Elements of waste from pigs : knowledge and control

Abstract

This paper summarises the contributions that the «Porcherie verte» programme makes to our knowledge and control of nitrogen, phosphorus and trace metals, which each have an impact on the environment.

The decrease in protein content of feed reduces the amount of nitrogen excreted by animals. An important part of nitrogen is eliminated as a gas when the animals are placed on floor litter or when the manure is used as compost. The importance of these nitrogen losses, however, varies according to the techniques used and the nature of the substrates. The biological treatment capts most of the nitrogen and some farms can capt the rest in potentially exportable co-products. The good agronomic valorisation of waste requires good knowledge of the nitrogen fertilisation value, which makes the typological approach developed within the framework of the programme easier.

A certain number of food levers decrease the fraction of food phosphorus that is excreted in waste : adjusting food intake by a better knowledge of the animal’s needs, improvement of digestibility of food phosphorus by improved knowledge of its availability in different raw materials or by adding exogenous phytase. Biological treatments with phase separation capt phosphorus in potentially exportable co-products and phosphate fertilising value of waste is generally very high and easy to predict.

Trace metals (copper and zinc) are often added to feed in concentrations much higher than those needed by the animal (to avoid deficiency) in order to benefit from their protective effects against digestive pathologies. Supplementation of trace metals is useful during the post-weaning phase, but not beyond and the addition of microbial phytase in feed improves the availability of zinc for the animal. Biological treatments with phase separation capture zinc and copper in the potentially exportable co-products. After spreading, trace metals are little mobile in the soil whereas soil supply generally exceeds the export capacities of plants. This can therefore lead to phytotoxicity situations on a short to long term.

Finally, the best way and most economic to manage breeding waste is the adapted use of fertilising value, which is obtained by a good balance between the quantity of animal products and the capacity of soil to absorb waste. Without such a balance, capting excess waste by using litter or composting manure has an important impact on the environment even though these solutions do not resolve anything in terms of phosphorus and trace metals. The most sophisticated biological treatments eliminate phosphorus and a part of the trace metals in the potentially exportable co-products but they have a very high cost for the economy and ecology.