办事指南

Four legs bad

点击量:   时间:2017-11-17 02:01:09

By Andy Coghlan WHAT’s good for pigs isn’t necessarily good for the countryside. When left to roam, sows create “hot spots” sodden with nitrogenous waste. A quarter of Britain’s pigs are now raised in the open. Left to their own devices, sows urinate and defecate in tightly defined areas. “Pigs don’t distribute their favours evenly,” says Sandra Edwards of the Scottish Agricultural College in Aberdeen. In these areas, she and her colleague Christine Watson have found that concentrations of nitrate and ammonium salts in the soil exceed 300 kilograms per hectare—several times greater than the concentrations produced by applying nitrogen fertilisers. Although the total amount of nitrogen deposited by the pigs is not excessive, its concentration into hot spots makes it more likely that autumn rains will wash the nitrogen away, creating problems in watercourses. When such fields are turned back to arable production, the crop grows unevenly. In the nitrogen-rich areas, some bloated plants collapse under their own weight, while elsewhere the plants are feeble and underfed. Farmers who keep their animals in intensive piggeries can store pig waste and spread it evenly on arable land. If sows are kept outside,