**1. Introduction**

The weaning process is a stressful event in the pig's life, characterized by intestinal and immune dysfunctions that result in impaired feed intake, health and growth [1]. The post-weaning growth check is characterized by an increased proliferation of pathogenic bacteria, in particular β-hemolytic enterotoxigenic *E. coli* serotypes, causing post-weaning diarrhea [2]. Antibiotic growth promoters are an effective means to reduce pathogenic bacteria such as *E. coli* and enhance growth rates in the immediate post-weaning period. However, the use of antibiotics as growth promoters was banned in the EU in 2006 (EC regulation no. 1831/2003). Since the ban on antibiotics, pharmacological doses of zinc oxide (ZnO) are used to reduce the incidence of diarrhea and improve performance in the post-weaning period. However, mounting concerns over the accumulation of ZnO in soils and a link between ZnO usage and antibiotic resistance have meant a phasing out of its usage by 2022 (Regulation (EU) 2019/61 on Veterinary Medicines and Regulation (EU) 2019/4 on Medicated Feed). This legislation also adds further restrictions to the use of antibiotics for prophylaxis or metaphylaxis. Therefore, there has been a focus on finding more natural means to improve performance and digestive health in pigs during the post-weaning period.

This search for natural alternatives has focused on a wide variety of substances such as organic acids, yeas<sup>t</sup> derivatives, glucans, mannans, herbs and spices amongs<sup>t</sup> many others [3]. One alternative that has been explored is the use of seaweed extracts such as laminarin (LAM) and fucoidan (FUC) as an alternative to antibiotics and ZnO [4]. The

**Citation:** Vigors, S.; O'Doherty, J.; Rattigan, R.; Sweeney, T. Effect of Supplementing Seaweed Extracts to Pigs until d35 Post-Weaning on Performance and Aspects of Intestinal Health. *Mar. Drugs* **2021**, *19*, 183. https://doi.org/10.3390/md19040183

Academic Editors: Irina M. Yermak and Viktoria Davydova

Received: 26 February 2021 Accepted: 24 March 2021 Published: 26 March 2021

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extract LAM is a β-glucan composed of β-1-3- D-glucan with β-1-6 branch chains that vary with species in distribution and length, while FUC represents a class of fucose-enriched sulfated polysaccharides extracted from the extracellular matrix of brown algae, with L-fucose 4-sulfate as the major component [4].

The feeding of seaweed extracts, such as laminarin and fucoidan, in the immediate post-weaning period illicits multiple beneficial physiological adaptions in the gastrointestinal tract, that can improve pig performance during this challenging time. A number of studies have examined the effects of feeding laminarin and fucoidan for 14 days postweaning [5–7]. In the study of Rattigan et al. [5], pigs were offered 3 levels of a laminarin rich extract to identify the most appropriate inclusion level. The 300 ppm laminarin supplemented group exhibited the greatest beneficial effects with improvements in animal performance with accompanying positive alterations in small intestinal morphology, microbial populations and gene expression profiles. Based on these positive findings, a more detailed analysis of the microbiome was conducted by Vigors et al. [7], where increased abundance of *Prevotella* and reductions in the family *Enterobacteriaceae* were reported in the laminarin supplemented group. In addition, while pigs supplemented with 250 ppm fucoidan did not see any improvement in performance, they had improved faecal scores and increased concentrations of total volatile fatty acids in the colon but had alterations in gene expression and microbial profiles which potentially could have negative connotations for animal health in the longer term [8]. Hence, in this experiment, the feeding of the experimental diets was continued past d14 until d35 to assess the longer-term effects on animal performance and intestinal health. This strategy will establish whether the feeding of LAM continues to be beneficial until d35 or beneficial effects are limited to the immediate post-weaning period. Therefore, the objective of this research was to examine the effects of feeding seaweed extracts (SWE) up to d35 post-weaning on animal performance with a focus on immune and microbial profiles in the gastrointestinal tract.
