Metabolizable energy (ME) is calculated from digestible energy (DE) using a constant conversion factor of 0.82. Methane and urine energy losses vary across diets and dry matter intake (DMI), suggesting that a static conversion factor fails to describe the biology. To quantify the effects of the forage-to-concentrate ratio (F:C) on the efficiency of conversion of DE to ME, 10 Angus steers were used in a 5 × 5 replicated Latin square. Dry-rolled corn was included in experimental diets at 0%, 22.5%, 45.0%, 67.5%, and 83.8% on a dry matter (DM) basis, resulting in a high F:C (HF:C), intermediate F:C (IF:C), equal F:C (EF:C), low F:C (LF:C), and a very low F:C (VLF:C), respectively. Each experimental period consisted of a 23-d diet adaption followed by 5 d of total fecal and urine collections and a 24-h gas exchange collection. Contrasts were used to test the linear and quadratic effects of the F:C. There was a tendency (P = 0.06) for DMI to increase linearly as F:C decreased. As a result, gross energy intake (GEI) increased linearly (P = 0.04) as F:C decreased. Fecal energy loss expressed as Mcal/d (P = 0.02) or as a proportion of GEI (P < 0.01) decreased as F:C decreased, such that DE (Mcal/d and Mcal/kg) increased linearly (P < 0.01) as F:C decreased. As a proportion of GEI, urine energy decreased linearly (P = 0.03) as F:C decreased. Methane energy loss as a proportion of GEI responded quadratically (P < 0.01), increasing from HF:C to IF:C then decreasing thereafter. The efficiency of DE to ME conversion increased quadratically (P < 0.01) as F:C decreased, ranging from 0.86 to 0.92. Heat production (Mcal) increased linearly (P < 0.04) as F:C decreased but was not different as a proportion of GEI (P ≥ 0.22). As a proportion of GEI, retained energy responded quadratically (P = 0.03), decreasing from HF:C to IF:C and increasing thereafter. DM, organic matter, and neutral detergent fiber digestibility increased linearly (P < 0.01) and starch digestibility decreased linearly (P < 0.01) as the F:C decreased. Total N retained tended to increase linearly as the proportion of concentrate increased in the diet (P = 0.09). In conclusion, the efficiency of conversion of DE to ME increased with decreasing F:C due to decreasing methane and urine energy loss. The relationship between DE and ME is not static, especially when differing F:C.

中文翻译：

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饲草与精料的比例对生长肉牛消化能转化为代谢能的影响。


可代谢能 ( ME ) 是使用恒定转换系数 0.82 根据消化能 ( DE ) 计算得出的。甲烷和尿液的能量损失因饮食和干物质摄入量（ DMI ）的不同而不同，这表明静态转换因子无法描述生物学特性。为了量化饲料与浓缩物的比率 ( F:C ) 对 DE 转化为 ME 的效率的影响，在 5 × 5 复制的拉丁方中使用了 10 头安格斯公牛。实验日粮中以干物质 ( DM ) 计，添加了 0%、22.5%、45.0%、67.5% 和 83.8% 的干玉米，产生高 F:C ( HF:C )、中等 F:分别为 C ( IF:C )、等于 F:C ( EF:C )、低 F:C ( LF:C ) 和非常低的 F:C ( VLF:C )。每个实验期包括 23 天的饮食适应、随后 5 天的粪便和尿液总收集以及 24 小时的气体交换收集。使用对比来测试 F:C 的线性和二次效应。随着 F:C 的降低，DMI 有线性增加的趋势（ P = 0.06）。结果，随着 F:C 减少，总能量摄入 ( GEI ) 线性增加 ( P = 0.04)。粪便能量损失以Mcal/d ( P = 0.02) 或GEI 比例( P < 0.01) 表示，随着F:C 的降低而降低，使得DE (Mcal/d 和Mcal/kg) 线性增加( P < 0.01)随着 F:C 减少。作为 GEI 的一部分，尿液能量随着 F:C 的降低而线性降低（ P = 0.03）。甲烷能量损失占 GEI 的比例呈二次方响应（ P < 0。01)，从 HF:C 增加到 IF:C，然后减少。随着 F:C 的降低，DE 到 ME 的转换效率呈二次方增加（ P < 0.01），范围从 0.86 到 0.92。随着 F:C 降低，产热量 (Mcal) 线性增加 ( P < 0.04)，但与 GEI 的比例没有差异 ( P ≥ 0.22)。作为 GEI 的一部分，保留能量呈二次方响应 ( P = 0.03)，从 HF:C 到 IF:C 递减，然后增加。随着F:C的降低，干物质、有机物和中性洗涤纤维的消化率线性增加（ P <0.01），淀粉消化率线性降低（ P <0.01）。随着日粮中精料比例的增加，保留的总氮呈线性增加（ P = 0.09）。总之，由于甲烷和尿液能量损失的减少，DE 转化为 ME 的效率随着 F:C 的减少而增加。 DE 和 ME 之间的关系不是静态的，尤其是当 F:C 不同时。