Effects of Ruminal Infusion of Acetic Acid onto Three Hay to Concentrate Ratios on Apparent Digestibilities and Rumen Parameters in Cattle1

Abstract

Three pairs of mature, dry, nonpregnant Holstein cows were fed at maintenance diets of 80:20, 50:50 or 20:80 hay (chopped timothy) to concentrate (82% corn meal, 15% soybean meal, 3% limestone) and, using a balanced switchback design, were either infused via fistula or not with acetic acid to supply the equivalent of an additional 10% of maintenance. The infusate was delivered at a constant rate in 2 liters of aqueous solution over 20 hr per day by way of a stainless steel device that assured its delivery into the ventral rumen 4 to 6 cm from the mucosa. Apparent digestibilities of dry matter, organic matter, gross energy acid detergent fiber, nitrogen and nitrogen retention were unaffected by the acetic acid infusion. Neutral detergent fiber, cellulose and hemicellulose digestibilities were reduced by acetic acid infusion at 20:80 hay to concentrate ratio, but were unaffected at rations of 80:20 or 50:50. In rumen fluid, acetic acid concentration was increased and pH was reduced by the acetic acid infusion at all hay to concentrate ratios. Osmolarity, sodium and potassium concentrations were unaffected by acetic acid infusion. Concentration of propionate, i-butyrate, i- and n-valerate, but not n-butyrate, was reduced by acetic acid infusion. Over a feeding cycle from 7.00 am to 5:00 pm, acetic acid infusion did not affect the postprandial rise in concentration of any of the volatile fatty acids or total volatile fatty acids, the reduction in pH, or potassium concentration. Osmolarity and sodium concentration were unaffected by time over the feeding cycle or by acetic acid infusion. These results are discussed in relation to the use of intraruminal infusion of acetic acid to measure its partial efficiency for body tissue deposition in cattle.