Validation of PigBal model predictions for pig manure production
A. G. Skerman A D , S. Willis A , E. J. McGahan B , M. G. Borgognone A and D. J. Batstone CA Department of Agriculture, Fisheries and Forestry, Toowoomba, Qld 4350, Australia.
B FSA Consulting, Toowoomba, Qld 4350, Australia.
C Advanced Water Management Centre, University of Queensland, St Lucia, Qld 4072, Australia.
D Corresponding author. Email: alan.skerman@daff.qld.gov.au
Animal Production Science 56(7) 1081-1090 https://doi.org/10.1071/AN14702
Submitted: 17 July 2014 Accepted: 1 December 2014 Published: 20 February 2015
Abstract
PigBal is a mass balance model that uses pig diet, digestibility and production data to predict the manure solids and nutrients produced by pig herds. It has been widely used for designing piggery effluent treatment systems and sustainable reuse areas at Australian piggeries. More recently, PigBal has also been used to estimate piggery volatile solids production for assessing greenhouse gas emissions for statutory reporting purposes by government, and for evaluating the energy potential from anaerobic digestion of pig effluent. This paper has compared PigBal predictions of manure total, volatile, and fixed solids, and nitrogen (N), phosphorus (P) and potassium (K), with manure production data generated in a replicated trial, which involved collecting manure from pigs housed in metabolic pens. Predictions of total, volatile, and fixed solids and K in the excreted manure were relatively good (combined diet R2 ≥ 0.79, modelling efficiency (EF) ≥ 0.70) whereas predictions of N and P, were generally less accurate (combined diet R2 0.56 and 0.66, EF 0.19 and –0.22, respectively). PigBal generally under-predicted lower N values while over-predicting higher values, and generally over-predicted manure P production for all diets. The most likely causes for this less accurate performance were ammonium-N volatilisation losses between manure excretion and sample analysis, and the inability of PigBal to account for higher rates of P uptake by pigs fed diets containing phytase. The outcomes of this research suggest that there is a need for further investigation and model development to enhance PigBal’s capabilities for more accurately assessing nutrient loads. However, PigBal’s satisfactory performance in predicting solids excretion demonstrates that it is suitable for assessing the methane component of greenhouse gas emission and the energy potential from anaerobic digestion of volatile solids in piggery effluent. The apparent overestimation of N and P excretion may result in conservative nutrient application rates to land and the over-prediction of the nitrous oxide component of greenhouse gas emissions.
Additional keywords: diet, effluent, nutrients, solids.
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