The physical and climatic conditions of a particular site are critical in determining whether it is suitable for outdoor pigs. However, the management of the site is also important, as marginal sites require a higher standard of management in order for them to be acceptable.
The main criterion in selecting a site must be to avoid waterlogged conditions. The main factors that effect this are as follows:
1.1. Physical factors
1.1.1 Soil type
A light, free-draining soil is ideal. Generally these will have a high content of sand, or possibly silt, but a low content of clay. Soils have varying proportions of these three ingredients so it is impossible to draw a fine line between suitable and unsuitable soils.
The underlying structure of the soil is also important. Free-draining subsoils such as gravel, chalk and limestone will allow topsoils to dry out more quickly and reduce the time spent waterlogged. Therefore a marginal soil type could be more suitable if the subsoil is free-draining.
Ideally an outdoor pig unit should be sited in a low rainfall area to avoid waterlogging of soils. However, there is an important interaction with soil types. A marginal soil type will be more suitable in an area of low rainfall than where the rainfall is higher.
Although this factor does not directly affect waterlogging, it is still an important consideration. Sloping fields can be an aid to drainage, but should be avoided for farrowing paddocks. On slopes, bedding and piglets can roll in the hut increasing the risk of overlying. Also, natural shelter is important especially in areas with a high risk of wind chill. The location of the site should take into account the risk of any natural hazards. In particular, sites susceptible to flooding should be avoided because it can take a considerable time to move pigs to safety.
1.2. Management factors
1.2.1 Ground cover
This has a very significant effect on the waterlogging properties of a soil. A good, well-established grass sward provides additional drainage and protects the soil from damage when it is waterlogged. Good ground cover can offer behavioural benefits, possibly through its insulation and therapeutic properties. Any nutritional value to pigs from eating grass or other ground cover is negligible.
1.2.2 Stocking Density
This is the factor over which the stockman has most control. Relatively high stocking rates are possible on the lighter, free-draining soils in the low rainfall areas. They need to be reduced on less suitable sites to avoid poaching damage to waterlogged soils. The practical maximum stocking density will depend on all the physical and management factors discussed in this section.
1.2.3 Paddock management
By rotating paddocks, being flexible with systems and allowing wetter areas to rest, less suitable sites can be managed to reduce the effect of poaching waterlogged soils.