Evolution in the fertilising value of pig slurry during storage pit emptying. Influence of mixing

The aim of the present study was twofold : firstly, to investigate the evolution in the composition of pig slurry in the spreader during the process of emptying the storage pit and secondly, to study mixing efficiency of slurry in the storage pit and during spreading. Thirty farms were selected and divided into 3 groups according to the level of mixing.Farmers from the first group did not mix slurry during spreading; in the second group, slurry was only mixed before spreading and in the last group, mixing was performed before and during spreading. During the emptying of the storage pit, the density changed significantly. After a slight decrease in density, observed during the middle phase of emptying (60 to 80% of the slurry had been removed from the pit), density increased by 7 points, which corresponds to a 31% increase in dry matter. During the spreading period, ammonia nitrogen concentration was stable, between 2.8 and 3.0 g/l, therefore, this soluble nutrient appears to be better distributed in slurry. In our survey, mixing did not significantly reduce the enrichment of slurry at the end of emptying. Nevertheless, we observed a large reduction in the variability of slurry composition during spreading. The standard deviation of the composition of the slurry spread was 29.9 g DM/l for the first group compared with 13.7 and 13.0 g DM/l for the second and the third groups, respectively.