True Absorption of Calcium and Phosphorus from Alfalfa and Corn Silage When Fed to Lactating Cows

Abstract

A crossover experimental design with an extra period was used with four lactating cows (645 kg BW) and two diets to measure the true absorption of Ca and P from alfalfa hay and com silage. True absorption was calculated after dosing cows intravenously with 45Ca and 32p to measure endogenous fecal losses. In alfalfa hay and alfalfa-com silage diets, the Ca and P that came from the hay or hay and silage fraction was 94 and 98% and 63 and 84%. respectively. Cows ate more DM (22.7 vs. 20.6 kg/d) and produced more FCM (35.2 vs. 32.0 kg/d) when consuming alfalfa-com silage compared with alfalfa hay. True absorption of Ca from alfalfa-com silage was greater (42.2%) than from alfalfa hay (24.6%). Partial true absorption of Ca from alfalfa hay was 23.5% and from corn silage 51.5%. True P absorption for total diet was similar for both alfalfa hay (64.4%) and alfalfa-com silage (74.6%). Partial true absorption of P from alfalfa hay (67.3%) was different from that from com silage (80.0%). Fecal endogenous Ca excretion was nearly double the value (31 vs. 16 mg/kg BW) currently used by NRC. True absorption of Ca from alfalfa was lower and from com silage was higher than currently used in NRC feed-