Farmers should aim to apply at least 75% of slurry in spring

Slurry has become a very valuable product which can enable a great saving in purchased fertiliser and if used correctly.

Teagasc researchers put a value of up to €30 per 1,000 gallons on undiluted slurry, or the equivalent to one 50kg bag of 6-5-38.

Approximately 85% of the value comes from the P and K, but if it is spread in spring, much of the nitrogen (worth €5 per 1,000 gallons) can be utilised.

The ideal time to spread slurry is as early as possible in spring. Most of the slurry should normally be spread on silage areas from where the slurry nutrients came from, ideally applied after early grazing and before closing up for first cut silage.

However, it is estimated that only one third is spread in spring, with half going out in the summer, mostly after first cut silage, and the remainder in the autumn.

Pig slurry and cattle slurry have similar total value, but have very different levels of nutrients.

Average cattle slurry dry matter contains 15% N, 17% P and 68% K, but this will vary with the level of nutrients being fed to cattle. Pig slurry DM contains 43% N, 22% P and 35% K, but often it is too diluted. It should have 4.5% solids (worth €27/1000 gls) but sometimes contain a lot of wash water.

Remember that 75% of the N, 71% of the P, and over 90% of the K consumed by animals is excreted in the slurry.

Teagasc estimate that the total nutrients excreted by the national cattle herd is 146,000 tonnes of N, 22,450 tonnes of P and 126,000 tonnes of K.

Soil fertility neglected

The results from 25,000 soil samples taken from dairy farms over the past few years show that 55% of soils are too low in lime, P and K for optimum grass growth, and fir utilisation of N.

These results indicate why most of our grasslands are producing far short of their potential.

In order to make the best use of slurry, and save on fertiliser costs, it is absolutely essential to know the P, K, and lime status of each separate area of your farm.

This involves having a proper slurry and fertiliser plan for the entire farm — which necessitates proper soil testing.

How many farmers have a map of their farm showing the nutrient levels of different areas? There are very few.

P and K availability

The P and K in slurry is almost 100% available to grass in normal soils.

Once the P and K soil levels are brought up to standard (based on a soil test), P and K levels can be maintained by applying all the slurry in the silage areas.

This minimises the amount of fertiliser P and K that is required on the entire farm.

Nitrogen availability

The availability of slurry N varies widely with factors such as timing and methods of application. Approximately half the slurry N is in organic form and is not available to crops in the year of application but can be released gradually over time.

The other half of the N is in the form of ammonium which is readily available to crops under certain circumstances.

With traditional methods of spreading slurry in the early spring, it is estimated that 25% of the N can be utilised, but only 5% is available if spreading during the summer, because it generally goes away in the air in dry summer conditions.

New developments in spreading techniques such as the trailing shoe or band spreading or the dribble bar increase availability further.

The target should be to apply at least 75% to 80% of slurry in spring before closing up for silage, and the remainder before mid-June, in cool moist conditions if possible. This will significantly cut the cost of fertiliser N for silage.

Research has shown that diluting slurry with water makes its nutrients more available to grass.

It is advised not to graze for six weeks after slurry — and don’t apply N within seven to 10 days of slurry.

Slurry should not be applied to ground until lime has been well washed into the soil. Urea should not be applied to ground that has been limed since last autumn.

Window of opportunity

The reason for so little slurry being applied in spring is because of a very narrow window of opportunity. Ground and weather conditions very often prevent spreading.

Pre-grazing of silage areas and contamination of grass are also major constraints to spring slurry application with the traditional splash plate. Ideally, slurry should be spread between early grazing and closing for first cut silage. This will not only give optimum use of slurry but also minimise disease risks.

By switching the application of 3,000 gallons per acre of cattle slurry from summer to spring, there is an economic advantage of €15 per acre in terms of extra N.

Unfortunately, this has been a bad year for early spring slurry spreading.

Band spreading and the trailing shoe method of application which separates the grass and applies the slurry in lines at the base of the sward have overcome many of the constraints to spring slurry spreading.

They minimise grass contamination and reduce odour emissions and losses of N to the air. They will allow farmers to apply slurry later in silage swards, when ground conditions are better.

Umbilical system

The umbilical system of slurry spreading is very useful, especially for early spring spreading and on heavy land.

The system lends itself to abuse in very wet conditions but there is no reason why it can’t be an effective and environmentally friendly system, with good management.

The system has great potential on all types of land. It is most suitable for contractors, being very expensive to set up.

I understand the contractor charges are around €105 per hour plus Vat, but the output is extremely high at over 30,000 gallons per hour, compared with a splash plate output of 7,000 to 9,000 per hour.

The system has one tractor and operator in the yard pumping under very high pressure up to 2.4 km to a container on a tractor which is continuously spreading through a dribble bar of up to 12 metres in the fields.

The system has many attractive advantages such as very high output, lower spreading costs for the farmer, and less compaction of ground, as there are no heavy tankers.

The system can very often be safely used when the ground is not suitable for tankers. These claims seem reasonable, with the emphasis on early spring application being so important.


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