Alternatives to antibiotics and trace elements (copper and zinc) to improve gut health and zootechnical parameters in piglets: A review

•Dietary strategies are the most popular alternatives.•Numerous feed materials/compounds and feed additives show promising results.•Gut health improvement is the tool with the highest potential to reduce the negative impact of antibiotics reduction.•In most cases the mechanisms of action and effective doses are far from be understood.•Further research in practical diets considering interactions with other ingredients is needed. The weaning period is a critical stage in pig production and the use of antibiotics and high concentrations of trace elements (copper and zinc) have proved to be very effective at reducing the economic impact of the post weaning diarrhoea that occurs during the first two weeks after weaning. However, the overuse of these substances has led to the development of bacterial resistance and accumulation in the environment, and thus has forced the pig/feed industry to search for alternatives. This paper presents a comprehensive review of the experimental data published in the scientific literature in the last decade focused on the alternatives to antibiotics/trace elements to improve gut health and growth performance in piglets. The diet appeared as the keystone to improve gut health and zootechnical parameters in weaned piglets. The critical points are the reduction of the protein content, the use of highly digestible protein sources to avoid undesirable fermentation of proteins in the hindgut, and the optimisation of the intake of different types of carbohydrates, mainly fibre. A large variety of feed additives were also examined, including probiotics, prebiotics, amino acids, enzymes, organic acids, plants extracts/essential oils, seaweed/seaweed extracts, nucleotides and antimicrobial peptides. The effects of these feed additives on gut health and the growth performance were tested, in most cases, against a commercial control diet, having shown promising results. The few studies that included a comparison of feed additives with a positive control diet, including antibiotics or trace elements (mainly zinc oxide), did not generally show statistically significant differences in gut health and growth performance parameters, which leads to suggest on their potential as alternatives to antibiotics/trace elements. The greatest uncertainties were found for probiotics and prebiotics, where no generalized conclusions can be drawn about their efficacy, as studies showed quite unsteady results. Moreover, the mechanisms responsible for the feed ad