Rumen by-pass soybean meal reduced ruminal ammonia but did not improve growth performance and nitrogen utilization in growing Hanwoo heifers

Abstract

Objective: The effects of heat-treated soybean meal (SBM) and citric acid (HCSBM) on the growth performance, rumen fermentation, blood metabolites, nutrient digestibility, and nitrogen (N) utilization were evaluated in growing Hanwoo heifers. Methods: Eight growing Hanwoo heifers (initial body weight: 228.5±11.3 kg; age: 9.3±0.3 months) were allocated to a crossover design with two dietary treatments: a control diet containing untreated SBM (Control) and a diet containing HCSBM. There were two 28- day phases in the trial, each containing 14-day measurements and an adaptation period. Growth performance, rumen fermentation parameters, blood metabolites, hematological parameters, apparent digestibility, energy, and N utilization were measured. Results: No significant differences were observed in the growth performance or energy utilization between the diets. The digestibility of dry matter and crude protein (CP) did not differ between the two diets. HCSBM supplementation increased neutral detergent fiber digestibility (p = 0.0874) and acetate molar proportions (p = 0.0748). The HCSBM diet resulted in lower ruminal ammonia nitrogen concentrations and iso-valerate proportions (p<0.05), indicating reduced ruminal protein degradation. Blood metabolites related to protein metabolism showed no significant differences between treatments. The control group exhibited higher red blood cell counts and hemoglobin and hematocrit levels (p<0.05). N excretion or retention did not significantly differ between the dietary groups. Conclusion: Despite enhanced protection against ruminal protein degradation, HCSBM supplementation did not improve N utilization efficiency or growth performance in growing Hanwoo heifers, possibly because of a sufficient N supply from the high CP content in both diets.