Effect of organic matter digestibility on obligatory faecal phosphorus loss in lactating goats, determined from balance data

Obligatory faecal phosphorus (P) losses in ruminants are commonly regarded as depending on dry-matter (DM) intake. This study was designed to investig ate whether it is the digestible or the indigestible fraction of the diet w hich causes the obligatory faecal P loss. Three groups of six lactating goa ts each were given a diet consisting of maize silage, chaffed barley straw and a low P concentrate achieving a DM intake of 1.76 kg/day. Goats of two groups were given approximately 0.4 kg/day supplementary DM either in the f orm of sawdust or in the form of potato starch. Organic matter digestibilit y, determined by difference, was 0.15 for sawdust and 0.79 for potato starc h. Goats were kept in balance crates and milk as well as faeces and urine w ere quantitatively collected. P intake was 2.9 g/day in unsupplemented gent s and slightly higher (3.2 and 3.0 g/day, respectively) when sawdust or sta rch were supplemented. Urinary P output was very low (23 mg/day) and not af fected by treatment. Milk yield and milk P output were 2.5 kg/day and 2.4 g /day, respectively, and not affected by treatment either Compared with the gents without supplementary DM, faecal P output was higher in gents given s upplementary starch but not in goats given supplementary sawdust. Increase in faecal P output due to starch supplementation exceeded the increase in P intake from starch by more than a factor of 4. When related to DM intake, faecal P excretion was significantly different between treatments (P = 0.05 8). The variation between treatments became even greater when faecal P outp ut was related to indigestible organic matter intake (P < 0.001). When rela ted to organic matter intake, however, differences in faecal P output betwe en treatments were not significant (P = 0.445). It is concluded, that oblig atory faecal P losses depend on the digestible fraction oft he diet rather than on the indigestible fraction.