One of the biggest factors affecting nutrient value of slurry is the degree to which slurry is diluted with water.
Slurry that is of a reasonably thick consistency, and that has not had rainwater or yard washings added to the tank over winter, will normally be in or around 6-8pc dry matter (solids) content.
Every 1000 gallons of this slurry could be considered to be equivalent in terms of fertiliser to a 50 kg bag of 2-5-30 in terms of N, P and K, respectively.
However, if slurry is more dilute, with perhaps half water from a dirty yard / milking parlour and half slurry, then the P and K content will be diluted, and so too will the fertiliser value. Therefore, where the slurry is only 3-4pc dry matter content, or is more watery, the value of 1000 gallons will be reduced to be equivalent of a 50 kg of 2-3-15 in terms of N, P and K .
Note that in the past, K levels in slurry were assumed to be higher than they are currently. Research conducted by Teagasc has shown that the average K levels in slurry has fallen in recent years, corresponding to decreased use of K fertilisers and soil K fertility levels.
Slurry and fertiliser on silage ground
It makes sense to target slurry applications for silage fields as the P and K content is well balanced to replace the high removal rates of P and K in harvested silage.
For second cut silage, the standard recommendation is to apply 80 units/acre of N, 10 units of P, and 60 units of K.
Thick slurry applied at 2,000 gallons per acre will supply all of this P and K requirement. Watery slurry should be applied at higher rates as the P and K content is diluted.
A straight N fertiliser to supply the remaining 75 units of N should then be sufficient to meet the full needs of the crop.
Low P or K soils will benefit from additional slurry or some P or K in the fertiliser.
Applying fertiliser that contains sulphur can also be beneficial, particularly on lighter soils.
To avoid the slurry having a negative impact of the N applied in fertiliser, slurry and fertiliser should be applied a week apart, ideally with slurry applied first.
Fields mowed for first cut that received no P and K in spring will need more P and K during the summer.
A crop of first cut silage will have removed approximately 16 units of P and 100 units of K per acre from the field. If this P and K was not applied earlier in the spring, then it will need to go out now in order to avoid soil fertility depletion.
Improving value of slurry in summer
Despite the N fertiliser value of slurry being considered lower in summer than spring, there are a number of steps that can be taken to improve the N availability. These include:
Slurry that is more watery will be more efficient in terms of N availability. While addition of water will dilute the total N content, it does increase the availability of N after application, as watery slurry will wash into the soil faster and have lower N losses to the air.
Opportunities for diluting slurry largely depend on the infrastructure of tanks on the farm. Dilution with dirty water, for example from dairy washings or silage effluents, may make sense as the overall volumes of material to be spread remains the same.
However, adding clean water to slurry, at levels in excess of what might be required for sufficient agitation, is not recommended as that will increase the overall volume of material to be applied, thus increasing spreading costs.
While timing of application in spring is targeted because of more favourable weather conditions, it is important to note that it is really the weather that is a bigger driver rather than any calendar date.
Therefore even in summer, target cool, overcast or misty days for spreading to improve the N availability.
lInjection/Trailing Shoe/ Band-spreader
Slurry applied by injection, trailing shoe or band-spreader will be worth approximately three units more N per 1000 gallons than slurry applied with splashplate.
The payback for using a contractor with this equipment will depend on the work rate (slurry applied per hour) relative to the additional cost per hour compared to using splashplate.
Additives for improving nutrient recovery from slurry can also improve the fertiliser value and the the grass growth response. Two products available from Grassland Agro are beneficial in this regard.
Actiglene is a product that is added to slurry as a powder applied to the slats during the housing period that helps retain more N in the slurry during storage and after application, and also makes the slurry more biologically active, thus improving overall availability of all nutrients. It also helps make slurry more easy to agitate.
Slurry Activ is a liquid product that is added to the slurry during agitation immediately prior to spreading. It works as a soil and plant stimulant to enhance the speed of nutrient release from slurry, and increase the regrowth of the grass following silage harvesting.
A scientific replicated research experiment conducted this year in Ireland has shown that both of these products offer significant grass yield benefits compared with untreated slurry.
Both products increased the grass yield under silage conditions compared with untreated slurry of equal nutrient content. The value of the additional yield response observed translates into up to three-fold return on investment.
Aim to spread earlier in the year to avoid tanks being full next Autumn. Every year, farmers get frustrated if weather conditions in September and October make slurry application difficult in the run up to the closed period in October.
With the more unpredictable weather in recent years, the advice is to take every opportunity to get slurry spread as early as possible in the year. This will help avoid the risk of having slurry tanks full in the Autumn.
Dr Stan Lalor is Head of Speciality Products with Grassland AGRO. He previously worked in Teagasc Johnstown Castle.