The Utilization of Acetic, Propionic and Butyric Acids by Growing Heifers.

Differing efficiencies of utilization of acetic, propionic and butyric acids in both fasting and fattening sheep have been demonstrated by Blaxter and his colleagues (Armstrong & Blaxter, 1957a, b ; Armstrong, Blaxter & Graham, 1957). In fasting sheep, when the acids were given singly as the sole source of energy, the heat increment with acetic acid (41 yo of the energy of the metabolized acid) was much higher than that with propionic and n-butyric acids ( 1 3 and 16%, respectively). With a mixture (3 : 2) of propionic and n-butyric acids the heat increment was reduced to 9 yo, and the addition of acetic acid to this mixture up to a molar proportion of 90 yo gave only a small increase in heat increment, up to 15 % . In fattening sheep, the efficiency of utilization of acetic acid (heat increment 67.1 yo) was much less than that of propionic and n-butyric acids (43-7 and 38.1 yo, respectively). In young growing animals, protein deposition may be of equal or greater importance than fat deposition and, because of the high water content of muscle, it may contribute more than fat to any gain in body-weight, but there is no information on the relative efficiencies of utilization of the fatty acids for protein deposition. This paper describes two experiments in which the effects of intraruminal infusions of acetic, propionic and butyric acids on nitrogen retention and on the gain in bodyweight have been determined. Information was also obtained on the effects of the infusions on the digestibility of the dry matter of the diet and the intake of hay when it was offered ad lib,, on the total concentration and relative proportions of the volatile fatty acids in the rumen and on the concentrations of total volatile fatty acids, glucose and ketone bodies in the blood plasma. A preliminary report of the results of the first experiment has been published (Rook, Balch & Campling, 1960).