An evaluation of various biological characteristics in assessing low phosphorus intake in weanling swine

A total of 204 weanling pigs were utilized in an experiment of 3 X 3 factorial design (in three replicates) to evaluate the effects of feeding low P diets for a 35 d period on various growth, serum and bone measurement characteristics. Total dietary P levels of .23, .35 and .55% were fed in a Ca:P ratio of 1:1, 1.5:1 or 3:1. The basal diet was formulated using cornstarch, dextrin and soy protein concentrate with monosodium phosphate and limestone used to supply appropriate dietary levels of Ca and P. Growth, serum minerals and serum alkaline phosphatase were evaluated at 21 and 35 d of the trial. At the end of the experiment six pigs/treatment group were killed and bones were collected for various measurement criteria. Daily gain, feed intake and feed conversion were not affected by Ca:P ratio, but there was an increased gain and feed intake response to dietary P level. Serum inorganic P decreased with increasing dietary Ca:P ratio and also increased linearly as dietary P level increased. Serum alkaline phosphatase activity was not influenced by dietary Ca:P ratio, but did decrease as dietary P level increased. Bone bending moment increased quadratically as Ca:P ratio increased, but also increased linearly with dietary P level. Percentage bone ash, bone ash and organic matrix weights and the accretion of both the mineral and matrix components were not affected by Ca:P ratio, but each was influenced as dietary P increased. These data suggest that percentage bone ash, bone component weights and accretion measurement characteristics were more sensitive indicators of dietary P levels than bone bending moment, serum alkaline phosphatase and serum P, which in turn were superior to serum Ca, Mg and growth performance.