Energy supplementation with starch and rumen-protected fat alters rumen epithelium but not marbling in grazing crossbred heifers

This study evaluated the effects of a marbling-oriented supplement on performance, carcass traits, rumen morphometrics, and gastrointestinal health of grazing ½ Angus × Nellore heifers. Forty heifers (375 ± 7.2 kg body weight, BW) were classified by ultrasound according to initial marbling potential and assigned to two treatments: Control (0.3% of BW) or Marbling+ (0.5% of BW, with greater starch and rumen-protected fat inclusion). The experiment lasted 112 days under grazing conditions. Heifers receiving the Marbling+ supplement showed greater (P = 0.003) final body weight (468.7 vs. 448.3 kg), average daily gain (0.66 vs. 0.47 kg/day), and hot carcass weight (249.7 vs. 231.5 kg) compared with Control. However, no differences were observed for marbling score, loin eye area, or fat thickness, despite numerically greater marbling gain in Marbling+ heifers (1.63 vs. 0.65 units; P = 0.113). Dressing percentage tended to increase with Marbling+ ( P = 0.067). No treatment effects were detected for rumenitis or cecal lesion scores, indicating that increased supplementation did not impair gastrointestinal health. Although papillae number and area were unchanged, absorptive surface area tended to be greater ( P = 0.052), while the proportion of papillary area was greater ( P = 0.021) in Marbling+ heifers, suggesting changes in rumen epithelial morphology. In conclusion, increasing dietary energy improved growth performance but did not enhance marbling deposition in grazing heifers, even in animals with greater initial potential. These results indicate that moderate supplementation in pasture-based systems may be insufficient to stimulate intramuscular fat deposition.