A simplified approach to calculate slurry production of growing pigs at farm level

A simplified approach to predict the amount of slurry produced by growing pigs at farm level is proposed. The inputs are initial (LWi) and final (LWf) live weights, production (t) and empty (empty) periods, feed consumption (FC), dry matter (DMD), N digestibilities and farm water consumption per pig (FWC). Estimates of the amount of water required (or arisen) per kg of feed for the various physiological functions were estimated by running a published mathematical model using data representing the ordinary conditions of rearing. Water excretion was estimated in two ways depending on: 1) free access (ad lib) to water; 2) restricted access (forced). In the first case, the proportion of water consumed (wi(ad) (lib)) and those excreted with the urine (wu(ad) (lib)) and the faeces (wfec) were quantified to be 2.9, 1.72 and 0.33 kg per kg of feed, respectively. From the urinary excretions of N and minerals, obtained as the difference between the digestible nutrient intakes and the retentions, the model predicted a urinary DM content of 2.1% (by weight). In the second case, for pigs receiving drinking water in forced ratio with the feed (wi(forced)), the urinary production was calculated as wu(forced)=(wi(forced)+wf+wo)-(wd+ws+wg+wfec+we), where wf=water content in feed (0.12 kg/kg), wo=water arising from nutrient oxidation (0.25 kg/kg), wd=water required for digestion (0.08 kg/kg), ws=water demand for protein and lipid synthesis (0.06 kg/kg), wg=water retained in body tissues (0.14 kg/kg) and we=water lost through evaporation (0.96 kg/kg). Estimates of fresh slurry production (faeces+urine) were regressed against the values resulting from empirical literature equations and referred to pigs fed water:feed ratios of 2.5:1, 2.9:1 and 4:1. The resulting regression (R(2)=0.97), with a slope close to unity (1.05), indicated that the approach can be extended to predict the farm fresh slurry production with pigs having free access to water or kept on different water:feed ratios. In agreement with international literature, but not with the current Italian national standards, estimates of mature slurry productions ranged from 1.5 to 2.9 ton/pig/year with DM contents ranging from 8.3 to 3.7%. At farm level the use water meters is recommended as from FWC minus the estimated water consumption (FWC(exp)) the farmer can evaluate the extra wastage and adjust the predicted mature slurry production.