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Implementation of animal and herd phosphorus balance in the Ruminant Farm Systems (RuFaS) model.

T. L. Hansen



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Implementation of animal and herd phosphorus balance in the Ruminant Farm Systems (RuFaS) model.
T. L. Hansen*1, M. A. Sotirova1, J. M. Tricarico2, K. F. Reed1. 1Cornell University Ithaca, NY, 2Dairy Management Inc Rosemont, IL.

Dairy cattle require P for growth, energy metabolism, and acid-base balance, but excess P in agricultural systems is related to P runoff and eutrophication. Tools to assess nutrient management decisions and environmental implications are often static and evaluated at single time points; thus, the use of a dynamic model would improve decision making and evaluation of P nutrient flow. The Ruminant Farm Systems (RuFaS) model is a modern, modular approach to dairy system simulations incorporating nutrient flows through soil, crop, feed storage, animal, and manure management modules. The objective of this study was to build a dynamic P tracking module that maintains P mass balance at the animal, herd, and barn level. Initial P values of animals are calculated from P gestation requirements for simulated calves or as 0.72% BW for purchased cattle. Two strategies to update daily animal P values were tested: (1) estimate fecal P as the difference of intake and absorbed P or (2) estimate retained P as the difference between intake, secretion and excretion. For the first approach, NRC requirements were used to calculate absorbed P, and the constraint of P mass balance was established at the animal level. In a simulation of 125 lactating dairy cows with P intakes from 25 to 179 g/d, fecal P excretion ranged from −3.3 to 150 g/d. Negative fecal P excretion occurred at very low P intakes (≤0.2% DMI). Therefore, a second approach to calculate fecal P excretion and retained P in the animal module was developed using empirical equations to estimate fecal P excretion from P intake. Retained P ranged from −18.2 to 39.6 g/d and was highly correlated with P intake (P < 0.001, r2 = 0.979). Animal P values are updated daily to account for individual animal P retention and excretion. Therefore, the second approach is more biologically relevant, accounts for BW changes, and maintains P mass balance at a herd level. Modeling the utilization and flow of P in addition to other nutrients within the animal module is critical to meet RuFaS objectives to identify and improve whole-farm production efficiency and minimize environmental impacts from dairy farms.

Keywords: cattle, mass balance, phosphorus.