Meta-analysis-based estimates of the efficiency of phosphorus utilisation by dairy cows

Phosphorus (P) has several vital functions, and its excretion may have environmental impacts. Thus, an adequate P supply is of utmost importance to meet the requirements of the animals while avoiding unnecessary excretion. Calculation of the P requirement of dairy cows considers factors such as P secretion via milk and P deposition in growing or pregnant animals. Furthermore, a value for the efficiency of P utilisation is applied, which should be determined in animals fed at a marginal P supply to maximise absorption. The objective of the present meta-analysis was thus to estimate the efficiency of P utilisation, accounting for P supply in dairy cows. The dataset included 399 observations published in 77 studies on dairy cows. Data on P intake and faecal P excretion were used to calculate standardised P digestibility, corrected for faecal endogenous P losses. Furthermore, the P requirement and the P supply relative to the requirement were calculated. Linear and non-linear regressions were performed to analyse the relationship between P excretion via faeces and urine and standardised P digestibility with P intake and P supply. Phosphorus intake exceeding the net requirement of dairy cows was excreted with faeces to a large extent, while urinary P excretion had only minor quantitative importance below 5.2 g P/kg DM of feed. The estimated standardised P digestibility at a marginal P supply was 83% and was interpreted as the estimate for efficiency of P utilisation. The data did not indicate that differentiation in P utilisation efficiency between mineral and organic P sources in the feed or by lactation stage is helpful for adequate P supply of dairy cows.