Effects of substituting soyabean meal and rapeseed meal with various microalgae species on in vitro rumen fermentation kinetics and methane production

The demand for protein crops to feed livestock is increasing, with soybean meal (SBM) and rapeseed meal (RSM) currently being two of the major protein sources. However, using land to grow protein crops for livestock competes with crops which can be consumed directly by humans. At the same time, there are increasing concerns about the environmental impact of growing some crops, especially soybean, due to the land-use change which is associated with it's growth in some countries. Thus, there is a need to identify alternative protein sources for livestock, with microalgae being one potential source. However, it is important to understand the impact of microalgae species and inclusion level on rumen fermentation and methane production. This study examined the effects of substituting SBM and RSM (replacing 5, 10 and 20% of SBM and RSM) with three different microalgae species ( Spirulina Platensis [SP], Chlorella Vulgaris [CLV] and Nannochloropsis Gaditana [NG]) on in vitro methane (CH 4 ) production and rumen fermentation characteristics. Samples were incubated for 4, 24 and 48-h with grass silage (GS) as the basal substrate in in vitro batch culture to mimic the rumen. While volatile fatty acid concentrations in the fermentation liquor and CH 4 concentrations did not differ between microalgae inclusion levels, CLV at 10 and 20% inclusions increased propionate concentration by 8.3% and 5.8%, respectively and decreased CH 4 concentration by 12.3% and 26.9%, respectively, compared with 0% inclusion. Ammonia-nitrogen (NH 3 -N) concentrations were comparable across microalgae inclusion levels, except for the 20% inclusion where NH 3 -N increased (within SP) and decreased (within CLV). The measured parameters demonstrated substrate-specific responses to microalgae species. For instance, SBM incubated with SP and CLV reduced CH 4 concentration by 23.5% and 18.5%, respectively, but increased propionate concentration by 8.8% and 12.1%, respectively. Similarly, RSM incubated with NG showed 26.3% lower CH 4 concentration and 14.3% higher propionate concentration relative to SBM at 24-h, indicating significant interactions between substrates and the microalgae species. This in vitro study highlights the importance of selecting suitable microalgae species, inclusion levels and substrates when examining the potential of microalgae as a protein replacement. In conclusion, SP, CLV and NG had no adverse effects on in vitro fermentation parameters suggesting that they may have potential as alternative protein sources, while CLV in particular demonstrated potential methane-mitigating effects. • Microalgae inclusions had no detrimental effects on in vitro rumen fermentation parameters. • The inclusion of CLV at 10 and 20% increased propionate levels and reduced CH 4 emissions. • The tested microalgae species exhibited substrate specific response for all in vitro fermentation parameters.