Estimating standardized ileal digestible methionine requirements for gilts during gestation using whole-body nitrogen retention and describing plasma creatine, glutathione, and taurine concentrations

A total of 70 gestating gilts (166 ± 13 kg initial BW on d 31 of gestation) were used to determine the standardized ileal digestible (SID) Met needed to maximize whole-body N retention and to describe the impact of dietary Met on indicators of Met utilization for roles beyond protein synthesis. Seven days prior to N balance measurements between d 38 and 41, d 53 and 56, d 87 and 90, and d 109 and 112 of gestation (periods 1, 2, 3, and 4, respectively), gilts were assigned to 1 of 7 dietary treatments (n = 10) that titrated SID Met between 50 and 150% of perceived requirements of 3.5 and 5.2 g SID Met/d (before and after d 90 of gestation, respectively) using DL-Met and in the presence of excess Cys in an incomplete randomized block design. Fasted blood samples were collected on d 38, 53, 87, and 109 to assess concentrations of plasma Met-derived metabolites. Contrast statements were used to determine linear and quadratic effects of the dietary inclusion level of SID Met and quadratic polynomial (QPM), broken-line linear, and broken-line quadratic (BLQ) ascending models were evaluated for the primary response variables. Whole-body N retention did not exhibit linear or quadratic relationships with increasing SID Met content in periods 1, 2 and 3, but inflection points were observed at 0.17% (3.8 g SID Met/d; QPM), 0.19% (4.2 g SID Met/d; BLQ), and 0.16% SID Met (3.5 g SID Met/d; QPM), respectively. Whole-body N retention increased (linear and quadratic; P < 0.001) with increasing SID Met content in period 4, optimized at 0.23% (6.0 g SID Met/d; QPM). Plasma creatine concentration did not exhibit linear or quadric relationships with increasing SID Met contents, but the QPM indicated inflection points at 0.17, 0.17, 0.22, and 0.18 % SID Met (3.8, 3.8, 4.9, and 4.7 g SID Met/d) on gestation d 38, 53, 87, and 109, respectively. Plasma concentrations of GSH tended to increase then decrease (quadratic; P = 0.096) on d 53 and tended to increase (linear; P = 0.095) on d 87 as SID Met increased; inflection points were observed at 0.16, 0.17, 0.21, and 0.16 % SID Met (3.5, 3.8, 4.6, and 4.2 g/d; QPM) on gestation days 38, 53, 87, and 109, respectively. Thus, the SID Met feeding recommendations provided by the NRC (2012) gestating sow model are insufficient to maximize whole-body N retention of gilts throughout gestation in current production conditions, while the supply of SID Met might also influence Met utilization for metabolic fates beyond protein retention.