The aim of the present study was to investigate differences in the apparent digestibility
of phosphorus (ADP) from various organic feed materials used in feeds for dairy cows,
in comparison with a highly available inorganic source. The P sources used in this trial
have a high P content and are commonly fed to dairy animals. Monosodium phosphate was
chosen as a reference source because of its known high TAC (Tillman and Brethour, 1958;
NRC, 2001). However, there are low values reported in sheep (Guéguen et al., 1976), but
in this trial the apparent digestibility was extremely low, indicating a large surplus of P in
the diet, which will also lead to a low TAC. In addition to the present trial, standard in vitro
procedure was tested for P availability as a possible complement to animal studies.