Comparative Study on Growth Performance and Rumen Digestive Metabolism Between Cull Beef Cows and Fattening Bulls

Culled beef cows may retain practical value for fattening after reproductive use, but the rumen and host physiological characteristics associated with this response remain incompletely characterized. This study compared culled beef cows (CBC; n = 9) with fattening bulls (CON; n = 9) using 18 healthy Simmental crossbred cattle fed the same experimental total mixed ration (TMR) under identical feedlot conditions. Growth performance, body condition score (BCS), apparent nutrient digestibility, economic outcomes, blood indices, rumen fermentation, ruminal enzyme activities, rumen metagenomes, and plasma untargeted metabolomes were assessed. Compared with CON, CBC had higher final body weight, average daily gain, feed intake, and apparent digestibility of dry matter, crude protein, and neutral detergent fiber, whereas the feed-to-gain ratio did not differ significantly between groups (p = 0.692). BCS increased significantly from day 0 to day 60 within both cattle categories. Because different, non-cross-calibrated BCS reference frameworks were used, no between-group comparison of BCS was made. Economic analysis indicated that the value attributable to actual live-weight gain was similar between groups when within-category prices were held constant, whereas the relative economic outcome varied with feed costs and changes in live-cattle market prices. CBC also showed more propionate-oriented fermentation, greater fibrolytic enzyme activities, distinct carbohydrate-active enzyme profiles, higher insulin and leptin, and lower non-esterified fatty acids and β-hydroxybutyrate. Because lean and fat mass were not directly measured, the tissue basis of the greater weight gain remains unresolved. Overall, CBC exhibited a stronger fattening response accompanied by coordinated differences in rumen function and host metabolic profiles.