Effect of feed wastage on piggery effluent characteristics

Across the Australian pork industry, a 5% change in feed wastage is 82 000 tonnes feed/year, with a current value of approximately $38m. While its importance is widely recognised, there are currently no practical robust methods to quantify feed wastage. Feed wastage influences feed efficiency, shed (manure), effluent management practices and effluent characteristics. Accordingly, the present study hypothesised that feed wastage could be estimated from effluent characteristics.

To relate feed wastage to effluent characteristics, the study used an innovative modelling and experimental approach to simulate different rates of feed wastage and assess its effects on piggery effluent. Quantities of pig feed (wheat and barley based grower diet), faeces, urine, flush water (clean bore water) and shed effluent (collected over 24 h in an agitated sump) were sampled from a commercial batch grower shed, housing 535 pigs (average 45 kg live weight at 13 weeks of age). Pre-determined proportions of these samples were mixed to simulate shed effluent having four different rates of feed wastage (Treatments A–D). Treatment A was composed of faeces, urine and flush water only, to simulate zero feed wastage. Treatment B was raw effluent discharged from the shed. Treatments C and D were composed of raw shed effluent with added feed, to simulate higher rates of feed wastage. The resulting samples were analysed to evaluate the total solids (TS), volatile solids (VS) and biochemical methane potential (BMP). Analyses of variance (ANOVA) followed by protected least significant difference (l.s.d.) testing were performed on the analysis results, at the 5% level, using Genstat 16.1 (VSN International, Hemel Hempstead, UK). The AUSPIG growth and production simulation model (Davies et al. 1998) was used to simulate the age, live weight, P2 back-fat and feed intake of the pigs in the trial shed over their entire growth cycle (wean-to-finish). The genotype settings and feed intake factors in the model were adjusted so that measured and predicted performance parameters (growth rate, P2 backfat and feed intake) were similar for the batch of pigs. For each of the four treatments, the extent of feed wastage was then estimated using the AUSPIG model (as reference for comparison) and separately by total solids mass balance.

Estimated feed wastage in the trial shed on the sampling day (Treatment B) was 4.2% from the mass balance calculations and 6.9% from the AUSPIG model. The difference between these two estimates would likely be indistinguishable with normal production data variability. This extent of wastage represents current industry best practice, supported by inspection of the shed indicating virtually no visible spilt feed. The analysis further confirmed that increasing levels of feed wastage resulted in an increased BMP due to the energy content of the waste feed, and also increased concentrations of TS and VS (Table 1). However, the increased feed costs associated with higher levels of feed wastage outweigh potential cost savings from increased methane recovery (higher BMP) for on-farm energy use because energy currently is a relatively minor contributor to total production costs (~4%) compared to feed (~60%).

Table 1.  Feed wastage and mean analysis results for treatments A, B, C and D

Overall, the results support the stated hypothesis, showing that feed wastage consistently affects and can be estimated from effluent characteristics. A logical next step is to adapt industry-standard effluent characteristic models such as PigBal (Skerman et al. 2013) to also estimate feed wastage.