| Summary: | This study was designed to analyze the chemical composition and in vitro rumen fermentation of eight seaweed species (Brown: <i>Alaria esculenta</i>, <i>Laminaria digitata</i>, <i>Pelvetia canaliculata</i>, <i>Saccharina latissima</i>; Red: <i>Mastocarpus stellatus</i>, <i>Palmaria palmata</i> and <i>Porphyra</i> sp.; Green: <i>Cladophora rupestris</i>) collected in Norway during spring and autumn. Moreover, the in vitro ruminal fermentation of seventeen diets composed of 1:1 oat hay:concentrate, without (control diet) or including seaweeds was studied. The ash and N contents were greater (<i>p</i> < 0.001) in seaweeds collected during spring than in autumn, but autumn-seaweeds had greater total extractable polyphenols. Nitrogen in red and green seaweeds was greater than 2.20 and in brown seaweeds, it was lower than 1.92 g/kg DM. Degradability after 24 h of fermentation was greater in spring seaweeds than in autumn, with <i>Palmaria palmata</i> showing the greatest value and <i>Pelvetia canaliculata</i> the lowest. Seaweeds differed in their fermentation pattern, and autumn <i>Alaria esculenta</i>, <i>Laminaria digitata</i>, <i>Saccharina latissima</i> and <i>Palmaria palmata</i> were similar to high-starch feeds. The inclusion of seaweeds in the concentrate of a diet up to 200 g/kg concentrate produced only subtle effects on in vitro ruminal fermentation.
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