Using slurry to reduce the cost of growing cereal crops

Shay Phelan, Mark Plunkett, John Mahon and Gerard McCutcheon tell us that slurry is a valuable nutrient source and its efficient use can help off-set rising fertiliser costs.

A major event to encourage farmers to make more efficient use of slurry and other organic fertilisers on tillage land took place on Tuesday, 28^th March, at the Signpost Farm of Mattie and Miriam Moore at Croghan, Rhode, Co. Offaly. The weather was dry and dull on the day and it was well worth making the effort to travel to this event.

Applying slurry in the spring to autumn-sown cereal crops leads to more efficient use of the nitrogen by greater uptake of nitrogen in it by the growing crops.

This event demonstrated the use of the umbilical application equipment and a tractor with a 12m band spreader to maximise slurry utilisation at farm level. The pig slurry being applied had a 4% dry matter content (good quality). A demonstration of the spreading of slurry on a small area was carried out to show the application of slurry onto growing cereal crops.

This farm has 1,600 sows and rears pigs up to 118kg liveweight on this site. There are also 95ha of cereal crops adjoining the pig unit (45% winter barley, 42% winter wheat and 13% winter rye). Pig slurry was applied at a rate of 33m³/ha (3,000 gallons per acre). Soil test results show that the phosphorous (P), potassium (K) and pH levels are good on this farm. No chemical P or K is applied to the land, as it is all supplied by the pig slurry.

The nutrient content of pig slurry is closely related to the dry matter (DM) content. The DM content is variable depending mainly on the amount of water added either in the feeding and watering of the pigs or from extraneous sources, such as washing of houses, leaks, spills or from roofs, open tanks or dirty yards. Good manure management on the pig farm will ensure minimal dilution with water. This will result in reduced storage and transport costs for the pig producer and a product with higher DM and nutrient content for the recipient farmers. Pig manure that contains 4.3% solids is of reasonable quality. Good quality pig manure will contain more than 5% solids.

The value of pig manure as a fertiliser depends on how much chemical fertiliser is replaced as well as the cost of the chemical nutrients replaced. The fertiliser value of pig manure at 4.3% solids is €10.99/m³ when there is a requirement for N, P and K (see Table 1 below). This translates into €50 per 1,000 gallons. A reasonable rule of thumb is that 1,000 gallons of pig slurry is equivalent to a bag of 19:7:20. Using reasonable costings, the umbilical system will cost €2.40/m³ or €11 per 1,000 gallons to spread.  This leaves a good return in terms of fertiliser saving. If using a slurry tanker, the transport and spreading costs are higher and should be calculated for each situation.

Table 1:  Nutrient content and value of pig slurry (4.3% solids)

Note: 1 m³ equals 220 gallons. *Based upon Chemical Fertiliser prices on 1^st April 2023

The dry matter of the pig slurry can be assessed very quickly using a slurry hydrometer following agitation. This can allow a reasonable prediction of the nutrient content of the slurry as shown in Table 2 below.

Table 2: Typical N,P & K value of Pig Slurry (available units per 1,000 gallons)

The nutrient content of the pig slurry should be known before anyone uses it to grow crops. The slurry should also be agitated before it is applied to land to get a more homogenous/consistent fertiliser response. There are savings to be made particularly if you can get the value from all the nutrients (N,P and K). Is this something you should consider for your farm?