Potential loss of nutrients from different rearing strategies for fattening pigs on pasture

Nutrient load and distribution on pasture were investigated with fattening pigs that: (1) spent a proportion of or their entire life on pasture; (2) were fed either restrictively or ad libitum; and (3) were weaned at different times of the year. N and P retention in pigs decreased the longer they were kept on pasture. The contents of soil inorganic N and exchangeable K were significantly raised compared with the soil outside the enclosures but with no differences between treatments. Pig grazing did not affect extractable soil P. Regular moving of huts, feeding and water troughs was effective in ensuring that nutrients were more evenly distributed on the paddocks. Grass cover, as determined by spectral reflectance, was not related to the experimental treatments but only to the time of year. During spring and summer, grass was present in parts of the paddocks, whereas during autumn and winter, the pigs kept grass cover below 10%. Fattening pigs on pasture carry a high risk of nutrient loss and it is concluded that the most environmentally acceptable way of keeping them on pasture involves a combination of reduced dietary N intake, reduced stocking rate and seasonal rather than round the year production.