The cecal microbiota is essential for intestinal health and performance. This study describes the succession patterns of the cecal microbiota in Japanese quail (Coturnix japonica) until 42 days of age. Sixty quails were raised using standard conditions and fed corn–soybean meal diets. Cecal contents were sampled from five birds weekly from 7 to 42 days of age and submitted to Illumina 16S rRNA sequencing for metabarcoding analysis. Diversity and functional prediction were carried out with QIIME2, PICRUSt2, STAMP and MicrobiomeAnalyst 2.0. Firmicutes increased from 50% at 7 days to more than 80% at 42 days, whereas Bacteroidota decreased from 45% to 12% in the same period. Alpha diversity progressively increased with age, indicating a richer and more balanced microbiota at later ages. Genera such as Bacteroides were predominant in the beginning and later were replaced by Lachnospiraceae, Ruminococcus and Faecalibacterium. These developmental taxonomic features aligned with significant shifts in ten metabolic pathways identified by prediction, revealing a transition from biosynthetic functions to complex carbohydrate metabolism and cell wall biosynthesis. The first seven days are considered a critical window for probiotics intervention, which may favor the establishment of a microbiota that is more stable and beneficial to quail performance.

The study evaluated the effect of supplying threonine (Thr) and glutamine (Gln) in ovo on the performance, intestinal morphometry, and mRNA expression of Muc2 and IgA in the intestine of broiler chickens. On day 17.5 of incubation, fertile eggs were injected into the amniotic fluid according to a completely randomized experimental design with four treatments: 0.9% saline; 35 mg/mL Thr; 35 mg/mL Gln; or Thr + Gln (17.5 mg/mL each). After hatching, the chicks were reared using four replicates of 10 birds per treatment until 21 days of age. Isolated Gln resulted in the highest villus:crypt ratio (V:C), whereas greater villus area and higher number of goblet cells were seen in the birds supplemented with isolated Thr or Thr + Gln. In ovo Thr increased Muc2 relative gene expression in the jejunum. Immunoglobulin A gene expression in the intestinal mucosa was higher in birds receiving either Gln or Thr. No significant differences were observed among treatments for the evaluated performance variables. In conclusion, in ovo supplementation with Thr + Gln can potentially exert trophic effects on the development of the intestinal mucosa. On the other hand, in ovo Gln+Thr reduces the expression of genes related to the protection of the intestinal mucosa. The isolated supply of Thr and Gln might be important for strengthening the protective mechanisms of the intestinal mucosa.