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 one of four drylot pens (30 × 12 m) equipped with GrowSafe automated feeding systems (Model 6000E, 4 bunks/pen) during the experiment (days −14 to 117). On day 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 day 118. Individual feed intake was recorded daily, and BW was recorded every 14 d and prior to slaughter (day 117). Blood samples were collected on days 0, 28, 56, 84, and 112, and hair samples were collected on days 0, 56, and 112 from the tail-switch. On day 42, eight 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; days 42 to 98). Rumen pH was measured on days 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 to 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 day 56 (P ≥ 0.25) and linearly decreased (P < 0.01) with MG inclusion on day 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.

中文翻译：

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向饲养场牛补充混合氧化镁：对瘤胃、生理和生产反应的影响

该实验评估了饲养场牛食用包含不同水平氧化镁 (MG) 混合物的以玉米为基础的育肥日粮后的瘤胃、生理和生产反应。一岁牛（58 头小母牛和 62 头阉牛）按性别和初始体重（BW；407±3.1 千克）排序，并分配到 4 组，每组 30 头动物。在实验期间（第 −14 至 117 天），各组被安置在配备 GrowSafe 自动喂食系统（型号 6000E，4 个铺位/围栏）的四个 drylot 围栏（30 × 12 m）之一中。在第 0 天，各组被随机分配接受完全混合日粮，不含（CON；n = 30）或含（喂食时）0.25%（MG25；n = 30）、0.50%（MG50） ; n = 30), 或 0.75% (MG75; n = 30) 直到第 118 天屠宰。每天记录个体采食量，每 14 天和屠宰前（第 117 天）记录体重。在第 0、28、56、84 和 112 天收集血液样本，并在第 0、56 和 112 天从尾部开关收集头发样本。在第 42 天，8 头瘤胃插管牛（BW = 492 ± 8.0 kg）与一岁牛（1 对/围栏）一起饲养。每 14 天在各组之间轮流配对，产生重复的 4 × 4 拉丁方设计（n = 8/处理；第 42 至 98 天）。在每个时期的第 7 天和第 14 天（0800、1200、1600 和 2000 小时）测量瘤胃 pH 值。正交对比用于确定是否包含 MG（饮食的 0%、0.25%、0.50% 或 0.75%）产生线性或二次效应，并探索 MG 补充的总体效果（CON 与 MG25 + MG50 + MG75 ). 在 BW 增加、采食量或饲料效率方面没有注意到处理差异 (P ≥ 0.31)。与 CON 相比，补充 MG 的牛往往具有较少的胴体大理石花纹 (P = 0.07)。包含MG线性增加(P＜0.01)镁的平均血浆浓度并且倾向于线性降低(P＝0.09)触珠蛋白的平均血浆浓度。与 CON 相比，补充 MG 的牛具有更高 (P < 0.01) 的皮质醇平均血浆浓度。毛发皮质醇浓度在第 56 天的治疗之间没有差异 (P ≥ 0.25)，并且在第 112 天（治疗×天；P = 0.02）加入 MG 后呈线性下降（P < 0.01）。加入 MG 会线性增加 (P = 0.03) 平均瘤胃 pH 值，但这些结果主要在当天的最后两次采样中注意到（处理×小时；P = 0.02）。总的来说，补充 MG 可有效控制接受以玉米为基础的肥育日粮的牛的瘤胃 pH 值，