Efficiency of Utilizing Standardized Ileal Digestible Lys for Whole Body Protein Retention in Pregnant Gilts during Early, Mid and Late Gestation

Nutrient requirement models for pregnant gilts are based on a paucity of data and assume constant efficiency of AA use throughout gestation. A total of 40 gilts (158.2 ± 7.9 kg at d 39.5 ± 1 of gestation) were used in a 3-period nitrogen (N)-balance study to evaluate the efficiency of utilizing standardized ileal digestible Lys (SID Lys) for whole body protein retention in pregnant gilts. Gilts were randomly assigned to one of 4 diets set to provide 60, 70, 80 and 90% of the model-predicted daily SID Lys for protein retention (NRC, 2012) in each of period I (d 41-52, 10.45 g/d), period II (d 68-79, 9.59 g/d) and period III (d 96-107, 16.04 g/d). Diets contained 3,300 kcal/kg and 11.6% CP and given at a rate of 2.13 kg/d in periods I and II and at 2.53 kg/d during period III to ensure energy was not limiting the response to Lys. The 12d balance period (7d adaptation and 5d urine and fecal collection) was based on total urine collection using urinary catheters and determination of fecal N-digestibility using indigestible marker. The SID Lys utilization for whole body protein retention was estimated using the NRC (2012) model and the predicted Lys content of each gestation pool. Lys efficiency (k_SIDLys) was calculated as the ratio of daily Lys retention and daily SID Lys intake. Data were analyzed as randomized complete block with diet as the fixed effect. The linear and quadratic response in whole body N and Lys retention was tested within each balance period. Measured dietary Lys levels were higher than formulated where 90% of SID Lys was 12.5, 11.7, and 18.2 g/d in period I, II, and III, respectively. Whole body N and Lys retention were not affected by the dietary SID Lys levels for periods I and II. In period III, there was a linear increase (P < 0.001) in whole body N and Lys retention. Efficiency of Lys use tended to decrease (linear, P = 0.074) in period I and decreased in period II (linear, P <0.001). Efficiency of Lys use in period I, II, and III ranged from 0.32 to 0.45, 0.41 to 0.59, and 0.50 to 0.56, respectively. The lack of response in whole body protein retention in periods I and II and the decreasing k_SIDLys may in part reflect excess Lys intake. However, efficiency of Lys use does not appear to be constant throughout gestation.