A model to predict water intake of a pig growing in a known environment ona known diet

A model to predict voluntary water intake (WI) of a pig fed a known diet in a known environment is described. The daily retentions of protein, lipid, water and ash were estimated over time using a published pig growth model. Food intakes were estimated using published methods. WI was estimated by ad ding the amounts required for digestion (WD), faecal excretion (W-fec), gro wth (WG), evaporation (WE), urinary excretion (WU) and by then subtracting the water arising from feed (WF), from nutrient oxidation (WO) and synthesi s of body constituents (WS). WD was predicted assuming an absorption of wat er of 0.10, 0.16 and 0.07 kg/kg digestible carbohydrate, crude protein and lipid respectively. W-fec was estimated taking into account the water assoc iated with the undigested protein (0.86 kg/kg), diethyl ether extract (-12. 11 kg/kg), crude fibre (1.86 kg/kg), ash (-0.42 kg/kg) and N-free extract ( 4.4 kg/kg). The basal level of WE was estimated from the heat production of the pig fed ad libitum (MJ/d) as: 0.25 x (metabolizable energy - energy re tained as protein and lipid) x 0.4, where 0.25 is the assumed proportion of the insensible heat loss at the comfort temperature and 0.4 is the water l ost per MJ dissipated heat. WE in a hot environment was predicted by assumi ng that evaporation increased up to three times the basal level to offset t he decreased sensible heat loss. To predict WU a water requirement for rena l excretion of 2.05 and 3.40 kg/osmol excreted N as urea and minerals respe ctively was assumed. The urinary load of N and minerals was predicted from the intake of digestible nutrients and their retention. From the oxidation of 1 kg carbohydrate, protein, and fat it was assumed that 0.6, 0.42 and 1. 07 kg water (WO) were released respectively. WS was predicted by assuming a release of 0.16, 0.07 and 0.57 kg water per kg retained protein, retained lipid coming from digestible lipid, and retained lipid coming from digestib le carbohydrate respectively. The model is strongly rooted in a theoretical structure. When its predictions were compared with data from suitable expe riments, the results were not significantly different. Both the pattern and the magnitude of responses of the model to changes in body weight, feed in take and environmental temperature are sensible and it allows a fuller pred iction of voluntary water intake than the methods currently available.