The effects of the forage-to-concentrate ratio on the conversion of digestible energy to metabolizable energy in growing beef steers

Authors: FULLER, AMANDA - Texas A&M University; WICKERSHAM, T - Texas A&M University; SAWYER, J - Texas A&M University; Freetly, Harvey; BROWN-BRANDL, TAMI - Former ARS Employee; HALES, KRISTIN - Former ARS Employee

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2020
Publication Date: 8/1/2020
Citation: Fuller, A.L., Wickersham, T.A., Sawyer, J.E., Freetly, H.C., Brown-Brandl, T.M., Hales, K.E. 2020. The effects of the forage-to-concentrate ratio on the conversion of digestible energy to metabolizable energy in growing beef steers. Journal of Animal Science. 98(8)1-10. https://doi.org/10.1093/jas/skaa231.
DOI: https://doi.org/10.1093/jas/skaa231

Interpretive Summary: Metabolizable energy of feed is used to predict the amount of feed needed to meet an animal’s energy requirement. It is difficult to measure and has often been estimated from the more easily measured digestible energy. Historically, it has been assumed that metabolizable energy was a proportion of digestible energy and could be calculated multiplying by 0.82. The diets fed beef cattle have changed since these previous assumptions were made, and recent research suggest that metabolizable energy is underestimated on modern diets. A study was conducted to determine the relationship between the forage:concentrate ratio of the diet and the relationship between the metabolizable:digestible energy ratio. We determined that on low forage:concentrate ratio diets the metabolizable:digestible ratio increased suggesting cattle get more energy per unit of feed weight on these diets. These differences were primarily being driven by a decrease in the digestible energy being lost as methane and in the urine. This study demonstrated the need for the development of higher order mathematical models to predict digestible energy from metabolizable energy.

Technical 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 (F:C) ratio on 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 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. 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. Dry matter, OM, and NDF 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.