Current AA recommendations for sows are to provide a fixed amount of AA intake throughout gestation based on the assumption that there is a constant demand for AA; however, the demand for nutrients changes from maternal lean tissue in early gestation to fetal and mammary growth in late gestation. The objective of this study was to determine the Thr requirement in early (d 35 to 53 and 25 to 55 for Exp. 1 and 2, respectively) and late (d 92 to 110 and 81 to 111 for Exp. 1 and 2, respectively) gestation using the indicator AA oxidation (IAAO) method with l-[1-13C]Phe as the tracer AA. A total of 14 multiparous sows were used: 6 in Exp. 1 and 8 in Exp. 2. Each sow received each of 6 diets in random order in both early and late gestation. A basal diet was formulated to contain Thr at 60% of the 1998 NRC recommendation in Exp. 1 and 20 and 60% of the 1998 NRC in Exp. 2 for early and late gestation, respectively. Crystalline l-Thr was added to create additional diets with approximately 10% incremental increases in Thr. Sows were placed in respiration chambers, and expired air and blood were collected every 30 min for 5.5 h. Tracer Phe [mg/(kg of BW・h)] was given orally over the last 4 h divided into eight 0.5-h meals. Expired air and plasma were measured for 13CO2 enrichment and free Thr concentration, respectively. Background 13CO2 was subtracted from plateau 13CO2 enrichment. Data were analyzed using a 2-phase nonlinear Mixed model. The overall litter size and litter weight were 13.5 and 20.5 kg, respectively. Based on IAAO, the Thr requirement in early gestation was 6.1 g/d (R2 = 0.59, Exp. 1) and 5.0 g/d (R2 = 0.71, Exp. 2). In late gestation, the Thr requirement based on IAAO was 13.6 g/d (R2 = 0.60, Exp. 1) and 12.3 g/d (R2 = 0.58, Exp. 2). Based on plasma Thr, the Thr requirement in early gestation was 7.0 g/d (R2 = 0.90, Exp. 1) and 3.9 g/d (R2 = 0.90, Exp. 2). In late gestation, the Thr requirement based on plasma Thr was 10.5 g/d (R2 = 0.67, Exp. 2). There was a linear response to increasing Thr intake in late gestation in Exp. 1. Feeding a single amount of AA throughout gestation results in overfeeding AA in early gestation and underfeeding AA in late gestation. The 2-fold increase in Thr requirement in the last third of gestation suggests that phase feeding sows in gestation will more closely meet the demands for nutrients and that the requirement for essential AA in gestating sows should be re-evaluated in early and late gestation separately.

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