You are correct with the stocking density you quote for outdoor sows, although this is based on welfare grounds and free draining soils. The rate would need to be reduced on poorly draining soils liable to poaching or areas of high rainfall.
The Defra Code of Recommendations for the Welfare of
Livestock - Pigs (2003) states the following:
"Field stocking densities must reflect the
suitability of the site and the system of management. A guidelines of 25 sows
per hectare overall is reasonable for suitable sites. You may need to reduce
stocking densities on less ideal sites or in extreme circumstances during
periods of adverse weather. More information can be found in Defra's booklet,
"Site suitability for outdoor pig farming"."
You can contact Defra Publications dept on: 0845 955
6000 or see http://www.defra.gov.uk
There are no published guidelines that we are aware
of for outdoor finishing stocking rates. This is because the majority of pigs
are finished indoors, and those that are finished outdoors are usually housed in
tents and small paddocks. Commercial scale finishing outdoors is often not
financially viable on account of feed use.
The stocking rate will also depend on the soil type
and the local climate.