Energetics of Sheep Concerned with the Utilization of Acetic Acid

Abstract

The efficiency of utilization for growth-fattening, of the energy of diets resulting in high (5.4:1) and low (3.1:1) ratios of acetic acid to propionic acid in the ruminal ingesta was determined in 24 intact male and 24 female sheep by means of a slaughter-analysis experiment. To establish the chemical composition and energy value of the body at the beginning of the feeding period, nine other animals of each sex were analyzed. Each of two basal diets (pelleted, ground hay (H), and a pelleted mixture of the same hay and corn meal (HC)) was supplemented singly with triacetin (Ac³) and glycerol (G). A given animal was fed continuously one of the four diets at one of two levels of intake (i.e., approximately 1 or 2.2 times the maintenance level) during the 175-day feeding period. For an increment of intake equivalent to the maintenance intake of gross energy (GE), the metabolizable energy (ME) value of the diets declined by the following rates: (in %) H + Ac³, 3.5; H + G, 3.0; HC + Ac³, 2.1; and HC +G, 3.6. The rate of depression in the digestible energy (DE) value was considerably greater than that of the ME value; thus, with increasing intake the energy loss in urine and methane declined. The amount of ME required to maintain body-energy equilibrium was 87.1 kcal/(kg of body weight)^0.73 per day, as the mean for all diets and both sexes; this value was not significantly different (P > 0.1) from that for any diet or either sex. The mean rates with which the ME ingested above the maintenance level of intake was utilized for body-energy gain, were: (in %) H + Ac³, 59.9; H + G, 59.5; HC + Ac³, 63.7; and HC + G, 61.8 (P > 0.3). Ignoring the kind of basal diet, the utilization rates were 62.0 and 61.2% for the ME provided by the diets containing triacetin and glycerol, respectively. The mean pooled net utilization of ME for body-energy gain by females (65.5%) was markedly greater (P < 0.01) than that by males (57.6%). In a series of respiration-calorimetric experiments, the net utilization of ME provided by the acetic acid moiety of triacetin was 76.4%, between days 50 and 70 of continuous feeding. On days 3, 15 and 30 after animals were changed abruptly from the continuous feeding of the H + G diet for 70 days to the H + Ac³ diet, the net utilization of the ME of the acetic acid was 47.9, 77.0, and 85.4%, respectively. These observations, in concert with those made in the 175-day slaughter-analysis experiment, suggest that the sudden influx of a high level of acetic acid results in a high heat increment soon after the change, but that this subsides by day 15 of continuous feeding.