Effects of dietary rumen-degradable protein on growth performance, nitrogen metabolism, and rumen microbiome in dairy buffalo heifers

Introduction Buffaloes are globally important dairy animals, but their low feed nitrogen utilization efficiency and excessive dietary rumen-degradable protein (RDP) results in aggravated nitrogen pollution and high breeding costs. Studies on the optimal RDP levels for 7–10-month-old dairy buffalo heifers remain scarce, limiting precise nutritional management. Materials and methods Dairy buffalo heifers ( n = 36, 7–10-month-old, 193.39 ± 4.10 kg) were selected, and randomly assigned to six groups ( n = 6 heifers/group, with one heifer in each replicate; Dietary RDP: 60.85–88.90 g/kg). The 73-day trial (15-day adaptation) included measurements of growth performance, nitrogen metabolism, serum indices, rumen parameters, and microbiome (16S rRNA/ITS sequencing). Results (1) No differences in initial/final body weight or dry matter intake were found among the groups ( p > 0.05). The low-RDP group (LP-1, 67.31 g/kg) had the highest average daily gain (0.79 kg/d) and lowest feed-to-gain ratio (7.88) ( p < 0.05). (2) With a decrease in dietary RDP levels, intake nitrogen (IN), urinary nitrogen (UN), digested nitrogen, and UN /IN efficiency decreased ( p < 0.05). The low-RDP group (LP-1) had the highest retention nitrogen/IN efficiency (32.31%) ( p < 0.05). (3) The serum total protein and urea levels decreased with decreasing dietary RDP levels (lowest in LP-2: 64.52 g/L and 5.15 mmol/L, p < 0.05), with no differences in liver or kidney function or glucose-lipid metabolism ( p > 0.05). (4) LP-1 had the highest rumen total volatile fatty acids, acetate, and butyrate levels ( p < 0.05), while rumen pH and NH 3 -N decreased with RDP ( p < 0.05). (5) Dietary RDP levels significantly altered the rumen microbial structure. Pichia in LP-1 was 28.81-fold and 39.68-fold higher than in HP-1 and MP-1 groups, respectively ( p < 0.05), along with the presence of group-specific taxa. Discussion An optimal dietary RDP level for 7–10-month-old dairy buffalo heifers was 67.31 g/kg, which improved the ADG and nitrogen utilization efficiency without compromising health, while also altering the rumen microbial structure. Therefore, when formulating diets for buffaloes, it is advisable to consider to note only meet the DCP requirements but also appropriately regulate the dietary RDP levels.