Efficiency of SID lysine utilization and maximum SID lysine retention for gilts in early, mid and late gestation

Efficiency of amino acid (AA) use is presumed constant across gestation but may not reflect changes in metabolic demand during gestation nor consider changes in efficiency depending on level of AA intake. Two experiments were conducted to determine efficiency of SID Lys utilization in gilts during early (d 48-52), mid (d 75-79) and late gestation (d 103-107). Each experiment provided 4 isocaloric (3,335 kcal ME/kg) and isoproteic (11.75 % CP) diets containing 4 SID Lys levels (Table 1). Diets were randomly assigned to 45 gilts (PIC 1050, 158.0 ± 8.0 kg at d 39.4 ± 1 of gestation) in Exp. 1 and 27 gilts (PIC 1050, 169.0 ± 7.5 kg at d 41 ± 1 of gestation) in Exp. 2. Dietary indispensable AA were set to meet or exceed 100% of AA:Lys ratios in both experiments. The SID Lys retention was estimated from whole body nitrogen (N) retention balance studies in each period (7 d diet adaptation, 5 d total urine collection and grab fecal sampling) according to the NRC (2012) equations. The relationship between SID Lys intake and SID Lys retention was determined by nonlinear regression models using the CurveExpert Professional software. According to the Hoerl regression model: E(y)=exp(ß0+ßjX)[Xß2] best-fitting line, maximum efficiency of SID Lys utilization (i.e. g SID Lys retention/g SID Lys intake) was 65%, 57%, and 53% in early, mid and late gestation and occurred at 6.6, 8 and 12 g of SID Lys intake/d, respectively. Maximum SID Lys retention occurred at 8.1 and 9.8 g of SID Lys intake/d for early and mid-gestation. The SID Lys retention did not reach a maximum value in late gestation. These results suggest that efficiency of SID Lys utilization is not constant across gestation and that maximal efficiency occurs at intake below current recommendations.