Nutrition and the Environmental Impact of Outdoor Pigs

By Professor Sandra Edwards, Newcastle University, School of Agriculture, Food and Rural Development. The growth of outdoor pig production in the UK has been driven by economic, marketing and agronomic advantages - and it has brought recognised niche marketing opportunities.
calendar icon 14 December 2007
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Outdoor pig production offers the producer a cost effective production, while the outdoor image is favourable with the consumer perception of animal welfare. They also offer the landowner a good "break crop" in an arable rotation, giving effective land clearance of volunteers and a significant manurial residue for the subsequent crop, allowing savings in inorganic fertiliser.

High density

Prof. Sandra Edwards

However, management of their environmental impact poses challenges under conditions of year round land occupation and nutrient deposition. At current commercial stocking rates, the level of deposition of excreted nutrients onto land can be high. The historic approach of using a single diet for dry and lactating sows - for simplicity of management - means that inputs could total as much as 600 kg Nitrogen and 150 kg Phosphorus per hectare per annum.

The free-draining soils best suited to pig production are also those with the greatest risk of nutrient leaching to ground water. Therefore, plant capture and utilisation of these deposited nutrients is highly desirable. However, the nature of rotation management and land rental arrangements often results in pigs moving onto autumn stubbles, where vegetation cover has little opportunity to establish. Even when pigs move on to established vegetation, their rooting and foraging activities can quickly remove ground cover, and so the vegetative uptake potential.

Bare soils also have a higher risk of gaseous nitrogen loss by volatilisation in warm dry conditions, and nitrogen and phosphorus run-off and leaching during periods of high rainfall. While nose-ringing of pigs can preserve vegetation, and reduce these risks, animal welfare considerations make this approach contentious.

Reducing excreted nutrients

There is no single, or simple, solution to preventing environmental impact of outdoor pigs. Producers must therefore combine nutritional, management and agronomic approaches to target key risk factors.

Modern nutritional expertise makes it possible to match dietary inputs more closely to requirements, and hence reduce excess nutrients excreted onto land. Historically, diet formulation for outdoor pigs has focussed more on ease of management and the physical quality (and durability) of feed than on minimising nutrient excretion. These objectives may sometimes be in conflict.

Limitations are now being imposed by raw material costs, lack of a wider range of commercially available synthetic amino acids and uncertainties about the safety margins needed to avoid detrimental effects on performance. However, significant reductions can be made by raw material selection, enzyme utilisation and dietary specification adjustment for season, stage of the reproductive cycle and, in the case of batch stocked sites, for sow parity.

Reducing nutrient loss from feed wastage is possible.

Vegetation cover reduces nutrient leaching, but it's not always available or easy to retain

Outdoor systems do require significantly higher feed inputs than indoor systems, typically by 10-15 per cent more. This is due to climatic penalties and waste through ground feeding losses and wildlife scavenging. The relative contribution of these sources of poorer feed utilisation is still uncertain, but better feeding techniques could reduce the losses.

But to target nutrient inputs more accurately, it is important to know and maximise actual intake, and minimise wastage of feed provided.

Evidence from the spatial distribution of soil nutrients after pig occupation suggests significant feed waste might be occurring around feeding areas. Management strategies to reduce this by trough feeding have improved feed efficiency, but they require care in implementation to minimise aggression/behavioural problems and ensure each animal receives an adequate level of food etc.

Other management factors can be manipulated to reduce environmental impact. For example:

  • Dunging behaviour
    Even when nutrient excretion is minimised by good diet formulation, the excretory behaviour of pigs can still result in localised areas of high soil nutrient loading and hence pollution risk.

    Understanding and modifying the natural behaviour of the animals is key to achieving better spatial distribution of excreta, and hence more even manuring of following crops.

  • Vegetation cover
    Vegetation cover prevent nutrient leaching, but its not always available or easy to retain Capture of excreted inorganic nutrients by vegetation and their fixation in slow release organic forms offers the greatest potential for minimising short term nutrient losses and maximising long term nutrient utilisation in rotations incorporating pigs.

  • Roots and fibre
    Dietary modifications to reduce foraging motivation of dry sows by increasing fibre content may slow vegetation destruction, and strategic use of root crops might also play a role. Predicting and adjusting for nutrient intake from pasture and forages will then become important in refining dietary inputs. Data on this subject are currently limited, but indicate that significant contributions to overall nutrition could be achieved.


Managing the nutrition of outdoor pigs for reduced environmental impact, in addition to maximal reproductive performance, requires more closely targeted nutrient inputs than in the past, in combination with skilled system management and careful rotation planning.

By achieving this, outdoor pigs can continue to be regarded as a valuable rotation option rather than an environmental hazard.

Work described in this paper has been funded by SEERAD and by the Defra "Ecopig" project (IS0215), a collaborative research project with Newcastle University, SAC, CEH and BQP, and the co-operation of commercial producers .

Further Reading

- Find out more about Professor Edwards research projects by clicking here.

December 2007
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