Settling characteristics of nursery pig manure and nutrient estimation by the hydrometer method

The hydrometer method to measure manure specific gravity and subsequently relate it to manure nutrient contents was examined in this study. It was found that this method might be improved in estimation accuracy if only manure from a single growth stage of pigs was used (e.g., nursery pig manure used here). The total solids (TS) content of the test manure was well correlated with the total nitrogen (TN) and total phosphorus (TP) concentrations in the manure, with highly significant correlation coefficients of 0.9944 and 0.9873, respectively. Also observed were good linear correlations between the TN and TP contents and the manure specific gravity (correlation coefficients: 0.9836 and 0.9843, respectively). These correlations were much better than those reported by past researchers, in which lumped data for pigs at different growing stages were used. It may therefore be inferred that developing different linear equations for pigs at different ages should improve the accuracy in manure nutrient estimation using a hydrometer. Also, the error of using the hydrometer method to estimate manure TN and TP was found to increase, from +/- 10% to +/- 50%, with the decrease in TN (from 700 ppm to 100 ppm) and TP (from 130 ppm to 30 ppm) concentrations in the manure. The estimation errors for TN and TP may be larger than 50% if the total solids content is below 0.5%. In addition, the rapid settling of solids has long been considered characteristic of swine manure; however, in this study, the solids settling property appeared to be quite poor for nursery pig manure in that no conspicuous settling occurred after the manure was left statically for 5 hours. This information has not been reported elsewhere in the literature and may need further research to verify.