Role of threonine as a potential secondary limiting amino acid in cereal-based substrates for Tenebrio molitor larvae: effects on larval composition and amino acid content

The effects of Thr supplementation in a barley-based substrate on growth performance and protein and amino acid (AA) content of Tenebrio molitor larvae were investigated, considering its potential role as a secondary limiting AA. Sixty larvae per tray (n = 6) were assigned to five dietary treatments over two 21–day periods: barley alone (B); barley combined with soybean meal (BS, 85:15); and barley supplemented with Lys (BL), Thr (BT), or a combination of both (BLT). Synthetic AA were supplied via agar cubes twice weekly. At the end of the experimental period, larval performance and whole-body composition and complete AA profile were determined. Final larval weight was higher (p < 0.001) in larvae fed the BS substrate (120.1 mg) than in those fed the other treatments (111.7, 110.2, 108.6 and 108.0 for BLT, BL, BT and B, respectively). In addition, larval crude protein (CP) and AA contents were greater in BS than in the other treatments (p < 0.001). Supplementation with Thr alone did not affect larval growth or CP and AA content, whereas combined supplementation with Lys and Thr increased CP and AA content compared with B, with values numerically higher than those obtained with Lys supplementation alone. These results suggest that Thr does not act as a primary limiting AA in barley-based substrates but may contribute as a secondary limiting factor once Lys deficiency is alleviated. The superior performance of BS suggests that additional nutritional constraints, potentially including other limiting AA, may still restrict protein deposition in cereal-based diets.HighlightsBarley as the unique substrate impairs protein deposition in Tenebrio molitor larvaeThreonine may limit protein deposition once lysine deficiency is correctedOther amino acids not evaluated may also limit larval protein production efficiency