Revision of the Simplified Balance Method to Evaluate Phosphorus Excretion by growing-finishing pigs

Guidelines have been developed in Quebec to maximize the benefits and minimize the risks associated with the application of livestock manure to agricultural soils. Within these guidelines, P content of manure has to be assessed and manure application adjusted to crop needs. However, the work required to obtain a representative sample of pig slurry is considerable and must be meticulously carried out to obtain accurate values. To simplify farmers work, the simplified balance method has been proposed. This method quantifies P excretion as the difference between farm P inputs and outputs. The objective of this study was to estimate the amount of P retained by growing-finishing pigs, which is a key parameter in this method, and to highlight the factors of variation. Results of three trials in which pig body P content was estimated by dual X-ray absorptiometry (DXA) after receiving different levels dietary Ca and P were used. Total body and head BMD are strongly related (R² = 94%, RMSE = 0.034). Therefore, total body P retention (g/kg of BWG) can be estimated from total body BMD (R² = 78%, RMSE = 0.263) but because pig heads have very low cost and can be easily obtained from slaughterhouses, the relationship between total body P and head BMD has been proposed for use in Quebec (P retained g/kg BWG = 2.28 + 1.32 x head BMD (g/cm²); n = 92; RMSE = 0.125; R² = 94%). Afterwards, 425 heads from 6 main Quebec slaughterhouses over 6 months were collected and total P retention estimated with this equation. Average P retention across slaughterhouses showed limited variation (ranging from 5.04 to 5.34 g P/kg BWG, average 5.15). However, the within slaughterhouse variation was high (SEM = 0.25 g of P/kg BWG). This later variation is an indication of the individual natural variation on bone mineralization, this observation further supported by the fact that pigs of one of the sampled slaughterhouses received all the same diets. Based on the current results showed that the utilization of a fixed 5.15 g of retained P per kg BWG is proposed for the application of the simplified balance method in Quebec growing-finishing pigs. The proposed method of estimating P retention in pigs can also be used for easy assessment of pig’s bone mineralization and for future verification and update of the P retention value to be used in the simplified balance method.