A rapid method for determining TN and TP concentrations in swine slurry using historical data

Quantifying nitrogen and phosphorus concentrations in liquid swine manure at the time of field application offers potential benefits that include reduced environmental impact from over-applying nutrients and better matching of manure nutrient levels with fertilizer recommendations or crop removal rates. This study developed and assessed a rapid on-farm method to predict total nitrogen (TN) and total phosphorus (TP) concentrations of swine slurries for a single farm. Swine slurry samples were collected from gestating sow and finishing barn deep pits during loading. A liquid ratio (liquid volume/filtrate volume) was generated by sieving a known manure volume through a 0.85 mm sieve. Gestation and finishing barn liquid ratios correlated well with total solids (TS) (R ² = 0.89). Regression analysis demonstrated that the liquid ratio and historical TN and TP concentrations of manure sample results could be used to predict TN and TP concentrations (R² = 0.92) prior to field application. Liquid ratio explained 82% of the TP variance, while a proxy variable for swine growth stage explained an additional 10% of the TP variance. The proxy variable relating to swine growth stage explained 89% of the TN variance, whereas adding the liquid ratio explained an additional 2% of the variance. Model predictions of TN and TP concentrations for finishing barn validation samples exhibited mean prediction errors of 11% and 10%. Gestation barn predictions indicated that the models were precise, but not accurate, with an average error of 61% for TN and 48% for TP; these errors were attributed to retaining slurry for an additional season. This study showed that liquid ratio can be used as a rapid method for predicting TP concentrations of swine slurries, whereas animal growth stage is a better predictor of TN.