Intake, digestion, and rumen microbial impacts of black soldier fly larvae and frass provided as protein supplements to cattle consuming forage.

Insects reared in large-scale, commercial settings may be a sustainable alternative to conventional livestock feeds. Black soldier fly larvae (BSFL) has been researched as a potential protein supplement for cattle, but knowledge gaps remain in how BSFL affects rumen microbial populations. Further, frass and larval sheddings (FRS) are produced in greater quantities than larval biomass and are N rich but have not been investigated as feed. Accordingly, the objective of our study was to compare the effects of isonitrogenous levels of cottonseed meal (CSM), BSFL, and FRS on forage utilization responses and rumen microbial communities in cattle consuming low-quality forage. Angus steers (n = 8; 240 ± 22.5 kg BW) with ad libitum access to low-quality forage (5.9% crude protein) were used in replicated 4 × 4 Latin squares. One of 4 treatments was provided daily: no supplemental protein (CON), CSM, partially defatted BSFL, or FRS all provided at 100 mg N/kg BW. There were four 16-d experimental periods which included an 8-d adaptation to treatments, 7-d measurement of intake and digestion, and 1-d for determination of ruminal fermentation and sampling of rumen content for determination of microbial populations. Rumen content samples were separated into liquid and solid fractions, extracted for genomic DNA, PCR amplified for the V3 to V4 region of the 16S rRNA gene, sequenced on a MiSeq platform, and analyzed using the QIIME2 pipeline. Protein supplementation increased forage organic matter (OM) intake and total digestible OM intake (TDOMI) relative to CON (P ≤ 0.01). For TDOMI, there were differences (P ≤ 0.03) between CSM (2.84 kg/d) and BSFL (3.07 kg/d) and CSM and FRS (3.05 kg/d). Treatment did not affect OM (P = 0.82) or neutral detergent fiber (P = 0.43) digestibility. Relative abundances of certain bacterial genera (i.e., Butyrvibrio, NK4A214, Prevotellacaeae UCG003, and Veillonellacaeae) were significantly affected by treatment in either the liquid or solid rumen fraction, but diversity indices, phyla, and dominant families were not affected. These data indicate that BSFL and FRS stimulate forage utilization to a similar or greater extent than a conventional protein supplement. The minor observed impacts on microbial community composition suggests no adverse effects on the rumen microbiome associated with supplementation of BSFL or FRS. Ultimately, BSFL and FRS may be suitable protein supplements for beef steers consuming low-quality forage.