Rearing mealworm larvae with wheat, barley or maize grains as main source of nutrients in unbalanced or balanced substrates

Feeding conditions of mealworm (T. molitor) larvae for livestock nutrition need to be optimised. The effects of the cereal offered as main nutrient source on growth performance and composition of T. molitor larvae were evaluated. In Experiment 1 (unbalanced diets), substrates included 80% of wheat (W1), barley (B1) or maize (M1) and 20% wheat straw. In Experiment 2 (balanced diets), substrates were formulated using wheat (W2), barley (B2) or maize (M2) as the main ingredient, combined with wheat bran or straw to contain similar N, starch and NDF contents (19.5, 520, and 270 g/kg DM, respectively). A control substrate based solely on wheat bran was also included in each experiment (C1 and C2). Each treatment was replicated four times in trays containing 24 g of substrate and 60 larvae, and the experimental period lasted for 28 (Experiment 1) or 35 (Experiment 2) days. Larval weight and residual substrate were monitored weekly, and larval chemical composition was analysed at the end of each experiment. In Experiment 1, feed intake was greatest for B1 and W1, intermediate for C1 and lowest for M1 (P < 0.001). However, larval growth was greater for C1 than for B1 and W1, with the lowest rates observed for M1 (P < 0.001). Consequently, the feed to gain (F:G) ratio was lowest for C1 and highest for M1 (P < 0.001). Mortality was also greatest for M1 (P < 0.001). In Experiment 2, feed intake (P < 0.001) and larval growth (P < 0.01) decreased in M2 compared to B2 and W2, which in turn showed similar growth performance to C2 (P < 0.001). Larval composition followed a similar trend in both experiments, with the highest N content observed in larvae fed C1 and C2 (P < 0.001), while DM and ether extract contents were higher in larvae reared on barley (P < 0.001). In summary, maize-based substrates may compromise feed intake and growth performance of T. molitor larvae compared to those based on wheat or barley, regardless of the nutrient content of the diet. Apparently, larval performance does not respond to the substrate starch content but it might be influenced by the endosperm physical structure. Additionally, a lower N content in the substrate could impair growth performance, an effect that is alleviated once larval protein requirements are achieved. Larval composition was affected by the main dietary ingredient, with substrates based on wheat bran increasing the N content of larvae, whereas those based on barley resulted in increased DM and Ether extract contents.