Effects of Dietary Threonine Levels on Growth Performance, Biochemical Parameters, Muscle Quality, and Intestinal Microflora of Rice Field Eel (Monopterus albus)

To evaluate the effects of dietary threonine levels on rice field eels (<i>Monopterus albus</i>), six experimental diets were formulated with graded threonine supplementation: 0 g/kg (T1), 3 g/kg (T2), 6 g/kg (T3), 9 g/kg (T4), 12 g/kg (T5), and 15 g/kg (T6). Rice field eels, with an initial body weight of 18.47 ± 0.11 g, were fed for 60 days. The weight gain ratio (WGR), specific growth rate (SGR), and protein efficiency ratio (PER) significantly increased in the T4 group, while feed conversion ratio (FCR) significantly decreased. Broken-line regression analysis determined the optimal dietary threonine requirement to be 7.5-9.0 g/kg. Concomitantly, whole-body crude protein content increased, while lipid content decreased; serum GOT and GPT activities declined, indicating reduced hepatic stress, whereas hepatic CAT and SOD activities markedly increased, enhancing antioxidant capacity. Digestive enzyme capacity was enhanced, as evidenced by elevated lipase and trypsin activities. Muscle texture properties, including cohesiveness, springiness, and chewiness, were significantly improved in the T4 and T5 groups. At the molecular level, MyoG, MyoD1, and MYF5 expression peaked in T4, while MRF4 expression increased progressively. LEfSe analysis of the microbiome, coupled with KEGG functional prediction, revealed that <i>Corynebacterium</i> and <i>Methylocella</i> were positively correlated with growth metrics; these genera likely promote muscle deposition through three enriched metabolic pathways that enhance energy and amino acid availability. Collectively, dietary threonine supplementation at 7.5-9.0 g/kg promotes growth, enhances digestive function, and improves muscle quality in rice field eels.