Cattle slurry is a valuable on-farm resource to reduce the impacts of high fertiliser prices.

A number of farm practice changes may be required when making decisions to maximise the fertiliser value of cattle slurry, such as soil test results, time of application, application techniques (trailing shoe/band spreader) and rates of applications.

Targeted early application of cattle slurry based on soil test results will ensure efficient use of slurry P and K and improved N recovery rates. The typical value of 4.5cubic metres (1,000 gallons) of cattle slurry applied by low-emission slurry spreading (LESS) in spring has an available N-P-K content equivalent to a 50kg bag of 9-5-32.

Laboratory analysis of slurry can be used to determine the nutrient values for different slurries on the farm

The nutrient content of cattle slurry may vary somewhat with animal type and diet, and especially with slurry dilution with water (see Table 2).

Knowing the nutrient content will help ensure that grassland receives the planned levels of N, P and K to maximise grass growth for either silage or grazing.

Laboratory analysis of slurry can be used to determine the nutrient values for different slurries on the farm.

A more practical approach may be to estimate the slurry dry-matter on the farm using a slurry hydrometer. This is a low-cost and useful tool to estimate the N-P-K value based on the dry-matter after agitating the slurry.

Slurry phosphorus (P), potassium (K) and sulphur (S)

Cattle slurry is a good source of P and K fertiliser and 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/maize silage.

Targeting these areas will help reduce fertiliser 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 away from the yard and often have low soil fertility levels, as well as the largest demand for both P and K.

The fertiliser replacement value of slurry has increased in line with increased fertiliser prices and the extra transport costs in moving slurry to fields further from the farmyard may offset the spreading charges associated with the longer transport distance.

A suitable fertiliser compound for grazing would be N-P-K: 18-6-12 or equivalent

Slurry is also a very well balanced fertiliser, as its P to K ratio is 1:6, which is ideal for grass silage crops. On grazing fields, the required P:K balance is 1:2 and pig slurry, for example, has a more suitable P:K ratio for grazing.

A suitable fertiliser compound for grazing would be N-P-K: 18-6-12 or equivalent. The P in organic manures such as cattle slurry is 100% available relative to chemical fertiliser when applied on soils at P index 3 status.

However, when applying on soil with P index 1 or 2 status, the availability of the P is assumed to 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.

Cattle slurry contains small amounts of S and continuous application will help build soils supply. For example, a typical application rate of 33m³/ha will supply around 1.0kg S/ha. For grass silage crops, ensure the fertiliser N+S type selected can also deliver between 15 to 20kg S/ha/cut.

Slurry nitrogen (N)

The form of N in cattle slurry is ammonium-N and is similar to the form of N that is made available in the soil after applying urea fertiliser. This form of N is readily available for plant uptake provided soil and weather conditions are favourable.

Applying slurry in spring on cool, overcast or misty days will reduce N losses and maximise N recovery.

Using LESS technology (trailing shoe/band spreader) will further improve N recovery by up to 50%. For example, cattle slurry applied using LESS application methods will deliver an extra 0.35kg N/m3 (3 units N/1,000 gals) compared to the splash-plate application method.

Therefore, aim to apply as much slurry as possible in the spring by LESS in order to maximise slurry N value.

Spring-applied slurry using LESS is worth an additional 6 units of N per 1,000 gallons (worth an additional €6.50 per 1,000 gallons) compared with summer application of slurry by splash-plate, due to better N recovery at that time of the year (Table 1).

Dilution of cattle slurry will improve the N uptake, as the slurry will infiltrate faster into the soil compared to thick slurry. Diluted slurry can also be washed off the grass faster, resulting in reduced grass contamination.

Typical cattle slurry has a dry matter of 6%

Remember that dilution of slurry can help to increase the N efficiency when applying during dry weather, but will reduce the P and K content of the slurry and needs to be accounted for when balancing the crop P and K requirements. Typical cattle slurry has a dry matter (DM) of 6% and a nutrient profile of slurry according to dry matter is shown in Table 2.