Apparent ileal digestibility (AID) of crude protein (CP) and amino acids (AA) is affected by CP and AA concentrations in diets because of the different contributions of endogenous N and AA to the ileal digesta from pigs fed diets with different concentrations of CP. Because of the influence of dietary CP and AA on calculated values for AID, values for AID obtained in individual feed ingredients are not always additive in mixed diets if the concentration of CP and AA in the mixed diet is different from that of the ingredients. Therefore, values for standardized ileal digestibility (SID) of CP and AA, which are corrected for the basal ileal endogenous losses of CP and AA, are used in diet formulations to avoid the influence of endogenous AA on digestibility values, and SID values are, therefore, additive in mixed diets.

Calculated values for SID of AA are influenced by the level of feed intake of animals and it is, therefore, recommended that animals used to determine SID of AA are fed close to ad libitum intake. However, when the direct procedure is used to determine AA digestibility, diets containing cereal grains are sometimes formulated to contain AA below the requirement. In contrast, if the SID of oilseed meals or other protein-rich ingredients are determined, concentrations of AA may be at or above the requirement. Because the influence of the basal endogenous losses of AA is eliminated in the calculations of SID values, it is assumed that values for SID are not influenced by dietary concentrations of AA. However, to our knowledge this hypothesis has not been experimentally verified, but it is an important question because it may influence how diets used in digestibility experiments are formulated. Therefore, two experiments were conducted to test the hypothesis that SID of AA by growing pigs is not influenced by the concentrations of CP and AA containing soybean meal (SBM) or soy protein concentrate (SPC).

Experimental design

In Exp. 1, thirty growing pigs (body weight = 31.4 ± 3.9 kg) with a T-cannula installed in the distal ileum were used. Pigs were allotted to a randomized complete block design with 5 diets and 6 replicate pigs per diet. Four diets were formulated to contain 4.7, 12.7, 22.1, or 30.4% CP, respectively, by including 10, 30, 50, or 70% SBM in the diets. An N-free diet was used to determine the basal endogenous losses of CP and AA.

In Exp. 2, twenty-four growing pigs (body weight = 22.8 ± 1.7 kg) with a T-cannula in the distal ileum were used. Pigs were allotted to a quadruplicated 6 × 2 incomplete Latin square design with 6 diets, 2 periods, and 4 replicate pigs per diet in each period for a total of 8 replicate pigs per dietary treatment. Five diets were formulated to contain 6.8, 14.1, 19.7, 26.6, or 31.8% CP, respectively, by including 10, 20, 30, 40, or 50% SPC in the diets. Ileal digesta were collected for 8 hours on the last two days of each feeding period following an adaptation period of 4 days (Exp. 1) or 5 days (exp. 2). Diets and ileal digesta were analyzed for CP, AA, and Cr to calculate the AID of CP and AA in each diet. The AID values were then corrected for basal endogenous losses of CP and AA to calculate SID values.

Data were analyzed using the PROC MIXED procedure (SAS Inst. Inc., Cary, NC, USA). The model included diet as a fixed effect. Mean values were calculated using the LSMeans statement, and polynomial contrasts were used to test for linear and quadratic effects of concentration of SBM or SPC on digestibility of CP and AA. Pig was the experimental unit and an α-value of 0.05 was used to determine statistical significances and results were considered a tendency if 0.05 ≤ P-value < 0.10.

Results

Results from Exp. 1 indicated that there were quadratic increases (P < 0.05) in the AID of CP and most AA as dietary CP increased. The SID of Arg, Ile, Leu, Met, Trp, and Val was not affected by dietary CP, but the SID of CP, Lys, Phe, Thr, Ala, Asp, Cys, Gly, and Ser linearly decreased (P < 0.05) as SBM in the diets increased (Table 1). However, this effect was primarily a result of the SID of AA in the diet containing 70% SBM and the SID of CP and most AA was not affected by dietary CP if diets contained 10, 30, or 50% SBM that correspond to 4.7, 12.7, or 22.1% CP in diets.

Results of Exp. 2 indicated that the AID of CP and all AA in SPC increased quadratically (P < 0.01) with increasing dietary CP, but the SID of His and Lys decreased quadratically (P < 0.05) and the SID of Ala, Asp, Cys, and Glu decreased linearly (P < 0.05) with increased inclusion of SPC in the diets (Table 2). However, if SPC was included at 40% or less (i.e., dietary CP was 26.6% or less), no effect of dietary CP on SID of CP and most AA except Lys, Cys, Glu, and Tyr was observed.

In conclusion, the SID of most AA by growing pigs is not influenced by dietary CP and AA if dietary CP from SBM or SPC does not exceed around 22 and 26%, respectively. This indicates that SID of AA in feed ingredients may be determined in diets that contain CP and AA below, at, or slightly above requirements without impacting result.

Key points

• Values for the AID of AA increased quadratically as the inclusion of SBM or SPC increased in diets fed to growing pigs.
• Values for the SID of most AA by growing pigs is not influenced by dietary CP and AA if dietary CP from SBM or SPC does not exceed 22 and 26%, respectively.

Table 1. SID of CP and AA in diets for growing pigs with different inclusions of SBM ^1,2 (Exp. 1)

¹Each least squares mean for each treatment represents 6 observations, respectively, except for the diet containing 50% SBM (n = 5).

²Values for SID were calculated by correcting the values for AID for basal ileal endogenous losses. Basal ileal endogenous losses (g/kg of dry matter intake) were determined as: CP, 10.58; Arg, 0.35; His, 0.13; Ile, 0.23; Leu, 0.39; Lys, 0.41; Met, 0.06; Phe, 0.25; Thr, 0.45; Trp, 0.09; Val, 0.38; Ala, 0.38; Asp, 0.54; Cys, 0.16; Glu, 0.65; Gly, 0.98; Pro, 1.96; Ser, 0.34; and Tyr, 0.17.

³Test of linearity for SID of CP and AA in 3 diets containing 4.7, 12.7, or 22.1% CP, respectively, by including 10, 30, and 50% SBM.

Table 2. SID of CP and AA in diets containing different inclusion rates of SPC fed to growing pigs^1,2 (Exp. 2)

¹Each least squares mean for each treatment represents 8 observations, respectively, except for the diet containing 20% SPC (n = 7).

²Values for SID were calculated by correcting the values for apparent ileal digestibility for basal ileal endogenous losses. Basal ileal endogenous losses were determined (g/kg dry matter intake) as: CP, 23.47; Arg, 0.84; His, 0.25; Ile, 0.42; Leu, 0.71; Lys, 0.66; Met, 0.10; Phe, 0.46; Thr, 0.70; Trp, 0.17; Val, 0.56; Ala, 0.77; Asp, 1.15; Cys, 0.27; Glu, 1.45; Gly, 2.17; Pro, 6.44; Ser, 0.60; and Tyr, 0.32.

³Test of linearity for SID of CP and AA in 4 diets containing 6.8, 14.1, 19.7, or 26.6% CP, respectively, by including 10, 20, 30, or 40% SPC.