In vitro rumen fermentation and gas production: influence of yellow grease, tallow, corn oil and their potassium soaps

The effect of addition of fat on in vitro gas production, volatile fatty acid (VFA) production, in vitro true digestibility (IVTD), and ammonia-N concentration was assessed by incubation of a simulated total mixed ration in buffered rumen fluid using an in vitro gas technique. Fat sources were corn oil (CO), tallow (TL), yellow grease (YG), and potassium soaps (K-soaps) of CO, YG, and TL. Addition of YG increased (P<0.001) in vitro gas production while TL had no effect. Neither YG nor TL affected IVTD and total VFA production. Addition of either YG or TL decreased acetate production and increased propionate production. Addition of CO increased in vitro gas production, but had no effect on IVTD and total VFA production. However, CO decreased acetate and increased propionate production with a concomitant decrease in the acetate to propionate ratio. Addition of K-soaps of CO, TL and YG depressed in vitro gas production, IVTD, VFA and ammonia-N concentrations. K-soaps also caused a marked decrease in acetate and increase in propionate production. The absence of negative effects on rumen fermentation parameters in response to the inclusion of fat in the form of triglycerides to an in vitro system at up to 25% DM suggests that triglycerides have much less effect on rumen fermentation parameters than corresponding free fatty acids. In contrast, the K-soaps of each respective fat had detrimental effects on in vitro rumen fermentations.