Effects of feed intake level on the additivity of apparent and standardized ileal digestibility of amino acids in diets for growing pigs

The present study was conducted to evaluate the additivity of apparent (AID) and standardized (SID) ileal digestibility of AA in diets at two feed intake (FI) levels. A total of 28 growing barrows were used in this study with 7 treatments and two 10-d experimental periods (7 d adaptation and 3 d ileal digesta collection). The seven treatments contained 3 experimental diets (Corn, SBM, and Corn + SBM mixed diet) at 2 feeding levels (low and high, represented 3% of BW per day and ad libitum, respectively) in a 3 × 2 factorial arrangement and a nitrogen-free diet (NFD) fed at low feeding level. There were 8 replicates for each of the 7 treatments. The AID of AA determined in low FI was greater than the AID determined in high FI levels for nitrogen and all AA except for Leu, Met, Cys, and Tyr in corn, SBM, and the mixed diet (P < 0.05). The SID of AA determined in low FI was greater than the SID determined in high FI levels for nitrogen, Phe, Thr, Trp, Ala, Asp, Glu, and Ser in corn, SBM, and the mixed diet (P < 0.05). When using AID of AA in corn and SBM determined at low FI level to predict the AID in the mixed diets at low FI level, the predicted AID underestimate the actual determined values of His, Phe, Trp, Vale, Asp, Glu, and Pro (P < 0.05). In low FI level, the predicted SID underestimated the determined values of Asp (P < 0.05) and overestimated Leu, and Cys (P < 0.05). When the digestibility derived at low FI level were used to predict the AID of AA in the mixed diets fed at high FI level, the predicted AID overestimated the determined values in nitrogen, Leu, Thr, and all non-essential AA except for Gly and Tyr (P < 0.05). Using the values derived from low FI level condition to estimate the SID of the mixed diet in high FI level condition, the determined SID of AA in the mixed diets was overestimated in His, Leu, Met, Thr, Val, Ala, Cys, Ser, and Tyr (P < 0.05). The determined AID of the mixed diet in high FI level were underestimated for His, Lys, Phe, Thr, and all dispensable AA except for Pro, Ser, and Tyr (P < 0.05), when AID of AA in corn and SBM derived from high FI level was used in the calculation. The predicted SID in the same calculation underestimated the SID of Asp and Glu in the mixed diet (P < 0.05). In conclusion, low FI level may increase the determined AID and SID of protein and AA in diets, which leads to the lack of additivity and overestimation of the actual AID and SID of AA in the complete diet when using the digestibility were derived from low FI level animals to predict the digestibility in high FI conditions.