In vitro evaluation of utilisable crude protein and methane production for a diet in which grass silage was replaced by different levels and fractions of extracted seaweed proteins

Utilisable crude protein (uCP), methane (CH₄) production and other fermentation parameters were analysed in vitro for a diet in which grass silage was replaced by different levels of seaweed protein fractions prepared from three seaweed species: Saccharina latissima, Alaria esculenta and Palmaria palmata. Ten fractions from these three species in which the protein content had been increased and the salt content reduced by simple processing were tested, with inclusion levels in the diet based on the nitrogen content of the fractions. Following an extraction procedure, four fractions from Saccharina latissima, three from Alaria esculenta and one from Palmaria palmata, were gradually included in the diet by replacing high quality silage with approximately 0, 0.15, 0.30 and 0.45 g/g DM, while two high-protein fractions of Palmaria palmata were tested at replacement levels of 0, 0.075, 0.15 and 0.225 g/g DM. To estimate fermentation parameters, 500 mg of each diet were incubated in bottles with 60 mL buffered rumen fluid. Estimated uCP increased linearly with increasing replacement rate of grass silage with seaweed protein fractions (from 158 g/kg DM to 206 g/kg DM on average for all fractions). Increasing protein fraction from the brown seaweed Saccharina latissima in the diet significantly increased true organic matter digestibility (OMD) (from on average 0.786 to 0.821). Organic matter digestibility decreased with increasing level of Alaria esculenta fractions (from on average 0.785 to 0.733), which also gave a linear decrease in CH₄ production (from on average 45.3 to 38.5 mL/g organic matter). As a result of decreased CH₄ production and OMD, total volatile fatty acid concentration decreased with increasing level of Alaria esculenta fractions (from on average 69.5 to 63.0 mmol/L). Thus, positive and species-specific effects of seaweed on estimated uCP and fermentation parameters were observed in vitro when protein fractions remaining after an extraction procedure on seaweed partly replaced grass silage in the feed ration.