The present study investigated the effect of P or phytase supplementation on the P-balance of dairy cows. 24 lactating German Holstein cows were used for a 5-weeks feeding trial and were allocated to three dietary treatments, P+MIN, P-MIN and P+PHY. All cows received a total mixed ration (TMR) composed of 63% corn silage and 37% concentrate on a dry matter (DM) basis for ad libitum intake. The concentrate of the P+MIN group was supplemented with dicalcium phosphate and represents the control

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... up. The concentrate of the P-MIN group was unsupplemented and the concentrate of the P+PHY group was supplemented with an exogenous phytase (0.1 g/kg DM in the TMR; 50 000 FTU/g). The P concentration in the TMR of the P+MIN, P-MIN and P+PHY groups were 3.98, 3.46 and 3.26 g P/kg DM, respectively. Dry matter intake (DMI) and milk yield were recorded daily. In the last two weeks samples of milk, urine and blood were collected. Samples of faeces were collected to determine the P-balances by using the acid insoluble ash (AIA) marker technique. No differences in P-concentration of milk, urine and faeces were observed between the treatments. The P-digestibility of Group P+MIN and P-MIN was 60 and 56%, respectively. These values were not different compared to the P-digestibility of 57% in the P+PHY-group. The P-balance in the P+MIN group (26 g/d) was higher compared to the P-MIN (16g/d) and P+PHY (17g/d) treatment. Overall, phytase supplementation had no effect on P-digestibility and P-balance of dairy cows in this trial.