Agriculture Reference
In-Depth Information
competitive bacteria. On pasture, the length of grass is
important, organisms surviving longer at the base of the
grass than at the top of the leaf. Although salmonellas, for
example, have been reported as surviving for many
months in soil, it is unusual for them to survive for more
than 14 days on grass (Findlay, 1972).
E. coli
have been
shown to survive for more than 11 weeks in slurry but for
only 7-8 days on the pasture (Rankin and Taylor, 1969).
Comparison of survival times for different organisms
is difficult, however, owing to the number of variables
to be considered and differences between experimental
design and measurement technique.
Experiments have shown that under normal farming
conditions, infection of adult grazing animals from
contaminated slurry on pasture is unlikely. However, if
pasture which has been spread with fresh slurry is grazed
within a few days by young or stressed susceptible animals,
infection may occur.
On the basis of current knowledge, the following
recommendations can be made:
Table 6.3
Quantities of excreta, as slurry, produced by livestock
Type of livestock
Output of livestock
(faeces and urine)
Dairy cow
41 kg/day
Pigs (fatteners)
a
4.5 kg/day
Poultry (1000 laying hens)
800 kg/week
a
Pigs fed dry. Use of swill or whey may increase this to
14-17 kg/day.
by surface run-off, slurry injection must be the method of
choice for application.
The survival of potentially pathogenic micro-organisms
in farmyard manure or slurry is dependent on several
factors including the following:
1
The micro-organism
. Some strains or serotypes of a
micro-organism, for example, salmonellas, survive
longer in the environment than others. Some can
assume resistant forms which may survive for several
years, for example, anthrax.
2
pH
. The pH of fresh slurry and farmyard manure var-
ies from 6.2 to 8.0 depending upon the species of ori-
gin and the constituents. In the case of slurry, the pH
drops to below 6.5 within the first 4 weeks of storage
and then gradually returns to zero. As a result, the
majority of micro-organisms, in the case of
Salmonella
over 90%, are destroyed in the first month of storage.
The low pH of peat and acid soils creates unfavourable
conditions for the survival of many pathogens.
Attempts have been made to sterilise slurry by altering
the pH. This has been achieved by the addition of
lime, formalin, ammonium persulphate and formic
acid, but the expense of the procedure makes these
techniques suitable only for situations where there has
been an outbreak of a serious disease. Slaked lime is
used to reduce the infectivity of slurry following
serious
Brucella
outbreaks in Northern Ireland.
3
Sunlight
. Ultraviolet light can have a bactericidal effect.
4
Temperature
. The rise in temperature which occurs
during composting of farmyard manure is generally
sufficient to destroy all bacterial and viral pathogens
except for bacterial spores. Shepherd
et al
. (2007)
demonstrated that while composting, with periodic
heap turning, was a practical approach to inactivating
E. coli
O157:H7 in cattle wastes on farm, the organ-
isms could survive for months at the heap surface if it
was not turned. However, since there is no discernible
temperature increase within stored slurry, microbio-
logical survival times are generally much longer.
1
Slurry should be stored for at least 60 days prior to
spreading on land.
2
Any disease hazard can be virtually eliminated by
spreading slurry or farmyard manure on arable land
or grassland used for conservation.
3
Pasture treated with slurry should not be grazed for at
least 30 days after spreading.
4
Since young animals are generally more susceptible to
disease, they should graze treated pasture only after
a prolonged period following application.
5
Utilisation of slurry should be related to the plant
nutrient requirements.
6
Slurry, manure and digested sewage sludge should be
ploughed in
immediately after application
. Slurry and
raw liquid sludge can be injected to a depth of
50-80 mm in grooves 200-300 mm apart.
7
Ground treated with slurry/manure should preferably
be ploughed immediately.
Sewage sludge
The solid material from human sewage sedimentation
tanks is available for agricultural use in raw and dry
digested forms. Raw sludge contains potentially harmful
bacteria and, on occasions, the eggs of tapeworms such
as
Taenia saginata
. In addition, sludge can contain many
undesirable heavy metals such as cadmium.
In Great Britain, the application of wastes from off-
farm sources on agricultural land is controlled by the
Control of Pollution (Silage, Slurry and Agricultural Fuel
Oil) Regulations 1991 and the Collection and Disposal of
Waste Regulations 1988, which allow their use, without
licensing, provided that they fertilise, or otherwise ben-
efit, the land. The use of sludge is governed by the Sludge
The survival of pathogenic micro-organisms in soil
is in addition influenced by the initial number of organ-
isms, the available moisture and the presence of
