Calcium, phosphorus and magnesium retention and excretion in pregnant sows during three parities

The validity of a theoretically minimal requirement of pregnant sows for digestible phosphorus was tested during three parities. The effect of two diets, differing in total and calculated apparently digestible phosphorus, on retention and excretion of Ca, P and Mg was studied in mid-pregnancy (days 50–60; n=22) and late pregnancy (days 105–112; n=60). The control diet contained 7.2 g P, 9.4 g Ca and 2.6 g Mg kg−1 dry matter and the experimental diet 4.8 g P, 7.6 g Ca and 2.1 g Mg. Calculated levels of digestible phosphorus were 3.5 and 1.9 g kg−1 DM, respectively. These values were based on phosphorus digestibilities that were measured in growing pigs. Daily feed allowance was 2.5 kg from day 0 to 85 of pregnancy and 3.0 kg from day 86 onwards. The estimated requirement for digestible phosphorus was 4.2 g day−1 in mid-pregnancy and 6.0 g day−1 in late pregnancy. For practical application a safety margin is required to cover higher litter size, piglet birth weights, maternal protein gain and to correct for a possible difference in phosphorus digestibility between sows and growing pigs. In this experiment, altering diet composition decreased the excretion of phosphorus by 35%.