True absorption of calcium and phosphorus from corn silage fed to nonlactating, pregnant dairy cows

A crossover design was implemented using four nonlactating dairy cows [mean body weight (BW) = 678 kg] and two rations to measure the true absorption of Ca and P from corn silage. True absorption was calculated after dosing cows intravenously with 45Ca and 32P to measure endogenous fecal losses. Rations consisted mainly of corn silage and were formulated to supply 32 g/d of Ca and 20 g/d of P or 16 g/d of Ca and 12 g/d of P. The percentages of total Ca and P that came from corn silage were 95 and 77%, respectively, for ration 1, and 98 and 79%, respectively, for ration 2. Cows ate more dry matter (10.9 vs. 10.2 kg/d) when consuming the corn silage in ration 1 than when consuming the corn silage in ration 2. Calcium intake was greater for cows fed ration 1 than for cows fed ration 2 (32.6 vs. 16.1 g/d), and P intake was greater for cows fed ration 1 than for cows fed ration 2 (20.1 vs. 11.7 g/d). True absorption of Ca was 34.4 and 43.7% for rations 1 and 2, respectively, and true absorption of P was 84.5 and 93.9% for rations 1 and 2, respectively. True absorption of Ca was about equal to values currently used in the National Research Council (NRC) feeding standards, but true absorption of P was higher than values currently used by the NRC. Fecal endogenous excretion of Ca (mean = 8.23 mg/kg of BW per d) was one-half of the value currently used by the NRC, and fecal endogenous excretion of P (mean = 7.23 mg/kg of BW per d) was only slightly less than NRC values.