Dr. Stan Lalor*, Grassland AGRO.
Many farmers are aiming towards applying slurry in spring when the nitrogen (N) value of slurry is known to be better. This has been well documented as being beneficial due to reduced losses of N as ammonia to air in cooler and moister spring conditions, with slurry applied in spring being typically worth about 3 units more N per 1000 gallons than if it were applied in summer. Applying early in the year also maximises the potential for the more slowly available organic N in slurry to be released over a longer growing season.
However, a large amount of slurry is still applied in the summer period, particularly to silage ground after cutting. Despite spring application being targeted on many farms, this is not always feasible due to limitations of ground conditions for spreading or high grass covers on fields in the spring.
It is also worth noting that even though N value may be reduced with summer application compared to spring, the value of other nutrients such as phosphorus (P) and potassium (K) will be unaffected by the timing of application, and therefore slurry should still be considered as a valuable resource on the farm, particularly for replenishing soil K levels in silage fields.
Fertiliser Value of slurry
If you were to go out and buy a fertiliser equivalent to typical cattle slurry applied in spring conditions with a splashplate, a 50 kg bag of 5-5-30 fertiliser would be equivalent to 1000 gallons of slurry.
One of the biggest factors affecting nutrient value of slurry is the degree to which it 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 6-8 % dry matter (solids) content.
Slurry applied in summer, or in dry and hot conditions, will reduce the nitrogen value of slurry by 3 units of N per 1000 gallons. However, the timing of application will not affect the P and K value. So, while 1000 gallons of slurry might be equal to a 50 kg of 5-5-30 in spring, it would be more like 2-5-30 in summer.
However, if slurry is more dilute (say 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 3-4% dry matter content (more watery), the value of 1000 gallons will be reduced to be equivalent of a 50 kg of 5-3-15 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 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.
Thick slurry applied at 3-3,500 gallons per acre for a first cut, or 2,500 gallons per acre for a second cut will supply the P and K maintenance requirements. Watery slurry should be applied at higher rates as the P and K content is diluted.
Low P or K soils will benefit from additional slurry or some P or K in the fertiliser to help build soil fertility levels.
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.
Slurry on grazing ground
Slurry is generally not well balanced in terms of P and K for grazing, and will normally apply more K than is required. The two exceptions to this for grazing are where 1) grazing ground has a low K Index, and/or 2) where grazing ground gets cut for surplus bales.
For paddocks with surplus grass, 1000 gallons of thick slurry will replace the P and K removed in every 4 bales of silage. If slurry is watery, 1000 gallons will be required per 2 bales. If slurry is not available immediately after cutting bales, it can be applied later in the autumn or the following spring.
Improving value of slurry in summer
Despite the N fertiliser value of slurry being normally lower in summer than spring, there are a number of steps that can be taken to improve the N availability where summer application is essential. 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.
3) Injection / Trailing Shoe / Band-spreader
Slurry applied by injection, trailing shoe or band-spreader will be worth approximately 3 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. Using these methods is now required on Nitrates Derogation farms for slurry spread after 15 June.
4) Slurry Enhancers
Additives for improving nutrient recovery from slurry can also improve the fertiliser value and 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 easier 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.
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 currently Head of Speciality Products with Grassland AGRO. He previously worked in Teagasc Johnstown Castle researching cattle slurry application to grassland and soil fertility and nutrient advice.