Can a mixture of Hermetia illucens and Tenebrio molitor meals be feasible to feed broiler chickens? A focus on bird productive performance, nutrient digestibility, and meat quality

As already highlighted by more than a decade of scientific research, using insects – mainly Hermetia illucens (HI) and Tenebrio molitor (TM) – as alternative feed ingredients represent a promising and feasible nutrition strategy in poultry production. On the one hand, the scientific community has increasingly focused its attention on the processing of insect larvae into meals or fats to be included in poultry diets as protein and energy sources, respectively. On the other hand, directly providing live or dehydrated larvae to chickens may allow exploring their function as environmental enrichments (Schiavone and Castillo, 2024). As a consequence of such an increasing “insect-driven” research, the EU Member States and the Italian Ministry of Health have recently authorized insect-processed animal proteins (Commission Regulation, 2021) and live insects (Ministero della Salute, 2023) in poultry feed in the EU and Italy, respectively.

However, despite these scientific and legislative foundations, insect-derived products are still facing difficulties in entering the feed market. In particular, availability and consistency of supply of insect meals represents one of the main limiting factors, as the estimated insect production is far below the protein requirements of poultry feeds, thus not allowing meal prices to lower and become competitive (Gasco et al., 2023). Furthermore, the scientific community is currently recommending not exceeding 10 % inclusion level of insect meals in diets for commercial strains (broilers and laying hens), in order to not impair nutrient digestibility, growth performance, and meat quality (Biasato et al., 2023; Dalmoro et al., 2023; Gasco et al., 2023). Indeed, including more than 10 % of insect meals (i.e., 15 %) may decrease diet nutrient digestibility and, in turn, negatively influence bird gut health and growth performance (Biasato et al., 2023). However, inconsistent results among the studies seem to be identified for high levels of HI meal (de Souza Vilela et al., 2021; Dörper et al., 2021), and recent meta-analyses also revealed that broiler growth can be preserved (Martínez Marín et al., 2023) or even improved with increasing insect meal rates (Lee et al., 2024), thus making a standardization in their application even more challenging. As a consequence of this scenario, novel formulation strategies could be beneficial to improve insect meal utilization by commercial strains and valorize this feed ingredient for the whole poultry industry.

An interesting, first glimpse of innovative approaches has recently been proposed by Hartinger et al. (2022), which tested the concomitant administration of HI meal (5.2 %) and fat (6.4 and 12.8 %) in diets for broiler chickens and observed improved BW, ADG and feed conversion ratio (FCR) in the starter phase. Another potential, still unexplored formulation strategy, could be represented by the utilization of a mixture of HI and TM meals, which may lead to several benefits. Firstly, adopting a mixture of insect meals could promote the competitiveness of the insect industry, as insect producers should provide less meal quantities to guarantee the same inclusion levels in poultry diets. As a second aspect to consider, the possibility of reducing the amount of single insect species-derived meal may attenuate their side effects, such as the reduction in nutrient digestibility (Biasato et al., 2023) or the worsening of meat fatty acid (FA) profile (Dalle Zotte, 2021), the latter being mainly highlighted for HI. Lastly, as Hartinger et al. (2022) previously identified improved fat digestibility in broiler chicks when HI meal was fed along with HI fat, a potential synergy between HI and TM could be exploited as well.

Based on the above-reported background, the present study aims to evaluate the feasibility of including a mixture of HI and TM meals in diets for broiler chickens, focusing on the assessment of growth performance, nutrient digestibility, carcass traits, and meat quality.

where (AA / TiO₂)d represents the ratio of the amino acid and TiO₂ concentrations in the diet, (AA / TiO₂)i represents the ratio of the amino acid and TiO₂ concentrations in the ileal digesta, and AAX represents each amino acid.

The growth trial was performed at the commercial poultry farm “Azienda Pozzo” (Riva presso Chieri, Turin, Italy). The experimental protocol was designed according to the guidelines of the current European Directive (2010/63/EU) on the care and protection of animals used for scientific purposes and approved by the Ethical Committee of the University of Turin (Italy) (Prot n. 15735). A total of 420 newly hatched male broiler chicks (Ross 308) were randomly allotted to 42 stainless steel mobile pens. Each pen (1.00 m wide × 1.00 m long × 1.00 m height) was equipped with a feeder and nipples, and rice hull as litter. At hatching (directly in the hatchery), all the chicks received subcutaneous vaccination against Newcastle and Gumboro diseases, ocular vaccination against infectious bronchitis, and spray vaccination against coccidiosis. The poultry shed was equipped with automatic ventilation (inlets and fans), humidification and feeding systems. The lighting schedule was 23 h light:1 h darkness until d 7 and then 18 h light:6 h darkness was adopted until the slaughtering age.

At the beginning of the growth trial, the newly hatched chicks were individually weighed and randomly allocated to the replicate pens to obtain a homogeneous initial BW among the seven dietary treatments (C: 45.8 ± 0.3 g, HI5: 45.9 ± 0.3 g, HI10: 45.7 ± 0.3 g, TM5: 45.7 ± 0.3 g, TM10: 45.7 ± 0.3 g, MIX5: 45.8 ± 0.3 g, MIX10: 45.7 ± 0.3 g). The BW was recorded at an individual level on d 10, 25, and 37 (feed change). The ADG, the ADFI and the FCR were determined at the pen level for each feeding phase (d 0−10, 11−25, and 26−37) and for the overall experimental period (d 0−37). All the measurements were made using an electronic scale (KERN PLE-N v. 2.2; KERN & SOHN, Sohn GmbH, Balingen-Frommern, Germany, d: 0.1).

At the end of each feeding phase of the growth trial, all the birds were removed from each pen and housed in wire-mesh cages ((100 cm width × 50 cm length) for 1 h per day for three consecutive days to collect fresh excreta samples (from d 8 to 10, d 23 to 25, and d 35 to 37 for the starter, grower and finisher periods, respectively). After collection, the cleaned excreta samples were frozen at −20 °C. At the end of each collection period, the excreta were pooled and stored at −20 °C for laboratory analyses, as reported in the “Chemical Analyses” section.

The ATTDC of the dietary nutrients were calculated as follows:

]), which determined differences in actual nutrient composition when compared to the theoretical assumptions.

Dietary insect meal inclusion did not influence the growth performance of the broiler chickens of the present study during the starter phase of the growth trial. However, the grower and the finisher phases revealed improved growth performance in the TM5 (and, to a lesser extent, MIX5) birds when compared to those fed the C diet and the highest inclusion level of all the insect meals (with more consistent effects being highlighted for HI10 and MIX10). These differences were also reflected in the whole experimental period, where the TM5 group overall performed more efficiently than the birds fed the 10 % of all the tested insect meals. This scenario confirms the better feasibility in using reduced percentages of insect meals in broiler diets, as already suggested by the scientific community (Biasato et al., 2023; Dalmoro et al., 2023; Gasco et al., 2023). Nevertheless, the results herein obtained seem to partially disagree with these guidelines as well, as including the recommended 10 % inclusion level of insect meals (mainly HI and MIX) worsened bird growth performance in the finisher phase (especially) and in the whole experimental period in comparison with those obtained using a standard diet. On the one hand, this can be attributed to the above-mentioned higher AA imbalances of the HI- and MIX-based diets than the TM ones, which, in turn, may be related to the less balanced AA profile of the HI and MIX meals herein used when compared to TM. On the other hand, the highest macronutrient digestibility of the TM meal could also have a key role in making its 10 % inclusion level still feasible for broiler diets. In the present study, the role of chitin – which has previously been reported to negatively affect nutrient digestibility (of CP mainly) in broiler chickens at 3 % inclusion level (Razdan and Pettersson, 1994) – may reasonably be excluded, as chitin dietary contents were relatively low (1.34-1.59 % on average) and CP digestibility was mostly unaffected.

Differently from the growth performance outcomes, diet digestibility results of the current research are less consistent and more difficult to be clearly attributed to the different dietary treatments. Insect-based diets showed increased or unaffected DM digestibility when compared to the standard diet in all the feeding phases, but the overall high ATTDC for all the dietary treatments (90-97 % on average) and the magnitude of the differences among them (3-5 % on average between the lowest and the highest ATTDC value) make these outcomes less biologically relevant. Differently, CP digestibility was improved or unaffected when insect meals were included in starter and grower diets, but reduced ATTCD were identified for TM10- and MIX-fed birds during the finisher phase. This may explain the worst growth performance highlighted in the MIX10 group in the same feeding phase, but, considering that the MIX5 and TM10 broilers performed similarly to those fed the standard diet, it is possible to hypothesize a more prominent influence of insect meal digestibility rather than diet one. Furthermore, as MIX5 diet was characterized by lower chitin content when compared to MIX10 and TM10 ones (1.41 % vs 1.56 % and 1.59 % on average, respectively), a significant role of chitin in shaping CP digestibility can herein be excluded as well. The EE digestibility was improved when the MIX meal was included in the starter diet when compared to the standard and TM5 ones, as a reasonable consequence of their lower SFA and, in particular, palmitic and stearic acids contents. Indeed, young chicks are characterized by a limited secretion of bile salts and pancreatic lipase, which are essential for the emulsification and breakdown of dietary fats (Smink et al., 2008). Insect-based diets also showed increased or unaffected EE digestibility when compared to the standard diet in the grower phase, but the magnitude of the differences among them (6 % on average between the lowest and the highest ATTDC value) make these outcomes less biologically relevant (as already previously discussed for DM digestibility). Interestingly, the finisher phase displayed increased EE digestibility for the diets containing 10 % inclusion level of insect meals than 5 %. This could be related to the higher EE content of the formers in comparison with the latter, as adult broiler chickens digest fats more efficiently than young chicks due to the development of their digestive enzymes, more advanced digestive tract structures, better bile secretion, and mature gut microbiota (Krogdahl, 1985).

Carcass traits outcomes of the present study logically reflected the animal growth performance, with higher RCW and CCW being mainly highlighted in TM5 and MIX5 groups when compared to HI10 and MIX10 birds. Indeed, improved feed efficiency is usually associated with increased energy and nutrients being allocated toward muscle growth rather than other body parts (Zuidhof et al., 2014). However, the carcass yields differences were less pronounced among the experimental treatments, thus confirming their stability in broiler chickens (Rohloff Junior et al., 2024). The identification of higher breast yield in TM5 and MIX5 animals than MIX10 – as well as lower thigh yield in TM5 birds in comparison with the MIX10 group – also represents a growth-related effect, as rapid growth rates are genetically linked with increased breast yield (Zuidhof et al., 2014), and higher breast yield can result in a decrease in other muscle regions such as the thigh (Santos et al., 2021). Despite commercial selection pressure having also reduced fat deposition in broilers (Zuidhof et al., 2014), the abdominal fat changes highlighted in the current research do not seem to be related to bird growth performance, thus confirming that fast-growing broilers may still be susceptible to fat deposition (abdominal fat included) due to differences in metabolism and nutrient partitioning (Griffin and Goddard, 1994). The HI10 birds interestingly displayed higher liver relative weight when compared to TM5, MIX5 and C groups. This can be related to the lower nutrient digestibility of the HI meal, which could have increased liver workload for metabolizing the undigested nutrients (proteins mainly) and, in turn, have led to liver hypertrophy as compensatory mechanism. Indeed, greater feed efficiency may reduce the relative weight of the broiler liver (Huang et al., 2022). Low nutrient digestibility may also be the reason behind the increased spleen relative weight (and the numerically higher bursa of Fabricius relative weight) identified in the HI10 chickens, as potentially harmful antigens (i.e., undigested nutrients or toxins) may have reached the intestine and triggered an immune response, with subsequent immune cells proliferation and, in turn, organ hypertrophy (Klasing, 2007).

As far as the meat physical quality is concerned, thigh pH from C and MIX5 birds of the current research resulted to be higher than that from TM10 and MIX10 groups. This can partially be attributed to the MIX5-related improved growth performance, as birds with better growth rates and feed efficiency are characterized by minimized pre-slaughter glycogen depletion due to energy use and, in turn, a more stable pH meat (Berri et al., 2007). Nevertheless, the absence of a clear insect-related pattern has already been reported by Malematja et al. (2024). In parallel, the lowest and the highest breast cooking loss were highlighted in C, HI5 and MIX5, and HI10 groups, respectively. Meat cooking losses are usually associated with a decrease in pH values, which, in turn, increase protein denaturation and decrease, as a consequence, water holding capacity (Petracci and Cavani, 2012). However, the absence of breast pH changes rules out the exclusive role of pH in cooking loss determination. Indeed, the higher ash content detected in the C and HI5 breast when compared to the HI10 one must be considered as well, as selected minerals have previously been reported to help improve water holding capacity by maintaining acid-base balance and osmotic pressure (Patel et al., 2019). Furthermore, the lowest breast DM content could have also contributed to the increase in cooking losses. Interestingly, the HI10 birds also displayed the highest thigh redness among all the dietary treatments, as a reasonable consequence of the already mentioned worst growth performance. Indeed, birds that grow less usually move more, and this increased physical activity can cause more blood flow to the muscles and the development of more red muscle fibers due to the increased oxygen demand (Guo et al., 2019). Previous studies assessing the influence of black soldier fly meal in broiler chickens also highlighted increased meat a* values (Schiavone et al., 2019; Altmann et al., 2020; Murawska et al., 2021), with these authors attributing such outcome to larvae-related pigments.

Overall, breast meat from the TM5 and HI10 birds of the present study displayed the lowest DM content, while the TM5 group was characterized by the lowest breast CP content. Such outcomes can be attributed to the optimal growth performance already highlighted for the broiler chickens fed the 5 % of TM meal, as high breast weights and yields, as well as ADG and SW, have recently been identified as risk factors for the development of wooden breast (Bordignon et al., 2022). Furthermore, reduced DM and CP contents can also be observed in commercial broilers with this meat quality defect (Dalle Zotte et al., 2020). Despite the incidence of wooden breast having not been object of investigation in the present study, this represents an interesting hypothesis to consider. The TM5 and HI10 groups also showed lower breast ash content when compared to the C and HI5 birds, as already mentioned before. The decrease in ash content of HI10 breast can logically be attributed to the lower dietary ash content, while the reduced TM5-related ash levels may be attributed to a potential wooden breast as well (Dalle Zotte et al., 2020). Last but not least, the lowest thigh CP content detected in the HI10-fed broiler chickens can logically be attributed to their poor growth performance and, in turn, less nutrients reserved for muscle growth (Zuidhof et al., 2014).

Breast FA analysis of the present study revealed higher SFA content in the TM10 birds when compared to the C, HI5 and TM5 groups (but analogous to that of HI10 chickens), as well as the highest and the lowest proportions of MUFA, and PUFA, n-3 and n-6 FAs, respectively. In parallel, the lowest and the highest amounts of MUFA, and PUFA and n-6, respectively, were highlighted in the C- and HI-fed birds. This meat FA profile – that logically reflects the dietary FA profile – may suggest a TM-related detrimental effect on meat healthiness (as already underlined for proximate composition outcomes), since PUFA are beneficial for cardiovascular health (Petracci and Cavani, 2012). However, the identification of unaffected n-6/n-3 ratio partially mitigates these negative changes. Interestingly, this scenario seems to overall disagree with the available scientific literature, as fat derived from HI usually increases SFA content (especially lauric and myristic acids) in poultry meat or eggs, while that from TM is commonly associated with increased unsaturated FA (mainly oleic and linoleic acids) (Toral et al., personal communication). However, the insect meals used in the present study were partially to highly defatted, thus firstly explaining this discrepancy. A second aspect to consider is that balancing the AA profile across all the experimental diets required significant adjustments in the inclusion rates of other feed ingredients (soybean oil mainly), which allowed achieving isonitrogenous and isoenergetic dietary treatments but prevented the attribution of changes in meat FA composition directly to the inclusion of insect meals. Nevertheless, small amounts of rumenic acid (C18:2 c9t11) were detected in the insect-based diets. Rumenic acid is the most abundant among conjugated linoleic acid (CLA) isomers found in nature (den Hartigh, 2019) and its presence in both black soldier fly and yellow mealworm larvae has recently been reported (Tognocchi et al., 2023). Renna et al. (2024) demonstrated that the rearing substrate significantly affects the amount of CLA isomers detectable in insect larvae and hypothesized that their presence may be the consequence of bioaccumulation from the rearing substrate, de novo biosynthesis mediated by the Stearoyl Co-A desaturase enzyme, or presence of lactic acid bacteria in the larvae gut. Rumenic acid possesses health-promoting effects (i.e., anti-carcinogenic, anti-obesogenic and anti-atherosclerotic properties) (den Hartigh, 2019) and production of CLA-enriched poultry meat could be desirable from a human health perspective. In the current research, no CLA isomers were detected in breast meat. This demonstrates that the low dietary inclusion levels (<10 %) of HI and TM meals typically used in poultry diets do not allow producing poultry meat naturally enriched in CLA. However, it is well known that the FA profile of insect larvae can be modulated via their rearing substrate (Georgescu et al., 2022), and CLA-enriched insect meals could be produced using such strategy. Therefore, further studies should be designed to verify the enhancement potential of the FA profile of meat incorporating CLA-enriched HI and TM larvae meals into poultry diets.

As far the sensory analysis is concerned, it is important to underline that the average scores for all the sensory parameters and for all the dietary treatments of the current research ranged from “neither like or dislike” and “like slightly”, thus overall indicating that consumers accepted the novel products. Nevertheless, panelists preferred the meat obtained from the C- and MIX-fed broiler chickens when compared to that derived from those fed diets containing 5 % and 10 % of TM and HI meal, respectively, in terms of tenderness, juiciness and overall liking. The reduced tenderness scores recorded for the TM5 breast can reasonably attributed to the above-mentioned, potential occurrence of wooden breast, as accumulation of interstitial connective tissue or fibrosis within the muscle represents one of its typical features (Bordignon et al., 2022). In parallel, the decrease in perceived juiciness highlighted for the same breasts is logically related to their higher cooking loss, since increase in water holding capacity enhances meat juiciness (Mir et al., 2017). Surprisingly, the reduced overall liking scores recorded for the HI10-fed group were not associated with other significant sensory changes, even if they displayed the highest cooking loss. It is well-known that meat flavor can be influenced by FA profile, with MUFA (oleic acid mainly) being positively correlated with it, and SFA and PUFA (n-3 mainly) showing the opposite trend (Hwang et al., 2017). Since HI10 breast was characterized by lower and higher, respectively, MUFA and PUFA than MIX-derived products, as well as numerically lower flavor scores, the potential role of FA profile cannot be ruled out.

In conclusion, including 5 % of TM and MIX meals in diets for broiler chickens led to improved growth performance and carcass traits, while the 10 % inclusion level of HI and MIX meals was associated with the worst productive outcomes. In parallel, including 10 % of HI meal or using TM meal (at 5 % inclusion level mainly) led to negative effects on meat quality and sensory analysis, in terms of increased cooking losses, decreased nutritional value, altered FA profile, and reduced tenderness, juiciness, and overall liking. This scenario opens up to three interesting considerations: 1) using a mixture of HI and TM meals appears to be feasible for broiler chickens, but does not lead to significant improvements when compared to the use of the single meals, 2) the response of broiler chickens to the use of insect meals seems to be mainly driven by the inclusion levels rather than the insect species (as already highlighted by the scientific community), and 3) productive and quality outcomes may deeply differ between each other, with the optimal diet for the growth performance being not necessarily the best one for obtaining a high-quality product as well. While the combination of HI and TM meals appears to be a feasible option for broiler chicken diets, the results obtained suggest that it does not offer significant advantages over using each meal individually. However, from a practical perspective, economic factors such as cost, availability, and sustainability could still justify their inclusion in feed formulations. Further research into alternative combinations or improved processing methods may help enhance their effectiveness. Since no real synergy or mitigation of negative effects was observed, optimizing how these insect meals are processed or incorporated into diets could be key to unlocking potential benefits. Additionally, given the crucial role of gut health in shaping broiler performance, future studies should not only investigate how insect meals—both individually and in combination—affect intestinal health but also establish a clear link between gut health features and bird growth performance. Understanding these connections could, indeed, provide practical insights for optimizing diet formulations and maximizing productivity.