Does the Optimal Dietary Methionine to Cysteine Ratio in Diets for Growing Chickens Respond to High Inclusion Rates of Insect Meal from Hermetia illucens?

Simple Summary Currently, several alternative protein sources are under investigation for replacing soybean meal in poultry diets. One alternative is larvae meal of the black soldier fly (Hermetia illucens) with a specific sulfur amino acid composition. The larvae meal is limiting in sulfur amino acids supply and provides a wide methionine:cysteine ratio of 61:39. Currently, it is not known whether the insect meal has an impact on the optimal ratio of methionine to cysteine in broiler chicken diets. The methionine:cysteine ratio significantly influences animal growth and welfare. Both methionine and cysteine excess and deficiency can lead to an impaired feed intake, growth, and feed efficiency. Therefore, the aim of this study was to investigate whether the optimal methionine:cysteine ratio is modulated when a high inclusion rate of partly defatted Hermetia illucens meal is applied. The results show that a methionine:cysteine ratio of 50:50 yields superior growth and dietary protein quality. It can be concluded that the insect meal under study is a promising alternative protein source without modulating the optimal methionine:cysteine ratio in broiler chicken diets. Abstract The dietary methionine:cysteine (Met:Cys) ratio (MCR) is an important factor influencing the optimal growth of chickens. Therefore, this study aimed to contribute to the assessment of the optimal dietary MCR in diets with the complete replacement of soybean meal (SBM) by a partly defatted larvae meal of Hermetia illucens (HM). A growth study with 240 male meat-type chickens (Ross 308) was conducted, also assessing the body nutrient deposition both at the end of the starter (day 21) and the grower (day 35) period. Birds were fed experimental diets based on wheat, maize, and insect meal (23%/21% HM in starter/grower diets). Sulfur amino acids were created as the limiting AA in diets with graded MCR (40:60; 45:55; 50:50; 55:45; 60:40). The control diet contained SBM instead of HM with a MCR of 50:50. The current results based on growth parameters, dietary protein quality, and Met efficiency data gave support to the previous assumption of an ideal MCR of 50:50, which was also valid in diets with a high proportion of insect meal. The lowest MCR of 40:60 led to significantly impaired feed intake and growth of the birds, while the response to the highest MCR (60:40) was moderate.