161. Performance and Carcass Responses of Feedlot Beef Heifers Fed a Magnesium Oxide Blend.

Abstract Subacute ruminal acidosis is a common challenge in high-energy feedlot diets and can negatively affect intake patterns, efficiency, and carcass yield. Magnesium-based buffering agents have been proposed as a nutritional strategy to stabilize ruminal conditions in high-starch diets. This study evaluated the effects of a magnesium oxide blend (pHix-up Terresis, France) on performance and carcass characteristics of beef heifers finished in a feedlot. The experiment was conducted at Rancho El 17 (Sonora, Mexico) using 480 beef heifers (initial BW = 436 kg). Animals were assigned to a randomized complete block design with 6 blocks and 4 dietary treatments (6 pens per treatment; 20 heifers per pen; pen as the experimental unit). Treatments were: 1) VM (virginiamycin), 2) VMMG (virginiamycin + magnesium oxide blend), 3) MG (magnesium oxide blend), and 4) MGF6 (magnesium oxide blend with 6% forage). All animals were implanted, and all treatments included monensin. The magnesium oxide blend was included at 0.3% of diet DM. Cattle were fed high-energy corn-based diets for 84 to 119 days (mean 101 days). Diets contained 10% forage, except for MGF6, which contained 6% forage. Body weight, dry matter intake (pen basis), average daily gain (ADG), feed efficiency (gain:feed), and carcass characteristics were recorded. Data were analyzed using mixed models including initial BW, frame, and class as covariates. Final body weight (P = 0.116) and ADG (P = 0.157) were not affected by treatment. Feed efficiency showed a tendency for treatment effects (P = 0.088), with MGF6 indicating improved efficiency compared with MG. Dressing percentage tended to be greater in MGF6 (63.0%) compared with VM (62.1%; P = 0.083). Ribeye area (P = 0.858) and marbling score (P = 0.918) were not affected by treatments. Under high-energy feedlot conditions, inclusion of a magnesium oxide blend maintained animal performance and carcass characteristics, while showing a tendency to improve feed efficiency and dressing percentage when forage level was reduced from 10 to 6%. These results indicate that magnesium-based buffering strategies may contribute to improved feedlot efficiency in high-starch finishing diets.