Empirical Calibration of Ideal SID Isoleucine to Lysine Profile in Growing Pigs (70-102 kg BW)

The ideal profile for standard ileal digestible (SID) isoleucine is not known with confidence. The NRC (2012) estimated, through factorial calculation, the SID isoleucine (ILE) profile to be 53% that of lysine (LYS) for 70 kg pigs. An empirical estimate was determined to be 60-62% at asymptote for 90 kg pigs, depending on criterion (D. Kendall, Ph D Dissertation, 2004). Our study was conducted to calibrate the NRC estimate for SID ILE profile in 70-100 kg pigs, so that amino acid (AA) displacement of SBM is not compromised by a deficit ILE level. A total of 1928 PIC terminal (TR-4 x Camborough) castrate and female pigs (70.9 ± 0.7 kg) were used in a 35 d growth assay to 102.4 kg (±0.9 kg). Pigs were placed in a commercial research facility, blocked by sex and weight then allocated (19 pigs/pen, with 0.78 m²/pig) to treatment. Treatments were arranged as a 2 x 5 factorial, involving sex and 5 ILE:LYS (SID) ratio’s (0.50, 0.56, 0.62, 0.68, 0.74). Pen was the experimental unit (10 pens/diet x sex subclass). Diets were composed of corn, soybean meal, choice white grease (1.5%) and AA. Two diets were milled and summit blended to create three additional diets using the Howeema system. Diets were isocaloric with SID LYS:Mcal NE set to approx. 96% asymptote, based on internal whole-body gain and G:F response curves: 2.684 and 2.301 g SID LYS:Mcal NE from 70 to 90 kg and 90-102 kg respectively. Ratio to LYS for key AA met or exceeded NRC minimums: Thr, 0.68, Trp, 0.22, Met, 0.32, Met:Cys, 0.58, Val, 0.68. The SID LYS:CP ratio ranged from 0.0588 to 0.0603. Average daily feed intake was not affected by diet (P=0.86) and sex x diet was not significant (P>0.50). The G:F response to ILE:LYS ratio was quadratic (0.329, 0.336, 0.338, 0.339, 0.323, SEM ±0.005, P<0.01) and described as: Y= -0.0135 + 0.0117x - 0.00010x² (R²=0.34, P<0.01). Average daily gain also responded in quadratic manner (0.904, 0.916, 0.924, 0.910, 0.887 kg/d, SEM ± 0.013, P=0.07); being represented as: Y= 9.1546 + 0.7682x - 0.00638x² (R²=0.16, P=0.06). Growth-derived estimates for ILE:LYS (SID), at asymptote, were 0.61 and 0.60 for G:F and total gain, respectively. This exceeds the NRC estimate for the 70-100 kg pig and illustrates the importance of empirical calibration of ideal AA patterns.