Effects of Replacing Oat Hay with Peanut Hull Depolymerization Product on Growth Performance, Serum Biochemical Parameters, and Rumen Fermentation in Holstein Dairy Bulls

Alternative roughage resources are increasingly important for sustainable ruminant production. This study evaluated the effects of replacing oat hay with a peanut hull depolymerization product (PHDP) on growth performance, apparent nutrient digestibility, serum biochemical variables, antioxidant status, and rumen fermentation characteristics in Holstein dairy bulls. Thirty-six Holstein dairy bulls (18–22 months old) were randomly assigned to four dietary treatments in a completely randomized design (n = 9 per treatment): T0, a basal diet containing 24% oat hay; T1, T2, and T3, in which 25%, 50%, and 75% of the oat hay was replaced with PHDP, respectively. The trial comprised a 14 d adaptation period and a 60 d measurement period. Growth performance and rumen fermentation data were analyzed using one-way ANOVA with linear and quadratic contrasts, while serum biochemical and antioxidant variables were analyzed using ANCOVA with baseline values as covariates. PHDP substitution significantly affected average daily gain (ADG), dry matter intake (DMI), and feed conversion ratio (FCR). Compared with the control group, the 75% replacement group showed greater ADG and lower FCR, whereas DMI was lower in T1 than in the other groups. Ruminal pH was increased in the 75% replacement group, whereas ammonia nitrogen concentration, major volatile fatty acid (VFA) concentrations, total VFA concentration, and apparent nutrient digestibility were not significantly affected by treatment. Most hematological and serum biochemical variables were not markedly influenced by PHDP substitution. However, total antioxidant capacity at d 60 was higher in T2 and T3 than in T1, catalase activity at d 30 was higher in T3 than in T0, and serum insulin-like growth factor-1 concentration at d 60 was highest in T3. In conclusion, PHDP could partially replace oat hay in diets for Holstein dairy bulls without impairing rumen fermentation, apparent nutrient digestibility, or major blood biochemical indicators. The 75% replacement level showed the most favorable response by improving growth performance and feed efficiency while supporting antioxidant status and serum IGF-1 concentration.