Feeding with a NaCl-Supplemented Alfalfa-Based TMR Improves Nutrient Utilization, Rumen Fermentation, and Antioxidant Enzyme Activity in AOHU Sheep: A Nutritional Simulation of Saline–Alkaline Conditions

Saline–alkaline soils are becoming prevalent across the globe, decreasing the availability of forage for animals and threatening sustainable animal production. This study evaluated the effects of a NaCl-supplemented alfalfa-based total mixed ration, simulating saline–alkaline soil conditions, on intake, the utilization of nutrients, antioxidant levels, and rumen fermentation. A 60-day feeding trial with 24 AOHU lambs (Australian White × Hu) compared a control diet (0.43% NaCl) with the NaCl-supplemented group (1.71% NaCl). Digestibility trials were conducted in metabolic cages for the collection of total feces and urine. Blood samples were taken at 0, 30, and 60 days for serum analysis, and slaughter samples (liver, kidney, rumen tissue, and rumen fluid) were taken for physiological, biochemical, and histological evaluation. The NaCl alfalfa-based TMR markedly increased liver and kidney weights. The rumen muscle layer thickened in the NaCl group. The ruminal ammonia nitrogen (NH3-N), ruminal microbial crude protein (MCP) synthesis, and glucogenic/branched-chain VFAs increased, indicating enhanced proteolysis, microbial protein synthesis, and energetically efficient fermentation. Serum total protein and albumin also rose over time in the NaCl group, reflecting increased nitrogen retention, while superoxide dismutase and glutathione peroxidase activity rose considerably by day 60, reflecting increased antioxidant defense. Furthermore, nitrogen intake, digestibility, and retention were improved in the NaCl group along with augmented digestible and metabolizable energy (28.47 vs. 13.93 MJ/d and 24.68 vs. 11.58 MJ/d, respectively) and gross energy digestibility (78.13% vs. 67.10%). Although NaCl-based alfalfa TMR cannot fully emulate naturally salt-stressed forages, these results indicate that the NaCl alfalfa-based diets improved rumen fermentation, energy yields, and antioxidant enzyme activity without impairing electrolyte balance. These findings suggest that NaCl-supplemented alfalfa-based TMRs, with a salt content comparable to that of alfalfa hay grown under saline–alkaline conditions, could support environmentally sustainable meat production in salt-stressed regions.