Dietary crude protein concentration oscillation can improve the nitrogen utilization efficiency of ruminants. However, little is known about the dynamic changes in microbiota and fermentation in the rumen of calves during the oscillation period. In this study, six calves were fed an oscillating diet at 2-day intervals, including a high-protein diet (HP) and a low-protein diet (LP). The rumen fermentation parameters, plasma urea-N concentration, and rumen bacterial diversity were characterized throughout the oscillation period. The concentrations of volatile fatty acids, NH₃-N, and plasma urea-N in rumen changed significantly with an oscillating diet. The abundance of Prevotella_1, Selenomonadales, Succiniclasticum, Clostridiales, Ruminococcaceae, Lachnospiraceae, and Rikenellaceae_RC9_gut_group showed significant changes with diet. Prevotella_1 was positively correlated, and Lachnospiraceae_AC2044_group and Saccharofermentans were negatively correlated with NH₃-N. The abundance of Amino Acid Metabolism, Metabolism of Other Amino Acids, and Glycan Biosynthesis and Metabolism pathways, annotated by bacterial functional genes, decreased when the diet changed from HP to LP. The abundance of the Carbohydrate Metabolism pathway increased after the two dietary changes. In conclusion, the plasma urea-N concentration was not as sensitive and quick to adapt to diet changes as the rumen fermentation parameters. Rumen bacteria were responsible for increasing the nitrogen utilization efficiency of calves fed an oscillating diet.

Defensins, one of the members of the antimicrobial peptide family, play a vital role in resisting microbial invasion and immune regulation. Porcine β-defensin 2 possesses excellent stability, making it an ideal antibiotic alternative for feed additives. In this study, a total of 15 piglets were used to investigate the effects of supplementing diets with 2.5 mg/kg (LP group) and 5 mg/kg (HP group) of pBD2 to weaned piglets. The results revealed that pBD2 significantly increased the total weight gain and average daily weight gain (p < 0.05), the contents of T-AOC, SOD, IgM, and IL-10 in serum (p < 0.05), the villus-to-crypt ratios, and the expression of tight-junction proteins ZO-1 and claudin-1 (p < 0.05) in the small intestine. Furthermore, pBD2 increased the abundance of beneficial bacteria related to nutrient and energy metabolism while decreasing the abundance of harmful bacteria associated with intestinal inflammation and diarrhea. Alterations in the gut microbiota were closely associated with the levels of T-AOC, SOD, IgM, and IL-10 in serum. pBD2 primarily enhanced the health of weaned piglets by influencing antioxidant capacity, intestinal barrier function, and the intestinal microbiota. Our research provides a novel perspective for addressing the issue of antibiotic residues in feed.