Interspecies and seasonal variations in macroalgae from the Nordic region: Chemical composition and impacts on rumen fermentation and microbiome assembly

Marine macroalgae may serve as sustainable feed resources for ruminant production due to their nutritional attributes, and enteric methane (CH4) mitigating potential. We aimed to characterize the anti-methanogenic properties of 12 Nordic macroalgae species (eight brown, three red, and one green). Differences in the chemical composition across two harvesting seasons and impacts of addition of macroalgae (20% dry matter basis) on in vitro rumen fermentability of maize silage (MS) and associated changes in the rumen microbiome composition were also evaluated. Green and red macroalgae contained twice as much crude protein (CP) as compared to brown macroalgae. The latter had higher mineral and total polyphenol content (TPC: 10 to 20 times). In some brown species, ash and CP contents were up to twice as high in spring than in autumn, but TPC was highest in autumn. The TPC content was inversely correlated with in vitro rumen fermentation characteristics: organic matter (OM) degradability (r = −0.85; P