The effects of the forage-to-concentrate ratio on the conversion of digestible energy to metabolizable energy in growing beef steers. (22nd July 2020)

• Record Type: Journal Article
• Authors: Fuller, Amanda L; Wickersham, Tryon A; Sawyer, Jason E; Freetly, Harvey C; Brown-Brandl, Tami M; Hales, Kristin E
• Is Part Of: Journal of animal science. Volume 98:Number 8(2020)
• Publication Date: 2020-07-22
• Subjects: beef cattle -- digestible energy -- metabolizable energy; Livestock -- Periodicals; Livestock; Electronic journals; Periodicals; 636.005
• DOI: 10.1093/jas/skaa231
• Languages: English
• ISSNs: 0021-8812

Abstract

Abstract: 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.