The effect of pH and polyunsaturated C18 fatty acid source on the production of vaccenic acid and conjugated linoleic acids in ruminal cultures incubated with docosahexaenoic acid

Previously, combining docosahexaenoic acid (DHA) with linoleic acid in rumen cultures enhanced vaccenic acid (VA) and conjugated linoleic acid (CLA) accumulations. The objective of this experiment was to examine the effect of two pH levels and two polyunsaturated C18 fatty acid (FA) sources on VA and CLA accumulations in rumen cultures incubated with DHA. High pH culture treatments consisted of 10 mg DHA + 20 mg linoleic acid (LOH), or 10 mg DHA + 20 mg linolenic acid (LNH). Low pH culture treatments consisted of 10 mg DHA + 20 mg linoleic acid (LOL), or 10 mg DHA + 20 mg linolenic acid (LNL). Treatments were incubated in triplicate in 125 ml flasks containing 500 mg finely ground TMR, 10 ml strained ruminal fluid, 40 ml media, and 2 ml reducing solution. Ruminal fluid was collected from fermenters fed high forage (pH 6.7) or high grain diets (pH 5.4). A 5-ml sample of culture contents was taken at 0 and 24 h for fatty acid analysis by gas liquid chromatography. After 24 h of incubation, VA was the predominant trans C18:1 FA isomer in the high pH cultures and its concentration was greater with LOH (20.9 mg/culture) than with LNH (9.3 mg/culture). Similarly, t10 C18:1 was the predominant trans C18:1 FA isomer in the low pH cultures and its concentration was greater with LOL (12.8 mg/culture) than with LNL (4.5 mg/culture). The c9t11 CLA (0.74 mg/culture) and tt CLA (0.67 mg/culture) were the predominant CLA isomers in the LOH cultures, while t10c12 CLA (1.47 mg/culture) and tt CLA (1.27 mg/culture) were the predominant CLA isomers in the LOL cultures. The t11t13 CLA increased in linolenic acid cultures, particularly when linolenic acid incubated under low pH condition (0.95 mg/culture). Additions of linolenic acid to rumen cultures caused a dramatic increase in the concentration of t11c15 C18:2 (9.84 and 8.45 mg/culture, for treatments LNH and LNL, respectively). The concentrations of VA and c9t11 CLA in rumen cultures were greatest when DHA was incubated with linoleic acid at high pH. In contrast, t10 C18:1 replaced VA and c9t11 CLA disappeared when linoleic and linolenic acids were incubated under the low pH condition. This study shows that the best approach to increase ruminal VA production, and potentially c9t11 CLA levels in ruminant food products, is by combining a DHA source (such as fish oil or algae) and linoleic acid source under high ruminal pH condition.