| Summary: | The aim of the present work was to investigate the potential of <i>Porphyra</i> sp. as an alternative source of protein to soybean meal in diets for sheep. Our experimental treatments included a control diet (CON) based on grass silage and crushed oats and three diets containing protein supplements, clover silage (CLO), soybean meal (SOY) or <i>Porphyra</i> sp. (POR) to increase dietary crude protein concentrations. We studied its effects on rumen fermentation, growth rate and methane emissions. Ruminal fermentation characteristics, kinetics of gas production and methane production were studied in vitro by using batch cultures inoculated with rumen inoculum from sheep. There were no differences among diets in total volatile fatty acids (VFA) production or in the VFA profile in vitro. Across treatments, we measured no differences in methane production either in vitro or in vivo, and we saw no noticeable antimethanogenic effect of <i>Porphyra</i> sp. The present in vivo trial with lambs showed no differences in average daily weight gain when fed diets including <i>Porphyra</i> sp. or soybean meal diets (250 and 254 g/d, respectively). We conclude that <i>Porphyra</i> sp. has a protein value similar to high-quality protein sources like soybean meal.
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