Manure analysis – is slurry just slurry?
Organic manures are an important resource for livestock farmers. They provide nutrients that could
reduce bagged fertiliser requirements as well as organic matter that improves soil condition and
help crop growth and performance. In order to make the most of them on-farm however they need
to be incorporated into nutrient management planning. Planning nutrient applications is a vital farm
management tool to ensure that applied nutrients (either in terms of bagged fertiliser or manures)
match the crop demand to optimise yield and minimise environmental losses. For reliable fertiliser
planning it is important to know the nutrient content of manures that are applied to land, so that
this can be taken into account when looking at fertiliser requirements. But what tools do you use?
Owing to farm specific feeding and manure handling practices, manures produced on different farms
may have a nutrient content that is consistently different from the published values. There are
various sources that can be used for nutrient management planning (including Think Manures,
PLANET, Manner NPK, Tried and Tested, and RB209). RB209 is the Defra Fertiliser Manual, and
alongside providing fertiliser recommendations for different crops grown on-farm, it contains
average nutrient values for the different types of organic manures produced. The RB209 values are
advocated as a useful nutrient management planning
tool to help assess the contribution that organic
manures make in terms of available nutrients for
crop growth. However how accurate are the figures
given what is happening on-farm?
To try and assess this, the SWARM Hub took slurry
from three different dairy farms in Devon and
Cornwall, and in association with Mole Valley
Farmers (www.molevalleyfarmers.com) and Lancrop
Laboratories (www.lancrop.com) compared the
results against the values found in RB209. The collection of samples was completed by Mole Valley
and the analysis by Lancrop. The analysis results reported on the nutrient content of the manures in
terms of the Total Nitrogen, Phosphate, Potash and Magnesium (kg/tonne) and the dry matter
percentage (expressed as kg/tonne) found in the submitted samples. Details of the three farm
scenarios are below.
Farm 1: A milking herd of 130 Holstein Friesians, housed in cubicles with mattresses, bedded on
sand and straw and fed through a TMR system. Average yield for the herd was 8,500 litres. 1
sample was taken from the slurry store for analysis (sample 1).
Farm 2: A dairy herd of 270 Holstein Friesians and 200 young stock, housed on cubicles with
mattresses and sawdust, fed through a TMR system and rotationally grazed. Average yield for the
herd was 8,000 litres. 2 samples were taken from this farm, 1 from the weeping wall slurry store
(sample 2) and one directly from the cubicle passageway (sample 3).
Farm 3: A herd with 60 cows milked through a robot. The cows were housed in cubicles bedded on
sand and fed on a TMR ration. The average yield was 7,300 litres. 2 samples were taken from this
farm, one from the lagoon that contains all the dairy cubicle scrapings (sample 4) and one from a
slurry tank under a shed housing young stock on a bedding free system (sample 5).
Results:
The results from the three farms can be seen in the table below.
Total nutrient
content (kg/t)
N
P
K
DM (%ge)
1
1.53
0.67
2.29
2
Sample number
3
3.17
1.54
4.50
11
2
1.69
0.91
2.66
6
4
2.38
1.10
3.71
11
5
0.94
0.43
0.98
2
Samples 3 and 4 both had a dry matter percentage of 11%. For comparison purposes the 10% dry
matter results from RB209 were used. The graph below shows the results from the farm samples
above (the bars on the chart) compared with the values that are in RB209 (the red diamonds). The
letter and numbers along the bottom axes refer to the farm sample number and the content of
Nitrogen, Phosphate or Potash (for example N1 refers to the total nitrogen content from sample 1).
Farm results compared with RB209 values
5
Farm
results
4.5
RB209
Results
kg/tonne total nutrient content
4
3.5
3
2.5
2
1.5
1
0.5
0
N1
N2
N3
N4
N5
P1
P2
P3
P4
P5
K1
K2
K3
K4
K5
From the table and graph, it is possible to see that there is variation between the farm samples. As
well as the variation when the samples are compared with RB209 all samples bar 2 fall below the
published nutrient values. The sample taken on farm 1 for phosphate and sample 3 (taken on farm
2) for potash are the only ones that are showing nutrient contents above the published RB209 levels.
These variations in the slurry analysed show that for these three farms, using the RB209 figures may
not be a true representation of what is being applied to the crops in terms of nutrient value. Other
interesting conclusions from the analysis highlight the importance of minimising rainwater entering
the slurry store. With the two samples taken from farm 2, it is possible to see that the amount of
nitrogen in the slurry halves from sample 2 (cubicles) to sample 3. Some of this loss will be due to
the effect of rainwater falling onto the store, which as well as diluting the nutrient content of the
slurry will also increase the volume of slurry that you need to store (especially relevant throughout
the recent weather). Sample 5 (from the third farm) was taken from the slurry tank when it hadn’t
been stirred. The results that have come back are representative of a very thin slurry, however if
this is just taken from the watery top level, it is not a truly representative sample of what you will be
spreading on the ground. Ensuring a homogenous sample is taken is vital if the results are going to
be fed into nutrient management planning, and optimal use of the nutrient content of organic
manure is going to be achieved.
This demonstration has highlighted that slurry is not “just slurry” and the nutrient content can vary
between farms and enterprises. Undertaking analysis can enhance the efficiency with which slurries
are used and can ensure optimal use of manures on-farm.
To read more about nutrient management planning and manure use on farm please visit the Think
Manures section on the SWARM Hub website
(http://www.swarmhub.co.uk/think_manure.php?id=3121).