[Effect of various sources of phosphorus on the apparent digestibility of crude nutrients, growing and slaughtering performance as well as selected parameters of metabolism in fattening bulls].

Two long-term individual feeding experiments (336 and 307 days) were carried out with 52 (experiment I; Black and White dairy cattle) and 22 (experiment II; Yellow cattle) growing bulls. Rations varied in P-content. In experiment I straw: concentrate-mixture (1:2.5) were fed, phosphorus content of rations amounted to 2, 3 or 4 g/kg DM. In experiment II maize silage was fed ad lib. supplemented with 2.4 kg concentrate per animal per day. P-content of rations amounted to 3 and 4 g/kg DM. Apparent digestibility of rations was determined during experiments. Blood samples were taken to analyse selected parameters of metabolism. Various P-supply did not significantly influence apparent digestibility of organic matter and crude nutrients in both experiments. Bulls fed 2 g P/kg DM decreased DMI in experiment I already after 50 days of experiment. After 168 experimental days daily weight gain of bulls fed with 2 g P/kg DM was 124 or 132 g lower than that of bulls fed with 3 or 4 g P/kg DM respectively. P-concentration of inorganic P in serum decreased to < 2 mmol/l, bone mineralisation was reduced. There exist no significant differences in feed intake, weight gain and metabolic parameters of bulls fed with 3 or 4 g P/kg DM. Increase of P-supply from 2 to 3 or 4 g/kg DM compensated partially the lower weight gain. Application of 3 or 4 g P/kg DM in experiment II effected daily weight gains of > 1200 g and did not significantly influence all investigated criterions.