Accurate manure and milk phosphorus (P) output estimates are important for monitoring P released to the environment and assist in quantifying P balance in lactating dairy cows. The objectives of the study were (1) to develop empirical models, and (2) evaluate extant models for estimating P output from lactating dairy cows. A meta-regression analysis was conducted using 191 fecal (Pf), 83 urine (Pu), and 110 milk (Pm) P output measurements from 39 studies. Dry matter intake (kg/cow/d), diet composition, milk yield and composition and DIM were used as predictor variables. The Pu was highly variable with a mean of 0.34 ± 0.027 g/cow/d. Two models; with and without DMI data were developed for each Pf and Pm (g/cow/d, Table 1). The DMI-based extant model by Weiss and Wyatt (2004) was the most successful in predicting manure P output [root mean square prediction error (RMSPE) as a percentage of average observed value = 19.6%]. Models developed in this study with and without DMI predicted Pf with RMSPE of 18.6 and 24.0%, respectively. The Pm predictions with and without DMI were related to RMSPE <17.0% of the average measured Pm. Our models also predicted total P output [Pf + Pm + Pu] from lactating dairy cows with good accuracy (RMSPE = 11.8 – 17.3%). The models can be used to improve our understanding of P utilization and excretion in dairy production systems. Table 1. Models developed in this study

Equation1	RMSPE (%)
With DMI data
Pf = −10.7(4.06) + 0.69(0.03) × Pintake + 0.56(0.24) × CP + 0.68(0.17) × GE –0.50(0.10)×Milk	18.6
Pm = 19.1(7.28) + 1.25(0.17) × DMI + 0.20(0.06) × Milk – 6.20(2.21) × MProt –0.02(0.01) × DIM	13.9
Without DMI data
Pf = −10.2(7.43) + 151.0(10.6) × P + 0.96(0.22) × GE – 0.52(0.22) × ADF	24.0
Pm = 36.6(8.46) + 0.45(0.06) × Milk – 0.03(0.01) × DIM – 5.20(2.7) × MProt	16.9

¹PIntake = P intake (g/cow/d), GE = dietary gross energy (MJ/kg DM), Milk = milk yield (kg/cow/d), MProt = milk protein (%), CP, P and ADF (% of dietary DM).