In Brittany where pig production is particularly intensive, copper (Cu) supplementation was as high as 125 ppm Cu in the fattening pigs diets. Since 5 years, the Cu supplementation tend to be lowered to 35 ppm as other growth factors have been introduced. Investigations were made to verify the Cu enrichment of soils in Brittany and 190 fields have been tested. Also pot and microplot experiments were done by incorporating CuSO4 to soil. A survey made over 6 years on those soils showed that during the survey period Cu enrichment and also zinc (Zn) enrichment could be observed, with an average increase of 1,23 ppm and 2,96 ppm respectively. On farms with the highest pig density the CU enrichment measured was 2,82 ppm and for Zn, 9,06 ppm. The pot and plot experiments showed that it would be dangerous to incorporate 500 kg/ha of Cu or Zn ( concentration higher to 120 ppm in the soil) into Finisterian soils for crops such as Italian rye-grass, maize, winter-wheat and barley. Considering a manure application rate of 50 ton manure/ha per year, manure coming from 70 pigs (represents a density of 70 pigs/ha) that would have received a diet supplemented with 125 ppm of Cu and 150 ppm of Zn, it would take 2 centuries before the concentration of these 2 elements would reach 500 kg/ha in the arable layer. As the phytotoxicity of Cu and Zn is complementary, it is believed that the dangers would appear after one century.

Cu and Zn accumulation can cause toxicity and attention has to be given particularly to the supplementation in the diet in order to provide those elements to the animal requirement level and not higher. For higher pig density of higher manure application rates, the phytotoxicity would appear sooner. The laboratory testing allowed them to verify their theoretical predictions.