Piglets performance, nutrient digestibility and gut health in response to feeding Ulva lactuca seaweed supplemented with a recombinant ulvan lyase or a commercial carbohydrase mixture.

<italic>Ulva lactuca</italic>, a green seaweed, may be an alternative source of nutrients and bioactive compounds for weaned piglets. However, it has a recalcitrant cell wall rich in a sulphated polysaccharide – ulvan – that is indigestible to monogastrics. The objective of this study was to evaluate the effect of dietary incorporation of 7% <italic>U. lactuca</italic>, combined with carbohydrases supplementation (commercial carbohydrase mixture or recombinant ulvan lyase), on growth performance, nutrient digestibility and gut health parameters (morphology and microbiota) of weaned piglets. The experiment was conducted over 14 days using 40 weaned piglets randomly allocated to one of four experimental diets: a control diet based on wheat–maize–soybean meal, a diet with 7% <italic>U. lactuca</italic> replacing the control diet (UL), a diet with UL supplemented with 0.005% Rovabio® Excel AP, and a diet with UL supplemented with 0.01% of a recombinant ulvan lyase. The dietary treatments had no major effects on growth performance, nitrogen balance and gut content variables, as well as histological measurements. Contrarily, dry matter and organic matter digestibility decreased with dietary seaweed inclusion, while hemicellulose digestibility increased, suggesting a high fermentability of this cell wall fraction independently of carbohydrases supplementation. Some beneficial microbial populations increased as a consequence of enzymatic supplementation (e.g., <italic>Prevotella</italic>), while seaweed diets as a whole led to an increased abundance of <italic>Shuttleworthia</italic>, <italic>Anaeroplasma</italic> and Lachnospiraceae_NK3A20_group, all related with a healthier gut. It also decreased <italic>Lactobacillus</italic> when compared to controls, which is possibly related to increased bioavailability of seaweed zinc. This study indicates that, under these experimental conditions, up to 7% dietary <italic>U. lactuca</italic> has no detrimental effect on piglet growth, despite decreasing acid detergent fibre digestibility. Carbohydrases supplementation of <italic>Ulva</italic> diets is not required at this incorporation level. [ABSTRACT FROM AUTHOR]