Supplementing a blend of magnesium oxide to feedlot cattle: effects on ruminal, physiological, and productive responses

This experiment evaluated ruminal, physiological, and productive responses of feedlot cattle consuming a corn-based finishing diet that included different levels of a magnesium oxide (MG) blend. Yearling cattle (58 heifers and 62 steers) were ranked by sex and initial body weight (BW; 407 ± 3.1 kg), and allocated to 4 groups of 30 animals each. Groups were housed in 1 of 4 drylot pens (30 × 12 m) equipped with GrowSafe automated feeding systems (Model 6000E, 4 bunks/pen) during the experiment (d -14 to 117). On d 0, groups were randomly assigned to receive a total-mixed ration without (CON; n = 30) or with the inclusion (as-fed basis) of MG at 0.25% (MG25; n = 30), 0.50% (MG50; n = 30), or 0.75% (MG75; n = 30) until slaughter on d 118. Individual feed intake was recorded daily, and BW were recorded every 14 d and prior to slaughter (d 117). Blood samples were collected on d 0, 28, 56, 84 and 112, and hair samples were collected on d 0, 56 and 112 from the tail-switch. On d 42, 8 rumen-cannulated steers (BW = 492 ± 8.0 kg) were housed with yearling cattle (1 pair/pen). Pairs rotated among groups every 14 d, resulting in a replicated 4 × 4 Latin square design (n = 8/treatment; d 42 to 98). Rumen pH was measured on d 7 and 14 of each period (0800, 1200, 1600 and 2000 h). Orthogonal contrasts were used to determine if inclusion of MG (0, 0.25, 0.50, or 0.75% of the diet) yielded linear or quadratic effects, and to explore overall effect of MG supplementation (CON vs. MG25 + MG50 + MG75). No treatment differences were noted (P ≥ 0.31) for BW gain, feed intake, or feed efficiency. Cattle supplemented with MG tended have less carcass marbling (P = 0.07) compared with CON. Inclusion of MG linearly increased (P < 0.01) mean plasma concentrations of magnesium, and tended to linearly decrease (P = 0.09) mean plasma concentrations of haptoglobin. Cattle supplemented with MG had greater (P < 0.01) mean plasma concentrations of cortisol compared with CON. Hair cortisol concentration did not differ between treatments on d 56 (P ≥ 0.25), and linearly decreased (P < 0.01) with MG inclusion on d 112 (treatment × day; P = 0.02). Inclusion of MG linearly increased (P = 0.03) mean rumen pH, but these outcomes were mostly noted during the last two sampling of the day (treatment × hour; P = 0.02). Collectively, supplemental MG was effective in controlling rumen pH in cattle receiving a corn-based finishing diet, but without improvements in feedlot performance and carcass merit.