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odour/fly nuisance and further increase the fertiliser N value of the broiler litter by
minimising ammonia volatilisation losses. MANNER estimated that ploughing broiler
litter into the soil within 24 hours of application would reduce ammonia emissions
by c. 85% compared with leaving the manure on the soil surface. This increased
the potential fertiliser N replacement value of the broiler litter by a further £3,550/
year. After autumn application, rapid soil incorporation would have little impact on
the fertiliser N replacement value of the broiler litter because any of the saved N
would subsequently be lost by overwinter nitrate leaching from the ‘leaky’ sandy soil
(Chambers et al., 2001).
CONCLUSIONS
Considerable capital investment (between £3,500 and £5,880/year over 20 years)
in extra slurry storage capacity was required to change slurry application timings
from autumn to spring on the model pig and dairy farms. The increased fertiliser
N replacement of the slurry as a result of the reduced nitrate leaching losses was
modest at between £180 and £380/year on the pig farms, and £280 and £760/year on
the dairy farms. The amortised (over 10 years) repayment cost of the bandspreading
equipment was c. £3,250/year on both the pig and dairy farms, compared with an
annual benefit in increased fertiliser N replacement value as a result of reduced
ammonia losses of £230/year on the pig farms and £330/year on the dairy farms.
The net costs of reducing nitrate leaching losses through the bandspreading of
slurry in spring compared with autumn surface broadcast applications ranged from
£6.46 to £14.09/kg of nitrate-N abated on the pig farms, and £4.25 to £12.35/kg of
nitrate-N abated on the dairy farms. The ammonia abatement costs were £5.60/kg
of ammonia-N saved on the pig farms and £3.96/kg of ammonia-N saved on the
dairy farms.
On the broiler farm, reducing nitrate leaching losses by changing the manure
application timing from autumn to spring increased the fertiliser N value of the manure
by £3,860/year. Moreover, the increased fertiliser N value of spring applied broiler
litter was around £1,000 greater than the annual amortised repayment costs of a
solid manure spreader capable of applying the broiler litter evenly to growing crops
in spring, although this policy would not be a practical option in most cases. Rapid
soil incorporation of the spring applied broiler litter reduced ammonia volatilisation
losses and increased the fertiliser N replacement value by a further £3,550 compared
with leaving the broiler litter on the soil surface.
Recent research has shown that changing slurry application timings from autumn to
spring/summer is likely to affect other N loss pathways (so called ‘pollution swapping’).
This highlights the need for an integrated approach to manure N management, so
that management practices which aim to reduce nitrate leaching losses (i.e. moving
from autumn application timings to spring/summer) do not exacerbate other N loss
pathways, either through ammonia losses to air or ammonium-N and other nutrient
losses in drainage waters from clay soils.
ACKNOWLEDGEMENT
Funding of this work by the UK Department for Environment, Food and Rural Affairs
is gratefully acknowledged.
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