The effect of high concentrations of minerals in pig diets on the convertibility to methane and in vitro digestibility of the resulting pig faeces

Abstract

Eight diets, based on either wheat and soybean meal or wheat and meat meal and containing low or high levels of trace and macroelements, were fed to growing pigs and the biodegradability of the resulting faeces was assessed in terms of in vitro digestibility and of gas production during anaerobic digestion in batch or semi-continuously fed digesters. The digestibility by the pigs of dietary components was also determined. The digestability of the dietary components, except for cell wall, was slightly higher for the diets containing soybean meal and low levels of ash than for the rest of the diets. Most of the zinc and copper in the semi-continuously fed digesters appeared bound to the solid material of the digester contents, while most of the sodium and potassium was found in the liquid phase. Increasing the loading rate in the semi-continuously fed digesters resulted in increased total daily gas production, but the efficiency of conversion of organic matter into gas was only slightly affected by the loading rates used. Gas production per gram of Volatile Solids added to the digester was significantly influenced by the type of the dietary protein supplement and, to a lesser extent, by the ash content in the faeces, but not by the trace element concentration. However, the faeces with high concentrations of trace elements had lower in vitro digestability and gave lower organic matter reductions, and rates and yield of methane per gram of Volatile Solids added, when digested in batch digesters, than did faeces with low concentrations of trace elements. These results indicated that higher levels of trace and macroelements than those normally included in pig diets as growth promotants or mineral supplements should pose no threat to anaerobic digestion of the resulting faeces.