Effects of Dietary Alfalfa Hay Supplementation on Growth Performance, Nutrient Digestion, Serum Biochemistry and Rumen Fermentation in Angus Cattle

Alfalfa (Medicago sativa L.) hay is a high-quality legume forage, but most previous studies have evaluated a single fixed inclusion level or alfalfa as a replacement for other forages. How growth performance, nutrient digestibility, serum biochemistry, rumen fermentation and rumen microbiota jointly respond to graded alfalfa supplementation in growing Angus cattle remains undefined. The objective of this study was to determine the effects of two fixed supplementation levels of alfalfa hay (1 and 2 kg/day) on growth performance, apparent digestibility of nutrients, serum biochemical parameters, rumen fermentation characteristics and rumen microbial community composition in growing Angus cattle. Fifteen Angus (407.4 ± 34.15 kg; 11–14 months) were randomly distributed to the three treatments: control (C; basal TMR), low alfalfa (AlfL; 1 kg/d), and high alfalfa (AlfH; 2 kg/d), and fed for 70 days with 10 days of adaptation. Intake, body weight, nutrient digestibility, serum parameters, rumen fermentation and microbiota were determined. Feed-to-gain ratio was improved (p = 0.002) with AlfH (8.04) being more efficient than C (8.93). The digestibility of DM (p = 0.013) and CP (p = 0.001) were significantly increased. The rumen pH and NH3-N tended to rise (p = 0.059 and 0.060, respectively) and acetate, propionate, and TVFA concentrations were significantly greater (p < 0.05) in the alfalfa groups. There was increase in serum total protein (p = 0.018) and serum GSH-Px activity (p < 0.001), and decrease in serum MDA (p < 0.001) in AlfH. The bacterial community structure in the rumen remained largely consistent across treatments, with Bacillota and Bacteroidota as the two dominant phyla, and only two low abundance phyla differing significantly, both higher in AlfH, Patescibacteria (p = 0.009) and Thermodesulfobacteriota (p = 0.003); the dominant phyla and genus level composition were unaffected. These findings provide new evidence that moderate supplementation with alfalfa hay can enhance nutrient utilization, rumen fermentation, and antioxidant status without adversely affecting metabolic health. From a practical perspective, supplementation with 2 kg/day of alfalfa hay may represent an effective feeding strategy for improving feed efficiency in growing Angus cattle, particularly in production systems relying on low-quality forage resources.