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

Abstract

AbstractObligatory faecal phosphorus (P) losses in ruminants are commonly regarded as depending on dry-matter (DM) intake. This study was designed to investigate whether it is the digestible or the indigestible fraction of the diet which causes the obligatory faecal P loss. Three groups of six lactating goats 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 form of sawdust or in the form of potato starch. Organic matter digestibility, determined by difference, was 0·15 for sawdust and 0·79 for potato starch. Goats were kept in balance crates and milk as well as faeces and urine were quantitatively collected. P intake was 2·9 g/day in unsupplemented goats and slightly higher (3·2 and 3·0 g/day, respectively) when sawdust or starch were supplemented. Urinary P output was very low (23 mg/day) and not affected 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 goats without supplementary DM, faecal P output was higher in goats given supplementary 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 = 0·058). The variation between treatments became even greater when faecal P output was related to indigestible organic matter intake (P < 0·001). When related to organic matter intake, however, differences in faecal P output between treatments were not significant (P = 0·445). It is concluded, that obligatory faecal P losses depend on the digestible fraction of the diet rather than on the indigestible fraction.