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Newsletter Nutrispices

April 2016 - Issue 17


Dear <<First Name>>,

During my last visit to France, I had the chance to meet and discuss with Dr Jean-Yves Dourmad from French National Institute of Agronomical Research. I asked him why the European authority regulated the use of zinc Oxide in piglet feed and whether we should expect other countries in Asia to follow similar strategies.

Zinc is traditionally included at 2,500-3,000ppm into prestarter and starter piglets feed to control bacterial population and help to maintain villi integrity in order to prevent the occurrence of diarrhea at the time of weaning. However, it has been demonstrated that at such high dosage in the animals for such a long period (6 to 8 weeks), zinc would cause serious environmental concern in areas of intensive pig farming.
It is important to take into account that at high dosage, zinc is not absorbed contrarily to the other ingredients in the diet. Therefore, the zinc is getting concentrated in the feces to a level higher than 3000ppm of dry matter. Moreover, zinc is not eliminated in the soil but is actually accumulating every time we apply manure. The total zinc concentration in soil will increase over the years to reach critical level that could become toxic for plants and soil microflora.
‘Total zn’ concentrations in soil usually fall in the range of 10 to 300 mg/kg, with concentrations above 150 mg/kg regarded as high (Landon, 1991), and likely to result in reduced plant growth.
High levels of zinc inhibit the uptake of iron, and it is common to find symptoms of severe iron deficiency induced by zinc toxicity. Iron deficiency is characterized by a pale yellow to white interveinal chlorosis on the younger leaves, and may eventually lead to necrosis of the leaf blades and growing point. Cultivars, which normally have purple pigmentation in the youngest leaves, become bright pink at the tip.
Soil microflora are even more sensitive than plants. The upper limit for microflora (150mg zinc / kg soil dry matter) is approximately half the level required to reach plant toxicity.
For the pig farms using manure as a source of energy through their biogas plant, the excess of zinc in the pig feces can also be an issue by reducing the growth of methanogenic bacteria.
Because of this environmental risk, several countries have reacted and set a maximum level of zinc in piglet diets.

This maximum level includes the 3 possible sources of zinc: zinc added in the premix for nutritional purpose, native zinc coming from feed ingredients (mostly plants) and zinc added as zinc oxide at pharmacologic level used to prevent diarrheas.
The figure above shows different scenarios depending on the amount of zinc fed to piglets. Scenario A is a situation in which weaning pigs are fed a diet with a 2,500ppm zn between 8-15kg bodyweight. When we are reducing the maximum dosage of zinc to 250ppm (scenario B), we are reducing zinc excretion by 33% and we can reach 53% reduction by not using zinc oxide as growth factor (scenario C). With the current use of 2,500 - 3,000ppm of Zinc Oxide, zn content in slurry (about 1,250ppm on dry matter) still exceeds the concentration allowed for organic fertilizers in Europe (600ppm).
At such dosage and assuming that 170 kg Nitrogen / ha of slurry are spread each year as fertilizer, it will take 25 to 30 years to reach toxicity for soil’s microorganisms. This means that if we do not change our current practices regarding high dosage of zinc oxide in piglet feed, the situation will become critical very soon.

It is highly likely that Asian local authorities anticipate such level of soil pollution. We can expect them to enforce regulation in the countries where the use of Zinc is already limited and to design a new regulations to limit the use of Zinc Oxide in the countries where it is legally used. The question is only when?

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