LAUSR

Abstract The aim of this study was to test the effect of exogenous dietary phytase on in vivo phytate P and total P excretion in dairy cows fed three different concentrate mixtures. Excretion and apparent total tract digestibility (ATTD) of phytate P and total P were measured in 24 Danish Holstein cows (18 multiparous and 6 primiparous), in early and mid-lactation. The cows were fed total mixed ration (TMR) with 60:40 forage to concentrate ratio. Three different concentrate mixtures were used, either with faba beans (FAB), rapeseed meal (RSM) or soybean meal (SBM) as the main protein source, constituting 270, 110 and 170 g/kg of dry matter (DM) in TMR, respectively. Addition of exogenous phytase enzyme was used to formulate 6 diets in a 3 × 2 factorial arrangement, used in a 6 × 4 incomplete balanced Latin square design with 4 periods. The six diets were formulated to be between 3.2 and 3.4 g P/kg of DM, below actual P recommendations (3.8 g P/kg of DM), and to have similar CP and Ca concentration. The main protein source accounted for 350−400 g/kg of total P supply. Obtained concentrations of P were 0.35 g/kg of DM higher than expected in RSM and SBM diets, and overall, intake of P was higher in cows fed RSM than in cows fed FAB and SBM diets. Cows fed RSM diets had lower P ATTD and excreted more P than cows fed FAB and SBM diets, likely because of a higher P intake. Cows fed FAB and SBM diets excreted similar amount of P, but cows fed SBM had higher P ATTD than cows fed FAB diet, indicating that P absorption was influenced by diet composition. Addition of phytase reduced phytate P excretion and increased phytate P ATTD independently of concentrate mixture in TMR. Addition of phytase tended to decrease faecal P concentration, tended to increase P ATTD and increased P balance in early lactation cows but not in mid lactation cows. This could have been caused by higher milk production and therefore higher P requirement in early latation cows than in mid lactation cows. Excess of dietary P reduced the effect of addition of phytase in mid-lactation cows, and possibly, to a lower extend, also in early lactation cows. Cows fed SBM had slightly lower dry matter intake than cows fed FAB and RSM, but no difference in milk production was observed between diets. Use of faba beans as main protein source can lower dietary P without compromising milk production, which can improve P digestibility and reduce P excretion.