In ruminant nutrition, metabolizable energy (ME) is calculated from digestible energy (DE) using a constant conversion factor of 0.82, but methane and urine energy losses vary across diets and dry matter intake (DMI) levels, suggesting that a static conversion factor fails to describe the biology, according to an article published recently in the Journal of Animal Science.
Researchers Amanda Fuller, Tryon Wickersham and Jason Sawyer with Texas A&M University and Harvey Freetly, Tami Brown-Brandl and Kristin Hales with the U.S. Department of Agriculture's Meat Animal Research Center conducted a study to quantify the effects of the forage-to-concentrate ratio on the efficiency of conversion of DE to ME.
They used 10 Angus steers in a 5 x 5 replicated Latin square. Dry-rolled corn was included in experimental diets at 0.0%, 22.5%, 45.0%, 67.5% and 83.8% on a dry matter basis, resulting in a high forage:concentrate (HF:C), intermediate forage:concentrate (IF:C), equal forage:concentrate, low forage:concentrate and very low forage:concentrate ratios, respectively. Each experimental period consisted of a 23-day diet adaption, followed by five days of total fecal and urine collections and a 24-hour gas exchange collection.
According to Fuller et al., there was a tendency (P = 0.06) for DMI to increase linearly as the forage:concentrate ratio decreased. As a result, gross energy intake (GEI) increased linearly (P = 0.04) as the ratio decreased.
Fecal energy loss expressed as megacalories per day (P = 0.02) or as a proportion of GEI (P < 0.01) decreased as the forage:concentrate ratio decreased such that DE (Mcal per day and Mcal/kg) increased linearly (P < 0.01) as the ratio decreased, the researchers reported.
They noted that as a proportion of GEI, urine energy decreased linearly (P = 0.03) as the forage:concentrate ratio decreased, but methane energy loss as a proportion of GEI responded quadratically (P < 0.01) — increasing from HF:C to IF:C and then decreasing thereafter.
Fuller et al. said the efficiency of DE to ME conversion increased quadratically (P < 0.01) as the forage:concentrate ratio decreased, ranging from 0.86 to 0.92.
Dry matter, organic matter and neutral detergent fiber digestibility increased linearly (P < 0.01) and starch digestibility decreased linearly (P < 0.01) as the forage:concentrate ratio decreased, Fuller et al. reported.
They concluded that the efficiency of conversion of DE to ME increased with a decreasing forage:concentrate ratio due to decreasing methane and urine energy losses. Therefore, the relationship between DE and ME is not static, especially when feeding beef cattle differing forage:concentrate ratios.