Far from waste, slurry must be viewed as a valuable asset
Teagasc researchers put a value of €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 spread in spring, much of the N (worth €5 per 1,000 gallons) can be utilised.
Therefore 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 and ideally applied after early grazing and before closing up for first cut silage.
However, it is estimated that only one-third of slurry is spread in spring — around half of this in summer, mostly after first-cut silage; and the remainder in autumn.
Pig slurry and cattle slurry have similar total value but have very different levels of nutrients.
Average cattle slurry DM contains 15% N, 17% P, and 68% K but this will vary with the level of nutrients being fed to cattle. Pig slurry contains 43% N, 22% P, and 35% K. 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 estimates that the total nutrients excreted by the Irish national cattle herd annually is 146,000 tonnes N, 22,450 tonnes P, and 126,000 tonnes K.
SOIL FERTILITY NEGLECTED
The results from 25,000 soil samples taken from dairy farms over the past 2 years show that 60% of soils are too low in lime, P and K for optimum grass growth, and utilisation of N. These results indicate why most of our grasslands are producing far short of expectations.
In order to make the best use of slurry and save in fertiliser costs, it is 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 entire farm which necessitates proper soil testing.
How many farmers have a map of their farm showing the nutrient levels of different areas? The answer is that 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 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 without the use of fertiliser. This minimises the amount of fertiliser P and K that is required on the entire farm.
N 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.
By spreading slurry in early spring by traditional methods, it is estimated that 25% of the N can be utilised but if spread during summer only 5% is available as it goes away in the air in dry summery conditions. New developments in spreading techniques such as the trailing shoe or band spreading or even 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 cut the cost of fertiliser N for silage. New research has shown that diluting slurry with soiled 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 SPREADING
Generally less than one-third of slurry is applied in spring. About half is applied in summer, mostly after first cut silage when it is most convenient to do so but with a loss of very valuable N fertiliser. A considerable amount of slurry is spread in autumn.
The reason for so little slurry being applied in spring is because of a very narrow window of opportunity. Ground and weather conditions are often a problem. Pre-grazing of silage areas and contamination of grass is also a major constraint 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.
The development of 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.
Not only will these methods of slurry application avoid grass contamination but will also reduce odour emissions and losses of N to the air.
They will allow farmers to apply slurry later in silage swards thereby giving better traffic ability.
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, environmentally friendly system with good management.
The system has great potential on all types of land especially for contractors, costing about €45,000 including hoses costing up to €30,000. I understand the contractor charges are around €100 per hour + Vat but the output is extremely high at over 30,000 gallons per hour versus 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 2.4km to a container on a tractor continuously spreading with up to a 12m dribble bar in the fields.
The manufacturers of the system claim 3 advantages: Very high output, lower spreading costs for farmers, and less compaction of ground as there are no heavy tankers. These seem reasonable claims with the emphasis on early spring application being so important.
