The objective of the current work was to use digestion coefficients from the Feng et al. (2015) model (Virginia Tech model) to calculate P bioavailability of common feeds used in dairy production. Compared with the bioavailability calculated by the Virginia Tech model, using the NRC (2001) P absorption coefficient of 0.64 for forages would underestimate the bioavailabilities of alfalfa hay, alfalfa silage, corn silage, grass hay, and mixed mainly legume silage. For concentrates, using the NRC (2001) P absorption coefficient of 0.70 would overestimate the bioavailabilities of corn grain but underestimate bioavailability of high-moisture corn. Two dairy diets were formulated using nutrient values from the NRC (2001): a standard diet that includes minimal by-products, and a by-product diet that has reduced corn and soybean meal which was replaced with corn gluten feed, distillers grains, hominy feed, and wet brewers grains. For each diet, total bioavailable P was calculated using availability values from the NRC (2001) and the Virginia Tech model. Comparison of P balance (the difference between required and bioavailable P) for each diet was made using the 2 sets of bioavailabilities for a reference cow weighing 682kg, producing 38.6kg of milk/d (3.5% fat and 3.0% true protein, 100d in milk), and consuming 23kg of dry matter/d, yielding an absorbed P requirement of 59.4g/d. The standard diet supplied 56.69 and 53.52g of bioavailable P per day using bioavailabilities from the NRC (2001) and Virginia Tech models, respectively, resulting in a P balance of −2.71 and −5.88±0.26g/d. The by-product diet provided 75.75 and 78.47g/d of bioavailable P, yielding P balances of 16.35 and 19.07±0.37g per day, respectively, using the 2 sets of bioavailabilities. Using P bioavailabilities for individual ingredients that were based on the Virginia Tech model resulted in different bioavailable P levels thus resulted in differing dietary P balances in a field setting.
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