P and K requirements of crops. Many farmers have seen the benefits of diluting cattle slurry with pig slurry rather than water.

The available N-P-K value of 70:30 and 50:50 mixtures of cattle and pig slurry are shown in Table 2. Before importing pig slurry, check your farm fertilizer plan to determine the volume that can be imported on to the whole farm.

Slurry application equipment

The method of slurry application (splash plate or trailing shoe/band spreader) will have a large effect on nitrogen losses.

The splash plate technique broadcasts slurry across the full spread width and, depending on timing/weather conditions, high levels of N loss may occur as a result. The trailing shoe/band spreader places the slurry in a narrow band close to the soil surface/below the grass canopy, reducing the risk of N loss.

Other benefits include a wider window of opportunity for application in better soil conditions and improved flexibility with application, as a result of reduced contamination of herbage. This leads to quicker return to grazing and the opportunity to apply slurry into larger grass covers.

The odours released during and after application are also usually reduced with trailing shoe or band spreader compared with splash plate.

The investment by an individual farmer in a trailing shoe or band spreader may be cost-prohibitive as the savings in N fertilizer may not cover the extra costs associated with farmer-owned equipment.

This will depend on the volume of slurry on the farm and the value placed on potential other benefits such as flexibility of timing into taller grass covers and reduced odours.

However, where a farmer is already using a contractor for applying slurry by splash plate, using a contractor with a band spreader, trailing shoe or shallow injector may be cost-effective.

The contractor price is usually higher per hour, but the value of slurry is increased by approximately €2 per 4,500 litres (1,000 gallons) by these methods. Depending on the volume spread per hour, a higher cost per hour of the contractor can be justified for using the modern techniques.

The draft GLAS scheme will reward farmers for the use of low-emission spreading equipment (trailing shoe/band spreader/ injection) at €1.20/m3/year.