INFLUENCE OF FEEDING ON FECAL PHOSPHORUS EXCRETION OF GROWING BULLS VARYING IN BODY-WEIGHT

Five smaller (body weight: 227.9 +/- 23.7 kg) and larger growing bulls (435.2 +/- 14.3 kg per animal) each were fed with rations rich (chopp ed wheat straw:concentrate=1:1) or poor in roughage (straw:concentrate =1:4). Animals were kept in balance cages: 20 days adaptation period w ere followed by 10 days collection period. Body weight of bulls did no t significantly influence apparent digestibility of rations, Digestibi lity of organic matter of fibre rich ration was significantly lower (6 6.8%) than those of concentrate rich ration (74.4%). Feeding and body weight did not significantly influence metabolic parameters of mineral status. The fecal P-excretion amounted to 0.94 and 1.08 g per kg DMI in bulls fed with rations rich and poor in roughage. No influence of b ody weight was measured. Feeding and body weight did not significantly influence fecal P-excretion per kg DOMI (between 1.44 and 1.58 g P\kg DOM). For calculation of P-requirements for growing cattle fecal P-ex cretion amounted to 1.0 or 1.5 g P\kg DM and DOMI. respectively.