One hundred thirty-nine pigs, with an average initial weight of 15.9 kg, were used in three trials to evaluate plasma alkaline phosphatase (AP) as a criterion for estimating available P in feedstuffs for swine. The first two trials were designed to determine the effect of adding graded levels of P (as KH2PO4) to a basal diet on the AP response pattern and degree of linear fit with time. Plasma AP increased (P less than .05) with time and, when regressed on the concentration of total P (.26 to .50%), provided the best linear fit by d 14 (r = -.996). This high degree of fit was maintained through d 21 (r = -.983). The poor correlation observed at 7 d (r = -.828) indicated that the AP pattern was not fully established, whereas departure from the close association by 28 d (r = -.913) may be related to a decline in the magnitude of P deficiency with increasing weight. In a third trial, available P was determined in high moisture corn (HMC, 24.6% moisture) using both AP and bone breaking strength (BKS) as response criteria. A standard curve was established by supplementing the basal diet (.26% P, .65% Ca) with graded levels of KH2PO4. High moisture corn was substituted for dextrose on an equal dry matter basis. Blood samples were acquired on d 14 and 21 for AP analysis and pigs were killed on d 28 for determination of metatarsal BKS. Phosphorus from HMC was determined (slope-ratio technique) to be 41.3% available using BKS as the criterion (KH2PO4, r = .961; HMC, r = .994) and 43.8% when AP at 14 d was used (KH2PO4, r = -.988; HMC, r = -.999). The data indicate that AP is a sensitive index potentially useful in P availability studies.

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