Efficiency of Energy Utilization by Young Cattle Ingesting Diets of Hay, Silage, and Hay Supplemented with Lactic Acid1,2

Abstract

Summary The purposes of this study were to determine whether the energetic efficiency of the dry matter of silage is different from that of hay harvested at the same time from the same source of forage, and whether the relative concentrations of the fatty acids in ruminal ingesta are associated with the utilization of energy. To pursue these objectives, the energy losses by growing cattle ingesting the following diets at the same level of dry matter per unit of metabolic size, were partitioned: (a) hay, (b) silage, and (c) hay supplemented by intraruminal infusions of lactic acid. These studies revealed no difference among treatments 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 kilogram of metabolic-body weight of steers ingesting hay, silage, and hay supplemented with lactic acid, respectively. As a consequence, the following percentages of the metabolizable energy were retained by the steers ingesting the three diets: hay, 18.0; silage, 33.4; and hay supplemented with lactic acid, 28.9. Although these differences were not mathematically significant, in only one instance was the retention of energy even slightly higher for the hay diet than it was for the two other diets. 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.