From waste to feed: In vitro assessment of spent silkworm by-products and manure-raised housefly as soybean meal substitutes in ruminant feed

The study aimed to evaluate the efficiency of utilizing insects-spent silkworm pupae, either fed naturally on mulberry leaves (SW-N) or on an artificial diet (SW-A), as well as housefly larvae (HF-L) and pupae (HF-P) reared on swine manure-as alternative feeds to replace soybean meal in ruminant diet (60% grass: 40% soybean meal) at inclusion levels of 5%, 10%, 20%, and 40%. This study assessed their nutritional value and investigated their effects on nutrient digestibility, rumen fermentation profile, and methane (CH₄) production using an in vitro batch culture technique. The results showed that the tested insects had higher protein and fat contents than soybean meal. Fatty acid analysis revealed that SW-N and SW-A were rich in unsaturated fatty acids (70.9% and 60.6%, respectively), whereas HF-L and HF-P contained 52.4% and 51.4%, respectively. The inclusion of SW-N and SW-A at levels up to 40% in the diet had no negative effects on volatile fatty acids (VFA) production, increased ammonia-nitrogen concentration and nutrient digestibility, and decreased CH₄ production by 20%. Although HF-L and HF-P supplementation reduced CH₄ production by up to 25%, their inclusion at levels exceeding 10% inhibited rumen fermentation. In conclusion, SW-N and SW-A can effectively replace soybean meal in ruminant diets; while the inclusion of HF-L or HF-P at levels exceeding 10% is not recommended. These insects represent a promising strategy for sustainable feed development with CH₄-reduction potential.