Copper Accumulation in Brittany Soils through Enriched Pig Slurry; Phytotoxic Risks

Abstract

In Brittany pig breeding is particularly intensive and the ‘Département du Finistère’ was the subject of our special attention. Copper complementation of feeds for fattening-pigs was often 125 ppm Cu five years ago but today the tendency is for a decrease to only 35 ppm because other growth-factors are incorporated. Zinc supplementation remains at about 150 ppm Zn.

Two methods were used for these investigations: 1) Direct measurement of Cu enrichment of soils for 190 fields (64 breeders); 2) Pot and microplot experiments with CuSO₄ incorporation into the soil. Soils analysed in 1979, 6 years, therefore₅ after the beginning of this survey, indicated a definite enrichment for Cu (NH₄Ac-EDTA method) and an even greater one for Zn. Averages of increases are respectively +1.23 ppm and +2.96 ppm. In farms with the highest pig density₃ enrichments are: +2.82 ppm for Cu and +9.06 ppm for Zn. Copper and zinc were also determined by 0.5 N, HN03 and by nitroperchloric digestion. From our pot and field experiments we believe it is dangerous to incorporate 500 kg/ha Cu or Zn into Finisterian soils for cultures such as Italian rye-grass, maize₃ winter-wheat, barley (≥ 120 ppm Cu or Zn by EDTA method). If we take for example, a pig fattener with moderate pig density (70 pigs produced/ha/year = 50 t slurry/ha/year) and with feeds containing Cu 125 ppm and Zn 150 ppm, copper and zinc incorporated in the arable layer would reach 500 kg/ha after two centuries; as we believe that the phytotoxicity of Cu and Zn are complementary₃ the dangers could appear after only one century.