Impact of zinc oxide levels and probiotic supplementation in weaning-to-finishing pig diets: Productivity, gut microbial composition, and environmental implications

The study aimed to assess the impact of dietary zinc oxide (ZnO) combined with probiotic supplementation on the performance, fecal characteristics, meat quality, noxious gas emissions, and microbiome composition in weaning-to-finishing pigs. The experiment was conducted using 200 weaned pigs (3 barrows and 2 gilts per pen) with an average body weight of 6.65 ± 0.66 kg, randomly distributed across four treatments, each repeated ten times. The dietary treatments were: 1) TRT1, basal diet + ZnO 75 ppm; 2) TRT2, Basal diet + ZnO 75 ppm + probiotic 0.3%→Basal diet + ZnO 75 ppm + probiotic 0.1%→Basal diet + ZnO 75 ppm + probiotic 0.1%; 3) TRT3, Basal diet + ZnO 2500 ppm→Basal diet + ZnO 2500 ppm→Basal diet + ZnO 75 ppm; 4) TRT4, Basal diet + ZnO 2500 ppm + probiotic 0.3%→Basal diet + ZnO 2500 ppm + probiotic 0.1%→Basal diet + ZnO 75 ppm + probiotic 0.1%,  TRT1 was consistent for all and variations in ZnO and probiotic doses in TRT2, TRT3, and TRT4 were applied phase-wise (weaning → growing → finishing). The treatment groups supplemented with probiotics (TRT2 and TRT4) exhibited significantly higher (<italic>p </italic>&lt; 0.05) body weight and average daily gain at weeks 18 and 22, as well as increased (<italic>p</italic> &lt; 0.05) average daily feed intake over the entire period compared to the TRT1. Additionally, these groups showed a marked reduction (<italic>p</italic> &lt; 0.05) in NH₃and H₂S emissions at weeks 18 and 22. Although no significant changes (<italic>p</italic> &gt; 0.05) were observed in fecal scores or meat quality, ZnO with probiotic supplementation significantly increased (<italic>p</italic> &lt; 0.05) gut microbiota diversity (alpha and beta), enhanced the abundance of beneficial bacteria such as <italic>Firmicutes</italic>, <italic>Prevotella</italic>, and <italic>Lactobacillus</italic>, and reduced pathogenic bacteria like <italic>Clostridium sensu_stricto_1</italic>. Taxonomic analysis also revealed significant changes (<italic>p</italic> &lt; 0.05) in bacterial composition. These findings demonstrate that combining probiotics with lower ZnO levels enhances growth performance, gut microbial composition, and environmental sustainability by reducing noxious gas emissions. This study highlights the potential of probiotic supplementation as a strategy to minimize reliance on high-dose ZnO while improving swine production efficiency and environmental impact.