The rationale for a maximum stocking density of 1500 chickens per hectare is quite clear. One thousand hens produce
approximately 20 tonnes of semi dry poultry manure each year.
Allowing the hens to free range over a pasture area has to be
designed around the need to maintain pasture cover which is vital for farm
sustainability, to limit dust and odour nuisances to neighbours and to
avoid off-site pollution caused by the nutrients in the manure.
Maintaining well managed
pasture over the range area is seen as the best method to handle
these issues and is a requirement of most Standards for the
production of free range poultry. A well managed pasture provides an
opportunity for retaining and utilizing the nutrients from the
poultry flock on site and avoiding the problems of leeching excess
nutrients into ground water and nutrient run off into waterways or
onto neighbouring properties. Excess poultry manure applied to
pasture has been shown to increase soil salinity.
The upper limits are
determined by the success in managing the rotation of the flock
around the pasture to maintain cover and growth of the pasture and
the nutrient load that the system can handle.
The maximum limit on
nutrient loads is seen as critical and assessable and this was a
major factor in formulating the recommended upper limit on stocking
rates for the range during the development of free range egg
production standards and the Model Code.
Agronomists assisted with
the exercise and they looked at their experience with highly
productive dairy pastures in the County of Cumberland (NSW) which had
been fertilized with poultry manure and irrigated. These perennial
pastures were mainly a Kikuyu Ryegrass Clover pasture which could
yield in excess of 20 tonnes of dry matter a year.
Such a pasture would
normally be recommended to be fertilized with 172 kg of N from Urea,
22kg of P from Single super and 60 kg of K from Muriate of potash.
Poultry manure application rates had traditionally been at a higher
rate resulting in high phosphate and potassium levels and an increase
in soil ph but it was felt that an application rate of 15 tonnes of
poultry manure per ha per annum would be sustainable in the longer
term. The dairy farms had been using poultry manure at these rates
for over twenty years.
Using semi dried poultry
manure as the calculation, 15 tonnes of manure per annum would be
applying 293 kg of N, 195 kg of P and 97.5 kg of K per ha per annum
to the pasture. This rate was equivalent to the output of 750
hens. However since the hens would spend the night in the laying
house from which the manure could be removed and used at another site
it was translated into supporting a maximum daily stocking rate of 1500 hens
per hectare.