Dont miss nutrient value

13 March 1998

Dont miss nutrient value

SWITCH on to the financial value of slurry through more careful estimation of its nutrient value.

Thats the message from researchers who believe farmers are underestimating the value of slurry in terms of its nutrient profile and its potential for savings on compound fertiliser costs.

Jonathan Fletcher, lecturer at Myerscough College, Preston, Lancashire, is overseeing long-term trial work on efficient use of slurry based on the output from the colleges 200-cow dairy herd.

"Slurry treatment, in whatever form, will improve its use and need not be as costly as many farmers think.

"Treatment is the starting point for making the best use of slurry. Unfortunately, problems encountered in the storing of large quantities of liquid slurry engender the wrong approach and often lead to application at a time when the land is cold and wet, and plants cannot make optimum use of the nutrients provided.

Nutrient value

"While many farmers consider the physical aspects of slurry disposal as the priority issue, these should not undermine its nutrient value and its impact on grassland management and subsequent fertiliser treatment," says Mr Fletcher.

Farmers must give greater consideration to the basic principles of application and ensure that the land is below field capacity to achieve maximum uptake of nutrients.

"If the soil is at field capacity the slurry will simply sit on top and most of the readily available nutrients will be lost to the atmosphere through volatilisation.

"This is likely to have far greater implications in the future from a pollution point of view if legislation is introduced to cover odour problems."

Grassland farmers who want to make more use of their slurry should be evaluating treatment systems in readiness for next winter and spring, and have an outline plan to achieve the most efficient method of application.

While slurry may not have a high economic value on a tonnage basis, its quite a different picture when estimated on an annual output a cow.

Broad calculations made by Mr Fletcher, based on MAFF figures, estimate slurrys potential value.

"Based on the assumption that a farmer can use all the slurry produced, the value of N is around £1/t, and P and K together is around £1/t. That does not appear to be very much but take it on a per cow basis and the real value becomes evident."

The calculations are based on an annual slurry output a cow of 18t/year – around 50 litres a day – at a dry matter of 10-12%. Thats worth approximately £70 a cow a year.

Even allowing for 50% manure loss at grass during summer grazing, it still leaves £35 in value.

"In reality most farmers will be doing very well if they get 50% of the value of their slurry out of every cow on the farm. In actual terms its around £15 worth of fertiliser value a cow," says Mr Fletcher.

While it is impossible to achieve the full potential of slurrys fertiliser value, there is scope for improvement which could lead to at least 60% of the value being used."

Application counts

Northumberland dairy farmers son Dave Woodcock, who has just started a PhD in slurry management systems at Myerscough College, says methods of application can have a profound effect on the nutrient value of slurry.

"Comparing a traditional tanker spreading system with an irrigation-style, rain-gun applicator shows that the more the liquid is "opened" during its application the smaller are the particles," says Mr Woodcock.

This leads to a high level of ammonia volatilisation – up to 80% – and subsequent loss of nitrogen that occurs as ammonia."

Consideration given to more careful handling of slurry and ensuring that it remains on the surface for as short a time as possible, will reduce losses through volatilisation.

Says Mr Fletcher: "Umbilical systems enable slurry to be applied when conditions are too wet for tanker spreading – that means land is probably at field capacity and slurry will remain on the surface and atmospheric losses will be higher." &#42

Umbilical systems enable slurry to be applied when conditions are too wet for tankers – Jonathan Fletcher.

Learned slurry: PhD student Dave Woodcock (left) and lecturer Jonathan Fletcher. Both are at Prestons Myerscough College.


1 Apply on a calm day with low wind speed.

2 Reduce volatilisation losses by spreading when air temperatures are low.

3 Ensure soil is below field capacity to take up the liquid.

4 Soil surface should show some structure to it and not be seriously compacted.

5 If slurry is injected it should be carried out on the land contour.

6 Lower the solid content to achieve a more rapid infiltration rate.

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