Efficiency of Energy Utilization by Young Cattle Ingesting Diets of Hay, Silage, and Hay Supplemented with Lactic Acid

Abstract

The utilization of energy provided by diets of (a) hay, (b) silage harvested from the same source at the same time, and (c) hay supplemented by intraruminal infusions of lactic acid was studied in growing cattle. No differences among treatments were observed in the size of the energy loss in feces (i.e. there was no difference in the digestibility of energy) urine, or in methane as estimated from digestible carbohydrates. However, the mean outputs of metabolic heat estimated from oxygen consumption and urinary nitrogen excretion were 110.3, 93.5, and 93.4 kcal per 24 hr per kg or metabolic-body weight of steers ingesting hay, silage, and hat supplemented with lactic acid, respectively. As a consequence, the following percentages of the metabolizable energy were retained by the steers ingesting the 3 diets: hay, 18.0; silage, 33.4; and hay supplemented with lactic acid, 28.9. The molar proportions of the following fatty acids as percentages of the total acids in the rumen ingesta of animals fed hay, silage and hay supplemented with lactic acid, respectively, were: acetic acid, 69.5, 58.3, and 57.4; propionic acid, 18.5, 20.7, and 28.7; butyric acid, 9.8, 15.4, and 11.0; and higher acids, 2.2, 5.6, and 2.8. Thus, the diet of hay alone was associated with a higher proportion of acetic acid and lower concentrations of propionic and butyric acids in ruminal ingesta than were the diets of silage or of hay supplemented with lactic acid. These observations indicate that the energetic efficiency of body gain in young cattle ingesting forage diets is favored by diets resulting in a high proportion of propionic and butyric acids relative to acetic acid in the rumen.