Schröder, J.J.

English

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Simulated nutrient management options for intensive livestock farms with a surplus of manure

Wageningen Yield 

 1 
Simulated nutrient management options for intensive livestock 
farms with a surplus of manure 
                     
Schröder, Jaap* 
 
*Agrosystems Research Wageningen University and Research Centre, P.O. Box 16, 6700 AA 
Wageningen, The Netherlands, tel: ++31 317 480578, jaap.schroder@wur.nl 
 
Abstract 
The output levels of intensive livestock farms are sustained with imports of feed. The imminent losses 
associated with the vast nitrogen (N) and phosphorus (P) fluxes through these farms need to be 
addressed. A blunt reduction of intensity (i.e. output levels) evidently alleviates the environmental 
pressure but does not necessarily contribute to a proper utilization of resources, among which N and 
P. The present paper shows some alternative nutrient management options. 
 
Introduction 
Resources such as rock phosphate and fossil fuels needed for the production of mineral fertilizers are 
finite. Moreover, the use of mineral and organic fertilizers can have a detrimental effect on our local 
and global environment. These aspects justify rethinking the current lax use of nitrogen (N) and 
phosphorus (P) in our societies (Neeteson et al., 2006; Schröder & Bos, 2008). These considerations 
apply not the least to intensive livestock farms. These farms are not self-sufficient in terms of feed 
production. Consequently they derive feed from elsewhere and together with this feed nitrogen (N) and 
phosphorus (P) are imported. The import of P commonly results in the accumulation of P in the soils of 
livestock farms and the eventual loss of P, as the P uptake of home-grown crops is often much lower 
than the amount of P available from manures. The utilization of the P in manure is also restricted by 
the amount of N that can be applied and supplemented within the limits of permissible N losses to the 
environment. To reduce the environmental pressure, this type of farms should reduce the import of P 
or export their excess manure. In any case these farms should use the N in manure as efficiently as 
possible, if alone to reduce the need for mineral fertilizer N supplements. The present paper gives 
model-based examples of how the inputs and outputs of N and P on livestock farms can be balanced. 
 
Figure 1. Simulated annual nutrient fluxes (kg N / P2O5 per ha per year) and, in brackets, conversion coefficients 
(kg per kg, for N / for P2O5) in a dairy farm producing approximately 14,000 liters of milk per ha per year 
(consult text for references and assumptions) 
 
 
Inbalances 
Figure 1 shows the imaginary N and P fluxes for a dairy farm producing 14,000 liter of milk per ha per 
year, as simulated with a simple model (Schröder, 2000; Schröder et al., 2003; 2005b). Assuming an 
achievable annual crop yield level of 250 kg N per ha (with a typical N/P2O5 ratio for forages of 
approximately 3), this type of farms must inevitably rely on imported concentrates of which the 
composition is mainly determined by the grain component with a typical N/ P2O5 ratio roundabout 2.5. 
 2 
Consequently, more P is added to the soil via manure than taken up by crops. Several measures can 
be taken to avoid the resultant accumulation of P in soils. Numerical effects of five of these measures 
are listed in Table 1. One obvious measure is to adjust the livestock density (i.e. feed requirements 
and milk production level) to the home grown amount of forage. P accumulation is stopped then, but, 
ceteris paribus, this measure implies an increased demand for mineral fertilizers, mainly N. Probably it 
is more profitable to maintain the milk production and spend money on the adjustment of the 
concentrate composition. P accumulation can be drastically reduced by switching to concentrates with 
a N/ P2O5 ratio of 3.6 instead of 2.5. Alternatively, the use of low-protein rations (either by importing 
low-protein concentrates or by avoiding harvests of forage in too young a stage) can improve the feed-
N conversion (Kebreab et al., 2001). Another option to balance P inputs with P outputs would be to 
remove P from the system by the export of manure. In the present example approximately 15% of the 
manure needs to be exported to arrive at a P surplus of 0 kg per ha. Again, this would lead to an 
increased demand for mineral fertilizer N, as some N is inevitably exported together with the manure 
P. All measures but the first one result in less manure P whilst maintaining milk and meat outputs. All 
measures but the first two ones would also reduce the loss of N, including ammonia, per unit milk 
produced. 
 
Table 1. Simulated fluxes (kg N / kg P2O5 per ha per year) and conversion coefficients (kg per kg) of dairy farms 
according to different strategies aiming at P surpluses close to 0 (all being alternatives to the systems reflected in 
Figure 1) to improve the utilization of inputs (consult text for references and assumptions) 
 Strategy:     
 Livestock 
density 
reduction 
Higher N/P2O5 
ratio in 
concentrates 
Low protein 
diets + higher 
N/P2O5 ratio in 
concentrates 
Manure export Manure export 
+ better N 
utilization* 
Fluxes:      
Imported feed 0 / 0 131 / 36 160 / 44 131 / 52 131 / 52 
Fertilizers, etc 245 / 24 180 / 0 78 / 0 216 / 0 162 / 0 
Milk and meat 58 / 24 89 / 37 89 / 37 89 / 37 89 / 37 
Exported manure 0 / 0 0 / 0 0 / 0 45 / 15 46 / 15 
Ammonia-N loss 35 / 0 51 / 0 38 0 47 / 0 45 / 0 
Leaching and runoff 151 / 0 171 / 0 142 / 7 166 / 0 113 / 0 
      
Coefficients:      
From herd to milk and meat 0.23 / 0.30 0.23 / 0.32 0.25 / 0.36 0.23 / 0.28 0.23 / 0.28 
From manure to soil 0.82 / 1.00 0.82 / 1.00 0.85 / 1.00 0.82 / 1.00 0.82 / 1.00 
From soil to crop 0.62 / 1.00 0.60 / 1.00 0.62 / 0.90 0.60 / 1.00 0.69 / 1.00 
From crop to herd 1.00 / 1.00 1.00 / 1.00 1.00 / 1.00 1.00 / 1.00 1.00 / 1.00 
*zero grazing, perfect timing and placement of slurry 
 
The permitted manure rates stipulated in the current Dutch Nitrates Directive Action Programme, 90 kg 
P2O5 and 250 kg N per ha for most dairy farms and 80 kg P2O5 and 170 kg manure-N per ha for other 
farms (Schröder & Neeteson, 2008), have strongly reduced the room for manure application and have 
indeed increased the necessity to take additional measures on livestock farms, including the export of 
manure. Wherever nearby arable spreading lands are scarce, however, the export of manure incurs 
costs for transport or even for the acceptance of manure as such. In any case livestock farmers will 
face the need to purchase more mineral fertilizer N, as indicated above. They are hence stimulated to 
increase the amount of available N per unit of applied manure P and improve the utilization of this 
available N from both manure and mineral fertilizers. 
 
Better utilization of N 
The amount of available N per unit manure P can be increased by limiting the gaseous N losses from 
housings and manure storage. Separation of manure can also increase the amount of N per unit 
manure P. The resultant solid fraction, rich in P, is less bulky and can be exported at lower costs to 
arable farms. The higher N (largely ammonia-N) to P ratio of the remaining liquid fraction matches 
better with the requirements of forage crops. Mineral fertilizer N could thus partly or largely be 
replaced by the liquid fraction, depending on the quality of the separation process (Sörensen & 
Thomsen, 2005; Birkmose et al., 2006; Schröder et al., 2007c). 
The greater the availability of N in manure, the more sensitive manures become for appropriate 
timings and methods of application. From this perspective spring injected manures are better utilized 
by crops than, for instance, manures that are surface applied in late summer. Note that autumn 
grazing too is therefore not conducive to a good utilization of N by crops, at least not in temperate 
regions where crop growth, including the growth of catch crops, is hampered by low temperatures 
(Schröder, 2005). 
 3 
 
 
Figure 2. Nitrogen fertilizer replacement value (kg N per 100 kg N applied) in the four years following application, 
as affected by the type of manure (Schröder et al., 2007b) 
0
10
20
30
40
50
60
70
1st year 2nd year 3rd year 4th year
Year
kg
 
N 
pe
r 
10
0 
kg
 
N 
ap
pl
ie
d
Solid manure
undigested cattle slurry
anaerobically digested cattle slurry
 
 
According to short term trials, anaerobic digestion (AD) of manure appears to contribute to a slightly 
better availability of N per unit manure P. However, long term trials have also shown that AD 
contributes little to cumulative availability of N over longer periods (Figure 2). Moreover, farmers are 
often inclined to stimulate the methane production of digesters by either importing substrates from 
food industries or sacrificing home grown crops. Schröder & Uenk (2006) have demonstrated that this 
deteriorates the N utilization at the whole farm level. 
Recent field trials have yet indicated that the long term N fertilizer replacement values (NFRVs) of 
slurries in particular, deserve an upgrade from the present 20-50 percent to 80 percent, provided that 
sufficient attention is given to an appropriate timing and method of application (Schröder et al. 2005a; 
2007b). The common short term trials tend to underestimate the NFRV of solid manures even stronger 
although it may take many decades before the NFRV of solid manures approximates the NFRV of 
slurries (Figure 3, Schröder et al., 2007b). 
 
Figure 3. Observed and simulated nitrogen fertilizer replacement value (kg N per 100 kg N applied), as affected 
by the number of consecutive applications, the type of manure and the adopted relative decomposition 
rate (RDR) (Schröder et al., 2007b) 
 
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70
Years
kg
 
N
 
pe
r 
10
0 
kg
 
N
 
ap
pl
ie
d
Cattle slurry, simulated, constant RDR
Cattle slurry, simulated, diminishing RDR
Solid manure, simulated, constant RDR
Solid manure, simulated, diminishing RDR
Cattle slurry, observed
Solid manure, observed
 
 
 4 
 
Environmentally sound manure rates 
Numerous factors including weather, soil type, manure composition, management skills, crop type, 
harvest regime, and environmental targets, determine which combinations of manure and mineral 
fertilizer are justifiable from an environmental point of view. We developed another model to handle 
this complexity (Schröder et al., 2007a). Simulations indicate that environmentally sound manure rates 
on dairy farms (i.e. rates associated with a P surplus of 0 kg per ha and a N concentration in the upper 
groundwater or nearby surface water < 11.3 mg N per litre) range from as much as 300 kg cattle slurry 
N (110 kg P2O5,) per ha, to as little as 190 kg cattle slurry N (70 kg P2O5) per ha, including the N and P 
in dung and urine excreted during grazing. Highest rates are, for instance, applicable to clay soils with 
only ‘zero grazing’ grasslands, whereas the lowest rates apply to dairy farms on dry sandy soils where 
silage maize is grown next to grazed grassland. In any case judicious supplements of either mineral 
fertilizer N or biologically fixed N are needed to exploit the manure P, thus avoiding P accumulation, 
whilst limiting risks of excessive N leaching. The simulated mineral fertilizer rates corresponding to the 
above manure rates and conditions, are 270 and 100 kg N per ha, respectively (Schröder et al., 2008). 
The simulated scenarios indicate that rates of 170 kg manure-N per ha as stipulated by the EU 
Nitrates Directive (Anonymous, 1991) are unnecessarily stringent for dairy farms using cattle slurry. If 
pig slurry or solid manures would be used instead of cattle slurry, however, rates of 170 kg manure-N 
per ha are generally associated with amounts of P that exceed the off-take in harvested crops 
(Schröder, 2005). 
 
 
Discussion and conclusions 
In intensive livestock systems feed requirements are partly met with imported feed stuffs. Under such 
conditions substantial losses are imminent due to an imbalance of imports (inputs) of N and P into the 
farm and exports (marketable outputs) of N and P from it. As potential local losses can thus be 
described by the following equations: 
 
Losses = input – output (kg/(ha.yr)) and output / input = efficiency (- / -) 
↔ 
Losses = input x (1 – efficiency)  
↔ 
Losses = output x ((1/efficiency) – 1), 
 
these losses can be limited by reducing the inputs whilst improving the efficiency, or by accepting a 
reduction of marketable outputs. The equations also show that efficient systems are not necessarily 
clean if their inputs are high, just as low input systems are not clean if their efficiency is low. The 
definition of environmentally sound livestock systems hence boils down to finding the right balance 
between intensities and efficiencies, as both factors may be interdependent. 
Our model explorations also indicate that there is little justification for uniform rules that would apply to 
all environments. We conclude that there is strong need to differentiate permitted manure application 
rates, unless policy makers do not mind that regulations are too slack in one situation and 
unnecessarily stringent in another situation. 
 
 
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2005a, Long term nitrogen fertilizer value of cattle slurry.

2005b, Improving the efficiency of nutrient use in cattle operations,

2007a, Permissible manure and fertilizer use in dairy farming systems on sandy soils in The Netherlands to comply with the Nitrates Directive target.

2007b, Long-term nitrogen fertilizer replacement value of cattle manures applied to cut grassland.

An evaluation of whole-farm nitrogen balances and related indices for efficient nitrogen use.

Bemestingswaarde van mestscheidingsproducten: theorie en praktijk,

Cattle slurry digestion does not improve the long term nitrogen use efficiency of farms. In:

Directive of the Council of December 12,

Koei’N 1.0: stroomdiagram en balans voor stikstof op melkveebedrijven.

Manure as a suitable component of precise nitrogen nutrition.

Need and opportunities to reduce phosphorus inputs, soil supply and loss from agriculture in The Netherlands.

Nitrates Directive requires limited inputs of manure and mineral fertilizer in dairy farming systems.

Nitrogen pollution by dairy cows and its mitigation by dietary manipulation,

Nutrient management regulations in The Netherlands. Geoderma 144:

Relationships between nutrient recycling, environmental impacts and agricultural production.

Separation of pig slurry and plant utilization and loss of nitrogen 15 labeled slurry nitrogen.

Utilization and losses of nitrogen and phosphorus from field-applied slurry separation products.

Simulated nutrient management options for intensive livestock farms with a surplus of manure

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