Muscle Amino Acid Composition of Pigs Is Influenced By the Feeding System and Amino Acid Intake

Pigs fed individually with daily tailored diets (PF: precision feeding) may respond differently to amino acid (AA) supply compared to pigs fed under conventional group-phase feeding (GF) systems. The response of growing pigs in PF and GF systems fed with different levels of threonine (70%, 85%, 100%, 115% and 130% of the ideal threonine-to-lysine ratio of 0.65) was studied in a 21‑d experiment. A total of 110 pigs of 25 (±0.8) kg BW were housed in the same pen and fed using electronic feeders. Individual pigs were the experimental unit. Five out of eleven pigs per treatment were slaughtered to obtain organs and muscle composition. The chemical composition of the longissimus dorsi and the pool of other carcass muscles were estimated by near-infrared transmittance, and AA were analyzed by gas chromatography. Data were analyzed by SAS mixed model procedures in a 2 × 5 factorial arrangement. Threonine intake increased linearly (P < 0.05) with increasing dietary Thr levels for PF (6.28 to 11.76 g/d) and GF pigs (6.85 to 11.01 g/d). Lysine intake was similar (12.5 g/d) across treatments. Dietary Thr supply did not affect longissimus dorsi CP concentration but increased Thr concentration (g/100 g of CP) in a quadratic manner in PF pigs without effect on GF pigs (interaction P < 0.05). Threonine concentrations in the muscle pool presented an interaction between feeding system and Thr level as it changed in a cubic manner (4.56 to 4.38 g; P < 0.05) in PF and GF pigs. Muscle pool CP concentration was greater for GF than PF pigs (18.06% vs. 17.79%; P = 0.05) and tended (P < 0.10) to increase linearly with increasing Thr levels in both systems. Threonine concentrations (g/100 g of CP) in the small intestine (4.62 g) and liver (4.41 g) were similar across dietary Thr levels confirming that these organs have priority over muscles under Thr restriction. However, Thr liver concentration tended (P < 0.10) to be greater in PF than GF pigs (4.44 vs. 4.39 g), showing that AA retention in organs may be more efficient in PF than GF pigs. Threonine restriction can, therefore, modify longissimus dorsi AA composition based on the AA intake. However, muscles respond differently to dietary Thr supply, and, under restriction, organs seem to be prioritized over muscles. These results indicate that feeding systems can affect the way pigs use Thr.